Is K2-18b really a potentially habitable exoplanet?

[ProtoJeb21]

On Wednesday, two separate teams of astronomers released their findings about water vapor detected in the atmosphere of the temperate Super-Earth/Mini-Neptune K2-18b. Naturally, the press went nuts with this story. I’ve seen a lot of articles calling K2-18b a “rocky” and “habitable” planet, and several annoyed astronomers in response to said articles. The thing is, even though the detection of water clouds in a small habitable zone planet is a landmark discovery, it does NOT immediately prove that K2-18b is habitable. 

Let’s start with the basics...which are already a little confusing. There are two sets of parameters for the host star; the first, which has been used most often and in the initial discovery and RV result papers on the planet, put K2-18 at around 39-42% the radius and mass of the Sun. The second, newer set uses Gaia and 2MASS data, and puts the star at about 47-49% the radius and mass of the Sun. This discrepancy is important because it radially changes the composition of K2-18b. All studies agree that the planet is in a 33-day orbit around a small red dwarf, gets similar sunlight as Earth, and is between 8 and 9 times the mass but is significantly less dense than Earth. 

Here we get into the first thing many of those news articles got wrong: K2-18b is NOT a purely rocky world. The older papers put the planet at 8-8.5 Earth masses and 2.2-2.4 Earth radii, with a density of roughly 3.5 g/cm3. This implies K2-18b has a large rocky interior, but also a significant fraction of volatiles, either a water mantle, a thin H/He envelope, or a combination of both. The H/He envelope for a planet of this size and density would be less than 1% of its total mass, likely below 0.5%. However, the newer radius estimate of 2.7x Earth (with a density down to 2.2-2.4 g/cm3) can allow for a thicker H/He up to 2-3% of the planetary mass. Even with this level of uncertainty, K2-18b can be ruled out as having a solid Earth-like surface. 

The detection of water vapor actually makes the chances of K2-18b habitable even worse. If the atmosphere was made of heavy molecules (N, O2, CO2, etc), water vapor would be hard to detect because such an atmosphere wouldn’t be as high up. However, a primarily hydrogen and helium atmosphere would allow for water to be detected more easily, even if it’s less than 0.1% of K2-18b’s atmospheric composition. The strong detection of water vapor means that K2-18b has retained at least most of its primordial H/He envelope, which is very bad for the development of life. 

One important thing to note is the exact abundance’s of water and H/He are still unknown. Could K2-18b only have a small amount of hydrogen on top of a water mantle? Sure, but it could also be a dry rocky core with a thicker H/He envelope and no chance of any sort of life. The point is, K2-18b is a pretty bad “potentially habitable” planet; it may be a great stepping stone to characterizing the atmospheres of small temperate planets, but it certainly isn’t the jackpot just yet. 

[sh1pman]

8 minutes ago, ProtoJeb21 said:

K2-18b has retained at least most of its primordial H/He envelope, which is very bad for the development of life. 

Why is it bad for the development of life? Maybe there's a liquid water ocean where heavier elements and ions are dissolved. Nitrogen and carbon would be much more difficult to get, but there could be geological sources. 

[kerbiloid]

1 hour ago, sh1pman said:

Why is it bad for the development of life? Maybe there's a liquid water ocean where heavier elements and ions are dissolved. Nitrogen and carbon would be much more difficult to get, but there could be geological sources. 

And what about an atmospheric oxidizer to breath, when it's full of hydrogen.

Spoiler

And how to take this life seriously when it speaks with cartoonish voices due to the Helium?

 

Edited September 16, 2019 by kerbiloid

[sh1pman]

3 minutes ago, kerbiloid said:

And what about an atmospheric oxidizer to breath, when it's full of hydrogen.

 

Using atmospheric oxidizer for living is a new fancy trick that life on Earth started doing not that long ago. It’s useful, but not necessary.

[kerbiloid]

3 minutes ago, sh1pman said:

Using atmospheric oxidizer for living is a new fancy trick that life on Earth started doing not that long ago. It’s useful, but not necessary.

But the hydrogen should prevent any proper oxidation, it would reduce everything it touches.

[electricpants]

5 minutes ago, kerbiloid said:

But the hydrogen should prevent any proper oxidation, it would reduce everything it touches.

life survived just fine in a reducing atmosphere for a long time on earth.

[kerbiloid]

1 minute ago, electricpants said:

life survived just fine in a reducing atmosphere for a long time on earth.

In the atmosphere or underwater?

[electricpants]

Just now, kerbiloid said:

In the atmosphere or underwater?

depends on if life started in a tidal pool or in underwater vents. in the latter, while life wasn't in the atmosphere per se it was still pretty near it.

[ProtoJeb21]

1 hour ago, sh1pman said:

Why is it bad for the development of life? Maybe there's a liquid water ocean where heavier elements and ions are dissolved. Nitrogen and carbon would be much more difficult to get, but there could be geological sources. 

The problem with a hydrogen envelope is that it would probably trigger a runaway greenhouse effect, and/or smother the “surface” under extremely high pressures that would make almost all life impossible. 

[Signo]

The planet is so close to its star that is probably tidally locked. The only funny thing about it is that a lookalike of this solar system was home to the Centaurs in the books by John W. Campbell jr. (The mightiest Machine and The Incredible Planet,  ever heard about Aarn Munro the Jovian?)

[Mitchz95]

Obligatory xkcd: