What We’re Learning about TRAPPIST-1

It’s no surprise that the James Webb Space Telescope’s General Observers program should target TRAPPIST-1 with eight different efforts slated for Webb’s first year of scientific observations. Where else do we find a planetary system that is not only laden with seven planets, but also with orbits so aligned with the system’s ecliptic? Indeed, TRAPPIST-1’s worlds comprise the flattest planetary arrangement we know about, with orbital inclinations throughout less than 0.1 degrees. This is a system made for transits. Four of these worlds may allow temperatures that could support liquid water, should it exist in so exotic a locale.

Image: This diagram compares the orbits of the planets around the faint red star TRAPPIST-1 with the Galilean moons of Jupiter and the inner Solar System. All the planets found around TRAPPIST-1 orbit much closer to their star than Mercury is to the Sun, but as their star is far fainter, they are exposed to similar levels of irradiation as Venus, Earth and Mars in the Solar System. Credit: ESO/O. Furtak.

The parent star is an M8V red dwarf about 40 light years from the Sun. It would be intriguing indeed if we detected life here, especially given the star’s estimated age of well over 7 billion years. Any complex life would have had plenty of time to evolve into a technological phase, if this can be done in these conditions. But our first order of business is to find out whether these worlds have atmospheres. TRAPPIST-1 is a flare star, implying the possibility that any gaseous envelopes have long since been disrupted by such activity.

Thus the importance of the early work on TRAPPIST-1 b and c, the former examined by Webb’s Mid-Infrared Instrument (MIRI), with results presented in a paper in Nature. We learn here that the planet’s dayside temperature is in the range of 500 Kelvin, a remarkable find in itself given that this is the first time any form of light from a rocky exoplanet as small and cool as this has been detected. The planet’s infrared glow as it moved behind the star produced a striking result, explained by co-author Elsa Ducrot (French Alternative Energies and Atomic Energy Commission):

“We compared the results to computer models showing what the temperature should be in different scenarios. The results are almost perfectly consistent with a blackbody made of bare rock and no atmosphere to circulate the heat. We also didn’t see any signs of light being absorbed by carbon dioxide, which would be apparent in these measurements.”

The TRAPPIST-1 work is moving relatively swiftly, for already we have the results of a second JWST program, this one executed by the Max Planck Institute for Astronomy and explained in another Nature paper, this one by lead author Sebastian Zieba. Here the target is TRAPPIST-1 c, which is roughly the size of Venus and which, moreover, receives about the same amount of stellar radiation. That might imply the kind of thick atmosphere we see at Venus, rich in carbon dioxide, but no such result is found. Let me quote Zieba:

“Our results are consistent with the planet being a bare rock with no atmosphere, or the planet having a really thin CO2 atmosphere (thinner than on Earth or even Mars) with no clouds. If the planet had a thick CO2 atmosphere, we would have observed a really shallow secondary eclipse, or none at all. This is because the CO2 would be absorbing all of the 15-micron light, so we wouldn’t detect any coming from the planet.”

Image: This light curve shows the change in brightness of the TRAPPIST-1 system as the second planet, TRAPPIST-1 c, moves behind the star. This phenomenon is known as a secondary eclipse. Astronomers used Webb’s Mid-Infrared Instrument (MIRI) to measure the brightness of mid-infrared light. When the planet is beside the star, the light emitted by both the star and the dayside of the planet reach the telescope, and the system appears brighter. When the planet is behind the star, the light emitted by the planet is blocked and only the starlight reaches the telescope, causing the apparent brightness to decrease. Credits: NASA, ESA, CSA, Joseph Olmsted (STScI)

What JWST is measuring is the 15-micron mid-infrared light emitted by the planet, using the world’s secondary eclipse, the same technique used in the TRAPPIST-1 b work. The MIRI instrument observed four secondary eclipses as the planet moved behind the star. The comparison of brightness between starlight only and the combined light of star and planet allowed the calculation of the amount of mid-infrared given off by the dayside of the planet. This is remarkable work: The decrease in brightness during the secondary eclipse amounts to 0.04 percent, and all of this working with a target 40 light years out.

Image: This graph compares the measured brightness of TRAPPIST-1 c to simulated brightness data for three different scenarios. The measurement (red diamond) is consistent with a bare rocky surface with no atmosphere (green line) or a very thin carbon dioxide atmosphere with no clouds (blue line). A thick carbon dioxide-rich atmosphere with sulfuric acid clouds, similar to that of Venus (yellow line), is unlikely. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI).

I should also mention that the paper on TRAPPIST-1 b points out the similarity of its results to earlier observations of two other M-dwarf stars and their inner planets, LHS 3844 b and GJ 1252 b, where the recorded dayside temperatures showed that heat was not being redistributed through an atmosphere and that there was no absorption of carbon dioxide, as one would expect from an atmosphere like that of Venus.

Thus the need to move further away from the star, as in the TRAPPIST-1 c work, and now, it appears, further still, to cooler worlds more likely to retain their atmospheres. As I said, things are moving swiftly. In the coming year for Webb is a follow-up investigation on both TRAPPIST-1 b and c, in the hands of the system’s discoverer, Michaël Gillon (Université de Liège) and team. With a thick atmosphere ruled out at planet c, we need to learn whether the still cooler planets further out in this system have atmospheres of their own. If not, that would imply formation with little water in the early circumstellar disk.

The paper is Zieba et al., “No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c,” Nature 19 June 2023 (full text). The paper on TRAPPIST-1 b is Greene et al., “Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST,” Nature 618 (2023), 39-42 (abstract).

Part II: Sherlock Holmes and the Case of the Spherical Lens: Reflections on a Gravity Lens Telescope

Part II: Sherlock Holmes and the Case of the Spherical Lens: Reflections on a Gravity Lens Telescope

Aerospace engineer Wes Kelly continues his investigations into gravitational lensing with a deep dive into what it will take to use the phenomenon to construct a close-up image of an exoplanet. For continuity, he leads off with the last few paragraphs of Part I, which then segue into the practicalities of flying a mission like JPL’s Solar Gravitational Lens concept, and the difficulties of extracting a workable image from the maze of lensed photons. The bending of light in a gravitational field may offer our best chance to see surface features like continents and seasonal change on a world around another star. The question to be resolved: Just how does General Relativity make this possible?

by Wes Kelly

Conclusion of Part I

At this point, having one’s hands on an all-around deflection angle for light at the edges of a “spherical lens” of about 700,000 kilometers radius (or b equal to the radius of the sun rS), if it were an objective lens of a corresponding telescope, what would be the value of the focal length for this telescopic component expressed in astronomical units?

The angle of 700,000 km solar radius observed from 1 AU, gives an arcsine of 0.26809 degrees. This is consistent with the rule of thumb solar diameter estimate of ~0.5 degrees.

Expressed in still another way, solar radius from this arcsine measure is 965 arc seconds. When the solar disc itself is observed to be about 1.75 arc seconds in radius, that’s where you will find the focus for this objective lens.

If we take the ratio of 965 to 1.75, we obtain a value 551.5. In other words, a focal point for the relativistic effect at 551.5 AU’s out. Thus, the General Relativity effect implies that light bent by the sun’s gravity near its surface radius is focused about 550 AUs out from the sun. And like the protagonist of Moliere’s 16th century comedy play, as I run off to tell everyone I know, I discover a feeling akin to, “For more than forty years I have been speaking prose while knowing nothing of it.”

This could be a primary lens for a very unwieldy telescope. True, but not unwieldy in all manners. When we consider the magnification power of a telescope system, we speak of the focal length of the objective lens over that for an eye piece or sensor lens focal length. And habitually one might assume it is enclosed in a canister – as most telescopes sold over the counter at hobby stars are. But it is not always necessary or to any advantage. Consider the largest ground-based optical reflectors, or the JWST and radio telescopes. Their objective focal lengths extend through the open air or space. The JWST focal length is 131.4 meters or taller than its Ariane V launch system. Its collected light reaches sensors through a succession of ricochets in its instrumentation package, but not through. a cylindrical conduit extending out from the reflector any significant distance to the front. [Note: The Jupiter deflection case mentioned above would make the focal length 100x longer.]

Continued Discussion

(Tables, Figures and References for Parts I and II are sequential).

In contrast with a 130-meter objective lens focal length, with 550 AU, any focal length for a conventionally manufactured “eyepiece” lens optical system of any size would have enormous magnification or light gathering potential. Were it a lens of 1 or 10 or 100 meter focal length at the instrument end of the telescope, with the “Oort Cloud radius sized” objective lens focal length (550 x 1.5xe8 meters = 8.2 x 10e8 meters) it would not matter much so far as interstellar mapping would be concerned now. We should add as well that the magnification is in terms of area rather than diameter or radius. In effect magnification is multiplication of projected surface area or surface light.

Given the above, issues that remain to be addressed related to the field of view.

1. The spherical lens (the sun) is a light source itself, which needs to be blocked out with a coronagraph on board the SGL spacecraft.

2. The signal obtained from the star (but especially the planet!) is “convoluted” by passage around the perimeter of the solar lens. This must be undone by a deconvolution process.

3. In application for examining an exoplanet in orbit around another star, the fix on the star must be either adjusted to center on the related planetary target or else the planet’s data must be extracted from an enormous extraneous data package.

On issue 1, there are many coronagraph techniques already applied in telescopes for blocking solar or stellar light sources. The Nancy Roman Space Telescope device when launched will be the state of the art and likely to influence SGL coronagraph design. For issue 2, it would be interesting to see a simple illustrative example (e.g., a sphere with a simple pattern such as broad colored latitudinal and longitudinal bands alternating in some patterns… a yellow or green smiley face?), transformed and then converted back. On issue 3, however, I believe that discussion below will provide more immediate insights.

Figure-6 As noted in [7], a meter class telescope with a coronagraph to block solar light is placed In the strong interference region of the solar gravitational lens (SGL) and is capable of imaging an exoplanet at a distance of up to 30 parsecs with a few 10-km scale resolution on its surface. The picture shows results of a simulation of the effects of the SGL on an Earth-like exoplanet image.

Left: Original RGB (red, green, blue) image with a 1024 x 1024 pixel array.

Center: Image blurred by the SGL, sampled at an SNR (signal to noise ratio) of 103 per color channel or overall SNR of 3 x 130.

Right: Result of image deconvolution.

In Reference 7 by Turyshev et al., with Figure-7, potential benefits of an SGL telescope are illustrated with a targeted planet similar to the Earth within a range of 100 light years. What follows is a reference point which we would like to examine as well; in this case, with a specific range (10 parsecs) to illustrate engineering and operational questions, concerns or trades. In archives, see also [ref. A12].

Figure-7 Contrast of benefits illustration with planet observed with an orbital plane in the line of sight of the GLT.

Figure-8 Observation of a target planet with an orbital plane inclined to the line of GLT line of sight.

Left side, with perpendicular to the orbital plane tipped forward, we can observe crescent phases similar to the planets orbiting the sun interior to the Earth, but at low angles, the illuminated exoplanet face is not illuminated. On the aft side of the sun it is in full phase, but perhaps experiencing significant glare. On the right side, with higher inclination, the exoplanet appears as a cat’s eye above the center point; below, as a crescent rotated at a right angle to its path.

What to Do about Slew?

As for deploying a telescope out into the Oort Cloud out to ~550 AUs: This seems explicable and feasible with a combination of conventional propulsion and orbital mechanics taken to a higher state of the art, nuclear thermal, nuclear fusion electric or thermal, sized based on constraints such as mass, mission duration, infrastructure and finance. It is assumed here, by this aerospace engineer, that trajectory, propulsion, navigation and guidance issues of deployment with resources not yet available but will be with larger spacecraft assembled and tested in the future. However, operational issues of this baseline or reference mission, I would still like to explore. In pursuit of this, we will add a reference target (perhaps the first of an enlarging set), an exoplanet similar to the earth in a solar system similar to ours at a viewing distance used to set stellar absolute magnitudes, a distance of ten parsecs.

Now if a stellar system were ten parsecs away or 32.26 light years off, the maximum radial offset of an Earth-like planet from a Sol like star (1 AU) would be 0.1 arc seconds. Hence, the Earth analog would be in the “nominal” field of view (FOV) but the FOV would encompass a radius of 175 AUs – If the center of the nominal FOV can be considered the center of the target star. The stellar absolute magnitude measure distance (10 parsecs ) is a middle distance for this exercise and a parsec (3.23 light years), also basic to astronomy, could be considered a minimum just below Alpha Centauri distance (4.3 light years).

However, FOV behind the sun used for now, might be misleading or unclear in these circumstances. Because it is not clear to me how much of the blocked celestial sphere is transferred back via the gravity lens phenomenon. In this analysis, without full understanding of how the coronagraph or convolutions will work, I am unsure whether there is any control over what the steradian field behind the sun will be; whether it can be entirely controlled. Focusing on the star could provide all the 175 AU radius in the field of view, or some fraction thereof. But if centering on a planetary target can limit the wasted scan area, I highly recommend such.

For argument’s sake, of this celestial “blockage” region, it could range from the infinitesimal to the whole. The image obtained might be treated akin to a point source from which we might extract image data somewhat akin to extracting the spectrum of a similar un-dimensioned source. Or there might be several different deconvolution methods which provide options. But the aspect that concerns me here is how one searches for a point source in this so-called FOV, more characterized by the blockage of the sun’s angular width. The FOV might be described as an area within an FOB, a field of blockage. Whether discerned directly without need of a deconvolution – or not – at ten parsec distance the field of “blockage” (FOB?) would include a radius of 175.5 AUs in the 17.5 arcsecond maximum field of view.

The diameter of a G2V sun like ours is about 0.01 AU and a terrestrial planet like our own is 0.01 of that. And then what kind of transformation or convolution would be required to take the information from the other side and convert it back into an image? An image we would recognize as a planet with continents, oceans and clouds. Not knowing for sure, I suspected that if the position of the target planet were known, it would make more sense to focus the telescope on it rather than the star itself. On the other hand, if obtaining a coronagraphic blocking of the star required centering on the star, and capturing the planet required processing the thick ring around the star, then the total amount of data processing could become enormous – as the following table shows.

In terms of terrestrial planet viewed area vs. that of the 1 AU radius region and the 175 AU radius encompassing the entire celestial pane blocked by the sun, the ratios are 1 to 500 million and 168 billion respectively. Depending on the resolution sought for the planetary analysis ( e.g., 10 kilometer features distinguishable), then data bits characterizing individual “squares” of smaller dimensions must be processed. For present purposes, we can select ten kilometers for illustration.

Table-3 Scanning the entire field of FOV of a target at 10 parsecs and for an exoplanet similar to the Earth orbiting a G2V star. At the distance selected for calibrating stellar absolute magnitude (about 33 light years) and a GLT placed 550 AUs from our sun, the geometrical area blocked by the solar disc is a region 17.5 AUs in radius or 17.5 arc seconds wide at 1 arc second distance, 10x wider at 10 parsecs. The sun-like star diameter ~ 0.01 AUs and an exoplanet Earth about 1/100th of that or 0.0001 AU wide.

As the NIAC Phase II Report and AIAA journal article [7 and 8] indicate, targeted resolution objectives are on the order of 10 kilometers, indicative of sampling cells of lower dimensions. A one-kilometer-wide sample cell we select for sake of argument. However, with each observed cell, the GLT telescope instrument suite will include 3 -5 color band sweeps (e.g., ultraviolet, blue, yellow, red, infra-red) which would include intensity levels. A spectrometer could also seek evidence of discrete spectral lines or molecular bands. So, for each square kilometer scanned, there could be considerable binary coded data for the telemetry link. More than one data-bit for sure associated with each polygon of space scanned by the SGL telescope. If each polygon has a location defined in a 2-dimensional grid, then that point likely has two 32 or 64-bit position assignments; then each color filter has an intensity. In addition, if spectral lines are tracked another databit code will be assigned to that point as well.

Processing the FOV indiscriminately with focus on the star is like searching for a needle (or data) in a haystack. Tracking the planet itself could eliminate orders of magnitude of excess data processing. On the other hand, slewing at 550 AU circular orbit entails 40,000 km magnitude oscillations over a year to follow the target, distances equivalent to a tenth of Earth-Moon separation, but an expenditure of propulsive resources. Consequently, this would become at least one resource trade between data handling and maneuverability. One possible solution would be multiple telescopes formation flying over “seasonal” tracking points a quarter of orbital revolution apart in the projected orbital track.

The scenario for deploying the telescope assumes considerable outbound velocity accumulated in the form of continuous low thrust acceleration. Consequently, on station a very large radial velocity will remain. Remarkably, at 550 AU distance, circular orbit velocities are still over a kilometer per second ( e.g., Earth orbital about 29.7 km/sec over square root of 550, about 1.27 km/sec). With the Earth-based example at 10 parsecs and the requirement to cover 40,000 km back and forth within about 6 months, the corresponding constant velocity would be 0.0025 km/sec to hold the alignment. This type of slewing would work better with a more rapidly orbiting exoplanet located in the HZ of a red dwarf. But the M star case would require more frequent reverses of direction. Significantly, were we to do this exercise for a target at 1 parsec such as the Alpha Centauri stars, the oscillations would be ten times larger (400,000 km) or about the distance to the moon.

Additionally, the rotation rate about the planetary axis could be star synchronous or, as with the Earth or Mars, much faster than the orbital revolution. There could be moons in its near vicinity. All these are natural considerations for a habitable zone exoplanet survey. And reasons that features on the exoplanet surface could become blurred. Other cases would generate different requirements, no doubt. And all this will affect how long it will take to process square kilometer data sets into each of their relevant maps.

Beside stellar glare, galactic background needs to be considered too. A dark field behind the target star would be preferable as well, achieving a higher signal to noise ratio. It would be a shame if threshold levels for observing a planet vs. magnified stellar backgrounds could not be assessed prior to flight. A potential problem making out the planet against the background would make a planetary ephemeris important; linkage to home base guide telescopes directing the GLT pointing, where in a sense the GLT will be blind. We have discussed just an Earth analog so far, but HZ targets at cooler K and M stars as well as hotter F main sequence stars could possess eye-opener properties too.

Several decades ago, during an undergraduate satellite design project I participated as the communications engineer – and then space navigation assignments called on putting on that hat again. An interesting experience each time and I found some overall equations that formulated relations among distance, signal to noise thresholds, signal rates and power required to stay in touch at both ends, spacecraft and the Deep Space Network. Unfortunately, I lost our first team’s final report in a flood, not of information like that discussed, but of tropical storm water. But it is not necessary to reconstruct the methods found then. At this date there is now an old literature base for communications with spacecraft situated in deep space, thanks to publications of the Jet Propulsion Laboratory, illustrative examples such as Voyager and other Jupiter bound spacecraft, even earlier spacecraft examined as if they were beaming from there and received with network capabilities of a given epoch (see Figure 9).

Figure-9 Figure-9 A diagram from Ref. 5 pegs down one end of the trade issues, chronological increases in data rates obtained from spacecraft in Jupiter vicinity. Reception is associated with 5.2 AU distance from the sun (varying with the Earth) vs. the 550 AUs or more anticipated for the GLT. On one axis, acquisition data rates are shown. For each spacecraft that sets out on these Jovian missions (some, of course, actually did not), a liftoff limit on power or data rate can be assumed for the spacecraft or observatory. Once launched, most of the growth was likely at the Earth based part of the communication link.

In comparison with attenuation of signals from the Jovian system at 5.2 AU for the various systems shown in the Figure-9 JPL diagram, signals 100x further out will be decreased in strength to ~1/10,000th or less with movement beyond 550 AU. Consequently, data rates shown in the diagram for various extent technologies will be dropped by a factor of 1/10,000th or 1.0 e-04 as well.

Depending on when such an SGL space observatory will be launched, some technologies will improve data transmission rates or storage capacities with respect to mass density or power required. Other technologies likely will not experience similar trends. For example, it is unclear what new Deep Space Network type tracking facilities will be employed in support of the SGL mission. However, if the data load is driven by a full scan of the equivalent of the solar angular area or FOV, the spacecraft system requirements for data storage and transmission are increased enormously.

On the other hand, as shown, slewing from the stellar focal point to a planetary position will require propellant resources and attitude control increases over those for the stellar fix. Even at 550 AU, there is a 1.28 km/sec characteristic circular orbit velocity. And depending on time of flight to outpost station delivery, in coast the spacecraft can be considered on an extremely hyperbolic heliocentric path. Consequently, fixed on a target planet, low thrust would be required without planet tracking even to maintain stellar focus.

My own quick assessment is that narrow field of view scanning in the planetary vicinity as it tracks around the star in some arbitrary orbital plane is the better procedure. The actual orbital plane’s normal could be inclined by some angle to the line of sight (See Figure 8). Hence, a circular path would be perceived as an elliptical projection; more complex if actually eccentric to a considerable fraction. But with a mean likelihood of 45-degree inclination and circular orbit, half phases would appear at greatest stellar elongation. Near the line of sight, a cat’s-eye would appear behind the star and a crescent in front with lowest elongation and greatest glare. With zero inclination of the planet, we are bound to learn much about its northern hemisphere and much less about its south, depending on its rotational axis alignment.

Now if a stellar system were ten parsecs away or about 33 light years off, the maximum radial offset of an Earth-like planet from a Sol like star (1 AU) would be 0.1 arc seconds. Hence, the Earth analog would be in the “nominal” field of view (FOV) but the FOV would encompass a radius of 17.5 AUs – If the center of the nominal FOV can be considered the center of the target star.

FOV, used for now, might be misleading in these circumstances. Because it is not clear to me how much of the blocked celestial sphere is transferred back via the gravity lens phenomenon.

For argument’s sake, of this celestial “blockage” region, it could range from the infinitesimal to the whole. The image obtained might be treated akin to a point source from which we might extract image data somewhat akin to extracting the spectrum of a similar un-dimensioned source. So the aspect that confuses me here is how one searches for a point source in this so-called FOV, more characterized by the blockage of the sun’s angular width. The FOV might be described as an area within an FOB, a field of blockage.

In this situation, there would have to be some fore-knowledge of where the target planet should be. You would need a tracker observatory probably closer to Terra home. You still need a means to locate a body orbiting an object about a hundredth of an AU in diameter and in turn a planet about 1/10,000 of an AU wide. To relay information from a stellar observatory not experiencing this occultation by the sun to 550 AUs out, the lag would be about 3.174 days based on the speed of light.

And then, presumably, the observatory would need to slew toward this planetary target from the reference point of the stellar primary – or perhaps even the center of mass in a binary system. Alpha Centauri could be such an example.

A Mission for One Star System and Exoplanet or More?

Additional trade issues to consider are related to completion of observation and characterization of one planetary system. Perhaps there is more than one planet (or a moon) in a target system to study. But there is also the issue of observing more than one planetary system. Minimal angular separation of two “good” candidate systems in the celestial sphere would have to be weighed against the “excellence” of an isolated stellar system with no potential for a phase II mission elsewhere, say within one degree of circular arc. Faced with such a dilemma I would hope that observing the isolated system over years until system deactivation will be well worthwhile.

At this writing we are aware of about 5000 exoplanets with attributable features, providing a range of reasons for continued or closer observation. Like the other design issues described above, eventually there will be the dilemma of which exoplanet or planets to select.

In terms of steradians, the whole celestial sphere has an area of 4 ? units. With some experimentation I discover that it is possible to determine the equidistant position of any number of stars – which can illustrate the dilemma of deciding how to deploy the SGL Telescope. The celestial arc A between equally spaced stars of a given number n can be described with the answer in radians convertible to degrees. Once n equals or exceeds 3, the equidistant points can be viewed as vertices to equilateral and equiangular spherical triangles of given arc segments, the latter the significant parameter. The total of 5000 exoplanets is not distributed with an equal spacing, but there is an element of likelihood with the fractional degree separation overall. And, of course, a smaller selection of select exoplanet systems will have wider individual separations overall, but perhaps a few will be less than a degree apart. For the case of ten parsec distant planetary system we noted that a traverse to cover 17.5 AUs encompassed about 400,000 kilometers at 550 AU. A one degree traverse is 205 times as large but it does not have tracking determined maneuver velocity requirements.

It is likely that by some set of selected parameters, several exoplanets can be selected for further scrutiny. However, if several parameters are involved and a couple of candidates or alternates can be identified in proximity, it is possible that two close star systems could outscore focus on one, even if it generally acknowledged the best, but located on the wrong side of the sky for total mission benefit.

Consequently, the mission analysis could become more complicated as time passes with a larger and larger selection of nearby systems identified with one or more planets.

What would the parameters tend to be to warrant such a trade? Even if there is no evidence of life, an exoplanet of exceptional nature could transcend parameters associated with habitable zone parameters or signs of life. And for examination of signs of life, our knowledge will have to exceed such identifications as diameter, albedo and placement in a habitable zone: atmospheric composition, nature of hydrosphere, traces of processes similar to terrestrial ones… Cost benefit issues too of propulsion and maneuver to survey two planets would need to have an identifiable threshold against the additional spacecraft weight budget for propulsion. If the two candidate systems are far apart, then the choice might be easier in a way if it requires launching two distinct missions.

Whether going for two exoplanets separated by a degree or more is worthwhile, is difficult to ascertain at this early stage. But the determination will depend on establishing criteria for a trade. To first order it will depend on how outstanding signs of life might be within a future database of exoplanets. And if not clear, which parameters of an exoplanet or a maneuverable spacecraft should be considered and with what weight. Reflecting on an earlier orbital application proposal, Arthur C. Clarke suggested geosynchronous orbit for a single communications relay station, elaborated as a call center with humans at switchboards. Instead, we have numerous geosats with no one aboard. It could be that SGL spacecraft will proliferate similarly and for several purposes. At the very least, we can be thankful to be able to consider such possibilities, coming from a time decades back when exoplanets were simply considered fantasy like Spock’s planet – or more locally – Lescarbault’s and Le Verrier’s Vulcan.

References for Part I and Part II

1.) Pais, Abraham, Subtle is the Lord … The Science and Life of Albert Einstein, Oxford University Press, 1982.

2.) https://www.stsci.edu/jwst/science-execution/observing-schedules

3.) Vallado, David A., Fundamentals of Astrodynamics and Applications, 2nd edition, Appendix D4, Space Technology Library, 2001.

4.) Moulton, Forest Ray, An Introduction to Celestial Mechanics, 2nd Edition, Dover, 1914 Text.

5.) Taylor, Jim et al. Deep Space Communications, online at https://descanso.jpl.nasa.gov/monograph/series13_chapter.html

6.) Wali, Kamshwar, C., Chandra – A Biography of S. Chandrasekhar, U. of Chicago Press, 1984.

7.) Turyshev et al., ”Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report, NASA Innovative Advanced, Concepts (NIAC) Phase II.

8.) Helvajian, H. et al., “Mission Architecture to Reach and Operate at the Focal Region of the Solar Gravitational Lens,” Journal of Spacecraft and Rockets, American Institute of Aeronautics and Astronautics (AIAA), February 2023, on line pre-print.

9.) Xu, Ya et al., ”Solar oblateness and Mercury’s perihelion precession”, MNRAS, 415, 3335-3343, 2011.

A1.) Archives: In the Days before Centauri Dreams… An Essay by WDK (centauri-dreams.org)

A2.) Archives: A Mission Architecture for the Solar Gravity Lens (centauri-dreams.org)

Here in Houston, the University of Houston, Clear Lake Physics and Astronomy Club had a recent meeting when the sky was obscured by clouds and the president had asked in advance, just in case of such circumstances, would I have any presentation I could give that night. There were some other ones that had grown all out of control, so I decided to start on a fresh topic. This article grew out of the evening presentation and consequently, it is dedicated to the club and its members.

13 April 23


Ring of Life? Terminator Habitability around M-dwarfs

It would come as no surprise to readers of science fiction that the so-called ‘terminator’ region on certain kinds of planets might be a place where the conditions for life can emerge. I’m talking about planets that experience tidal lock to their star, as habitable zone worlds around some categories of M-dwarfs most likely do. But I can also go way back to science fiction read in my childhood to recall a story set, for example, on Mercury, then supposed to be locked to the Sun in its rotation, depicting humans setting up bases on the terminator zone between broiling dayside and frigid night.

Addendum: Can you name the science fiction story I’m talking about here? Because I can’t recall it, though I suspect the setting on Mercury was in one of the Winston series of juvenile novels I was absorbing in that era as a wide-eyed kid.

The subject of tidal lock is an especially interesting one because we have candidates for habitable planets around stars as close as Proxima Centauri, if indeed a possibly tidally locked planet can sustain clement conditions at the surface. Planets like this are subject to extreme conditions, with a nightside that receives no incoming radiation and an irradiated dayside where greenhouse effects might dominate depending on available water vapor. Even so, moderate temperatures can be achieved in models of planets with oceans, and most earlier work has gone into modeling water worlds. I also think it’s accurate to say that earlier work has focused on how habitable conditions might be maintained in the substellar ‘eye’ region directly facing the star.

But what about planets that are largely covered in land? It’s a pointed question because a new study in The Astrophysical Journal finds that tidally locked worlds mostly covered in water would eventually become saturated by a thick layer of vapor. The study, led by Ana Lobos (UC-Irvine) also finds that plentiful land surfaces produce a terminator region that could well be friendly to life even if the equatorial zone directly beneath the star on the dayside should prove inhospitable. Says Lobo:

“We are trying to draw attention to more water-limited planets, which despite not having widespread oceans, could have lakes or other smaller bodies of liquid water, and these climates could actually be very promising.”

Image: Some exoplanets have one side permanently facing their star while the other side is in perpetual darkness. The ring-shaped border between these permanent day and night regions is called a “terminator zone.” In a new paper in The Astrophysical Journal, physics and astronomy researchers at UC Irvine say this area has the potential to support extraterrestrial life. Credit: Ana Lobo / UCI.

The team’s modeling simulates both water-rich and water-limited planet scenarios, even as the question of how much water to expect on a habitable zone M-dwarf planet remains open. After all, water content likely depends on planet formation. If a habitable zone planet formed in place, it likely emerged with lower water content than one that formed beyond the snowline (relatively close in for M-dwarfs) and migrated inward. We also have to remember that flare activity could trigger water loss for such worlds.

Water’s effects on climate are abundant, from affecting surface albedo to the production of clouds and the development of greenhouse effects. They’re also tricky to model when we move into other planetary scenarios. As the paper notes:

Due to water’s various climate feedbacks and its effects on the atmospheric structure, the habitable zone of a water-limited Earth twin is broader than that of an aquaplanet Earth (Abe et al. 2011). But while water’s impact on climate is well understood for Earth, many of these fundamental climate feedbacks behave differently on M-dwarf planets, due to the lower frequency of the stellar radiation.

To perform the study, Lobo’s team considered a hypothetical Earth-class planet orbiting the nearby star AD Leonis (Gliese 388), an M3.5V red dwarf, using a 3D global climate model to find out whether a tidally locked world here could sustain a temperature gradient large enough to make the terminator habitable. The study uses a simplified habitability definition based solely on surface temperature. The researchers deployed ExoCAM, a modified version of the Community Atmosphere Model (CAM4) developed by the National Center for Atmospheric Research and used to study climate conditions on Earth. Their software tweaked the original code to adjust for factors such as planetary rotation.

The results are straightforward: With abundant land on the planet, terminator habitability increases dramatically. A water-rich world like Earth, with land covering but 30 percent of the surface, is not necessarily the best model for habitability here, as we consider the factors involved in tidal lock, with extensive land offering viable options in at least part of the surface. A ‘ring’ of habitability may prove to be a common outcome for such worlds. But it’s interesting to consider how these initial conditions might complicate the early development of biology. Here I return to the paper:

There are still many uncertainties regarding the water content of habitable-zone M-dwarf planets. Based on our current understanding, it is possible that water-limited planets could be abundant and possibly more common than ocean-covered worlds. Therefore, terminator habitability may represent a significant fraction of habitable M-dwarf planets. Compared to the temperate climates obtained with aquaplanets, terminator habitability does offer reduced fractional habitability. Also, while achieving a temperate terminator is relatively easy on water-limited planets, constraining the water availability at the terminator remains a challenge. Overall, the lack of abundant surface water in these simulations could pose a challenge for life to arise under these conditions, but mechanisms, including glacier flow, could allow for sufficient surface water accumulation to sustain locally moist and temperate climates at or near the terminator.

The paper is Lobo et al., “Terminator Habitability: The Case for Limited Water Availability on M-dwarf Planets,” Astrophysical Journal Vol. 945, No. 2 (16 March 2023), 161 (full text).


Interstellar Research Group: 8th Interstellar Symposium Second Call for Papers

Abstract Submission Final Deadline: April 21, 2023

The Interstellar Research Group (IRG) in partnership with the International Academy of Astronautics (IAA) hereby invites participation in its 8th Interstellar Symposium, hosted by McGill University, to be held from Monday, July 10 through Thursday, July 13, 2023, in Montreal, Quebec, Canada. This is the first IRG meeting outside of the United States, and we are excited to partner with such a distinguished institution!

Topics of Interest

Physics and Engineering

Propulsion, power, communications, navigation, materials, systems design, extraterrestrial resource utilization, breakthrough physics


Exoplanet discovery and characterization, habitability, solar gravitational focus as a means to image exoplanets

Human Factors

Life support, habitat architecture, worldships, population genetics, psychology, hibernation, finance


Sociology, law, governance, astroarchaeology, trade, cultural evolution


Technosignature and biosignature identification, SETI, the Fermi paradox, von Neumann probes, exoplanet terraformation

Submissions on other topics of direct relevance to interstellar travel are also welcome. Examples of presentations at past symposia can be found here:

Confirmed Speakers

Dr. Stephen Webb (University of Portsmouth)
“Silence is Golden: SETI and the Fermi Paradox”

Dr. Kathryn Denning (York University)
“Anthropological Observations for Intestellar Aspirants”

Dr. Rebecca M. Rench (Planetary Science Division, NASA Headquarters)
“The Search for Life and Habitable Worlds at NASA: Past, Present and Future”

Dr. Frank Tipler (Tulane University)
“The Ultimate Rocket and the Ultimate Energy Source and their Use in the Ultimate Future”

Contributed Plenary Lectures

The primary submissions for the Interstellar Symposium are plenary lectures. The lectures will be approximately 20 minutes in length and be accompanied by a manuscript prior to the Symposium. The early bird deadline for abstract submission, which ensures expedited consideration and notification of acceptance, is January 15, 2023. Submitted abstracts will continue to be considered until April 21, 2023, if space in the program permits. The submitted abstract should follow the format described in the Abstract Submission section below. Abstracts should be emailed to: registrar@irg.space

No Paper, No Podium: Contributed plenary lectures are to be accompanied by a written paper, with an initial draft due June 23, 2023. You will have an opportunity to revise and extend your draft before the publication deadline of September 8, 2023. If a paper is not submitted by the final manuscript deadline, authors will not be permitted to present their work. Papers should be original work that has not been previously published.

Work in Progress Posters

Contributors wishing to present projects still in progress or at a preliminary stage may submit an abstract for a Work in Progress poster presentation. The deadline for abstract submission for Work in Progress posters is May 20, 2023. The abstract describing the work to be presented should follow the format described in the Abstract Submission section below. The poster should not exceed 36 inch (width) by 48 inch (height). The presenters are responsible for printing their own posters and would need to bring their poster to the Interstellar Symposium. Abstracts should be emailed to: registrar@irg.space

Sagan Meetings

An interested Sagan Meeting organizer is given the option to define a particular question for an in-depth panel discussion. The organizer would be responsible for inviting five speakers to give short presentations staking out a position on a particular question. These speakers will then form a panel to engage in a lively discussion with the audience on that topic. Carl Sagan famously employed this format for his 1971 conference at the Byurakan Observatory in old Soviet Armenia, which dealt with the Drake Equation. A one-page description (format of your choosing) of the panel topic, the questions to be addressed, and the suggested panel members should be emailed by January 15, 2023 to: registrar@irg.space


Seminars are 3-hour presentations on a single subject, providing an in depth look at that subject. Seminars are held before the Symposium begins, on Sunday, July 9, 2023, with morning and afternoon sessions. The content must be acceptable to be counted as continuing education credit for those holding a Professional Engineer (PE) certificate.

Other Content

Other content includes, but is not limited to, posters, displays of art or models, demonstrations, panel discussions, interviews, or public outreach events. IRG recognizes the importance of a holistic human cultural experience and encourages the submission of non-academic works to be involved with the symposium program.


The IRG serves as a critical incubator of ideas for the interstellar community. Following the success of the 7th Interstellar Symposium, papers may be submitted for consideration in publication within a special issue of Acta Astronautica. Papers from the 7th Symposium (September 2021) have now been published in the August 2022 issue of Acta Astronautica. Contributors who wish to publish their papers elsewhere may do so. Abstracts and papers not published elsewhere will be compiled into a complete Symposium proceedings in book form.

Video and Archiving

All symposium events may be captured on video or in still images for use on the IRG website, in newsletters and social media. All presenters, speakers, and selected participants will be asked to complete a Release Form that grants permission for IRG to use this content as described.

Abstract Submission

Abstracts for the 8th Interstellar Symposium must relate to one or more of the many interstellar mission related topics. The previously listed topics are not exclusive but represent a cross-section of possible categories. All abstracts must be submitted online via email to: registrar@irg.space.

Acceptable formats are text, Microsoft Word, and PDF only. Submissions of Contributed Plenary Lectures and Work in Progress Posters must follow the format described below.

Presenting Author(s)

Please list only the author(s) who will actually be in attendance and presenting at the conference. (First name, last name, degree – for example, Susan Smith, MD)

Additional Author(s)

List all authors here, including Presenting Author(s) – (first name, last name, degree(s) – for example, Mary Rockford, RN; Susan Smith, MD; John Jones, PhD)


Abbreviations within the body should be kept to a minimum and must be defined upon first use in the abstract by placing the abbreviation in parenthesis after the represented full word or phrase. Non-proprietary (generic) names should be used.

Abstract Length

The entire abstract (excluding title, authors, presenting author’s institutional affiliation(s), city, state, and text) including any tables or figures should be a maximum of 350 words. It is your responsibility to verify compliance with the length requirement.

Abstract Structure

Abstracts must include the following headings:

  • Title = The presentation title.
  • Background = Describes the research or initiative context.
  • Objective = Describes the research or initiative objective.
  • Methods = Describes research methodology used. For initiatives, describes the target population, program or curricular content, and evaluation method.
  • Results – Summarizes findings in sufficient detail to support the conclusions.
  • Conclusion – States the conclusions drawn from results, including their applicability.

Questions and responses to this call for papers, workshops, and participation should be directed to:


For updates on the meeting, speakers, and logistics, please refer to the website:



Chasing nomadic worlds: Opening up the space between the stars

Ongoing projects like JHU/APL’s Interstellar Probe pose the question of just how we define an ‘interstellar’ journey. Does reaching the local interstellar medium outside the heliosphere qualify? JPL thinks so, which is why when you check on the latest news from the Voyagers, you see references to the Voyager Interstellar Mission. Andreas Hein and team, however, think there is a lot more to be said about targets between here and the nearest star. With the assistance of colleagues Manasvi Lingam and Marshall Eubanks, Andreas lays out targets as exotic as ‘rogue planets’ and brown dwarfs and ponders the implications for mission design. The author is Executive Director and Director Technical Programs of the UK-based not-for-profit Initiative for Interstellar Studies (i4is), where he is coordinating and contributing to research on diverse topics such as missions to interstellar objects, laser sail probes, self-replicating spacecraft, and world ships. He is also an associate professor of space systems engineering at the University of Luxembourg’s Interdisciplinary Center for Security, Reliability, and Trust (SnT). Dr. Hein obtained his Bachelor’s and Master’s degree in aerospace engineering from the Technical University of Munich and conducted his PhD research on space systems engineering there and at MIT. He has published over 70 articles in peer-reviewed international journals and conferences. For his research, Andreas has received the Exemplary Systems Engineering Doctoral Dissertation Award and the Willy Messerschmitt Award.

by Andreas Hein

If you think about our galaxy as a vast ocean, then the stars are like islands in that ocean, with vast distances between them. We think of these islands as oases where the interesting stuff happens. Planets form, liquid water accumulates, and life might have emerged in these oases. Until now, interstellar travel has been primarily thought in terms of dealing with how we can cross the distances between these islands and visit them . This is epitomized by studies such as Project Daedalus and most recently Breakthrough Starshot, Project Daedalus aiming at reaching Barnard’s star and Breakthrough Starshot at Proxima Centauri. But what if this thinking about interstellar travel has missed a crucial target until now? In this article, we will show that there are amazing things hidden in the ocean itself – the space between the stars.

It is frequently believed that the space between the stars is empty, although this stance is incorrect in several ways, as we shall elucidate. The interstellar community is firmly grounded in this belief. It is predominantly focused on missions to other star systems and if we talk about precursors such as the Interstellar Probe, it is about the exploration of the interstellar medium (ISM), the incredibly thin gas long known to fill the spaces between the stars, and also features of the interaction between the ISM and our solar wind, such as the heliosheath, or with its interaction with microscopic physical objects or phenomena linked to our solar system. However, no larger objects between the stars are taken into account.

Image: Imaginary scenario of an advanced SunDiver-type solar sail flying past a gas-giant nomadic world which was discovered at a surprisingly close distance of 1000 astronomical units in 2030 by the LSST. The subsequently launched SunDiver probes spotted several potentially life-bearing moons orbiting it. (Nomadic world image: European Southern Observatory; SunDivers: Xplore Inc.; Composition: Andreas Hein).

Today, we know that the space between the stars is not empty but is populated by a plethora of objects. It is full of larger flotsam and smaller “driftwood” of various types and different sizes, ejected by the myriads of islands or possibly formed independently of them. Each of them might hold clues to what its island of origin looks like, its composition, formation, and structure. As driftwood, it might carry additional material. Organic molecules, biosignatures, etc. might provide us with insights into the prevalence of the building blocks of life, and life itself. Most excitingly, some important discoveries have been made within the last decade which show the possibilities that could be obtained by their exploration

In our recent paper (Lingam, M., Hein, A.M., Eubanks, M. “Chasing Nomadic Worlds: A New Class of Deep Space Missions”), we develop a heuristic for estimating how many of those objects exist between the stars and, in addition, we explore which of these objects we could reach. What unfolds is a fascinating landscape of objects – driftwood and flotsom – which reside inside the darkness between the stars and how we could shed light on them. We thereby introduce a new class of deep space missions.

Let’s start with the smallest compound objects between the stars (individual molecules would be the smallest objects). Instead of driftwood, it would be better to talk about sawdust. Meet interstellar dust. Interstellar dust is tiny, around one micrometer in diameter, and the Stardust probe has recently collected a few grains of it (Wetphal et al., 2014). It turns out that it is fairly challenging to distinguish between interstellar dust and interplanetary dust but we have now captured such dust grains in space for the first time and returned them to Earth.

The existence of interstellar dust is well-known, however, the existence of larger objects has only been hypothesized for a long time. The arrival of 1I/’Oumuamua in 2017 in our solar system changed that; 1I is the first known piece of driftwood cast up on the beaches of our solar system. We now know that these larger objects, some of them stranger than anything we have seen, are roaming interstellar space. There is still an ongoing debate on the nature of 1I/’Oumuamua (Bannister et al., 2019; Jewitt & Seligman, 2022). While ‘Oumuamua was likely a few hundred meters in size (about the size of a skyscraper), larger objects also exist. 2I/Borisov, the second known piece of interstellar driftwood, was larger, almost a kilometer in size. In contrast to ‘Oumuamua, it showed similarities to Oort Cloud objects (de León et al., 2019). The Project Lyra team we are part of has authored numerous papers on how we can reach such interstellar objects, even on their way out of the solar system, for example, in Hein et al. (2022).

Now comes the big driftwood – the interstellar flotsam. Think of the massive rafts of tree trunks and debris that float away from some rivers during floods. We know from gravitational lensing studies that there are gas planet-sized objects flying on their lonely trajectories through the void. Such planets, unbound to a host star, are called rogue planets, free-floating planets, nomads, unbound, or wandering planets. They have been discovered using a technique called gravitational microlensing. Planets have enough gravity to “bend” the light coming from stars in the background, focusing the light, brightening the background star, and enabling the detection even of unbound planets.

Until now, about two hundred of these planets (we will call them nomadic worlds in the following) have been discovered through microlensing. These detections favor the more massive bodies, and so far objects with a large mass (Jupiter-sized down to a few Earth masses) have been detected. Although our observational techniques do not yet allow us to discover smaller nomadic worlds (the smallest ones we have discovered are a few times heavier than the Earth), it is highly likely that smaller objects, say between the size of the Earth and Borisov, exist. Fig. 1 provides an overview of these different objects and how their radius is correlated with the average distance between them according to our order of magnitude estimates. Note that microlensing is good at detecting planets at large interstellar distances, even ones thousands of light years away, but it is very inefficient (millions of stars are observed repeatedly to find one microlensing event), and with current technology is not likely to detect the relative handful of objects closest to the Sun.

Fig. 1: Order of magnitude estimates for the radius and average distance of objects in interstellar space

We have already explored how to reach interstellar objects (similar to 1I and 2I) via Project Lyra. What we wanted to find out in our most recent work is whether we can launch a spacecraft towards a nomadic world using existing or near-term technology and reach it within a few decades or less. In particular, we wanted to find out whether we could reach nomadic worlds that are potentially life-bearing. Some authors have posited that nomadic worlds larger than 100 km in radius may host subsurface oceans with liquid water (Abramov & Mojzsis, 2011), and larger nomadic planets certainly should be able to do this. Now, although small nomadic worlds have not yet been detected, we can estimate how far such a 100 km-size object is from the solar system on average. We do so by interpolating the average distance of various objects in interstellar space, ranging from exoplanets to interstellar objects and interstellar dust. The size of these objects spans about 13 orders of magnitude. The result of this interpolation is shown in Fig. 2. We can see that ~100 km-sized objects have an average distance of about 2000 times the distance between the Sun and the Earth (known to astronomers as the astronomical unit, or AU).

Fig. 2: Radius of nomadic world versus the estimated average distance to the object

This is a fairly large distance, over 400 times the distance to Jupiter and about five times farther away than the putative Planet 9 (~380 AU) (Brown & Batygin, 2021). It is important to keep in mind that this is a rough statistical estimate for the average distance, meaning that the ~100 km-sized objects might be discovered much closer or farther away than the estimate. However, in the absence of observational data, such an estimate provides us with a starting point for exploring the question of whether a mission to such an object is feasible.

We use such estimates to investigate further whether a spacecraft with an existing or near-term propulsion system may be capable of reaching a nomadic world within a timeframe of 50 years. The result can be seen in Table 1.

Table 1: Average radius of nomadic world reachable with a given propulsion system in 50 years

It turns out that chemical propulsion combined with various gravity assist maneuvers is not able to reach such objects within 50 years. Solar sails and magnetic sails also fall short, although they come close (~75 km radius of nomadic object).

However, electric sails seem to be able to reach nomadic worlds close to the desired size and already have a reasonably high technology readiness level. Electric sails exploit the interaction between charged wires and the solar wind. The solar wind consists of various charged particles such as protons which are deflected by the electric field of the wires, leading to a transfer of momentum, thereby accelerating the sail. Proposed by Pekka Janhunen in 2004 (Janhunen, 2004), electric sails have also been considered for interplanetary travel and even into interstellar space (Quarta & Mengali, 2010; Janhunen et al., 2014). Up to 25 astronomical units (AU) per year seem to be achievable with realistic designs (Janhunen & Sandroos, 2007). Electric sail prototypes are currently being prepared for in-space testing (Iakubivskyi et al., 2020). Previous attempts to deploy an electric sail by the ESTCube-1 CubeSat mission in 2013 and Aalto-1 in 2022 were not successful (Slavinskis et al., 2015; Praks et al., 2021).

It turns out that more advanced propulsion systems are required, if we want to have a statistically good chance of reaching nomadic worlds significantly larger than 100 km radius. Laser electric propulsion and magnetoplasmadynamic (MPD) thrusters would get us to objects of 150 and 230 km respectively. Laser electric propulsion uses lasers to beam power to a spacecraft with an electric propulsion system, thereby removing a key bottleneck of providing power to an electric propulsion system in deep space (Brophy et al., 2018). MPD thrusters would be capable of providing high specific impulse and/or high thrust (the VASIMR engine is an example), although it remains to be seen how sufficient power can be generated in deep space or sufficient velocities be reached in the inner solar system by solar power.

Reaching even larger objects (i.e., getting to significantly further distances) requires propulsion systems which are potentially interstellar capable: nuclear fusion and laser sails, as the closest such objects might be at distances of as much as a light year off. These propulsion systems could even reach nomadic worlds of a similar size as Earth, nomadic worlds comparable to those we have already discovered. The average distance to such objects should still be a few times smaller than the distance to other star systems (~105 AU from the solar system, versus Proxima Centauri, for example, at about 270,000 AU). Hence, it is no surprise that the propulsion systems (fusion and laser sail) have a sufficient performance to reach large nomadic planets in less than 50 years, although the maturity of these propulsion system is at present fairly low.

Laser electric propulsion and MPD propulsion are also on the horizon, although there are significant development challenges ahead to reach sufficient performance at the system level, integrated with the power subsystem.

What does this mean? The first conclusion we draw is that while we develop more and more advanced propulsion systems, we become capable of reaching larger and larger (and potentially more interesting) objects in interstellar space. At present, electric sails appear to be the most promising propulsion system for nomadic planet exploration, possessing sufficient performance and a reasonably high maturity at the component level.

Second, instead of seeing interstellar space as a void with other star systems as the only relevant target, we now have a quasi-continuum of exploration-worthy objects at different distances beyond the boundary of the solar system. While star systems have been “first-class citizens” so far with no “second-class citizens” in sight, we might now be in a situation where a true class of “second-class citizens” has emerged. Finding these close nomads will be a technological and observational challenge for the next few decades.

Third, and this might be controversial, the boundary defining interstellar travel is destabilized. While traditionally interstellar travel has been treated primarily as travel from one star system to another, we might need to expand its scope to include travel to the “in-between” objects. This would include travel to aforementioned objects, but we might also discover planetary systems associated with free-floating brown dwarfs. It seems likely that nomadic worlds are orbited by moons, similar to planets in our solar system. Hence, is interstellar travel if and only if we travel between two stars, where stars are objects maintaining sustained nuclear fusion? How shall we call travel to nomadic worlds then? Shall we call this type of travel “transstellar” travel, i.e. travel beyond a star, or in-between-stellar travel?

Furthermore, nomadic worlds have likely formed in a star system of origin (although they may have formed at the end, rather than at the beginning, of the stellar main sequence). To what extent are we visiting that star system of origin by visiting the nomadic world? Inspecting a souvenir from a faraway place is not the same as being at that place. Nevertheless, the demarcation line is not as clear as it seems. Are we visiting another star system if and only if we visit one of its gravitationally bound objects? While these are seemingly semantic questions, they also harken back to the question of why we are attempting interstellar travel in the first place. Is traveling to another star an achievement by itself, is it the science value, or potential future settlement? Having a clearer understanding of the intrinsic value of interstellar travel may also qualify how far traveling to interstellar objects and nomadic worlds is different or similar.

We started this article with the analogy of driftwood between islands. While the interstellar community has been focusing mainly on star systems as primary targets for interstellar travel, we have argued that the existence of interstellar objects and nomadic worlds opens entirely new possibilities for missions between the stars, beyond an individual star system (in-between-stellar or transstellar travel). The driftwood may become by itself a worthy target of exploration. We also argued that we may have to revisit the very notion of interstellar travel, as its demarcation line has been rendered fuzzy.


Abramov, O., & Mojzsis, S. J. (2011). Abodes for life in carbonaceous asteroids?. Icarus, 213(1), 273-279.

Bannister, M. T., Bhandare, A., Dybczy?ski, P. A., Fitzsimmons, A., Guilbert-Lepoutre, A., Jedicke, R., … & Ye, Q. (2019). The natural history of ‘Oumuamua. Nature astronomy, 3(7), 594-602.

Brophy, J., Polk, J., Alkalai, L., Nesmith, B., Grandidier, J., & Lubin, P. (2018). A Breakthrough Propulsion Architecture for Interstellar Precursor Missions: Phase I Final Report (No. HQ-E-DAA-TN58806).

Brown, M. E., & Batygin, K. (2021). The Orbit of Planet Nine. The Astronomical Journal, 162(5), 219.

de León, J., Licandro, J., Serra-Ricart, M., Cabrera-Lavers, A., Font Serra, J., Scarpa, R., … & de la Fuente Marcos, R. (2019). Interstellar visitors: a physical characterization of comet C/2019 Q4 (Borisov) with OSIRIS at the 10.4 m GTC. Research Notes of the American Astronomical Society, 3(9), 131.

Hein, A. M., Eubanks, T. M., Lingam, M., Hibberd, A., Fries, D., Schneider, J., … & Dachwald, B. (2022). Interstellar now! Missions to explore nearby interstellar objects. Advances in Space Research, 69(1), 402-414.

Iakubivskyi, I., Janhunen, P., Praks, J., Allik, V., Bussov, K., Clayhills, B., … & Slavinskis, A. (2020). Coulomb drag propulsion experiments of ESTCube-2 and FORESAIL-1. Acta Astronautica, 177, 771-783.

Janhunen, P. (2004). Electric sail for spacecraft propulsion. Journal of Propulsion and Power, 20(4), 763-764.

Janhunen, P., Lebreton, J. P., Merikallio, S., Paton, M., Mengali, G., & Quarta, A. A. (2014). Fast E-sail Uranus entry probe mission. Planetary and Space Science, 104, 141-146.

Janhunen, P., & Sandroos, A. (2007, March). Simulation study of solar wind push on a charged wire: basis of solar wind electric sail propulsion. In Annales Geophysicae (Vol. 25, No. 3, pp. 755-767). Copernicus GmbH.

Jewitt, D., & Seligman, D. Z. (2022). The Interstellar Interlopers. arXiv preprint arXiv:2209.08182.

Lingam, M., & Loeb, A. (2019). Subsurface exolife. International Journal of Astrobiology, 18(2), 112-141.

Praks, J., Mughal, M. R., Vainio, R., Janhunen, P., Envall, J., Oleynik, P., … & Virtanen, A. (2021). Aalto-1, multi-payload CubeSat: Design, integration and launch. Acta Astronautica, 187, 370-383.

Quarta, A. A., & Mengali, G. (2010). Electric sail mission analysis for outer solar system exploration. Journal of guidance, control, and dynamics, 33(3), 740-755.

Slavinskis, A., Pajusalu, M., Kuuste, H., Ilbis, E., Eenmäe, T., Sünter, I., … & Noorma, M. (2015). ESTCube-1 in-orbit experience and lessons learned. IEEE aerospace and electronic systems magazine, 30(8), 12-22.

Westphal, A. J., Stroud, R. M., Bechtel, H. A., Brenker, F. E., Butterworth, A. L., Flynn, G. J., … & 30714 Stardust@ home dusters. (2014). Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft. Science, 345(6198), 786-791.


Super Earths/Hycean Worlds

Dave Moore is a Centauri Dreams regular who has long pursued an interest in the observation and exploration of deep space. He was born and raised in New Zealand, spent time in Australia, and now runs a small business in Klamath Falls, Oregon. He counts Arthur C. Clarke as a childhood hero, and science fiction as an impetus for his acquiring a degree in biology and chemistry. Dave has kept up an active interest in SETI (see If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare) as well as the exoplanet hunt, and today examines an unusual class of planets that is just now emerging as an active field of study.

by Dave Moore

Let me draw your attention to a paper with interesting implications for exoplanet habitability. The paper is “Potential long-term habitable conditions on planets with primordial H–He atmospheres,” by Marit Mol Lous, Ravit Helled and Christoph Mordasini. Published in Nature Astronomy, this paper is a follow-on to Madhusudhan et al’s paper on Hycean worlds. Paul’s article Hycean Worlds: A New Candidate for Biosignatures caught my imagination and led to this further look.

Both papers cover Super-Earths, planets larger than 120% of Earth’s radius, but smaller than the Sub-Neptunes, which are generally considered to start at twice Earth’s radius. Super-Earths occur around 40% of M-dwarf stars examined and are projected to constitute 30% of all planets, making them the most common type in the galaxy. Hycean planets are a postulated subgroup of Super-Earths that have a particular geology and chemistry; that is, they have a water layer above a rocky core below a hydrogen–helium primordial atmosphere.

We’ll be hearing a lot more about these worlds in the future. They are similar enough to Earth to be regarded as a good target for biomarkers, but being larger than Earth, they are easier to detect via stellar Doppler shift or stellar transit, and their deep atmospheres make obtaining their spectra easier than with terrestrial worlds. The James Webb telescope is marginal for this purpose, but getting detailed atmospheric spectra is well within the range of the next generation of giant, ground-based telescopes: the 39-meter Extremely Large Telescope and the 24.5-meter Giant Magellan Telescope, both of which are under construction and set to start collecting data by the end of the decade (the status of the Thirty Meter Telescope is still problematic).

Earth quickly lost its primordial hydrogen-helium atmosphere, but once a planet’s mass reaches 150% of Earth’s, this process slows considerably and planets more massive than that can retain their primordial atmosphere for gigayears. Hydrogen, being a simple molecule, does not have a lot of absorption lines in the infrared, but under pressure, the pressure-broadening of these lines makes it a passable greenhouse gas.

If the atmosphere is of the correct depth, this will allow surface water to persist over a much wider range of insolation than with Earth-like planets. With enough atmosphere, the insulating effect is sufficient to maintain temperate conditions over geological lengths of time from the planet’s internal heat flow alone, meaning these planets, with a sufficiently dense atmosphere, can have temperate surface conditions even if they have been ejected from planetary systems and wander the depths of space.

Figure 1: This is a chart from Madhusudhan et al’s paper showing the range where Hycean planets maintain surface temperatures suitable for liquid water, compared with the habitable zone for terrestrial planets as derived by Kopparapu et al. ‘Cold Hycean’ refers to planets where stellar insolation plays a negligible part in heating the surface. Keep in mind, that Lous et al regard the inner part of this zone as unviable due to atmospheric loss.

Madhusudhan et al’s models were a series of static snapshots under a variety of conditions. Lous et al’s paper builds on this by modeling the surface conditions of these planets over time. The authors take a star of solar luminosity with a solar evolutionary track and, using 1.5, 3 and 8 Earth mass planets, model the surface temperature over time at various distances and hydrogen overpressures, also calculating in the heat flow from radiogenic decay.

Typically, a planet will start off too hot. Its steam atmosphere will condense, leaving the planet with oceans; and after some period, the surface temperature will fall below freezing. The chart below shows the length of time a planet has a surface temperature that allows liquid water. (Note that, because of higher surface pressures, water in these scenarios has a boiling point well over 100°C, so the oceans may be considered inhospitable to life for parts of their range.)

Planets with small envelope masses have liquid water conditions relatively early on, while planets with more massive envelopes reach liquid water conditions later in their evolution. Out to 10 au, stellar insolation is the dominant factor in determining the surface temperature, but further out than that, the heat of radiogenic decay takes over. The authors use log M(atm)/log M(Earth) on their Y axis, which I didn’t find very helpful. To convert this to an approximate surface pressure in bars, make the following conversions: 10-6 = 1 bar, 10-5 = 10 bar, 10-4 = 100 bar and so on.

Figure 2: Charts a-c are for core masses of 1.5 (a), 3 (b) and 8 M? (c). The duration of the total evolution is 8 Gyr. The color of a grid point indicates how long there were continuous surface pressures and temperatures allowing liquid water, ?lqw. These range from 10 Myr (purple) to over 5 Gyr (yellow). Gray crosses correspond to cases with no liquid water conditions lasting longer than 10 Myr. Atmospheric loss is not considered in these simulations. d is the results for planets with a core mass of 3 M?, but including the constraint that the surface temperature must remain between 270 and 400 K. Every panel contains an ‘unbound’ case where the distance is set to 106 AU and solar insolation has become negligible.

The authors then ran their model adjusted for hydrodynamic escape (Jeans escape is negligible). This loss of atmosphere mainly affects the less massive, closer in planets with thinner atmospheres.

To quote:

The results when the hydrodynamic escape model is included are shown in Fig. 3. In this case, we find that there are no long-term liquid water conditions possible on planets with a primordial atmosphere within 2au. Madhusudhan et al. found that for planets around Sun-like stars, liquid water conditions are allowed at a distance of ~1 au. We find that the pressures required for liquid water conditions between 1 and 2au are too low to be resistant against atmospheric escape, assuming that the planet does not migrate at a late evolutionary stage.

Figure 3: Charts a-c are for core masses of 1.5 (a), 3 (b) and 8 M? (c). d is the results for planets with a core mass of 3 M?, but including the constraint that the surface temperature must remain between 270 and 400 K. Note: escape inhibits liquid water conditions by removing the atmosphere for close-in planets with low initial envelope masses. Lower core masses are more affected.

The authors also note that their simulations indicate that, unlike terrestrial planets which require climatic negative feedback loops to retain temperate conditions, Hycean worlds are naturally stable over very long periods of time.

The authors then go on to discuss the possibility of life, pointing out that the surface pressures required are frequently in the 100 to 1000 bar range, which is the level of the deep ocean and with similar light levels, so photosynthesis is out. This is a problem searching for biomarkers because photosynthesis produces chemical disequilibria, which are considered a sign of biological activity, whereas chemotrophs, the sort of life forms you would expect to find, make their living by destroying chemical disequilibria.

The authors hope to do a similar analysis with red dwarf stars as these are the stars where Super-Earths occur most frequently. Also, they are the stars where the contrast between stellar and planetary luminosity gives the best signal.

Thoughts and Speculations

The exotic nature of these planets lead me to examine their properties, so here are some points I came up with that you may want to consider:

i) The Fulton Gap—also called the small planet mass-radius valley. Small planets around stars have a distinctly bimodal distribution with peaks at 1.3 Earth radii and 2.4 Earth radii with a minimum at 1.8 Earth radii. Density measurements align with this distribution. Super-Earth densities peak, on average, at 1.4 Earth radii with a steady fall off above that. Planets smaller than about 1.5 Earth radii are thought to contain a solid core with shallow atmospheres, whereas planets above 1.8 Earth radii are thought to have deep atmospheres of volatiles and a composition like an Ice-Giant (i.e. they are Sub-Neptunes.)

Taking Lous et al’s planets, a 3 Earth mass planet would have an approximate radius of 1.3 Earth radii. An 8 Earth mass planet would have an approximate radius of 1.8 Earth radii (assuming similar densities to Earth.) This would point towards the 8 Earth mass planets having an atmosphere too deep to make a Hycean world. The atmosphere would probably transition into a supercritical fluid.

ii) I compared the liquid water atmospheric pressures from our solar system’s giant planets with the expectations of the paper. I had trouble finding good figures, as the pressure temperature charts peter out at water ice cloud level, but here are the approximate figures for the giant planets compared with the range on the 270°K-400°K graph that Lous et al produced:

Jupiter: 7-11 bar / 8-30 bar

Saturn: 10-20 bar / 25-100 bar

Neptune: 50+ Bar (50 bar is the level at which ice clouds form) / 200-500 bar

Our giant planets appear to be on the shallow side of the paper’s expectations. This could be attributed to our giant planets having greater internal heat flow than the Super-Earths modeled, but that would make the deviation greatest for Jupiter and least for Neptune. The deviation, however, appears to increase in the other direction.

The authors of the paper note that their models did not take into consideration the greenhouse effect of other gasses such as ammonia and methane likely to be found in Hycean planets’ atmospheres, which would add to the greenhouse effect and therefore give a shallower pressure profile for a given temperature. And from looking at our giant planets, this would appear to be the case.

This could mean that an unbound world would maintain a liquid ocean under something like 100+ bars of atmosphere rather than the 1000 bars originally postulated.

iii) Next, I considered the chemistry of Hycean worlds. Using our solar system’s giant planets as a guide, we can expect considerable quantities of methane, ammonia, hydrogen sulfide and phosphine in the atmospheres of Hycean worlds. The methane would stay a gas, but ammonia, being highly hydrophilic, would dissolve into the ocean. If the planet’s nitrogen to water ratio is similar to Earth’s, this would result in an approximately 1% ammonia solution. A ratio like Jupiter’s would give a 13% solution. (Ammonia cleaning fluids are generally 1-3% in concentration.) A 1% solution would have a pH of about 12, but some of this alkalinity may be buffered by the hydrosulfide ion (HS) from the hydrogen sulfide in solution.

It then occurred to me to look at freezing point depression curves of ammonia/water mixtures, and they are really gnarly. An ammonia/water ocean, if cooled below 0°C, will develop an ice cap, but as the water freezes out, this increases the ammonia concentration, causing a considerable depression in the freezing point. If the ocean reaches -60°C, something interesting starts to happen. The ice crystals forming in the ocean and floating up to the base of the ice cap start to sink, as the ocean fluid, now 25% ammonia, is less dense than ice. This will result in an overturn of the ocean and the ice cap. Further cooling will result in the continued precipitation of ice crystals until the ocean reaches a eutectic mixture of approximately 2 parts water to 1 part ammonia, which freezes at -91°C. (For comparison, pure ammonia freezes at -78°C.) Note: all figures are for 1 bar.

When discussing the possibility of liquid water on planets, we have to include the fact that water under sufficient pressure can be liquid up to its critical point of 374°C. The paper takes this into account; but what we see here is that, aside from showing that the range of insolation over which planets can have liquid water is larger than we thought, the range that water can be liquid is also larger than we assumed.

While some passing thought has been given to the possibility of ammonia as a solvent for life forms, nobody appears to have considered water/ammonia mixtures.

iv) Turning from ammonia to methane, I began to wonder if these planets would have a brown haze like Titan. A little bit of research showed that the brown haze of Titan is mainly made of tholins, which are formed by the UV photolysis of methane and nitrogen. Tholins are highly insoluble in hydrocarbons, which is why Titan’s lakes are relatively pure mixtures of hydrocarbons. However, tholins are highly soluble in polar solvents like water. So a Hycean planet with a water cycle would rain out tholins that formed in the upper atmosphere, but if the surface was frozen like Titan’s, they would stay in the atmosphere, forming a brown haze.

This points to the possibility that there are significant differences in the composition of a Hycean planet’s atmosphere depending on whether its surface is frozen or oceanic. and this may be detectable by spectroscopy.

I’m looking forward to finding out more about these planets. In some ways, I feel that in respect to exosolar planets, we are now in a position similar to that of our own solar system in the early 60s – eagerly awaiting the first details to come in.


Marit Mol Lous, Ravit Helled and Christoph Mordasini, “Potential long-term habitable conditions on planets with primordial H–He atmospheres,” Nature Astronomy, 6: 819-827 (July 2022). Full text.

Nikku Madhusudhan, Anjali A. A. Piette, and Savvas Constantinou, “Habitability and Biosignatures of Hycean Worlds,” The Astrophysical Journal, (Aug. 2021). Preprint.

Fulton et al, “The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets,” The Astronomical Journal, 154 (3) 2017. Abstract.

Christopher P. McKay, ”Elemental composition, solubility, and optical properties of Titan’s organic haze,” Planetary Space Science, 8: 741-747 (1996). Abstract.