Last Friday’s post on K-dwarfs as home to what researchers have taken to calling ‘superhabitable’ worlds has caught the eye of Dave Moore, a long-time Centauri Dreams correspondent and author. Readers will recall his deep dives into habitability concepts in such essays as The “Habitability” of Worlds and Super Earths/Hycean Worlds, not to mention his work on SETI (see If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare). Dave sent this in as a comment but I asked him to post it at the top because it so directly addresses the topic of habitability prospects around K-dwarfs, based on a quick survey of known planetary systems. It’s a back of the envelope overview, but one that implies habitable planets around stars like these may be more difficult to find than we think.
by Dave Moore
To see whether K dwarfs made a good target for habitable planets, I decided to look into the prevalence and type of planets around K dwarfs and got carried away looking at the specs for 500 systems of dwarfs between 0.6 mass of the sun and 0.88.
Some points:
i) This was a quick and dirty survey.
ii) Our sampling of planets is horribly skewed towards the massive and close, but that being said, we can tell if certain types of planets are not in a system. For instance Jupiter and Neptune sized planets at approximately 1 au show up, so if a system doesn’t show them after a thorough examination, it won’t have them.
iii) I had trouble finding a planet list that was configurable to my needs. I finally settled on the Exoplanets Data Explorer configured in reverse order of stellar mass. This list is not as comprehensive as the Exosolar Planetary Encyclopedia.
iv) I concocted a rough table of the inner and outer HZ for the various classes of K dwarfs. Their HZs vary considerably. A K8 star’s HZ is between 0.26 au and 0.38 au while a K0’s HZ is between 0.72 au and 1.04 au. This means that you can have two planets orbiting at the same distance around a star and one I will classify as outside the HZ and the other inside the HZ.
v) Planets below 9 Earth mass I classified as Super-Earth/Sub-Neptune. Planets between 9 Earth masses and 30 are classified as Neptunes. Planets over that size are classified as Jupiters.
Image: An array of planets that could support life are shown in this artist’s impression. How many such worlds orbit K-dwarf stars, and are any of them likely to be ‘superhabitable’? Credit: NASA, ESA and G. Bacon (STScI).
What did I find:
By far the most common type are hot Super-Earths/Sub-Neptunes (SE/SNs). These are planets between 3 EM (Earth mass) and 6 EM. It is amazing the consistency of size these planets have. They are mostly in close (sub 10 day) orbits. There also appears to be a subtype of sub 2EM planets in very tight orbits (some quoted in hours) and given some of these were in multi-planet systems of SE/SNs, I would say these were SE/SNs, which have been evaporated down to their cores.
I also found 7 in the HZ and 2 outside the HZ.
I found 52 hot Jupiters and what I classified as 43 elliptical orbit Jupiters. These were Jupiter-sized planets in elliptical orbits under 3 au.
There were also 10 Jupiter classification planets in circular orbits under 3 au. and 3 outside that limit in what could be thought of as a rough analog of our system.
There were also 46 hot Neptunes and 14 in circular orbits further out, only one outside the habitable zone.
Trends:
At the lower mass end of the scale, K dwarf systems start off looking very much like M dwarfs except that everything, even those in multi-planet systems, is inside the habitable zone.
As you work your way up the mass scale, there is a slight increase in the average mass of the SE/SNs with 7-8 EM planets becoming more prevalent. More and more Jupiters appear, and Neptune-sized planets appear and become much more frequent. Also, you get the occasional monster system of tightly packed Jupiters and Neptunes like 55 Cancri.
An interesting development begins at about the mid mass range. You start getting SE/SNs in nice circular longer period orbits but still inside the HZ (28 in 20-100d orbits.)
Conclusions:
If we look at the TRAPPIST-1 system around an M-dwarf, its high percentage of volatiles (20% water/ice) implies that there is a lot of migration in from the outer system. If a planet has migrated in from outside the snow line, then there’s a good chance that even if it’s in the habitable zone, it will be a deep ocean planet.
Signs of migration are not hard to find. Turning back to the K-dwarfs, if we look at the Jupiters, only three show signs of little migration (analogs of our system). Ten migrated in smoothly but sit at a distance likely to have disrupted a habitable planet. Forty-three are in elliptical orbits, which are considered signs of planet-planet scattering.
Hot Jupiters can be accounted for by either extreme scattering or migration. As to inward migration, Martin Fogg did a series of papers showing that as Jupiter mass planets march inwards they scatter protoplanets, but these can reform behind the giant, and so Earth-like planets may occur outside of the hot Jupiter.
Neptunes in longer period circular orbits and the longer period SE/SNs all point to migration. These last groups are intriguing as they point to a stable system with the possibility of smaller planets further out. I would include the 7 planets in the habitable zone in this group. But if these planets all migrated inwards they may well be ocean planets.
K dwarfs have an interesting variety of systems, so they’d be useful to study, but I don’t see them as the major source of Earth analogs—at least not until we learn more.
Nice work, thanks. By the way, were you able to get any information of the stars mettalicity, ? I recall that stars with metalicities appreciable higher as the Sun have a higher chance of having Hot Jupiters than those without but probably this may be a factor relevant to the higher mass K stars.
The metallicity of the stars is listed, so it would be possible to do a correlation.
I had a thought after I wrote my post. Most of the SE/SNs between 3-6 EM (the most common planet) were detected via transit. If the plane of a planetary system is even a bit off the sight line from Earth, then subsequent planets would not transit–and most of these stars do not appear to have RV studies done on them. RV studies, I’m pretty sure, would pick up similar-sized planets further out, so a lot of these single super-Earths could well be the innermost planet of a system.
To get a better idea of the composition of these common systems, RV studies will have to be done on them, and the longer the study goes on for the better idea we will have of the system’s composition, be that picking up more planets or eliminating the possibility of certain classes of planets.
There is the KOBE experiment https://arxiv.org/abs/2209.05205?utm_source=chatgpt.com which started in 2021 and is scheduled to finish up final measurements in 2025. “The KOBE (K-dwarfs Orbited By habitable Exoplanets) experiment is a dedicated survey initiated in 2021 to search for habitable planets around late-type K-dwarf stars. Utilizing the CARMENES instrument at the Calar Alto Observatory in Spain, the project monitors (the radial velocities of) 50 carefully selected K-dwarfs over five semesters, with an average of 90 data points per target.“
I can imagine that this will be a beautiful addition to the transit data that we already have.
This will be a good addition to characterizing planetary distributions. What I think we need is to do careful RV measurements of every K system that has a transiting SE/SN–or a large number of them–to characterize this type of system. There were several multi-planet systems in my count. Are these transiting planets we’re seeing the only super Earth’s in the inner system, or are they part of multi planet systems. We can detect SE/SNs by RV to outside of the habitable zone.
The other part of the survey would be definitively exclude certain sizes of planets. If we can characterize planetary system types and their frequency, then we can get a better idea of how common/rare systems that have Earth-like planets in the habitable zone are.
https://astrobiology.com/2025/01/x-ray-activity-of-nearby-g-k-and-m-type-stars-and-implications-for-planet-habitability-around-m-stars.html
X-ray Activity Of Nearby G-, K-, And M-type Stars And Implications for Planet Habitability Around M stars
By Keith Cowing
Status Report
astro-ph.SR
January 14, 2025
Context. The intense X-ray and UV emission of some active M stars has raised questions about the habitability of planets around M-type stars. Aims. We aim to determine the unbiased distribution of X-ray luminosities in complete, volume-limited samples of nearby M dwarfs, and compare them to those of K and G dwarfs. Methods.
We constructed volume-complete samples of 205 M stars with a spectral type ≤ M6 within 10 pc of the Sun, 129 K stars within 16 pc, and 107 G stars within 20 pc. We used X-ray data from Chandra, XMM-Newton, eROSITA, and ROSAT to obtain the X-ray luminosities of the stars.
Results. Our samples reach an X-ray detection completeness of 85%, 86%, and 80% for M, K, and G stars, respectively. The fractional X-ray luminosities relative to the bolometric luminosities, log(LX/Lbol), of the M stars show a bimodal distribution, with one peak at around -5, mostly contributed by early M stars (M0–M4), and another peak around -3.5, contributed mainly by M4–M6 stars.
The comparison of the different spectral classes shows that 63% of all M stars in our sample (80% of the M stars with a spectral type < M4) have LX/Lbol values that are within the central 80% quantile of the distribution function for G stars. In addition, 55% of all M stars in our sample (and 72% of the M stars with a spectral type < M4) have LX/Lbol less than 10 times the solar value.
Conclusions. The X-ray activity levels of the majority (≥60%) of nearby M dwarfs no later than M6 are actually not higher than the typical (80% quantile) levels for G-type stars. The X-ray irradiation of habitable-zone planets around these stars should therefore not present a specific problem for their habitability.
E. Zhu, T. Preibisch
Comments: 18 pages, 11 figures, 6 tables, accepted for publication in A&A
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP); High Energy Astrophysical Phenomena (astro-ph.HE)
Cite as: arXiv:2501.07313 [astro-ph.SR] (or arXiv:2501.07313v1 [astro-ph.SR] for this version)
https://doi.org/10.48550/arXiv.2501.07313
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Submission history
From: Enyi Zhu
[v1] Mon, 13 Jan 2025 13:25:18 UTC (4,382 KB)
https://arxiv.org/abs/2501.07313
Astrobiology
“Conclusions. The X-ray activity levels of the majority (≥60%) of nearby M dwarfs no later than M6 are actually not higher than the typical (80% quantile) levels for G-type stars. The X-ray irradiation of habitable-zone planets around these stars should therefore not present a specific problem for their habitability.”
Apart from the HZ planets being much closer to the star, with a correspondingly higher UV and X-ray flux.
https://astrobiology.com/2025/01/the-tess-keck-survey-xxiv-outer-giants-may-be-more-prevalent-in-the-presence-of-inner-small-planets.html
The TESS-Keck Survey XXIV: Outer Giants May be More Prevalent in the Presence of Inner Small Planets
By Keith Cowing
Status Report
astro-ph.EP
January 14, 2025
We present the results of the Distant Giants Survey, a three-year radial velocity (RV) campaign to search for wide-separation giant planets orbiting Sun-like stars known to host an inner transiting planet.
We defined a distant giant to have a = 1–10 AU and Mpsini=70−4000 mearth~ = 0.2-12.5mj, and required transiting planets to have a <1 AU and Rp=1−4rearth. We assembled our sample of 47 stars using a single selection function, and observed each star at monthly intervals to obtain ≈30 RV observations per target.
The final catalog includes a total of twelve distant companions: four giant planets detected during our survey, two previously known giant planets, and six objects of uncertain disposition identified through RV/astrometric accelerations. Statistically, half of the uncertain objects are planets and the remainder are stars/brown dwarfs. We calculated target-by-target completeness maps to account for missed planets.
We found evidence for a moderate enhancement of distant giants (DG) in the presence of close-in small planets (CS), P(DG|CS) = 30+14−12%, over the field rate of P(DG) = 16+2−2%. No enhancement is disfavored (p∼ 8%). In contrast to a previous study, we found no evidence that stellar metallicity enhances P(DG|CS).
We found evidence that distant giant companions are preferentially found in systems with multiple transiting planets and have lower eccentricities than randomly selected giant planets. This points toward dynamically cool formation pathways for the giants that do not disturb the inner systems.
Judah Van Zandt, Erik A. Petigura, Jack Lubin, Lauren M. Weiss, Emma V. Turtelboom, Tara Fetherolf, Joseph M. Akana Murphy, Ian J. M. Crossfield, Greg Gilbert, Teo Mocnik, Natalie M. Batalha, Courtney Dressing, Benjamin Fulton, Andrew W. Howard, Daniel Huber, Howard Isaacson, Stephen R. Kane, Paul Robertson, Arpita Roy, Isabel Angelo, Aida Behmard, Corey Beard, Ashley Chontos, Fei Dai, Paul A. Dalba, Steven Giacalone, Michelle L. Hill, Lea A. Hirsch, Rae Holcomb, Steve B. Howell, Andrew W. Mayo, Mason G. MacDougall, Daria Pidhorodetska, Alex S. Polanski, James Rogers, Lee J. Rosenthal, Ryan A. Rubenzahl, Nicholas Scarsdale, Dakotah Tyler, Samuel W. Yee, Jon Zink
Comments: 32 pages, 20 figures, 4 tables. Comments welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2501.06342 [astro-ph.EP] (or arXiv:2501.06342v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2501.06342
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Submission history
From: Judah Van Zandt
[v1] Fri, 10 Jan 2025 20:51:58 UTC (11,038 KB)
https://arxiv.org/abs/2501.06342
Astrobiology
I am biased against planet migration unless there are planets are highly elliptical orbits or more than one gas giant. It does not seem like there are a lot of Earth sized planets in the sample of K stars. It will be interesting so see their spectra if we find some.
Nice summary, but the conclusion is affected by the detection bias of the methods and sensitivity we got, and a somewhat small sample.
Large planets such as these super-E’s and mini Neptunes will give a clear signal and be confirmed at K-type stars, but terrestrial ones with about the same mass as Earth will be more difficult to spot. This while such a mass is enough to drag a small M-class star around enough to get detected – and also give a very clear signal if it’s one that transit from our vantage point.
Some of the planets found that way orbit very close to M-dwarfs which barely is larger than Jupiter. So we see a lot of such at the M-dwarfs currently, but fewer around other stars.
Eventually I expect more Earth type worlds will be recorded and confirmed, and perhaps even something that resemble the Trappist system but a bit wider and now at a K-type star, if and when such a system is found it will be time to give those worlds a very close look for habitability.
Hi David
the trends you found were very interesting and make our own solar system that more unusual.
Thanks Edwin