Centauri Dreams
Imagining and Planning Interstellar Exploration
‘Hycean’ Worlds: A New Candidate for Biosignatures?
We’ve just seen the coinage of a new word that denotes an entirely novel category of planets. Out of research at the University of Cambridge comes a paper on a subset of habitable worlds the scientists have dubbed ‘Hycean’ planets. These are hot, ocean-covered planets with habitable surface conditions under atmospheres rich in hydrogen. The authors believe they are more common than Earth-class worlds (although much depends upon their composition), and should offer considerable advantages when it comes to the detection of biosignatures.
Hycean worlds give us another habitable zone, this one taking in a larger region than the liquid water habitable zone we’ve always considered as the home to Earth. In every respect they challenge our categories. Not so long ago a Cambridge team led by Nikku Madhusudhan found that K2-18b, 2.6 times Earth’s radius and 8.6 times its mass, could maintain liquid water at habitable temperatures beneath its hydrogen atmosphere. The team has now generalized this work with a full investigation of the planetary and stellar properties making life possible on such planets.
Planets between the size of Earth and Neptune are thus far the most common type of planet we’ve found, generally being labeled as ‘super-Earths’ or ‘mini-Neptunes.’ There are no analogues to planets like this in the Solar System; they are classed as super-Earths or mini-Neptunes largely on the basis of their density as inferred by their mass and radius. Some may be predominantly rocky, while others are closer to the ice giants in our system. Some may be water worlds. Some of them are in the habitable zones of nearby M-dwarf stars, making them good candidates for atmospheric studies and possible biosignature detections.
For planets with a hydrogen atmosphere surrounding a layer of high-pressure water covering an inner core of rock and iron may become astrobiologically interesting. It’s true that too dense a hydrogen envelope would create temperature and pressure at the surface that would preclude life. But if the atmosphere is not too thick, life-sustaining temperatures can exist.
The Hycean planets thus represent a new category of potentially habitable worlds, and can be up to 2.6 times the size of Earth, with atmospheric temperatures up to 200 degrees Celsius, while still remaining habitable. They are defined not only by size but also by mass, temperature and atmospheric pressure. Conditions in their oceans may allow at least microbial life.
We are looking at a wide habitable zone as well. Its range takes in planets with orbital separations so large that the only energy source would be internal heat. It also extends to planets orbiting so close to the host star that they are tidally locked, but can support life on their dark sides. The span of possible temperatures allowing life to exist is thus substantial. About the tidally locked worlds the authors refer to as ‘Dark Hycean,’ for example, we learn this:
…we nominally consider the planet-wide average surface and atmospheric temperature to be 500 K. The choice of this temperature is motivated by the atmospheric models for nightsides of Dark Hycean planets…. In particular, we find that planets with equilibrium temperatures of ?510 K with inefficient day-night energy redistribution can lead to dayside temperatures of ?500-600 K but nightside surface temperatures ?400 K. Therefore, while a 510 K temperature is not considered to be habitable, it represents a planet wide average and still allows a nonnegligible fraction of the nightside ocean surface to be at habitable surface temperatures, i.e., below 400 K.
Image: Artist’s conception of the surface of a Hycean planet. Credit: Amanda Smith, Nikku Madhusudhan.
The researchers believe these interesting worlds are common. In their paper, they present a sample of potential Hycean targets that could be useful fodder for next-generation telescopes. All of these orbit red dwarf stars close enough to be suitable targets for the James Webb Space Telescope; none are more than 150 light years away. JWST observations of K2-18b are already being considered and could conceivably provide a biosignature detection. For having looked at five potential biomakers in Hycean atmospheres, the authors note:
Hycean atmospheres may offer even better opportunities for detecting these biomarkers than those of rocky super-earths… For a 10 M? planet, the Hycean radius range is ?2-2.6 R? compared to the super-Earth radius of 1.75 R? considered in Seager et al. (2013b). The increased radii and lower gravities lead to larger, more easily detectable spectral signatures for Hycean planets. Second, considering that prominent sources of the above biomarkers are thought to be aquatic microorganisms, we expect them to be even more abundant on Hycean worlds compared to predominantly rocky worlds.
The fact that Hycean planets open up the discovery space for worlds that could support life makes them noteworthy. We begin to consider planets of higher mass and radius than before, provided they have a rocky core that, according to the paper, is at least 10% of planetary mass and is of Earth-like composition. Their wide habitable zone expands the area for detection, while presenting a range of challenges that is likewise wide. Mass and radius help us spot a Hycean candidate, but they alone are not sufficient. We also need to learn more about temperatures and pressures in the ocean, and basic properties of the atmosphere:
,,,even if a candidate Hycean planet is in the Hycean HZ it may not necessarily have the right conditions for habitability, e.g., the internal structure and atmospheric properties may be such that the ocean surface pressure and/or temperature is too high… [T]he detection of H2O in the atmosphere does not guarantee the presence of an ocean on the planet, as H2O can be naturally occurring in H2-rich atmospheres as the prominent oxygen bearing species. Conversely, the nondetection of H2O does not preclude the presence of an ocean, since at low atmospheric temperatures H2O can rain out and not be detectable in the atmosphere. Nevertheless, in all these aspects Hycean candidates offer better prospects for establishing their habitability compared to habitable rocky exoplanets, which are inherently harder to characterize.
Given the wide range of transiting worlds we’ve discovered between 1 and 2.6 Earth radii, Hycean worlds offer no shortage of targets and if nothing else provide opportunities for atmospheric characterization that, according to the authors, should be less challenging than similar work on rocky exoplanets. Their large radius and thick atmospheres seem made to order for JWST and future instruments like the Extremely Large Telescope. Although not similar to Earth, Hycean planets can be valuable venues for detecting trace biosignatures. That alone contributes to the larger quest of finding life on planets more similar to our own.
The paper is Madhusudhan et al., “Habitability and Biosignatures of Hycean Worlds,” in process at The Astrophysical Journal (2021). Preprint. The paper on K2-18b is “The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b,” Astrophysical Journal Letters Vol. 891, No. 1 (27 February 2020), L7 (abstract). And I’ve just become aware of (but haven’t yet read) Benncke et al., “Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b,” Astrophysical Journal Letters 887:L14 (10 December 2019). Abstract.
A Huge Population of Interstellar Comets in the Oort Cloud
TAOS II is the Transneptunian Automated Occultation Survey, designed to spot comets deep in our Solar System. It may also be able to detect comets of the interstellar variety, of which we thus far have only one incontrovertible example, 2I/Borisov. And TAOS II, as well as the Vera C. Rubin Observatory (both are slated for first light within a year or so) could have a lot to work with, if a new study from Amir Siraj and Avi Loeb (Center for Astrophysics | Harvard & Smithsonian) is correct in its findings.
I cite Borisov as thus far unique in being an interstellar comet because the cometary status of ‘Oumuamua is still in play. On my way to looking at his paper on Borisov, I had an email exchange with Avi Loeb, from which this:
Observations with the Spitzer Space Telescope of `Oumuamua placed very tight limits on carbon-based molecules in its vicinity, implying that it was not made of carbon or oxygen. This led to suggestions that perhaps it is made of pure hydrogen or pure nitrogen, but these would be types of objects we had never seen before. Borisov appeared to be just like a regular comet that we had seen many times before. Clearly, `Oumuamua and Borisov are of very different composition and origin (irrespective of whether `Oumuamua is natural or artificial in origin).
Image: Comet 2I/Borisov. Credit: NASA, ESA and D. Jewitt (UCLA).
The paper refers to Borisov as “the first confirmed interstellar comet with a known composition,” but if this comet is alone in our catalog, it’s unlikely to remain that way long. Siraj and Loeb argue that there exist more interstellar objects in the Oort Cloud than objects born in the Solar System. Indeed, Loeb in his email cited “a hundred trillion Borisov-like interstellar comets” in this vast space, which extends from roughly 2,000 AU perhaps as far out as 50,000 AU, with some sources citing an outer edge as far as 200,000 AU. That should ring a few bells — Alpha Centauri is 268,000 AU from the Sun, meaning our Oort Cloud could mingle with any similar cloud in that system.
The prospect of studying interstellar objects without leaving our own system is enhanced by these results, even if the calculations contain significant uncertainties. There should be many Borisovs, a small number of which should enter the inner system. This is a reversal of earlier thinking that interstellar visitors should be rare, all part of a reevaluation of the subject forced by the detection of ‘Oumuamua and 2I/Borisov in recent years, and the coming upgrades in equipment and surveys mentioned above.
We are only now getting into position to be able to see these objects and identify their true nature. The detection of Borisov in 2019 allowed scientists to calculate a number density for such objects per star based on a statistical analysis of the likelihood of a single object like this being within 3 AU of the Sun. Other researchers had applied this kind of calibration to ‘Oumuamua, with the number density implied by both being approximately the same. Similarly, the population of bound Oort Cloud comets can be inferred through observations of long-period comets. Figure 1 in the paper shows the comparison.
Image: This is Figure 1 from the paper. Caption: Comparison of the relative abundance per star of bound Oort cloud objects, as implied by the observed rate of long-period comets (Brasser & Morbidelli 2013), and interstellar objects, as implied by the detection of Borisov (Jewitt et al. 2020), with a differential size distribution for power-law index, q, values of 2.5, 3, and 3.5, displayed for reference. The error bar indicates the 3? Poisson error bars for the implication of a singular interstellar object detection on the abundance. The shaded band correspond[s] to the plausible range of nucleus radii for Borisov, given the central value for Borisov’s abundance. The error bounds on the abundance of bound Oort cloud objects are not resolvable on this plot. Credit: Siraj and Loeb.
These are calibrations with, as the paper notes, uncertainties of several orders of magnitude, but even adjusting for these, interstellar objects still prevail in the Oort. They also come with limits that can be tested by observation. Interstellar objects experience negligible gravitational focusing because of their speed (on the order of 30 kilometers per second) and the nature of their orbits as related to their distance from the Sun. Bound Oort Cloud objects should have characteristic orbits that can be differentiated from the orbits of objects that have entered the Oort from elsewhere.
Note: ‘Gravitational focusing’ refers not to gravitational lensing but to the likelihood that two particles will collide based on their mutual gravitational attraction. The authors are saying that bound Oort objects are significantly affected by gravitational focusing. We wind up with a wide dispersion in these two populations:
Given that the number density of interstellar objects may be ?103 larger than that of bound Oort cloud objects far from the Sun, the Oort cloud objects may be still a factor of ?10 more abundant than interstellar objects in the inner Solar system, due to the unequal influence of gravitational focusing on the two populations. The fact that interstellar objects outnumber Oort cloud objects per star is consistent with the Oort cloud having lost most of its initial mass. However, the degree to which interstellar objects outnumber Oort cloud objects is still very uncertain. Stellar occultation surveys of the Oort cloud will be capable of confirming the results presented here, by differentiating between the two populations through speed relative to the Sun…
Thus we can look to planned surveys of the sort mentioned above to test the abundances of the two classes of objects, and can expect more visitors of the Borisov kind, even if such comets are far more common in the Oort Cloud than in the inner system. Siraj points out that such an abundance of interstellar objects indicates that planetary formation leaves a great deal more debris than previously thought:
“Our findings show that interstellar objects can place interesting constraints on planetary system formation processes, since their implied abundance requires a significant mass of material to be ejected in the form of planetesimals. Together with observational studies of protoplanetary disks and computational approaches to planet formation, the study of interstellar objects could help us unlock the secrets of how our planetary system — and others — formed.”
The paper is Siraj & Loeb, “Interstellar objects outnumber Solar system objects in the Oort cloud,” Monthly Notices of the Royal Astronomical Society Vol. 507, Issue 1 (October, 2021) L-16-L18 (abstract).
Enter the ‘Belatedly Habitable’ Zone
The most common objection I hear about what we call the ‘habitable zone’ is that it specifies conditions only for life as we know it. It leaves out, for example, conceivable biospheres under the ice of gas giant moons, examples of which we possibly have here in the Solar System. But there is another issue with defining habitability in terms of atmospheric pressures that can support liquid water on the surface. As Jason Wright and Noah Tuchow (both at Penn State) point out in a recent paper, the classic habitable zone concept does not take the evolution of both planet and star into account.
It’s a solid point. A planet now residing in the habitable zone could have remained habitable since the earliest era of its formation. Or it could have become habitable at a later time. Thus Tuchow and Wright make a distinction between what they refer to as the Continuous Habitable Zone (CHZ) and a class of planets they refer to as ‘belatedly habitable.’ These worlds may benefit from changes in the location of the habitable zone as stellar properties change, or they may enter the habitable zone through planetary migration. They may represent a substantial fraction of planets in the habitable zone. But are they truly habitable?
As the authors see it, there is not a single belatedly habitable zone (let’s refer to this as the BHZ), but rather two. The outer consists of the planets whose stars become more luminous over time, thus moving the habitable zone outward. The question here would be whether planets like this can successfully thaw and become habitable. I like James Kasting’s term for these worlds, coined as long ago as 1993. He calls them ‘cold start’ planets, and they represent a lively area of current research.
The inner belatedly habitable zone holds stars around which the habitable zone moves inward as the star dims. These inner BHZ planets are an intriguing lot because they orbit a wide range of lower-mass objects. Both brown and white dwarfs dim with time as they cool, making previously uninhabitable worlds more clement, though the authors note that these may lose many of their volatiles before achieving temperate conditions.
And because of their ubiquity in the Milky Way, we should pay special attention to M-dwarf planets. These worlds may spend millions of years in a greenhouse phase, with the possible loss of water, before their host star has finished the contraction that will eventually place it on the main sequence, dimming enough for habitability.
Given these distinctions, the liquid water habitable zone is actually a combination that includes the Continuous Habitable Zone as well as the inner and outer belatedly habitable zones, and as the authors point out, at any specific time in a star’s history, these regions will have different sizes and as the star evolves, may disappear entirely.
Image: This is Figure 1 from the paper. Caption: Habitable zone evolution for a 0.5?M? M dwarf (left) and a 1.0?M? solar analog (right). Continuous habitability is considered to start at the dashed vertical line, roughly representing the planet formation timescale. The green regions on the plots represent the continuously habitable zone, while the orange and blue regions represent the inner and outer belated habitable zones respectively. Credit: Tuchow & Wright.
To consider what the authors call ‘belated habitability,’ the star’s evolutionary history must be considered along with the presence of volatiles and their origins, the rates of cooling and outgassing as a young planet evolves, its related geophysical processes and more. Thus the complexity of the habitable zone deepens, taking the edge off quick claims for habitability in any given system. The fact that a planet is in the habitable zone today does not necessarily mean that liquid water exists on its surface:
A large portion of exoplanets that we find in the habitable zones of other stars will lie in the belatedly habitable zones, and future missions will greatly benefit by considering belated habitability and not assuming these planets are habitable. For example, in a search for biosignatures, the target stars and the search strategy will be affected by whether or not one considers the habitability of these planets. While the special circumstances of their habitability have been overlooked in the past, belatedly habitable planets could have major implications for future mission design and warrant future study.
I think these are useful distinctions that should come into play as our new generation telescopes come online. It’s certainly true that the press often exaggerates new discoveries of ‘habitable zone planets’ (and our friend Andrew Le Page is a shrewd judge of such claims), but from the standpoint of creating a catalog of best targets for further investigation, we need to be able to winnow the list efficiently and accurately. The study of ‘belated habitability’ should prove a productive research path.
The paper is Tuchow & Wright, “Belatedly Habitable Planets,” Research Notes of the AAS,” Volume 5, No. 8 (August, 2021). Full text.
How to Explain Unusual Stellar Acceleration?
Anomalies in our models are productive. Often they can be explained by errors in analysis or sometimes systematic issues with equipment. In any case, they force us to examine assumptions and suggest hypotheses to explain them, as in the case of the unusual acceleration of stars that has turned up in two areas. Greg Matloff has written about one of them in these pages, the so-called Parenago’s Discontinuity that flags an unusual fact about stellar motion: Cool stars, including the Sun, revolve around galactic center faster than hotter ones.
This shift in star velocities occurs around (B-V) = 0.62, which corresponds to late F- or early G-class stars and extends down to M-dwarfs. In other words, stars with (B-V) greater than 0.61 revolve faster. The (B-V) statement refers to a color index that is used to quantify the colors of stars using two filters. One, the blue (B) filter, lets only a narrow range of wavelengths centered on blue colors through, while the (V) visual filter only passes wavelengths close to the green-yellow band. Hot stars have (B-V) indices closer to 0 (or negative); cool M-dwarf stars come out closer to 2.0. Proxima Centauri’s (B-V) number, for example, is 1.90. The (B–V) index for the Sun is about 0.65.
What the Soviet space scientist Pavel Parenago (1906-1960) had observed is that cooler stars, at least in the Sun’s neighborhood, revolve about 20 kilometers per second faster around the galactic center than hotter, more massive stars. Work in the 1990s using Hipparcos data on over 5,000 nearby main sequence stars clearly showed the discontinuity. An early question was whether or not this was an effect local to the Sun, or one that would prove common throughout the galaxy.
Image: Soviet space scientist Pavel Parenago. Credit: Visuotin? Lietuvi?.
The answer came in the form of a paper by Veniamin Vityazev (St. Petersburg State University) and colleagues in 2018. The scientists hugely expanded the stars in their sample by using the Gaia Data Release 1 dataset,. The Gaia release included some 1,260,000 stars on the main sequence and a further 500,000 red giants. As we saw yesterday, the Gaia mission, which launched in 2013, is continuing to gather data at the Earth-Sun L2 Lagrange point, using astrometric methods to determine the positions and motions of a billion stars in the galaxy.
You may remember that the Vityazev paper (citation below) and its consequences was the subject of a 2019 dialogue in these pages between Greg Matloff and Alex Tolley (see Probing Parenago: A Dialogue on Stellar Discontinuity). And for good reason: The Gaia dataset allowed the Russian scientist and colleagues to demonstrate that Parenago’s Discontinuity was not a local phenomenon, but was indeed galactic in nature. All of this naturally leads to questions about the mechanisms at work here — Matloff’s interest in panpsychism as one explanation is well known and discussed in his book Starlight, Starbright (Curtis, 2019).
But in a new paper with Matloff, Roman Ya. Kezerashvili (New York City College of Technology) and Kelvin Long (Interstellar Research Centre, UK) point to another noteworthy datapoint. The extensive Gaia dataset showed a feature that turned up in the Hipparcos data as well. There was what the authors call a ‘strange bump in the curve of galactic revolution velocity versus (B-V) color index between (B-V) ? 0.55 and (B-V) ? 0.9…’ This would take in stars of the spectral classes G1 through K2, and the authors add that the bump is apparent only in the direction of stellar galactic revolution, like Parenago’s Discontinuity itself. Is this to be construed as a subset of the discontinuity or a separate effect? We don’t yet know.
What we do have is apparent acceleration in the direction that stars rotate around the galaxy, an unusual effect that adds up: In a billion years, the positional shift between a star without this acceleration vs. with the acceleration is about 1,500 light years.
This second apparent acceleration has yet to be confirmed by other researchers, as the authors acknowledge. But as we await that result, the new paper investigates the physical processes that could account for it. If we consider a star expelling material in a specific direction, we can model the system by analogy to a rocket and its expelled fuel, and thus examine velocity change with the Tsiolkovsky rocket equation. From the paper:
There are a number of physical mechanisms responsible for expelling mass during the stars’ life. These include but may not be limited to: unidirectional or focused electromagnetic (EM) flux, solar wind, accelerated solar wind, coronal mass ejections (CMEs), unidirectional neutrino flux, and solar wind thermonuclear fusion. Each mechanism can be treated as an exhaust velocity Vex with a mass flow rate so that the acceleration of the star is analogous to a rocket thrust F = VexMs.
What stands out here is how big the force F = 1016 N — the force needed to accelerate stars like the Sun to this level — is from our terrestrial perspective. It is the equivalent of the force exerted by two billion 106 kg rockets, each accelerating at 3g. Backing out to cosmic scale, though, the force appears less daunting, being 5 orders of magnitude less than the mutual gravitational force between the Earth and the Sun.
The mechanisms of mass loss are many and varied, as noted above, and the paper goes to work on eight of them:
- Unidirectional or focused stellar electromagnetic flux
- Galactic cannibalism
- Stellar mass loss by thermonuclear fusion
- Non-isotropic stellar wind
- Accelerated unidirectional stellar wind
- Coronal mass ejections
- Unidirectional neutrino flux
- Solar wind thermonuclear fusion
Note how many of these demand an existing advanced technology to implement them. In fact, of the eight, only two have solutions that are what Matloff calls ‘mechanistic;’ i.e. operational within nature without technological intervention. These are galactic cannibalism and stellar sass loss by thermonuclear fusion. Neither show promise as a cause for stellar acceleration of this order.
Galactic cannibalism refers to interactions between dwarf galaxies and large galaxies like the Milky Way, with the satellite galaxies being absorbed. This adds to the mass of the primary galaxy and may increase star orbiting velocities, but the authors discount the process because stellar nurseries are also accelerated by galaxy mergers, just as pre-existing stars. Stellar mass loss by thermonuclear fusion due not only to fusion within the core but via the stellar wind — which is in any case omnidirectional — likewise fails to produce the observed acceleration effect across classes of stars.
The other six options all involve the application of advanced technologies to create stellar motion in a specific direction, and some of these don’t work either. Unidirectional coronal mass ejections, for example, even if they could be shaped as thrust, fail to provide the needed energies. What emerge as viable candidates are the combined effects of a unidirectional neutrino jet and an accelerated unidirectional stellar wind “in conjunction with the unidirectional photon jet ejected from the star and non-isotropic stellar wind.” Thus on the accelerated unidirectional stellar wind:
Consider the case of a dual-purpose Dyson/Stapledon megastructure constructed around a star. Confirmation of the existence of such a megastructure would constitute evidence for the existence of a Kardashev Level 2 or higher civilization. A fraction of the star’s radiant output is used to accelerate a fraction of the star’s stellar wind as a unidirectional jet.
Obviously these options take us into science fiction territory, such as the recent Greg Benford/Larry Niven collaboration beginning with Bowl of Heaven (Tor, 2012), where a star in directed motion carries with it a vast environmental shell. So as an example of just one way to do all this, let me quote from an article Benford wrote for Centauri Dreams in 2014 (see Building the Bowl of Heaven):
Our Bowl is a shell more than a hundred million miles across, held to a star by gravity and some electrodynamic forces. The star produces a long jet of hot gas, which is magnetically confined so well it spears through a hole at the crown of the cup-shaped shell. This jet propels the entire system forward – literally, a star turned into the engine of a “ship” that is the shell, the Bowl. On the shell’s inner face, a sprawling civilization dwells…
The jet passes through a Knothole at the “bottom” of the Bowl, out into space, as exhaust. Magnetic fields, entrained on the star surface, wrap around the outgoing jet plasma and confine it, so it does not flare out and paint the interior face of the Bowl — where a whole living ecology thrives, immensely larger than Earth’s area.
Image: Moving a star as depicted in Bowl of Heaven. Credit: Don Davis.
But the idea of moving stars via technology has a long pedigree. As just one example, let me quote Fritz Zwicky, that fiercely independent and prescient thinker, from an article he published in June, 1961 in Engineering and Science:
In order to exert the necessary thrust on the sun, nuclear fusion reactions could be ignited locally in the sun’s material, causing the ejection of enormously high-speed jets. The necessary nuclear fusion can probably best be ignited through the use of ultrafast particles being shot at the sun. To date there are at least two promising prospects for producing particles of colloidal size with velocities of a thousand kilometers per second or more. Such particles, when impinging on solids, liquids, or dense gases, will generate temperatures of one hundred million degrees Kelvin or higher-quite sufficient to ignite nuclear fusion. The two possibilities for nuclear fusion ignition which I have in mind do not make use of any ideas related to plasmas, and to their constriction and acceleration in electric and magnetic fields…
So could an advanced culture pull this off? The authors are quick to point out that the eight mechanisms they evaluate do not exhaust the range of possibilities, but thus far the physical mechanisms under investigation fail the test unless assisted by a technology. The first order of business, of course, will be to verify the apparent stellar acceleration and then to ponder stellar kinematics in their combined light.
But while we can envision advanced cultures developing the tools to move individual stars, as breathtaking as the concept seems, it appears unlikely in the extreme that entire stellar classes are being moved about simultaneously by megastructures or other technologies.
I think the existence of an explanation within the realm of nature has to be assumed. The question, of course, is just where it is to be found. This spirited paper, cheerfully speculative, contains an open call to the community to investigate anomalous stellar accelerations and produce other hypotheses explaining this curious behavior.
The paper is Kezerashvili, Matloff & Long, “Anomalous Stellar Acceleration: Causes and Consequences,” JBIS Vol. 74 (2021), pp. 269-275 (full text). The Vityazev paper is V. V. Vityazev et al., “New Features of Parenago’s Discontinuity from Gaia DR1 Data,” Astronomy Letters 44, 629-644 (2018). Abstract.
Star-Forming Regions Trace a New Galactic Structure
Infrared imagery drawn from Spitzer Space Telescope data, coupled with the massive Gaia Early third Data Release (EDR3), have just given us a new insight into our galaxy’s spiral structure. The Milky Way’s Sagittarius Arm is now shown to have a ‘spur’ of star-forming gas and young stars emerging at a steep angle and stretching some 3,000 light years. The authors of the paper on this work refer to it as “unprecedented in the context of the generally adopted model of the Milky Way spiral structure.”
The spur was a tricky catch, because from our position within the galactic disk we can only see the full spiral structure in galaxies other than our own. But the authors point out that in these galaxies, spiral arms often show smaller-scale structures including ‘spurs,’ which are luminous groupings of stars, and ‘feathers,’ which are dust features. We also find branching in the main arms. Now we’ve identified a spur structure in the Milky Way.
Image: Artist’s concept of the Milky Way. The galaxy’s two major arms (Scutum-Centaurus and Perseus) can be seen attached to the ends of a thick central bar, while the two minor arms (Norma and Sagittarius) are less distinct and located between the major arms. The major arms consist of the highest densities of both young and old stars; the minor arms are primarily filled with gas and pockets of star-forming activity. The new structure (not shown here) has been found to extend from the Sagittarius Arm. Credit: NASA/JPL-Caltech.
Early work on galactic structure going back to the 1950s homed in on nearby star-forming regions in work that helped to define the Sagittarius Arm in the first accepted galactic map. Today we have a great deal more data to work with than earlier researchers. Michael Kuhn (Caltech) and collaborators homed in on the Sagittarius Arm, looking for data on infant stars — the paper refers to ‘young stellar objects’ (YSO) — still nestled in the molecular clouds of their birth nebulae; these are thought to align with the shape of the arms in which they are found. Indeed, in other spiral galaxies star formation follows the features of the spiral pattern.
The researchers drew on a Spitzer-derived catalog called SPICY (Spitzer/IRAC Candidate YSO) to study a nearby portion of the Sagittarius Arm, using the information to map out star-forming regions and compare their actual distribution with models of the arm. They also used a catalog derived from Spitzer data called the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) containing more than 100,000 newborn stars. But it took Gaia to map the result in 3-D.
The European Space Agency’s mission tracks the motions, luminosity, temperature and composition of stars down to magnitude 20. Its most recent data release, EDR3, contains astrometry information on the distances and velocities of these objects. Using EDR3, the authors inferred the parallax and proper motions for their star-forming regions and estimated their radial velocities. The team rightly points to the “renaissance in investigations of Galactic spiral structure within a few kiloparsecs of the Sun” that Gaia’s positional and radial velocity measurements have produced. The Gaia achievement is not exaggerated: Thanks to this mission, we can now examine the position and kinematics of over a billion stars.
Out of all this comes evidence for a long, thin structure of young stars moving roughly with the Sagittarius Arm and in the same direction. Kuhn points out that as the structure of a spiral galaxy becomes more open, its pitch angle increases. If a circle has a pitch angle of 0 degrees, the most recent models of the Milky Way show that the Sagittarius Arm has a pitch angle of 12 degrees. The new structure breaks radically here, with an angle of nearly 60 degrees.
Previous studies have suggested a linear structure in this region, but the high pitch angle of this spur has not previously been noted. According to the authors’ models, the structure would appear as a bright stellar feature from another galaxy, paralleling the same kind of high pitch-angle structures found in many. Gravitational instabilities may account for their formation, with mass concentrations within the arms shearing as a consequence of the galaxy’s rotation. In any case, the authors see the structure as “an excellent laboratory for examining star formation on scales large enough to be compared to extragalactic observations, but with the ability to resolve the mass function, spatial distribution, and kinematics of the individual sources.”
Robert Benjamin (University of Wisconsin-Whitewater) is principal investigator for the GLIMPSE survey:
“Ultimately, this is a reminder that there are many uncertainties about the large-scale structure of the Milky Way, and we need to look at the details if we want to understand that bigger picture. This structure is a small piece of the Milky Way, but it could tell us something significant about the Galaxy as a whole.”
Image: These four nebulae (star-forming clouds of gas and dust) are known for their breathtaking beauty: the Eagle Nebula (which contains the Pillars of Creation), the Omega Nebula, the Trifid Nebula, and the Lagoon Nebula. In the 1950s, a team of astronomers made rough distance measurements to some of the stars in these nebulae and were able to infer the existence of the Sagittarius Arm. Their work provided some of the first evidence of our galaxy’s spiral structure. In the new study, astronomers have shown that these nebulae are part of a substructure within the arm that is angled differently from the rest of the arm. Credit: NASA/JPL-Caltech.
The paper is Kuhn et al., “A high pitch angle structure in the Sagittarius Arm,” Astronomy & Astrophysics Vol. 651, L10 (July 2021). Full text.
A Landing Site for Dragonfly
Rotorcraft have certainly been in the news lately, with Ingenuity, the Mars helicopter, commanding our attention. The Dragonfly mission to Titan involves a far more complex rotorcraft capable of visiting numerous destinations on the surface. In fact, Dragonfly makes use of eight rotors and depends upon an atmosphere more helpful than what Ingenuity has to work with on Mars. Titan’s atmosphere is four times denser than what we have on Earth, allowing Dragonfly to move its entire science payload from one location to another as it examines surface landing zones while operating on a world whose gravity is but one-seventh that of Earth.
I want to call your attention to the publication of the science team that just appeared in the Planetary Science Journal, because it lays out the rationale for the various decisions made thus far about operations on and above Titan’s surface. It’s a straightforward, interesting read, and makes clear how much work we have to do here. Yes, we had Cassini for a breathtaking tour that lasted 13 years, with repeated flybys and investigations on Titan using radar, but while we know a lot about structures like lakes and mountains on the surface, we know all too little about their composition.
In fact, as Alex Hayes points out, we didn’t know at the time Cassini launched whether the Huygens probe would find a global ocean at Titan or a solid surface of ice and organics. Because of the uncertainty, the Huygens science experiments were primarily atmospheric, meant to function during the descent phase. Hayes (Cornell University) is a co-investigator for Dragonfly. He adds:
“The science questions we have for Titan are very broad because we don’t know much about what is actually going on at the surface yet. For every question we answered during the Cassini mission’s exploration of Titan from Saturn orbit, we gained 10 new ones.”
Image: What we do know. This is Figure 1 from the paper. Caption: Dragonfly will image from the surface to provide context for sampling and measurements, as well as in flight to identify sites of interest at a variety of locations. (Left) Huygens image of Titan’s surface; cobbles are 10-15 cm across and may be water ice (Tomasko et al. 2005; Keller et al. 2008; Karkoschka & Schröder 2016a). (Right) Huygens aerial view of terrain akin to the diverse equatorial landscapes that Dragonfly will traverse and image at higher resolution. Credit: NASA/ESA/Barnes et al.
Dragonfly’s 2.7 year mission, starting upon arrival at Titan in 2034 during winter in the northern hemisphere, will commence at a landing site that was chosen for its safety factors (broad, relatively flat terrain) as well as its proximity to nearby interesting scientific targets. The goal is to set down at the equatorial dune fields called Shangri-La, which NASA notes are similar to the dunes found in Namibia on Earth. A series of short flights will explore this area before longer flights of up to eight kilometers begin, the beauty of the design being that Dragonfly will be able to sample interesting surface areas along the route to its destination, the Selk impact crater.
As the mission now stands, the lander should log on the order of 175 kilometers across Titan enroute to Selk. The latter is an interesting place because there is evidence here of past liquid water as well as organics, complex molecules containing carbon, along with hydrogen, oxygen and nitrogen. Methane rain and a snow of organics keep Titan’s weather systems complex amidst a landscape containing the building blocks of life.
But let’s get back to that landing site. The paper refers to Shangri-La as an “organic sand sea,” with the touchdown site located 134 kilometers south of Selk Crater, and approximately 175 kilometers north-northwest of the Huygens Landing Site. The image below is Figure 7 from the paper, giving the landing site in context.
Image: Dragonfly landing site. Credit: Barnes et al.
As the paper notes, a ‘sand sea’ is only partially sand. Dunes can be separated by flat sand-free areas called ‘interdunes,’ a feature likewise common to Namibia, where the Namib sand sea is covered only 40 percent by sand. The interdunes that make up the balance are primarily gravel. Cassini was able to resolve Titan’s interdunes to reveal their predominantly icy character, one that matches the spectral properties at the Huygens landing site. The authors find the correlation interesting because it implies the Shangri-La interdunes will include water-ice gravels, “potentially a fine-grained layer damp with condensed methane,” and thus offering a chance for Dragonfly to sample both Titan’s organic sands and materials with a water ice component.
Image: This illustration shows NASA’s Dragonfly rotorcraft-lander approaching a site on Saturn’s exotic moon, Titan. Taking advantage of Titan’s dense atmosphere and low gravity, Dragonfly will explore dozens of locations across the icy world, sampling and measuring the compositions of Titan’s organic surface materials to characterize the habitability of Titan’s environment and investigate the progression of prebiotic chemistry. Credit: NASA/JHU-APL
The path to Selk Crater should take in a variety of terrain with different compositions, which will include the edge of the crater’s ejecta deposits. From Cassini data, the authors believe this material is similar in composition to the Huygens landing site, representing an area likely to feature water ice. Selk itself is 80 kilometers in diameter. Cassini data along with the Dragonfly team’s modeling show the spectral signature of organic sand in the interior and water-ice around the edges of the crater floor.
As you can see, the astrobiological examinations Dragonfly will engage in are both water-based and hydrocarbon-based, meaning a potential biosignature is possible from impact melt deposits or interactions with the interior ocean — this would be life as we know it — or from a form of life we have yet to discover that draws on liquid hydrocarbons within Titan’s lakes, seas and aquifers. The mission is designed around the ability to seek out both, as the paper explains:
We designed the science of Dragonfly around the themes of prebiotic chemistry, habitability, and the search for biosignatures, with an explicit consideration of both water and hydrocarbon solvents. To address prebiotic chemistry, we will determine the inventory of prebiotically relevant organic and inorganic molecules and reactions on Titan. In the realm of habitability, we will determine the role of Titan’s tropical atmosphere and shallow subsurface reservoirs in the global methane cycle, determine the rates of processes modifying Titan’s surface and rates of material transport, and constrain what physical processes mix surface organics with subsurface ocean and/or melted liquid-water reservoirs. Our search for biosignatures will entail a broad-based search for signatures indicative of past or extant biological processes.
The paper is Barnes et al., “Science Goals and Objectives for the Dragonfly Titan Rotorcraft Relocatable Lander,” Planetary Science Journal Vol. 2, No. 4 (full text).
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