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.