Ring of Life? Terminator Habitability around M-dwarfs

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

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

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

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

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

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

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

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

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

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

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

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

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


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

Abstract Submission Final Deadline: April 21, 2023

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

Topics of Interest

Physics and Engineering

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


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

Human Factors

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


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


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

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

Confirmed Speakers

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

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

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

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

Contributed Plenary Lectures

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

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

Work in Progress Posters

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

Sagan Meetings

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


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

Other Content

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


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

Video and Archiving

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

Abstract Submission

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

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

Presenting Author(s)

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

Additional Author(s)

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


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

Abstract Length

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

Abstract Structure

Abstracts must include the following headings:

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

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


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



Chasing nomadic worlds: Opening up the space between the stars

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

by Andreas Hein

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


Super Earths/Hycean Worlds

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

by Dave Moore

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

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

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

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

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

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

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

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

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

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

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

To quote:

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

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

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

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

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

Thoughts and Speculations

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

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

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

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

Jupiter: 7-11 bar / 8-30 bar

Saturn: 10-20 bar / 25-100 bar

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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


137496 b: A Rare ‘Hot Mercury’

We haven’t had many examples of so-called ‘hot Mercury’ planets to work with, or in this case, what might be termed a ‘hot super-Mercury’ because of its size. For HD 137496 b actually fits the ‘super-Earth’ category, at roughly 30 percent larger in radius than the Earth. What makes it stand out, of course, is the fact that as a ‘Mercury,’ it is primarily made up of iron, with its core carrying over 70 percent of the planet’s mass. It’s also a scorched world, with an orbital radius of 0.027 AU and a period of 1.6 days.

Another planet, non-transiting, turns up at HD 137496 as well. It’s a ‘cold Jupiter’ with a minimum mass calculated at 7.66 Jupiter masses, an eccentric orbit of 480 days, and an orbital distance of 1.21 AU from the host star. HD 137496 c is thus representative of the Jupiter-class worlds we’ll be finding more of as our detection methods are fine-tuned for planets on longer, slower orbits than the ‘hot Jupiters’ that were so useful in the early days of radial velocity exoplanet discovery.

The discoverers of the planetary system at HD 137496, an international group led by Tomas Silva (University of Porto, Portugal), found HD 137496 b, the hot Mercury, in K2 data, its transits apparent in the star’s light curve. The gas giant HD 137496 c was then identified in radial velocity work using the reliable HARPS and CORALIE spectrographs.

The primary is a G-class star a good bit older than the Sun, its age calculated at 8.3 billion years, but with a comparable mass (1.03 solar masses), and a radius of approximately 1.50 solar radii.

Image: HARPS (orange) and CORALIE (blue) radial velocities. In this figure, we present our RV time series. As is clearly seen, the data show a long-term and high-amplitude trend (semiamplitude of ~ 200 m s-1), typical of the signature of a long period giant planet. Credit: Silva et al.

A hot Mercury should turn out to be a useful find in a variety of ways. As the paper notes:

HD 137496 b (K2-364 b) joins the small sample of well characterized dense planets, making it an interesting target for testing planet formation theories, density enhancing mechanisms, and even the possible presence of an extended cometlike mineral rich exosphere. Together with HD 137496 c (K2-364 c), a high-mass (mass ratio…, high-eccentricity planet, this system presents an interesting architecture for planetary evolution studies. Future astrometric observations could also provide significant constraints on the relative inclination of the planetary orbits, unraveling new opportunities to discover the system’s dynamical history.

Keep in mind that most of the planets we now know about have radii somewhere between that of Earth and Neptune. In this range, numerous different system architectures are in play, and a wide variety of possible formation scenarios. As the authors note, high-density planets like HD 137496 b are distinctly under-sampled, which has been a check on theories of planet formation that would accommodate them.

And the theorists are going to have their hands full with this one. HD 137496 b’s parent star shows too little iron to form a planet with this density. I’m going to quote Sasha Warren on this. Working on a PhD at the University of Chicago, Warren focuses on how planetary atmospheres have evolved, particularly those of Mars and Venus. Of HD 137496 b, she has this to say in a recent article on astrobites about how such planets can become more iron-rich:

Firstly, the protoplanetary disks of dust and gas within which planets form around young stars can change in composition as a function of distance from the star. So, it is possible that a combination of high temperatures and magnetic interactions between the host star and the protoplanetary disk concentrated iron-rich materials where HD 137496 b originally formed. This could mean star compositions might not be very useful to help understand what short period rocky planets are made of. Secondly, planets close to their stars like HD 137496 b are so hot that their rocky surfaces can sometimes just evaporate away!

It will be fascinating to see how our theories evolve as we begin to expand the catalog of hot Mercury planets. 137496 b is only the fifth world in this category yet discovered.

The paper is Silva et al., “The HD 137496 system: A dense, hot super-Mercury and a cold Jupiter,” in process at Astronomy & Astrophysics (preprint).


Exomoons: The Binary Star Factor

Centauri Dreams readers will remember Billy Quarles’ name in connection with a 2019 paper on Alpha Centauri A and B, which examined not just those stars but binary systems in general in terms of obliquity — axial tilt — on potential planets as affected by the gravitational effects of their systems. The news for habitability around Centauri B wasn’t good. Whereas the Moon helps to stabilize Earth’s axial tilt, the opposite occurs on a simulated Centauri B planet. And without a large moon, gravitational forcing from the secondary star still causes extreme obliquity variations.

Orbital precession induced by the companion star is the problem, and it may be that Centauri A and B are simply too close together, whereas more widely separated binaries are less disruptive. I’ll send you to the paper for more (citation below), but you can get an overview with Axial Tilt, Habitability, and Centauri B. It’s exciting to think that our ongoing investigations of Centauri A and B will, one of these days, be able to confirm these results or cause them to be reassessed, assuming we find planets there.

Exomoons in Binary Systems

Quarles (Georgia Institute of Technology) continues to use numerical methods to look at the dynamics in both single and multiple star systems, with his interest in Centauri A and B undiminished. His most recent paper looks at exomoon possibilities at binary star systems, homing in on orbital stability in systems where a companion star forces greater eccentricity. We can look for such moons using transit-timing variations as well as variations in the durations of a transit. All of this gives us no more than hints of a moon, which is too small to be seen, but it opens up new space for such detections.

Co-author Siegfried Eggl (University of Illinois Urbana-Champaign) explains:

“We first had to determine the orbital resonances in the systems we looked at. When moons and planets have slightly elliptical orbits, they don’t always move at the same speed. The more eccentric an orbit, the more frequencies can be excited, and we see these resonances become more and more important. At some point there will be overlapping resonances that can lead to chaos in the system. In our study we have shown, however, that there is enough stable ‘real estate’ to merit a thorough search for moons around planets in double star systems.”

Image: In this map of overlapping orbital resonances, the regions between resonances are colored black and could allow for stable satellite orbits under optimal conditions. The light green curve connects the first point of intersection between adjacent resonances and marks a stability boundary within the three body problem. Credit: Quarles et al.

Transit timing variations (TTV) and variations in the actual duration of the transit (TDV) are the most readily observable effects on the table. TTVs are variations in timing as the planet transits its star. Does the transit show strict periodicity, or is there some variation from one transit to the next? TTVs can be used to demonstrate the presence of other gravitational influences, an unseen planet, or a moon. Transit duration variation measures the time during which any part of the planet obscures the stellar disk. Variations in duration occur as planet and moon orbit a common center of mass.

David Kipping (Columbia University) has been looking at transit timing variations and other factors for a long time in connection with the quest for an exomoon detection, a quest he began as a grad student and continued with his project The Hunt for Exomoons with Kepler. HEK uses dynamics and Kepler photometry in combination, modeling observable effects of a moon on a transit as well as the dynamical perturbations that can be revealed by transit timing and transit duration variations.

Quarles and team have taken the exomoon hunt explicitly into the realm of binary stars, where a stellar companion forces its own perturbations on moons orbiting planets there, affecting their occurrence and orbital evolution. The researchers have applied their findings to hypothetical Earth-Moon analogues at Centauri A and B, and have set up orbital stability limits for exomoons in binary star systems in general.

Gravitational interactions with a companion star can foster greater eccentricity in planetary orbits, with resulting stability issues for moons and implications for detecting them through TTVs. In typical binary systems, “TTV (rms) amplitudes induced by exomoons in binary systems are ?10 minutes and appear more likely for planets orbiting the less massive stellar component.”

In some systems, say the researchers, we would expect the Hill radius — the region around a planetary body where its own gravity, as compared to that of other nearby bodies, is the dominant force in attracting satellites — to shrink, which could cause moons to become unstable. If too close to the host star, the moon could be ejected from its planetary orbit and flung outwards. Zeroing in on Alpha Centauri:

The truncation of the Hill radius through secular eccentricity oscillations and outward tidal migration can influence potential observations of exomoons through TTVs… The TTV (RMS) amplitude is largest when satellites are close to their outer stability boundaries. These mechanisms limit the outer stability limit and can constrain the range of tidal dissipation allowed. The maximum TTV amplitude in a system like ? Cen AB is ?40 min, where we find that an Earth-Moon analog would exhibit ?2 min TTV signature.

The acronym RMS above stands for ‘root mean square,’ a reference to the value of the total waveform of the transit data, but let’s not get too deep into the weeds. The point is that a delicate balance needs to be struck so that the moon can survive. This is what Eggl refers to above as ‘stable real estate.’ But to detect an exomoon, we first have to find planets in the Alpha Centauri system, about which the authors have this to say:

The primary star of ? Cen AB would be a good candidate for searching for TTV inducing exomoons if transiting Earth-analogs were present. However, surveys of ? Cen AB for planets are difficult because of pixel saturation in photometric observations (Demory et al. 2015) and astrophysical noise in radial velocity observations…

And further on:

Observations of ? Cen AB with the Very Large Telescope (VLT) have suggested that any exoplanets there need to be ?20 M? (Kasper et al. 2019), which bodes well for the potential for terrestrial planets. The first results of the New Earths in the ? Centauri Region (NEAR) experiment on VLT uncovered a direct imaging signature of a roughly Neptune-sized planet orbiting ? Centauri A (Wagner et al. 2021), but these early results still await confirmation. Detecting exoplanets in binary star systems is a crucial step in the search for exomoons, where a wide array of methods (including TTVs) can be employed.

A crucial step indeed, but detecting an exomoon is an even tougher task, whether in a binary system or not. In 2020, for example, Chris Fox and Paul Wiegert (University of Western Ontario) theorized that six exoplanets found by Kepler could be hosting exomoons. Studying TTVs in the data, the astronomers noted that these were indirect detections, and that nearby planets could also be responsible for the TTVs.

We’re reminded that this is truly a frontier. Having examined the data, Quarles found that four of these six systems would tidally disrupt their exomoons or lose them to outward migration. He is quoted elsewhere as saying of the six possible moons:

“Could they (exomoons) exist physically? Four (candidate systems) of the six could not, two of the six are possible but the signature they produced aren’t produced by the data. Those two probably aren’t moons.”

Exomoon hunter Kipping found no compelling evidence for any of the six exomoons based on his own work. Moons around gas giants could be interesting venues for habitability, and we know the investigation of such will continue. You’ll recall evidence of a moon forming around the planet PDS 70c, an encouraging sign that a confirmed exomoon is getting closer. So it’s a fascinating part of the process that we now examine forced resonances in binary systems as another way into this daunting problem.

The paper is Quarles et al., “Exomoons in Systems with a Strong Perturber: Applications to ? Cen AB,” Astronomical Journal Vol. 162, No. 2 (14 July 2021) 58 (abstract / preprint). The Quarles paper on orbital obliquity is “Obliquity Evolution of Circumstellar Planets in Sun-like Stellar Binaries,” Astrophysical Journal Vol. 886, No. 1 (19 November 2019). Abstract / Preprint. The Fox & Wiegert paper on exomoon detection cited above is “Exomoon candidates from transit timing variations: eight Kepler systems with TTVs explainable by photometrically unseen exomoons.” Monthly Notices of the Royal Astronomical Society Vol. 501, Issue 2 (February 2021) 2378-2393 (abstract).