Centauri Dreams regular Dave Moore just passed along a paper of considerable interest for those of us intrigued by planetary systems around red dwarf stars. The nearest known exoplanet of roughly Earth’s mass is Proxima Centauri b, adding emphasis to the question of whether planets in an M-dwarf’s habitable zone can indeed support life. From the standpoint of system dynamics, that often comes down to asking whether such a planet is not so close to its star that it will become tidally locked, and whether habitable climates could persist in those conditions. The topic remains controversial.
But there are wide variations between M-dwarf scenarios. We might compare what happens at TRAPPIST-1 to the situation around Proxima Centauri. We have an incomplete view of the Proxima system, there being no transits known, and while we have radial velocity evidence of a second and perhaps a third planet there, the situation is far from fully characterized. But TRAPPIST-1’s superb transit orientation means we see seven small, rocky worlds moving across the face of the star, and therein lies a tale.
The paper Dave sent, by Cody Shakespeare (University of Nevada Las Vegas) and colleague Jason Steffen, picks up on earlier work Shakespeare undertook that probes the differences between such scenarios. We know that conditions are right for a solitary planet, unperturbed by neighbors, to orbit with a spin rate synchronous with its orbital rate, the familiar ‘tidal lock.’ On such a world, we probe questions of climate, heat transport, the effects of an ocean and so on, to see if a planet with a star stationary in its sky could sustain life.
Image: This illustration shows what the TRAPPIST-1 system might look like from a vantage point near planet TRAPPIST-1f (at right). Credit: NASA/JPL-Caltech.
But as TRAPPIST-1 shows us in exhilarating detail, multi planet systems are not uncommon around this type of star, and now we have to factor in mean motion resonance (MMR), where the very proximity of the planets (all well within a fraction of Mercury’s orbit of our Sun) means that these effects can perturb a particular planet out of its otherwise spin-orbit synchronization. Call this ‘orbital forcing,’ which breaks what would have been, in a single-planet system, a system architecture that would inevitably lead to permanent tidal lock.
The results of this breakage produce the interesting possibility that planets like TRAPPIST-1 e and f may retain tidal lock but exhibit sporadic rotation (TLSR). Indeed, another recent paper referenced by the authors, written by Howard Chen (NASA GSFC) and colleagues, makes the case that this state can produce permanent snowball states in the outer regions of an M-dwarf planetary system. What is particularly striking about TLSR is the time frame that emerges from the calculations. Consider this, from the Shakespeare paper:
The TLSR spin state is unique in that the spin behavior is often not consistently tidally locked nor is it consistently rotating. Instead, the planet may suddenly switch between spin behaviors that have lasted for only a few years or up to hundreds of millennia. The spin behavior can occasionally be tidally locked with small or large librations in the longitude of the substellar point. The planet may flip between stable tidally locked positions by spinning 180°so that the previous substellar longitude is now located at the new antistellar point, and vice versa. The planet may also spin with respect to the star, having many consecutive full rotations. The spin direction can also change, causing prograde and retrograde spins.
Not exactly a quiescent tidal lock! Note the term libration, which refers to oscillations around the rotational axis of a planet. What Shakespeare and Steffen are analyzing is the space between long-lasting rotation and pure tidal lock. Indeed, the authors identify a spin scenario within the TLSR domain they describe as prolonged transient behavior, or PTB. Here the planet moves back and forth in a ‘spin regime’ that is essentially chaotic, so that questions of habitability become fraught indeed. Instead of a persistent climate, which we usually assume when assessing these matters, we may be looking at multiple states of climate determined by present and past spin regimes, and their necessary adaptation to the ever changing spin state.
Such global changes are reminiscent, though for different reasons, of Asimov’s fabled story “Nightfall,” in which scientists on a world in a system with six stars must face the social consequences of a ‘night’ that only appears every few thousand years. For here’s what Shakespeare and Steffen say about a scenario in which TSLR effects kick in, a world that had been tidally locked long enough for the climate to have become stable. The scenario again involves TRAPPIST-1:
Such a planet in the habitable zone around a TRAPPIST-1-like star could have an orbital period of around 4-12 Earth days – the approximate orbital periods of T-1d and T-1g, respectively. Due to the TLSR spin state, this planet may, rather abruptly, start to rotate, albeit slowly – on the order of one rotation every few Earth years. The previous night side of the planet, which had not seen starlight for many Earth years, will now suddenly be subjected to variable heat with a day-night cycle lasting a few years. The day side would receive a similar abrupt change and the climate state that prevailed for centuries would suddenly be a spinning engine with momentum but spark plugs that now fire out-of-sync with the pistons. In this analogy, the spark plugs and the subsequent ignition of fuel correspond to the input of energy from starlight. The response of ocean currents, prevailing winds, and weather patterns may be quite dramatic.
Not an easy outcome to model in terms of climate and habitability. The authors use a modified version of an energy release modeling software called 1D EBM HEXTOR as well as a model called the Hab1 TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) Protocol as they analyze these matters. I send you to the paper for the details.
Science fiction writers take note – here is rich material for new exoplanet environments. Notice that the TLSR spin state is different from the one-way change that occurs when a rotating planet gradually becomes tidally locked over large timescales. This is a regime of sudden change, or at least it can be. The authors consider spin regimes lasting less than 100 Earth years, with the longest regimes (these are classified as ‘quasi-stable’) lasting for 900 years or more and perhaps reaching durations of hundreds of thousands of years. The point is that “TLSR planets are able to be in both long-term persistent regimes and PTB regimes -– where frequent transitions between behaviors are present.”
We learn that all tidally locked bodies experience libration to some degree even if no other bodies are found in the system being examined. Four spin regimes are found within the broader spin state TLSR. Tidal lock with libration can occur around the substellar point, as well as around the substellar or antistellar point, or as noted a planet may be induced into a slow persistent rotation. Much depends upon how long any one of these ‘continuous’ states lasts; given enough time, a stable climate could develop. The chaotic behavior of the fourth state, prolonged transient behavior (PTB), induces frequent transitions in spin. Such transitions would be expected to produce extreme changes in climate.
The spin history of a given system will depend upon that system’s architecture and the key parameters of each individual planet, an indication of the complexity of the analysis. What particularly strikes me here is how fast some of these changes can occur. Here’s a science fiction scenario indeed:
The more extreme change is in the temperature of different longitudes as the planet transitions from a tidally locked regime to a Spinning regime or after the planet flips 180 and remains tidally locked. Rotating planets experience temperature changes at the equator of 50K or more over a single rotation period. The exact effects require more robust climate models, like 3D GCMs [Global Circulation Model], to properly examine. However, using comparisons with climate changes on Earth, it is likely that erosion of land masses would increase and major climate systems would experience significant changes.
As if the issue of habitability were not complex enough…
The paper is Shakespeare & Steffen, “Day and Night: Habitability of Tidally Locked Planets with Sporadic Rotation,” in process at Monthly Notices of the Royal Astronomical Society and available as a preprint. The Chen paper referenced above is “Sporadic Spin-Orbit Variations in Compact Multi-planet Systems and their Influence on Exoplanet Climate,” accepted at Astrophysical Journal Letters (preprint).
A search for liquid water on a planetary surface may be too confining when it comes to the wide range of possibilities for supporting life. We see that in our own Solar System. Consider the growing interest in icy moons like Europa and Enceladus, where there is no possibility of surface water but a potentially rich environment under a thick layer of ice. Extending these thoughts into the realm of exoplanets reminds us that our calculations about how many life-bearing worlds are out there may be in need of revision.
This is the thrust of work by Lujendra Ojha (Rutgers University) and colleagues, as developed in a paper in Nature Communications and presented at the recent Goldschmidt geochemistry conference in Lyon. What Ojha and team point out is that radiogenic heating can maintain liquid water below the surface of planets in M-dwarf systems, and that added into our astrobiological catalog, such worlds, orbiting a population of stars that takes in 75 percent or more of all stars in the galaxy, dramatically increase the chances of life elsewhere. The effect is striking. Says Ojha:
“We modeled the feasibility of generating and sustaining liquid water on exoplanets orbiting M-dwarfs by only considering the heat generated by the planet. We found that when one considers the possibility of liquid water generated by radioactivity, it is likely that a high percentage of these exoplanets can have sufficient heat to sustain liquid water – many more than we had thought. Before we started to consider this sub-surface water, it was estimated that around 1 rocky planet every 100 stars would have liquid water. The new model shows that if the conditions are right, this could approach 1 planet per star. So we are a hundred times more likely to find liquid water than we thought. There are around 100 billion stars in the Milky Way Galaxy. That represents really good odds for the origin of life elsewhere in the universe.”
Image: This is Figure 2 from the paper. Caption: Schematic of a basal melting model for icy exo-Earths. a Due to the high surface gravity of super-Earths, ice sheets may undergo numerous phase transformations. Liquid water may form within the ice layers and at the base via basal melting with sufficient geothermal heat. If high-pressure ices are present, meltwater will be buoyant and migrate upward, feeding the main ocean. The red arrows show geothermal heat input from the planet’s rocky interior. b Pure water phase diagram from the SeaFreeze representation illustrating the variety of phases possible in a thick exo-Earth ice sheet. Density differences between the ice phases lead to a divergence from a linear relationship between pressure and ice-thickness. Credit: Ohja et al.
The effect is robust. Indeed, water can be maintained above freezing even when planets are subject to as little as 0.1 Earth’s geothermal heat produced by radiogenic elements. The paper models the formation of ice sheets on such worlds and implies that the circumstellar region that can support life should be widened, which would take in colder planets outside what we have normally considered the habitable zone.
But the work goes further still, for it implies that planets closer to their host star than the inner boundaries of the traditional habitable zone may also support subglacial liquid water. We also recall that the sheer ubiquity of M-dwarfs in the galaxy helps us, for if water from an internal ocean does reach the surface, perhaps through cracks venting plumes and geysers, we may find numerous venues relatively close to the Sun on which to search for biosignatures.
The key factor here is subglacial melting through geothermal heat, for oceans and lakes of liquid water should be able to form under the ice on Earth-sized planets even when temperatures are as low as 200 K, as we find, for example, on TRAPPIST-1g, which is the coldest of the exoplanets for which Ojha’s team runs calculations.
Such water is found to be buoyant and can migrate through this ‘basal melting,’ a term used, explain the authors, for “any situation where the local geothermal heat flux, as well as any frictional heat produced by glacial sliding, is sufficient to raise the temperature at the base of an ice sheet to its melting point.” Subglacial ice sheets are found on Earth in the West Antarctic Ice Sheet, Greenland and possibly the Canadian Arctic, and the paper points out the possibility of the mechanism at work at the south pole of Mars.
The authors’ modeling uses a software tool called SeaFreeze along with a heat transport model to investigate the thermodynamic and elastic properties of water and ice at a wide range of temperatures and pressures. Given the high surface gravity of worlds like Proxima Centauri b, LHS 1140 b and some of the planets in the TRAPPIST-1 system, water ice should be subjected to extreme pressures and temperatures, and as the paper points out, may evolve into high-pressure ice phases. In such conditions, the meltwater migrates upward to form lakes or oceans. Indeed, this kind of melting and migration of water is more likely to occur on planets where the ice sheets are thicker and there is both higher surface gravity as well as higher surface temperatures.
Image: A frozen world heated from within, as envisioned by the paper’s lead author, Lujendra Ojha.
Beyond radiogenic heating, tidal effects are an interesting question, given the potential tidal lock of planets in close orbits around M-dwarfs. Yet planets further out in the system could still benefit from tidal activity, as the paper notes about TRAPPIST-1:
…the age of the TRAPPIST-1 system is estimated to be 7.6 ± 2.2 Gyr; thus, if geothermal heating has waned more than predicted by the age-dependent heat production rate assumed here, tidal heating could be an additional source of heat for basal melting on the TRAPPIST-1 system. On planets e and f of the TRAPPIST-1 system, tidal heating is estimated to contribute heat flow between 160 and 180 mW m−2. Thus, even if geothermal heating were to be negligible on these bodies, basal melting could still occur via tidal heating alone. However, for TRAPPIST-1 g, the mean tidal heat flow estimate from N-body simulation is less than 90 mW m−2. Thus, ice sheets thinner than a few kilometers are unlikely to undergo basal melting on TRAPPIST-1 g.
So we have two mechanisms in play to maintain lakes or oceans beneath surface ice on M-dwarf planets. The finding is encouraging given that one of the key objections to life in these environments is the time needed for life to evolve given that the young planet should be bombarded by ultraviolet and X-ray radiation, a common issue for these stars. We put in place what Amri Wandel (Hebrew University of Jerusalem), who writes a commentary on this work for Nature Communications, calls ‘a safe neighborhood,’ and one for which forms of biosignature detection relying on plume activity will doubtless emerge building on our experience at Enceladus and Europa.
The paper is Ojha et al., “Liquid water on cold exo-Earths via basal melting of ice sheets,” Nature Communications 13, Article number: 7521 (6 December, 2022). Full text. Wandel’s excellent commentary is “Habitability and sub glacial liquid water on planets of M-dwarf stars,” Nature Communications 14, Article number: 2125 (14 April 2023). Full text.
It would be useful to have a better handle on how and when water appeared on the early Earth. We know that comets and asteroids can bring water from beyond the ‘snowline,’ that zone demarcated by temperatures beyond which volatiles like water, ammonia or carbon dioxide are cold enough to condense into ice grains. For our Solar System, that distance in our era is 5 AU, roughly the orbital distance of Jupiter, although the snowline would have been somewhat closer to the Sun during the period of planet formation. So we have a mechanism to bring ices into the inner Solar System but don’t know just how large a role incoming ices played in Earth’s development.
Knowing more about the emergence of volatiles on Earth would help us frame what we see in other stellar systems, as we evaluate whether or not a given planet may be habitable. Usefully, there are ways to study our planet’s formation that can drill down to its accretion from the materials in the original circumstellar disk. A new study from Caltech goes to work on the magmas that emerge from the planetary interior, finding that water could only have arrived later in the history of Earth’s formation.
Published in Science Advances, the paper involves an international team working in laboratories at Caltech as well as the University of the Chinese Academy of Sciences, with Caltech grad student Weiyi Liu as first author. When I think about studying magma, zircon comes first to mind. It appears in crystalline form as magma cools and solidifies. I’m no geologist, but I’m told that the chemistry of melt inclusions can identify factors such as volatile content and broader chemical composition of the original magma itself. Feldspar crystals are likewise useful, and the isotopic analysis of a variety of rocks and minerals can tell us much about their origin.
So it’s no surprise to learn that the Caltech paper uses isotopes, in this case the changing ratio of isotopes of xenon (Xe) as found in mid-ocean ridge basalt vs. ocean island basalt. Specifically, 129Xe* comes from the radioactive decay of the extinct volatile 129I, whose half-life is 15.7 million years, while 136Xe*Pu comes from the extinction of 244Pu, with a halflife of 80 million years. So the 129Xe*/136Xe*Pu ratio is a useful tool. As the paper notes, this ratio:
…evolves as a function of both time and reservoirs compositions (i.e., I/Pu ratio) early in Earth’s history. Hence, the study of the 129Xe*/136Xe*Pu in silicate reservoirs of Earth has the potential to place strong constraints on Earth’s accretion and evolution.
The ocean island basalt samples, originating as far down as the core/mantle boundary, reveal this ratio to be low by a factor of 2.8 as compared to mid-ocean ridge basalts, which have their origin in the upper mantle. Using computationally intensive simulations drawing on what is known as first-principles molecular dynamics (FPMD), the authors find that the low I/Pu levels were established in the first 80 to 100 million years of the Solar System (thus before 129I extinction), and have been preserved for the past 4.45 billion years. Their calculations assess the I/Pu findings under different accretion scenarios, drawing on simulated magmas from the lower mantle, which runs from 680 kilometers below the surface, to the core-mantle boundary (2,900 kilometers), and also from the upper mantle beginning at 15 kilometers and extending downward to 680 kilometers.
The result: The lower mantle reveals an early Earth composed primarily of dry, rocky materials, with a distinct lack of volatiles, with the later-forming upper mantle numbers showing three times the amount of volatiles found below. The volatiles essential for life seem to have emerged only within the last 15 percent, and perhaps less, of Earth’s formation. In the caption below, the italics are mine.
Image: This is Figure 4 from the paper. Caption: Schematic representation of the heterogeneous accretion history of Earth that is consistent with the more siderophile behavior of I and Pu at high P-T [pressure-temperature] conditions (this work). As core formation alone does not result in I/Pu fractionations sufficient to explain the ~3 times lower 129Xe*/136Xe*Pu ratio observed in OIBs [ocean island basalt] compared to MORBs [mid-ocean ridge basalt], a scenario of heterogeneous accretion has to be invoked in which volatile-depleted differentiated planetesimals constitute the main building blocks of Earth for most of its accretion history (phase 1), before addition of, comparatively, volatile-rich undifferentiated materials (chondrite and possibly comet) during the last stages of accretion (phase 2).Isolation and preservation, at the CMB [core mantle boundary], of a small portion of the proto-Earth’s mantle before addition of volatile-rich material would explain the lower I/Pu ratio of plume mantle, while the mantle involved in the last stages of the accretion would have higher, MORB-like, I/Pu ratios. Because the low I/Pu mantle would also have an inherently lower Mg/Si, its higher viscosity could help to be preserved at the CMB until today. Credit: Liu et al.
We’re a long way from knowing in just what proportions Earth’s water has derived from incoming materials from beyond the snowline. But we’re making progress:
…our model sheds light on the origin of Earth’s water, as it requires that chondrites represent the main material delivered to Earth in the last 1 to 15% of its accretion. Independent constraints from Mo [molybdenum] nucleosynthetic anomalies require these late accreted materials to come from the carbonaceous supergroup. Together, these results indicate that carbonaceous chondrites [the most primitive class of meoteorites, containing a high proportion of carbon along with water and minerals] must have represented a non-negligible fraction of the volatile-enriched materials in phase 2 and, thus, play a substantial role in the water delivery to Earth.
All this from the observation that mid-ocean ridge basalts had roughly three times higher iodine/plutonium ratios (inferred from xenon isotopes) as compared to ocean island basalts. The key to this paper, though, is the demonstration that the ratio difference is likely from a history of accretion that began with dry planetesimals followed by a secondary accretion phase driven by infalling materials rich in volatiles.
Thus Earth presents us with a model of planet formation from dry, rocky materials, one that presumably would apply to other terrestrial worlds, though we’d like to know more. To push the inquiry forward, Caltech’s Francois Tissot, a co-author on the paper, advocates looking at rocky worlds within our own Solar System:
“Space exploration to the outer planets is really important because a water world is probably the best place to look for extraterrestrial life. But the inner solar system shouldn’t be forgotten. There hasn’t been a mission that’s touched Venus’ surface for nearly 40 years, and there has never been a mission to the surface of Mercury. We need to be able to study those worlds to better understand how terrestrial planets such as Earth formed.”
And indeed, to better measure the impact of ices brought from far beyond the snowline to the infant worlds of the inner system. Tissot’s work demonstrates how deeply we are now delving into the transition between planetary nebulae and fully formed planets. working across the entire spectrum of what he calls ‘geochemical problematics,’ which includes studying the isotopic makeup of meteorites and their inclusions, the reconstruction of the earliest redox conditions in the Earth’s ocean and atmosphere, and the analysis of isotopes to investigate ancient magmas. At Caltech, he has created the Isotoparium, a state-of-the-art facility for high-precision isotope studies.
That we are now probing our planet’s very accretion is likely not news to many of my readers, but it stuns me as another example of extraordinary methodologies driving theory forward through simulation and laboratory work. And as we don’t often consider work on the geological front in these pages, it seems a good time to point this out.
The paper is Weiyi Liu et al., “I/Pu reveals Earth mainly accreted from volatile-poor differentiated planetesimals,” Science Advances Vol. 9, No. 27 (5 July 2023) (full text).
I’m collecting a number of documents on gravitational wave detection and unusual concepts regarding their use by advanced civilizations. It’s going to take a while for me to go through all these, but as I mentioned in the last post, I plan to zero in on the intriguing notion that civilizations with abilities far beyond our own might use gravitational waves rather than the electromagnetic spectrum to serve as the backbone of their communication system. It’s a science fictional concept for sure, though there may be ways it could be imagined for a sufficiently advanced culture.
For today, though, let’s look at a new survey that targets highly unusual planets. Binaries Escorted by Orbiting Planets has an acronym I can get into: BEBOP. It awakens the Charlie Parker in me; I can almost smell the smoky air of a mid-20th century jazz club and hear the clinking of glasses above Parker’s stunning alto work. I was thinking about the great sax player because I had just watched, for about the fifth time, Clint Eastwood’s superb 1988 film Bird, whose soundtrack is, of course, fabulous.
On the astronomy front, the BEBOP survey is a radial velocity sweep for circumbinary planets, those intriguing worlds, rare but definitely out there, that orbit around two stars in tight binary systems. Beginning in 2013, BEBOP targeted 47 eclipsing binaries, using data from the CORALIE spectrograph on the Swiss Euler Telescope at La Silla, Chile. This is intriguing because what we know about circumbinary planets has largely come from detections based on transit analysis. Radial velocity work has uncovered planets orbiting one star in a wide binary configuration but until now, not both.
Image: Artist’s visualization of a circumbinary planet. Credit: Ohio State University / Getty Images.
The new work adds data from the HARPS spectrograph at La Silla and the ESPRESSO spectrograph at Paranal to confirm one of two planets at TOI-1338/BEBOP-1. Thus we have radial velocity evidence for the gas giant BEBOP-1 c, massing in the range of 65 Earth masses, in an orbit around the binary of 215 days. A second world, referenced as TOI-1338 b because it shows up only in transit data from TESS, complements the RV find, making this only the second circumbinary system known to host multiple planets. TOI-1338 b is 21.8 times as massive as the Earth and as a transiting world could well be a candidate for atmospheric studies by the James Webb Space Telescope.
But BEBOP-1 c is the planet that stands out. I think I am safe in calling a co-author on this paper, David Martin (Ohio State University), a master of understatement when he describes the problems in extracting radial velocity data on a circumbinary world. After all, we’re relying on the tiniest gravitational effects flagged by minute changes in wavelength, and now we have to factor in multiple sets of stellar spectra. Here’s Martin:
“When a planet orbits two stars, it can be a bit more complicated to find because both of its stars are also moving through space. So how we can detect these stars’ exoplanets, and the way in which they are formed, are all quite different. Whereas people were previously able to find planets around single stars using radial velocities pretty easily, this technique was not being successfully used to search for binaries.”
Nice work indeed. Circumbinary planets are what the paper describes as ‘harsh environments’ for planet formation given the gravitational matrix in which such formation occurs, and thus we should be able to use the growing number of such systems (now 14 including this one) in the study of how planets form and also migrate. BEBOP should be a useful survey in providing accurate masses for planets in systems we’ve already discovered with the transit method.
Image: This is Figure 3 from the paper, offering an overview of the BEBOP-1 system. Caption: The BEBOP-1 system is shown along with the extent of the system’s habitable zone (HZ) calculated using the Multiple Star HZ website. The conservative habitable zone is shown by the dark green region, while the optimistic habitable zone is shown by the light green region. The binary stars are marked by the blue star symbols in the centre. The red shaded region denotes the instability region surrounding the binary stars as described by Holman and Wiegert. BEBOP-1 c’s orbit is shown by the red orbit models…shaded from the 50th to 99th percentiles. TOI-1338 b’s orbit is shown by the yellow models, and is also based on 500 random samples drawn from the posterior in its discovery paper. Credit: Standing et al.
Learning more about how planets in such perturbed environments emerge should advance the study of planet growth around single stars. It’s likely that the increased transit probabilities of circumbinary planets should play into our efforts to study planetary atmospheres as well. And while transits should provide the bulk of new discoveries in this space, radial velocity follow-ups should expand our knowledge of individual systems, being less dependent on orbital periods and inclination. BEBOP presages a productive use of these complementary observing methods.
The paper is Standing et al., “Radial-velocity discovery of a second planet in the TOI-1338/BEBOP-1 circumbinary system,” Nature Astronomy 12 June 2023 (full text). See also Martin et al., “The BEBOP radial-velocity survey for circumbinary planets I. Eight years of CORALIE observations of 47 single-line eclipsing binaries and abundance constraints on the masses of circumbinary planets,” Astronomy & Astrophysics Vol. 624, A68 (April 2019), 45 pp. Abstract.
Work on the Centauri Dreams internals continues, with the unwelcome result that the site has been popped offline twice because of a possible security problem. Needless to say, this has to be resolved before I can move forward on other aspects of the rebuild. While I deal with that issue, let me respond to a few backchannel questions about yesterday’s post on gas giants in red dwarf planetary systems. What I’m being asked about is my comment that gas giants like Jupiter, at similar distances and installation, around other classes of stars are common compared to what we see at red dwarfs.
This has been a problematic issue, and the matter is a long way from achieving a consensus among researchers. A moment’s reflection yields the reason: Finding gas giants in outer system orbits around a star like the Sun is no easy matter. Radial velocity is most sensitive when dealing with large planets in tight orbits, which is why the first detections in main sequence stellar systems, beginning back in 1995 with 51 Pegasi b, were of the ‘hot Jupiter’ variety. That in itself offered new insights into planetary formation and dynamics. As physicist Isidor Isaac Rabi cogently asked when the muon was first detected, “Who ordered that?”
We’re making all kinds of advances in radial velocity as we use ever more sophisticated instruments to measure the motion induced by orbiting bodies around distant stars, but if we back out to, say, 5 AU, Jupiter’s distance from the Sun, we’re still dealing with extremely tiny effects. Transits are problematic because a planet on a five-year orbit obviously transits its host on long timeframes. Gravitational microlensing is an interesting prospect, because here we can detect planets at the needed distances, but even so the catalog isn’t large and there is much we don’t know.
Fortunately, resources like the California Legacy Survey (719 stars over three decades) are available and have produced data on what we can call ‘cold giants.’ I made my comment because of a paper in the Astrophysical Journal Supplement Series that I learned about through the Pass et al. paper we looked at in the previous post. This is from Caltech’s Lee Rosenthal and colleagues, and it examines the combination of small rocky planets with outer gas giants using the CLS for the bulk of its data. The result is a look at the occurrence of close-in planets with outer giant companions.
The Rosenthal paper addresses radial velocity work on F-, G-, K- and M-class stars and targets both categories of planets, finding that roughly 41 percent of systems with a close-in small planet also host an outer giant. By close-in small planet, the authors mean planets orbiting from 0.023–1 AU with a mass twice to 30 times that of Earth. And the giant planets examined are from 0.23–10 AU and 30 to 6000 Earth masses.
The implication is that stars hosting small inner planets are more likely to have an outer gas giant, for the number is roughly 17 percent for stars irrespective of small planet presence. There is much to be done with data from the California Legacy Survey (the baseline of RV observations goes back to 1988, and is invaluable), but studies like these lead to the conclusion that planets in Jupiter-like orbits are not uncommon among F-, G- and K-class stars. As to the M-dwarfs, the Pass paper indicates the scarcity of gas giants around them, with all that may imply about inner planet habitability. Note that the CLS is made up mostly of FGK stars, with 98% of stars in the sample having stellar masses above 0.3 solar masses..
I haven’t had time to dig into a previous paper using the California Legacy Survey data, this one from Benjamin Fulton (Caltech) with Rosenthal as a co-author, but do note that the authors find that the occurrence of planets less massive than Jupiter (from 30 Earth masses up to 300 as per RV data) is enhanced near 1–10 AU “in concordance with their more massive counterparts.” The complete citation is below.
We still have much to learn about exoplanet system architectures, but we’re making progress as the inflowing current of high-quality data grows ever more powerful.
The paper is Rosenthal et al., “The California Legacy Survey. III. On the Shoulders of (Some) Giants: The Relationship between Inner Small Planets and Outer Massive Planets,” Astrophysical Journal Supplement Series, Vol. 262, No. 1 (17 August 2022), 262 1 (abstract). The Fulton paper is “California Legacy Survey. II. Occurrence of Giant Planets beyond the Ice Line,” Astrophysical Journal Supplement Series Vol. 255, No. 1 (9 July 2021), 255, 13 (abstract).
Gas giant worlds like Jupiter may be uncommon around red dwarf stars, as a number of recent studies have found. It would be useful to tighten up the data, however, because many of the papers on this matter have used stellar samples at the high end of the mass range of M-dwarfs. At the Center for Astrophysics | Harvard & Smithsonian (CfA), Emily Pass and colleagues have gone to work on the question by looking at lower-mass M-dwarfs and working with a lot of them, some 200 in their sample, all within 15 parsecs.
The question is not purely academic, for some scientists suggest that the presence of a Jupiter-class planet – not uncommon around G-class stars like the Sun – is a factor in the development of life. Migrating inward from a formation in the first few hundred million years of the Solar System’s existence, Jupiter would have stirred up plenty of icy cometary bodies through gravitational interactions. Impacts from this infall into the inner system likely delivered a great deal of water and organic molecules to the young Earth, thus becoming a factor in the development of life.
Thus a system like TRAPPIST-1, with its seven rocky planets orbiting a nearby red dwarf, raises the question of whether such a system would have gone through this kind of mixing. No one knows whether life would have begun on Earth without these effects, but the suggestion that systems without a gas giant are barren is plausible. So just how common are red dwarf systems with gas giants equivalent to Jupiter based on what we know so far? It’s telling that only two of the known gas giants orbiting a red dwarf occur around stars of less than 30 percent of the Sun’s mass: LHS 252 b and GJ 83.1 b.
Image: A gas giant around an M-class dwarf, as visualized by artist Melissa Weiss, CfA.
What Pass and team deliver is a statistical analysis, using spectroscopic surveys and radial velocity data on nearby M-dwarfs in the mass range of 0.10–0.30 stellar masses. The data are presented in a paper now in process at The Astronomical Journal. The results confirm the belief that red dwarfs are seldom the hosts for Jupiter-class worlds. In fact, in the entire sample, not a single Jupiter-equivalent planet occurred, allowing the authors to conclude that Jupiter analogues must be found in fewer than 2 percent of low-mass red dwarf systems:
Planets that are Jupiter-like in mass and instellation are rare around low-mass M dwarfs, consistent with expectations from core accretion theory. Compared with previous radial-velocity and microlensing studies that consider broader distributions of M-dwarfs with higher mean stellar masses, our results are consistent with a decrease in giant planet occurrence with decreasing M-dwarf mass…
The authors note the complications of comparing occurrence rate between the various surveys that have so far attempted it, but add:
…the picture of giant planet occurrence from microlensing is still unclear. If Poleski et al. (2021) are correct in their assertion that every microlensing star has a wide-orbit giant planet, our results imply that the distribution of giant planets around low-mass M dwarfs must differ dramatically from more massive stars, whose giant planets are more prevalent near the water snow line than on wide orbits.
These are interesting findings especially in terms of habitability. Rather than assuming that red dwarf planets are unlikely to have life, they could just as easily point to the differences between these systems and our own as offering other avenues for life to develop. CfA’s David Charbonneau makes the point explicitly: “We don’t think that the absence of Jupiters necessarily means rocky planets around red dwarfs are uninhabitable.”
What we do have are planetary systems different enough from ours to encourage speculation on what factors might produce life in different ways than our own system. Consider that the lack of gas giants also indicates more raw material for planetary formation on the scale of smaller rocky worlds. Given the proximity of red dwarf stars with rocky planets, they’ll be at the forefront of astrobiological investigation as we develop the ability to study their atmospheres. The possibilities remain open, and perhaps exotic, as we continue the hunt for life elsewhere. Adds Pass:
“We have shown that the least massive stars don’t have Jupiters, meaning Jupiter-mass planets that receive similar amounts of starlight as Jupiter receives from our Sun. While this discovery suggests truly Earth-like planets might be in short supply around red dwarfs, there still is so much we don’t yet know about these systems, so we must keep our minds open.”
The paper is Pass et al., “Mid-to-Late M Dwarfs Lack Jupiter Analogs,” in process at The Astronomical Journal (preprint).