Free-Floating Planets as Interstellar Targets

Just a few weeks ago I wrote about stellar interactions, taking note of a concept advanced by scientists including Ben Zuckerman and Greg Matloff that such stars would make for easier interstellar travel. After all, if a star in its rotation around the Milky Way closes to within half a light year of the Sun, it’s a more feasible destination than Alpha Centauri. Of course, you have to wait for the star to come around, and that takes time. Zuckerman (UCLA), working with Bradley Hansen, has written about the possibility that close encounters are when a civilization will attempt such voyages.

I have a further idea along the lines of motion through the galaxy and its advantages to explorers, and it’s one that may not require tens of thousands of years of waiting. We’d like to get to another star system because we’re interested in the planets there, so what if an interstellar planet nudges into nearby space? I’ll ignore Oort Cloud perturbations and the rest to focus on a ‘rogue’ or ‘free-floating’ planet as the target of a probe, and ask whether we may not already have some of these in nearby space.

After all, finding free-floating planets – and I’m now going to start calling them FFPs, because that’s what appears in scientific papers on the matter – are hard to find. There being no reflected starlight to look for, the most productive way is to pick them out by their infrared signature, which means finding them when they’re relatively young. This is what Núria Miret Roig (University of Vienna) and team did a couple of years ago, working with data from the Very Large Telescope and other sources. Lo and behold, over one hundred FFPs turned up, all of them infants and still warm.

Image: The locations of 115 potential FFPs [free-floating planets] in the direction of the Upper Scorpius and Ophiuchus constellations, highlighted with red circles. The exact number of rogue planets found by the team is between 70 and 170, depending on the age assumed for the study region. This image was created assuming an intermediate age, resulting in a number of planet candidates in between the two extremes of the study. Credit: ESO/N. Risinger (skysurvey.org).

But young FFPs are most likely to be found in star-forming regions, two of which (in Scorpius and Ophiuchus) were subjected to Miret Roig and team’s searches. What’s likely to amble along in our rather more sedate region is an FFP with enough years on it to have cooled down. The WISE survey (Wide-Field Infrared Survey Explorer) showed how difficult it is to pin down red dwarfs in the neighborhood, although it can be done. But even there, when you get down to L- and T-class brown dwarfs, uncertainty persists about whether you can find them. With planets the challenge is even greater.

Sometimes FFPs are found through microlensing toward the galactic core, but I don’t think we can rely on that method for finding a population of such worlds within, say, half a light year. Nonetheless, Miret Roig is not alone in pointing out that “there could be several billions of these free-floating planets roaming freely in the Milky Way without a host star.” Indeed, that number could be on the low side given what we’re learning about how these objects form. Given the excitement over ‘Oumuamua and other interstellar interlopers that may appear, I’m surprised that there hasn’t been more attention paid to how we might detect planet-sized objects near our system.

The ongoing search for Planet 9 demonstrates how difficult finding a planet outside the ecliptic can be right here at home. While pondering the best way to proceed, I’ll divert the discussion to rogue planet formation, which has always been central to the debate. Are the processes rare or common, and if the latter, do most stellar systems including our own, have the potential for ejecting planets? The last two decades of study have been productive, as we have refined our methods for modeling this process.

Recent work on the Trapezium Cluster in the Orion Nebula shows us how the catalog of FFPs is growing. The Trapezium Cluster is helpfully located out of the galactic plane, and there is a molecular cloud behind it that reduces the problems posed by field stars. I was startled to learn about this study (conducted at the European Space Agency’s ESTEC facility in the Netherlands by Samuel Pearson and Mark J McCaughrean) because of the sheer number of FFPs it turned up. Some 540 FFP candidates are identified here, ranging in mass from 0.6 to 13 Jupiter masses, although the range is an estimate based on the age of the cluster and our current models of gas giant evolution.

Image: A total of 712 individual images from the Near Infrared Camera on the James Webb Space Telescope were combined to make this composite view of the Orion Nebula and the Trapezium Cluster. Credit: NASA, ESA, CSA/Science leads and image processing: M. McCaughrean, S. Pearson, CC BY-SA 3.0 IGO.

What stopped me cold about this work is that among the 540 candidate FFPs, 40 are binaries. Two free-floating planets moving together without a star, and enough of them that we have to learn a new term: JuMBOs, for Jupiter-mass binary objects. How does that happen? There are even two triple systems in the data. Digging into the paper:

…we can compare their statistical properties…with higher-mass systems. The JuMBOs span the full mass range of our PMO [planetary-mass object] candidates, from 13 MJup down to 0.7 MJup. They have evenly distributed separations between ∼25–390 au, which is significantly wider than the average separation of brown dwarf-brown dwarf binaries which peaks at ∼ 4 au [42, 43]. However, as our imaging survey is only sensitive to visual binaries with separations > 25 au, we can not rule out an additional population of JuMBOs with closer orbits. For this reason we take 9% as a lower bound for the PMO multiplicity fraction. The average mass ratio of the JuMBOs is q = 0.66. While there are a significant number of roughly equal-mass JuMBOs, only 40% of them have q ≥ 0.8. This is much lower than the typical mass ratios for brown dwarfs, which very strongly favour equal masses.

That last line is interesting. Our FFP binary systems tend to have planets of distinctly different masses, which implies, according to the authors, that if the JuMBOs formed through core collapse and fragmentation – like a star – “then there must be some fundamental extra ingredient involved at these very low masses.” But the binary systems here go well below the mass where this formation method was thought to work. That opens up the ‘ejection’ hypothesis, with the planets forming in a circumstellar disk only to be ejected by gravitational interactions. So note this:

In either case, however, how pairs of young planets can be ejected simultaneously and remain bound, albeit weakly at relatively wide separations, remains quite unclear. The ensemble of PMOs and JuMBOs that we see in the Trapezium Cluster might arise from a mix of both of these “classical” scenarios, even if both have significant caveats, or perhaps a new, quite separate formation mechanism, such as a fragmentation of a star-less disk is required.

Ejection is a rational thing to look at considering that gravitational scattering is a well-studied process and may well have occurred in the early days of our own system. On the other hand, in star-forming regions like Trapezium the nascent systems are so young that this scenario may be less likely than the core-collapse model, in which the process is similar to star formation as a molecular cloud collapses and fragments. The open question is whether a scenario like this, which seems to work for brown dwarfs, is also applicable to considerably smaller FFPs in the Jupiter-mass range.

In any case, it seems unlikely that binary planets could survive ejection from a host system. As co-author Pearson puts it, “Nine percent is massively more than what you’d expect for the planetary-mass regime. You’d really struggle to explain that from a star formation perspective…. That’s really quite puzzling.”

All of which triggered a new paper from Fangyuan Yu (Shanghai Jiao Tong University) and Dong Lai (Cornell University), which takes an entirely different tack when it comes to formation of binary FFPs:

The claimed detection of a large fraction (9 percent) of JuMBOs among FFPs (Pearson & McCaughrean 2023) seems to suggest that core collapse and fragmentation (i.e. scaled-down star formation) channel plays an important role in producing FFPs down to Jupiter masses, since we do not expect the ejection channel to produce binary planets. On the other hand, (Miret-Roig et al. 2022) suggested that the observed abundance of FFPs in young star clusters significantly exceeds the core collapse model predictions, indicating that ejections of giant planets must be frequent within the first 10 Myr of a planetary system’s life.

Yu and Lai look at close stellar flybys as a contributing factor to FFP binary formation. If we’re talking about dense young star clusters, encounters between stars should be frequent, and there has been at least one study advancing the idea that bound binary planets could be the result of such flybys. Yu and Lai model two-planet systems to study the effects of a flyby on single and double-planet systems. Will an FFP result from a close flyby? A binary FFP? Or will the flyby star contribute a planet to the system it encounters?

These numerical experiments yield interesting results: The production rate of binary pairs of FFPs caused by stellar flybys is always less than 1 percent in their modeling, even when parameters are adjusted to make for tightly packed stellar systems. Directly addressing the JWST work in Trapezium and the large number of JuMBOs found there, Yu and Lai deduce that they cannot be caused by flybys, and because ejection scenarios are so unlikely, they see “a scaled-down version of star formation” at work “via fragmentation of molecular cloud cores or weakly-bound disks or pseudo-disks in the early stages of star formation.”

The matter remains unresolved, producing much fodder for future observations and debate. And while we figure out how to detect free-floating planets that may already be far closer than Proxima Centauri, we can create science fictional scenarios of journeys not just to a single rogue planet, but to a binary or even a triple system cohering despite the absence of a central star. I can only imagine how much Robert Forward, the man who gave us Rocheworld, would have enjoyed working with that.

The paper is Pearson & McCaughrean, “Jupiter Mass Binary Objects in the Trapezium Cluster” (preprint). The Miret-Roig paper is “A rich population of free-floating planets in the Upper Scorpius young stellar association,” published online at Nature Astronomy 22 December 2021 (abstract). The Fangyuan Yu & Dong Lai paper is, “Free-Floating Planets, Survivor Planets, Captured Planets and Binary Planets from Stellar Flybys,” submitted to The Astrophysical Journal (preprint).

New Angles on Planet Formation

Planet formation is a fascinating subtopic of the exoplanet hunt, and it may just have produced the first exoplanet detection in data that go back as far back as 1981, though the event in question has never been confirmed as being caused by a planet. I learned this through a paper sent me recently by Jean Schneider (Observatoire de Paris), who along with colleague Danielle Briot wrote about the early days of transit searches in a chapter for the Handbook of Exoplanets (Springer, 2018).

I want to dig deeper into that chapter in a later post, but for now, I note that the planet Beta Pictoris b, discovered in 2008 and orbiting an infant star 63 light years from Earth, may have transited in 1981, according to subsequent papers on the matter. The debris disk around the primary has long fascinated astronomers and it has been investigated for the possible presence of comet-like bodies and subjected to direct imaging searches, which revealed Beta Pictoris b and confirmed it in 2009. But the 1981 data show light variations that could be interpreted as a transit.

Image: Various planet formation processes, including exocomets and other planetesimals, around Beta Pictoris, a very young type A V star. Credit: NASA/FUSE/Lynette Cook.

Testing the matter involved a nano-satellite called PicSat designed by astronomers at the Paris-Meudon observatory to look for a possible 2018 transit. Unfortunately, the mission failed when communications were lost. As Schneider and Briot write:

If the transit is confirmed in the near future, we could obtain better observations of the following transit in 2053, when the period will be known more accurately and when we could use very large and extraordinary outstanding future instruments. So, will Beta Pictoris b win the title of the first detected exoplanet?

The massive debris disk at Beta Pictoris is asymmetric (likely the result of perturbations by another star), and seen edge-on from Earth. But the beauty of the circumstellar disks out of which planets form is that if we peer into planet-forming regions, we can find them oriented in all kinds of ways, as we learn in a new series of images from the European Southern Observatory’s Very Large Telescope. They’re part of a survey of how planetary systems form, and they mark a transition between the intense study of individual star systems to a broad swath of planets and stars. As Christian Ginski (University of Galway, Ireland) puts it, “We’ve gone from the intense study of individual star systems to this huge overview of entire star-forming regions.”

Let’s home in on a single system to begin with to ponder how much we can learn from these disks.

Image: This composite image shows the MWC 758 planet-forming disc, located about 500 light-years away in the Taurus region, as seen with two different facilities. The yellow colour represents infrared observations obtained with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on ESO’s Very Large Telescope (VLT). The blue regions on the other hand correspond to observations performed with the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner. These facilities allow astronomers to map how dust is distributed around this and other stars in different but complementary ways. SPHERE captures light from the host star that has been scattered by the dust around it, whereas ALMA registers radiation directly emitted by the dust itself. These observations combined help astronomers understand how planets may form in the dusty discs surrounding young stars. Credit: ESO/A. Garufi et al.; R. Dong et al.; ALMA (ESO/NAOJ/NRAO).

The new ESO imagery takes in numerous young stars like this one. What leaps out here is the wide range of formation scenarios. Working with the star-forming regions at Orion, Taurus and Chamaeleon I, an international team of astronomers investigated 86 stars using SPHERE, the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument mounted on the VLT. The quality of the imagery is attributable to SPHERE’s adaptive optics, which allows disks to be imaged around stars down to about half the Sun’s mass while correcting for the distortions produced by looking through the Earth’s atmosphere. The use of ALMA, the Atacama Large Millimeter/submillimeter Array, helped quantify the amount of dust in these systems.

Image: This research brings together observations of more than 80 young stars that might have planets forming around them in spectacular discs. This small selection from the survey shows 10 discs from the three regions of our galaxy observed in the papers. V351 Ori and V1012 Ori are located in the most distant of the three regions, the gas-rich cloud of Orion, some 1600 light-years from Earth. DG Tau, T Tau, HP Tau, MWC758 and GM Aur are located in the Taurus region, while HD 97048, WW Cha and SZ Cha can be found in Chamaeleon I, all of which are about 600 light-years from Earth. The discs have been scaled to appear roughly the same size in this composition. Credit: ESO/C. Ginski, A. Garufi, P.-G. Valegård et al.

The imagery is stunning. Per-Gunnar Valegård (University of Amsterdam), who led the work on Orion, noted “It is almost poetic that the processes that mark the start of the journey towards forming planets and ultimately life in our own Solar System should be so beautiful.” I think we can drop that word ‘almost’ as we are reminded that the sheer wonder of celestial vistas at ever increasing scales is what drew many of us into astronomy at an early age. We’re a long way from those grainy Palomar images of Jupiter and Saturn that I acquired decades ago at Adler Planetarium in Chicago.

Image: Planet-forming discs around young stars and their location within the gas-rich cloud of Chamaeleon I, roughly 600 light-years from Earth. The team observed 20 stars in the Chamaeleon I region, detecting discs around 13. The background image shows an infrared view of Chamaeleon I captured by the Herschel Space Observatory. Credit: ESO/C. Ginski et al.; ESA/Herschel.

Meanwhile, in a separate investigation of the young star T Chamaeleontis, we gain insights into planet formation in its latter stages. Here we have the James Webb Space Telescope to thank. Naman Bajaj (University of Arizona) and Uma Gorti (SETI Institute) have been able to extract data on the four lines of the noble gases neon (Ne) and argon (Ar), with the neon detection showing processes that are occurring over an extended area. The observations show dispersing gasses from a planet-forming disk that is in its latter stages, completing the formation process. These ‘winds’ seem to be driven by stellar photons or, according to Bajaj, by the magnetic field within the disk itself.

The find is significant, says Richard Alexander (University of Leicester):

“We first used neon to study planet-forming discs more than a decade ago, testing our computational simulations against data from Spitzer, and new observations we obtained with the ESO VLT. We learned a lot, but those observations didn’t allow us to measure how much mass the discs were losing. The new JWST data are spectacular, and being able to resolve disc winds in images is something I never thought would be possible. With more observations like this still to come, JWST will enable us to understand young planetary systems as never before.”

The paper on the history of transit studies is Briot & Schneider, “Prehistory of Transit Searches,” in Handbook of Exoplanets, 2nd edition.

The papers on the SPHERE work are Ginski et al., “The SPHERE view of the Chamaeleon I star-forming region: The full census of planet-forming disks with GTO and DESTINYS programs”, Astronomy & Astrophysics (abstract).

And two other papers in the same issue: Garufi et al., “The SPHERE view of the Taurus star-forming region: The full census of planet-forming disks with GTO and DESTINYS programs,” (abstract) and Valegard et al., “Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): The SPHERE view of the Orion star-forming region” (abstract).

The paper on T Chamaeleontis is Bajaj et al., “JWST MIRI MRS Observations of T Cha: Discovery of a Spatially Resolved Disk Wind,” The Astronomical Journal Vol. 167, No. 3 (4 March 2024), 127 (abstract).

Musings on Red Dwarf Planets

I’m going to start in the Kuiper Belt this morning before going further out, because the news that the Belt may extend much further than expected reminds us of the nature of exploration. The New Horizons spacecraft, well beyond Pluto’s orbit and approaching 60 AU from the Sun, is finding more dust than expected. Our theoretical models didn’t see that coming. In fact, the dust produced by collisions between Kuiper belt objects was thought to decline as we approached the Belt’s outer edge.

So just where is that outer edge? It had been pegged around 50 AU but now looks more like 80 AU, if not further out, a finding corroborated by the fact that New Horizons scientists have used Earth-based resources like the Subaru Telescope in Hawaii to find numerous KBOs beyond the assumed boundary. Is this a new population of Solar System objects, or are we actually seeing something more mundane, such as radiation pressure pushing inner belt dust further out than would be expected? It takes patient observation to decide, and to re-shape our notions according to hard data.

Which gets me into exoplanet territory, and specifically our understanding of red dwarf stars. Theory is always malleable and yields to observation which, in turn, re-energizes theory. Michaël Gillon (University of Liége), who is among other things the discoverer of the TRAPPIST-1 system, made this point in a recent email exchange. He was responding to my article What We Know Now about TRAPPIST-1 (and what we don’t) with a much needed note of caution. The question of whether rocky planets orbiting M-dwarf stars can retain atmospheres is one of the hottest controversies going. Observations, says Gillon, will tell the tale, not theory, no matter how elegant the latter. After all, we now have JWST, and a new generation of telescopes already under construction to help.

Image: An artist’s concept of Kepler-438b, shown here in front of its violent parent star, a red dwarf. It is regularly irradiated by huge flares of radiation, which could render the planet uninhabitable and possibly strip it of its atmosphere entirely. A variety of mechanisms for depleting the atmosphere of such a planet are now under discussion, using a wide range of models. Image credit: Mark A Garlick / University of Warwick.

On that score, it’s useful to look at a paper that Gillon suggests in his email, a study by Ignasi Ribas and collaborators that appeared in Astronomy & Astrophysics in 2016. The paper does a deep dive into the question of the habitability of Proxima Centauri b, that tantalizingly close Earth-mass planet in the star’s habitable zone. A key issue is whether the extreme dosing of X-ray and ultraviolet radiation such a planet would suffer would compromise a young atmosphere or prevent its existence at all.

There is a lot going on in the Ribas paper, but Gillon pointed me to its discussion of what we might learn from the lesson of our own Earth. Here the essential fuzziness of theory manifests itself, as Ribas and team point out how many uncertainties exist in our estimates of volatile loss, including the telling line “…they rely on complex models that were never confronted to actual observations of massive escape.” Even more telling is the lack of sufficient information about what might prevent such escape:

None of the available models include all the mechanisms controlling the loss rate, for example, the photochemistry of the upper atmosphere and its detailed interaction with the wavelength-dependent stellar emission, non-LTE [Non-Local Thermodynamic Equilibrium] cooling processes, and an accurate description of the outflow beyond the exobase where hydrodynamics no longer apply. Some key data are not known, like the intrinsic planetary magnetic moment, now and in the past, the detailed evolution of the atmospheric composition, of the high-energy spectrum and of stellar wind properties.

Nor do we have hard data on these things even now. Note the term ‘non-LTE cooling’ above. An LTE system is one in thermal equilibrium, maintaining a single temperature. Poking around in the literature, I learn that the lack of thermal equilibrium involves processes that have to be carefully measured to build an accurate profile of an atmosphere, with ramifications for any discussion of its long-term survival. In the absence of such data, it’s telling that other factors remain unknown, including local magnetic conditions and the actual properties of the stellar wind affecting the planet. And we have no good modeling for how volatiles may be distributed between the atmosphere of an M-dwarf planet and the internal processes that can replenish it.

Thus the Ribas paper, although eight years old, remains pertinent to this ongoing discussion, as scientists attack volatile retention in such systems. The authors point out that the protoplanets that built up the early Earth were exposed to a young Sun that was blasting our planet with X-rays, ultraviolet and stellar wind conditions that may equal, and perhaps surpass, what occurred at Proxima b. Note this (italics mine):

The XUV irradiation and stellar wind on the proto-Earth was therefore comparable, and possibly higher, than that of Proxima b. Proxima b spent 100–200 Myr in runaway before entering the HZ, which is longer than the runaway phases experienced by the proto-Earth by a small factor only (<10). Models predict that early Earth suffered massive volatile losses: hydrodynamic escape of hydrogen dragging away heavier species and non thermal losses under strong stellar wind exposure and CMEs (Lammer et al. 2012). Nonetheless, no clear imprint of these losses is found in the present volatile inventory.

The authors point out, then, that geochemical evidence alone shows us no signs of significant depletion of Earth’s inventory of volatiles, which can lead to the possibility that volatile loss was extremely limited under conditions that some of our models would suggest should deplete them radically. If this analysis is correct, then the idea that the planets of red dwarf stars will likely be barren rock stripped of atmospheres is questionable. I come back to Gillon’s point. We’re only going to know from observation, just as we can only know about the extent of the Kuiper Belt through hard data.

Now comes a new paper from Ofer Cohen (University of Massachusetts) and colleagues. Writing in The Astrophysical Journal, the authors again address the TRAPPIST-1 question, this time with a new twist. They’re looking at electric currents that would be produced in the ionosphere of TRAPPIST-1e, a planet that may be in the star’s habitable zone. The question is whether such currents would produce atmospheric heating that would contribute to dissipating the atmosphere entirely.

So this is another stripping mechanism to consider, one produced by the planet’s upper atmosphere encountering the star’s changing magnetic field as the planet proceeds along its orbit. The operative term is ‘Joule heating.’ I only received the paper this morning, so I won’t go too deeply into it. But my early reading suggests that the results from the models used in it point to serious atmospheric loss. This adds to earlier modeling involving the stellar wind and ionized upper atmosphere, some of this conducted by the same authors. The conclusion draws naturally from the modeling:

The JH [Joule heating] is the result of a dissipation of electric current, which is driven by the rapidly varying magnetic field along the planetary orbit. We estimate the JH energy flux on the exoplanet Trappist-1e as well as similar planets orbiting the Sun in close-in orbits. We find that the JH energy flux is larger than the anticipated EUV energy flux at the planet, and it may reach a few percent of the stellar constant energy flux. Such an intense heating could drive a strong atmospheric escape and could lead to a rapid loss of the atmosphere. Thus, the rapid orbital motion of short-orbit exoplanets may exhaust a significant portion of their atmospheres over time.

Again we find useful theories painting a landscape of possibilities. But it’s also true that we lack observational data on the properties of the stellar wind, its evolution over time, and the magnetic fields affecting the planet. The authors call attention to this fact:

VDJH [voltage-driven Joule heating] depends on the variations of the interplanetary magnetic field (IMF) strength along the planetary orbit. Such detailed IMF data are not available for exoplanets (some observations were made for the stellar wind interaction with the interstellar medium; e.g., Wood et al. 2021), nor it is available for short orbits around the Sun (limited data at specific locations are available from the Parker Solar Probe; Raouafi et al. 2023). Due to the lack of observational constraints, we must rely on models to estimate the relevant stellar wind conditions.

Thus energy output, stellar wind and magnetic field changes all factor into a model that suggests atmospheric escape and, like other models, is in need of confirmation with future instrumentation. We can only turn to such observation to begin to understand how diverse theories mesh. I think all the scientists involved in the study of planetary atmospheres around M-dwarfs would agree with this. And headlines in the popular media announcing barren rocks at TRAPPIST-1 are making ongoing investigations into settled science.

Getting too comfortable with theory can mislead us. Recently we saw that the astronomer Otto Struve proposed detecting Jupiter-class worlds in tight orbits around their host stars, only to have the suggestion ignored for decades because ‘hot Jupiters’ simply didn’t fit into then current thinking.

For that matter, nobody thought ‘super-Earths’ were likely, especially in the kind of numbers we’ve found them, nor ‘mini-Neptunes,’ and I doubt many were expecting tiny compact systems of numerous planets, like those Gillon identified at TRAPPIST-1. All in all, I appreciate Gillon’s reminder that patience and data gathering are needed as we explore the question of life around small red stars, an issue that is under deep study but has been by no means resolved. Perhaps the Habitable Worlds Observatory (Habex) will allow a definitive answer for TRAPPIST-1 in the not so distant future.

The Ribas paper is “The habitability of Proxima Centauri b I. Irradiation, rotation and volatile inventory from formation to the present,” Astronomy & Astrophysics 506 (2016), A111 (full text). The Cohen paper is “Heating of the Atmospheres of Short-orbit Exoplanets by Their Rapid Orbital Motion through an Extreme Space Environment,” The Astrophysical Journal 962 (16 February 2020), 157 (full text).

Otto Struve: A Prescient Look at Exoplanet Detection

Some things just run in families. If you look into the life of Otto Struve, you’ll find that the Russian-born astronomer was the great grandson of Friedrich Georg Wilhelm von Struve, who was himself an astronomer known for his work on binary stars in the 19th Century. Otto’s father was an astronomer as well, as was his grandfather. That’s a lot of familial energy packed into the study of the stars, and the Struve of most recent fame (Otto died in 1963) drew on that energy to produce hundreds of scientific papers. Interestingly, the man who was director at Yerkes and the NRAO observatories was also an early SETI advocate who thought intelligence was rife in the Milky Way.

Of Baltic-German descent, Otto Struve might well have become the first person to discover an exoplanet, and therein hangs a tale. Poking around in the history of these matters, I ran into a paper that ran in 1952 in a publication called The Observatory titled “Proposal for a Project of High-Resolution Stellar Radial Velocity Work.” Then at UC Berkeley, Struve had written his PhD thesis on the spectroscopy of double star systems at the University of Chicago, so his paper might have carried more clout than it did. On the other hand, Struve was truly pushing the limits.

Image: Astronomer Otto Struve (1897-1963). Credit: Institute of Astronomy, Kharkiv National University.

For Struve was arguing that Doppler measurements – measuring the wavelength of light as a star moves toward and then away from the observer – might detect exoplanets, if they existed, a subject that was wildly speculative in that era. He was also saying that the kind of planet that could be detected this way would be as massive as Jupiter but in a tight orbit. I can’t call this a prediction of the existence of ‘hot Jupiters’ as much as a recognition that only that kind of planet would be available to the apparatus of the time. And in 1952, the idea of a Jupiter-class planet in that kind of orbit must have seemed like pure science fiction. And yet here was Struve:

…our hypothetical planet would have a velocity of roughly 200 km/sec. If the mass of this planet were equal to that of Jupiter, it would cause the observed radial velocity of the parent star to oscillate with a range of ± 0.2 km/sec—a quantity that might be just detectable with the most powerful Coudé spectrographs in existence. A planet ten times the mass of Jupiter would be very easy to detect, since it would cause the observed radial velocity of the star to oscillate with ± 2 km/sec. This is correct only for those orbits whose inclinations are 90°. But even for more moderate inclinations it should be possible, without much difficulty, to discover planets of 10 times the mass of Jupiter by the Doppler effect.

Struve suggested that binary stars would be a fertile hunting ground, for the radial velocity of the companion star would provide a “reliable standard of velocity.”

Imagine what would have happened if the discovery of 51 Pegasi (the work of Michel Mayor and Didier Queloz in 1995) had occurred in the early 1960s, when it was surely technically possible. Joshua Winn (Princeton University) speculates about this in his book The Little Book of Exoplanets (Princeton University Press, 2023). And if you start going down that road, you quickly run into another name that I only recently discovered, that of Kaj Aage Gunnar Strand (1907-2000). Working at Sproul Observatory (Swarthmore College) Strand announced that he had actually discovered a planet orbiting 61 Cygni in 1943. Struve considered this a confirmed exoplanet.

Now we’re getting deep into the weeds. Strand was using photometry, as reported in his paper “61 Cygni as a Triple System.” In other words, he was comparing the positions of the stars in the 61 Cygni binary system to demonstrate that they were changing over time in a cycle that showed the presence of an unseen companion. Here I’m dipping into the excellent Pipettepen site at the University of North Carolina, where Mackenna Wood has written up Strand’s work. And as Wood notes, Strand was limited to using glass photographic plates and a ruler to make measurements between the stars. Here’s the illustration Wood ran showing how tricky this would have been:

Image: An example of a photographic plate from one of the telescopes used in the 1943 61 Cygni study. The plate is a negative, showing stars as black dots, and empty space in white. Brighter stars appear as larger dots. Written at the bottom of the plate are notes indicating when the image was taken (Nov. 10, 1963), and what part of the sky it shows. Credit: Mackenna Wood.

Strand’s detection is no longer considered valid because more recent papers using more precise astrometry have found no evidence for a companion in this system. And that was a disappointment for readers of Arthur C. Clarke, who in his hugely exciting The Challenge of the Spaceship (1946) had made this statement in reference to Strand: “The first discovery of planets revolving around other suns, which was made in the United States in 1942, has changed all ideas of the plurality of worlds.”

Can you imagine the thrill that would have run up the spine of a science fiction fan in the late 1940s when he or she read that? Someone steeped in Heinlein, Asimov and van Vogt, with copies of Astounding available every month on the newsstand and the great 1950s era of science fiction about to begin, now reading about an actual planet around another star? I have a lot of issues of Astounding from the late 1930s in my collection though few from the late ‘40s, but I plan to check on Strand’s work to see if it appeared in any fashion in John Campbell’s great magazine in the following decade. Surely there would have been a buzz at least in the letter columns.

Image: Kaj Aage Gunnar Strand (1907-2000) was director of the U.S. Naval Observatory from 1963 to 1977. He specialized in astrometry, especially work on double stars and stellar distances. Credit: Wikimedia Commons / US Navy.

We’re not through with early exoplanet detection yet, though, and we’re staying at the same Sproul Observatory where Strand did the 61 Cygni work. It was in 1960 that another Sproul astronomer, Sarah Lippincott, published work arguing that Lalande 21185 (Gliese 411) had an unseen companion, a gas giant of ten Jupiter masses. A red dwarf at 8.3 light years out, this star is actually bright enough to be seen with even a small telescope. And in fact it does have two known planets and another candidate world, the innermost orbiting in a scant twelve days with a mass close to three times that of Earth, and the second on a 2800-day orbit and a mass fourteen times that of Earth. The candidate planet, if confirmed, would orbit between these two.

Image: Swarthmore College’s Sarah Lippincott, whose work on astrometry is highly regarded, although her exoplanet finds were compromised by faulty equipment. Credit: Swarthmore College.

The work on Lalande 21185 in exoplanet terms goes back to Peter van de Kamp, who proposed a massive gas giant there in 1945. Lippincott was actually one of van de Kamp’s students, and the duo used astrometrical techniques to study photographic plates taken at Sproul. It turns out that Sproul photographic plates taken at the same time as those Lippincott used in her later paper on the star were later used by van de Kamp in his claim of a planetary system at Barnard’s Star. It was demonstrated later that the photographic plates deployed in both studies were flawed. Systematic errors in the calibration of the telescope were the culprit in the mistaken identifications.

Image: Astronomer Peter van de Kamp (1901-1995). Credit: Rochester Institute of Technology newsletter.

We always knew that exoplanet hunting would push us to the limits, and today’s bounty of thousands of new worlds should remind us of how the landscape looked 75 years ago when Otto Struve delved into detection techniques using the Doppler method. At that time, as far as he knew, there was only one detected exoplanet, and that was Strand’s detection, which as we saw turned out to be false. But Struve had the method down if hot Jupiters existed, and of course they do. He also reminded us of something else, that a large enough planet seen at the right angle to its star should throw a signal:

There would, of course, also be eclipses. Assuming that the mean density of the planet is five times that of the star (which may be optimistic for such a large planet) the projected eclipsed area is about 1/50th of that of the star, and the loss of light in stellar magnitudes is about 0.02. This, too, should be ascertainable by modern photoelectric methods, though the spectrographic test would probably be more accurate. The advantage of the photometric procedure would be its fainter limiting magnitude compared to that of the high-dispersion spectrographic technique.

There, of course, is the transit method which has proven so critical in fleshing out our catalogs of exoplanets. Both radial velocity and transit techniques would prove far more amenable to early exoplanet detection than astrometry of the sort that van de Kamp and Lippincott used, though astrometry definitely has its place in the modern pantheon of detection methods. Back in 1963, when van de Kamp announced the discovery of what he thought were planets at Barnard’s Star, he relied on almost half a century of telescope observations to build his case. No one could fault his effort, and what a shame it is that the astronomer died just months before the discovery of 51 Pegasi b. It would be fascinating to have his take on all that has happened since.

What We Know Now about TRAPPIST-1 (and what we don’t)

Our recent conversations about the likelihood of life elsewhere in the universe emphasize how early in the search we are. Consider recent work on TRAPPIST-1, which draws on JWST data to tell us more about the nature of the seven planets there. On the surface, this seven-planet system around a nearby M-dwarf all but shouts for attention, given that we have three planets in the habitable zone, all of them of terrestrial size, as indeed are all the planets in the system. Moreover, as an ultracool dwarf star, the primary is both tiny and bright in the infrared, just the thing for an instrument like the James Webb Space Telescope to harvest solid data on planetary atmospheres.

This is a system, in other words, ripe for atmospheric and perhaps astrobiological investigation, and Michaël Gillon (University of Liége), the key player in discovering its complexities, points in a new paper to how much we’ve already learned. If its star is ultracool, the planetary system at TRAPPIST-1 can also be considered ‘ultracompact’ in that the innermost and outermost planets orbit at 0.01 and 0.06 AU respectively. By comparison, Mercury orbits at 0.4 AU from our Sun. The stability of the system through mean motion resonances means that we’re able to deduce tight limits on mass and density, which in turn give us useful insights into their composition.

Image: Measuring the mass and diameter of a planet reveals its density, which can give scientists clues about its composition. Scientists now know the density of the seven TRAPPIST-1 planets with a higher precision than any other planets in the universe, other than those in our own solar system. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).

Because we’ve been talking about SETI recently, I’ll mention that the SETI Institute has already subjected TRAPPIST-1 to a search using the Allen Telescope Array at frequencies of 2.84 and 8.2 gigahertz. The choice of frequencies was dictated by the researchers’ interest in whether a system this compact might have a civilization that had spread between two or more worlds. Searching for powerful broadband communications when planetary alignments between two habitable planets occur as viewed from Earth is thus a hopeful strategy, and as is obvious, the search yielded nothing unusual. A broader question is whether life might spread between such worlds through impacts and subsequent contamination.

What I’m angling for here is the relationship between a bold, unlikely observing strategy and a more orthodox study of planetary atmospheres. Both of these are ongoing, with the investigation of biosignatures a hot topic as we work with JWST but also plan for subsequent space telescopes like the Habitable Exoplanet Observatory (HabEx). The gap in expectations between SETI at TRAPPIST-1 and atmosphere characterization via such instruments highlights what a shot in the dark SETI can be. But it’s a useful shot in the dark. We need to know that there is a ‘great silence’ and continue to poke into it even as we explore the likelihood of abiogenesis elsewhere.

But back to the Gillon paper. Here you’ll find the latest results on planetary dynamics at TRAPPIST-1 and the implications for how these worlds form, along with current data on their densities and compositions. Another benefit of the compact nature of this system is that the planets interact with each other, which means we get strong signals from Transit Timing Variations that help constrain the orbits and masses involved. No other system has rocky exoplanets with such tight density measurements. The three inner planets are irradiated beyond the runaway greenhouse limit, and recent work points to the two inner planets being totally desiccated, with volatiles likely in the outer worlds.

What we’d like to know is whether, given that habitable zone planets are found in M-dwarf systems (Proxima Centauri is an obvious further example), such worlds can maintain a significant atmosphere given irradiation from the parent star. This is tricky work. There are models of the early Earth that involve massive volatile losses, and yet today’s Earth is obviously life supporting. Is there a possibility that rocky planets around M-dwarfs could begin with a high volatile content to counterbalance erosion from stellar bombardment? Gillon sees TRAPPIST-1 as an ideal laboratory to pursue such investigations, one with implications for M-dwarfs throughout the galaxy. From the paper:

Indeed, its planets have an irradiation range similar to the inner solar system and encompassing the inner and outer limits of its circumstellar habitable zone, with planet b and h receiving from their star about 4.2 and 0.15 times the energy received by the Earth from the Sun per second, respectively. Detecting an atmosphere around any of these 7 planets and measuring its composition would be of fundamental importance to constrain our atmospheric evolution and escape models, and, more broadly, to determine if low-mass M-dwarfs, the larger reservoir of terrestrial planets in the Universe, could truly host habitable worlds.

Image: Belgian astronomer Michaël Gillon, who discovered the planetary system at TRAPPIST-1. Credit: University of Liége.

Thus the early work on TRAPPIST-1 atmospheres, conducted with Hubble data and sufficient to rule out the presence of cloud-free hydrogen-dominated atmospheres for all the planets in the system. But now we have early papers using JWST data, and the issues become more stark when we turn to work performed by Gwenaël Van Looveren (University of Vienna) and colleagues. While previous studies of the system have indicated no thick atmospheres on the two innermost planets (b and c), the Van Looveren team focuses specifically on thermal losses occurring as the atmosphere heats as opposed to hard to measure non-thermal processes like stellar winds.

Here the situation clarifies. Working with computer code called Kompot, which calculates the thermo-chemical structure of an upper atmosphere, the team has analyzed the highly irradiated TRAPPIST-1 environment, modeling over 500 photochemical reactions in light of X-Ray, ultraviolet and infrared radiation, among other factors. The results show strong atmospheric loss in the early era of system development, but take into account losses through the different stages of the system’s evolution. It’s important to keep in mind that a star like this takes between 1 and 2 billion years to settle onto the main sequence, a period of high radiation. It’s also true that even main-sequence M-dwarfs can show high levels of radiation activity.

The upshot: X-ray and UV activity declines very slowly in the first several billion years on the main sequence, and stellar radiation in these wavelengths is the main driver of atmospheric loss. Things look dicey for atmospheres on any of the TRAPPIST-1 planets, and the Van Looveren model generalizes to other stars. From the paper:

The results of our models tentatively indicate that the habitable zone of M dwarfs after their arrival on the main sequence is not suited for the long-term survival of secondary atmospheres around planets of the considered planetary masses owing to the high ratio of spectral irradiance of XUV to optical/infrared radiation over a very long time compared to more massive stars. Maintaining atmospheres on planets like this requires their continual replenishment or their formation very late in the evolution of the planets. A further expansion of the grid and more detailed studies of the parameter space are required to draw definitive conclusions for the entire spectral class of M dwarfs.

Image: This is Figure 8 from the paper. Caption: Overview of the planets in the TRAPPIST-1 system and the estimated habitable zone (indicated by the green lines, taken from Bolmont et al. 2017). We added vertical lines at the minimum distances at which atmospheres of various compositions could survive for more than 1 Gyr. Credit: Van Looveren et al.

Note the term ‘primary atmosphere.’ Primary atmospheres of hydrogen and helium give way to secondary atmospheres that are the result of later processes like volcanic outgassing and molecules breaking down under stellar radiation on the planet’s surface. The paper, then, is saying that the kind of secondary atmospheres in which we might hope to find life are unlikely to survive in this environment, although active processes on a given planet might still allow them. The paper ends this way:

Our conclusion from this work is therefore significant for terrestrial planets with a mass that is similar to the Earth’s mass that orbit mid- to late-M dwarfs such as TRAPPIST-1 near or inside the (final) habitable zone. For these planets, substantial N2/CO2 atmospheres are unlikely unless atmospheric gas is continually replenished at high rates on timescales of no more than a few million years (the loss timescales estimated in our work), for example, through volcanism.

I wouldn’t call this the death knell for atmospheric survival at TRAPPIST-1, nor do the authors, but the work points to the factors that have to be addressed in further study of the system, and the results certainly challenge the possibility of life-sustaining atmospheres on any of these planets. The Van Looveren work isn’t included in Michaël Gillon’s paper, which appeared just before its release, but I hope you’ll look at both and keep the Gillon available as the best current overview of TRAPPIST-1.

As to M-dwarf prospects in general, it’s one thing to imagine a high-radiation environment, with the possibilities that life might find an evolutionary path forward, but quite another to strip a planet of its atmosphere altogether. If that is the prospect, then the census of ‘habitable’ worlds drops sharply, for M-dwarfs make up somewhere around 80 percent of all the stars in the Milky Way. A sobering thought to close the morning as I head upstairs to grind coffee beans and rejuvenate myself with caffeine.

The papers are Gillon, “TRAPPIST-1 and its compact system of temperate rocky planets,” to be published in Handbook of Exoplanets (Springer) and available as a preprint. The Van Looveren paper is “Airy worlds or barren rocks? On the survivability of secondary atmospheres around the TRAPPIST-1 planets,” accepted at Astronomy & Astrophysics (preprint).

Forbidden Worlds? Theory Clashes with Observation

Back before we knew for sure there were planets around other stars, the universe seemed likely to be ordered. If planet formation was common, then we’d see systems more or less like our own, with rocky inner worlds and gas giants in outer orbits. And if planet formation was a fluke, we’d find few planets to study. All that has, of course, been turned on its head by the abundant discoveries of exoplanets galore. And our Solar System turns out to be anything but a model for the rest of the galaxy. In today’s essay, Don Wilkins looks at several recent discoveries that challenge planet formation theory. We can bet that the more we probe the Milky Way, the more we’ll find anomalies that challenge our preconceptions.

by Don Wilkins

The past few decades have not been easy on planet formation theories. Concepts formed on the antiquated Copernican speculation, the commonality of star systems identical to the Solar System, have given way to the strangeness and variety uncovered by Kepler, Hubble, and the other space borne telescopes. The richness of the planetary arrangements defies easy explanation.

Penn State University researchers uncovered another oddity challenging current understanding of stellar system development. [1] Study of the LHS 3154 system reveals a planet so massive in comparison to its star that generally accepted theories of planet formation cannot explain the existence of the planet, Figure 1. LHS 3154, an “ultracool” star with a “chilly” surface temperature of 2,700 °K (2,430 °C; 4,400 °F), is an M-dwarf, a category that comprises three quarters of the stars in the Milky Way. Most of the light of LHS 3154 is in the infrared band. The M- dwarf star is nine times less massive than the Sun yet it hosts a planet 13 times more massive than Earth.

Figure 1. An artist rendition of the mass comparison between the Earth and Sun and the star LHS 3154, and its companion, LHS 3154b. Credit: Pennsylvania State University.

In current theories, stars form from condensing large clouds of gas and dust into smaller volumes. After the star forms, the left-over gas and dust which is a much smaller fraction of the original cloud, settles into a disk around the new star. From this much smaller mass, planets will condense, completing the star system. In these theories, the star consumes the major proportion of the progenitor clouds.

The Sun, for example, contains an estimated 99.8% of the mass of the Solar System. Only 0.2% is left over for the eight planets, various moons and asteroids.

The mass ratio comparing LHS 3154b to LHS 3154 is 117 times greater than mass ratio comparing the Earth to the Sun. LHS 3154b probably is Neptune-like in composition, completes its orbit in 3.7 Earth days and, the researchers believe, is a very rare world. Typically M-dwarves host small rocky bodies rather than gas giants.

According to current theories, once the star formed, there should not have been enough mass to form a planet as large as LHS 3154b. A young LHS 3154 disk dust-mass and dust-to-gas ratio must be ten times greater than what is typically observed surrounding an M-dwarf star to birth a giant such as LHS 3154b.

“The planet-forming disk around the low-mass star LHS 3154 is not expected to have enough solid mass to make this planet,” Suvrath Mahadevan, the Verne M. Willaman Professor of Astronomy and Astrophysics at Penn State and co-author on the paper said. “But it’s out there, so now we need to reexamine our understanding of how planets and stars form.”

Mahadevan’s team built a novel spectrograph, the Habitable Zone Planet Finder (HPF), with the intention of detecting planets orbiting the coolest of stars. Planets orbiting low temperature stars might have surfaces cool enough to support liquid water and life. In looking for planets with liquid water, the team found, as often happens in research, something new, a massive planet to challenge current theories of stellar system formation.

Another discovery, this time by a Carnegie Institution for Science team, uncovered another challenging world. [2]

Figure 2. Artist’s conception a small red dwarf star, TOI-5205, and its out-sized companion TOI-5205b. Credit: Katherine Cain, the Carnegie Institution for Science.

“The host star, TOI-5205, is just about four times the size of Jupiter, yet it has somehow managed to form a Jupiter-sized planet, which is quite surprising,” observed Shubham Kanodia, who led the team which found TOI-5205b.

When TOI-5205b crosses in front of TOI-5205, the planet blocks about seven percent of the star’s light—a dimming among the largest known exoplanet transit signals.

The rotating disk of gas and dust that surrounds a young star gives birth to its planetary companions. More massive planets require more of the gas and dust left over as the star ignites. Gas planet formation, in the accepted theories, requires about 10 Earth masses of rocky material to produce the massive rocky core of the gas giant. Once the core is formed, it gathers gas from the surrounding clouds, resulting in the mammoth atmosphere of the giant planet.

“TOI-5205b’s existence stretches what we know about the disks in which these planets are born,” Kanodia explained. “In the beginning, if there isn’t enough rocky material in the disk to form the initial core, then one cannot form a gas giant planet. And at the end, if the disk evaporates away before the massive core is formed, then one cannot form a gas giant planet. And yet TOI-5205b formed despite these guardrails. Based on our nominal current understanding of planet formation, TOI-5205b should not exist; it is a ‘forbidden’ planet.”

Not all mysteries are confined to M-dwarfs. A sun-like star, an infant of 14 million years some 360 light years from Earth, hosts a gas giant six times more massive than Jupiter, that orbits the star at a distance twenty times greater than the distance separating Jupiter and the Sun, Figure 3. [3]

Figure 3. A direct image of the exoplanet YSES 2b (bottom right) and its star (center). The star is blocked by a coronagraph. Credit: ESO/SPHERE/VLT/Bohn et al.

The large distance from YSES 2b to the star does not fit either of the two most well-known models describing large gaseous planet formation. If YSES 2b formed by means of core accretion at such an enormous distance far from the star, the planet should be much lighter than what is observed as a result of scarcity of disk material at that distant location. YSES 2b is too massive to satisfy this theory.

Gravitationally instability, the second theorized method for producing gas giants, postulates very massive protostellar disks that are unstable, splintering into large clumps from which gas giants are directly formed. YSES 2b appears not massive enough to have been formed in this fashion.

In a third possibility, YSES 2b might have formed by core accretion much closer to its host star and migrated outwards. A second planet is needed to pull YSES 2b into the outer regions of the system, but no such planet has been located.

Observations by the current generation of space-borne telescopes have upset the theories of planet formation. Hot Jupiters, worlds orbiting pulsars, odd arrangements of worlds, super Earths, and wandering worlds flung close to a star then flying back have complicated the ideas of Laplace, See, Chamberlin and Moulton. Further study by the James Webb Space Telescope and its successors will only enliven the debate surrounding the origin of the planets.

References

[1] Guðmundur Stefánsson, Suvrath Mahadevan, Yamila Miguel, et al, “A Neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models,” Science, 30 Nov 2023, Vol. 382, Issue 6674, pp. 1031-1035, DOI: 10.1126/science.abo0233.

[2] Shubham Kanodia et al, “TOI-5205b: A Short-period Jovian Planet Transiting a Mid-M Dwarf,” The Astronomical Journal (2023). DOI: 10.3847/1538-3881/acabce

[3] Alexander J. Bohn et al. “Discovery of a directly imaged planet to the young solar analog YSES 2.” Accepted for publication in Astronomy & Astrophysics, www.aanda.org/10.1051/0004-6361/202140508