Celestia Forums

Hello! I haven't been online in the forums in a while...but I've been working a lot lately on my hard sci-fi scenario, and one of the planets I'm developing is a superearth that is covered in a global ocean. It isn't a major setting, but it's in the same system as a human settlement and I'm trying to "flesh out" all of the planets in that system for a comprehensive "guide". The planets orbit around Beta Hydri. Here are the stats I've come up for it:

Mass: 2.8 eM
Density: 5.53 g/cm^3
Radius: 9037 km
Surface Pressure: 3.27 atms
Surface Gravity: 13.66 m/s^2
Semi-Major Axis: 2.2 AU
Period: 3.263 Earth Years
Day Length: 17.58 Hours
Escape Velocity: 15.716 km/s

Atmosphere:
81.3% N2
12.4% H
5.3% H2O
0.8% He
0.2% CO2

From what I've researched about "ocean planets" and super earths in general, a super-earth's high gravity would make it possible to have much more hydrogen in it's atmosphere, also allowing it to have larger amounts of water vapor. The percent compositions are, of course, more or less randomly contrived, based off of Earth's and Jupiter's atmospheres. The surface pressure is also a contrived number, as is the day length (I've never been able to find a good source to help calculate what a planet's rotation period would be...can anyone help with that?) The water would be able to be in a liquid state due to the high atmospheric pressure, and at the bottom of the hundreds-of-kilometers deep oceans would exist exotic high-pressure ices.
My questions are: What would be a realistic percent composition for such a planet's atmosphere, how much of an atmosphere can a planet of this mass actually hold onto, would there be global cloud cover as well, similar to Venus but with water vapor instead of sulfuric acid?

Any help would be greatly appreciated!

In terms of atmosphere, it depends on the mass of the planet. Super-Earths range from 2 to 10 times the mass of Earth (higher masses are possible if the star is exceptionally metal-rich). Since you're dealing with a planet covered in a global ocean of water, then the planet would be in the habitability zone of the star. A super-Earth would have a limited ability to hold onto helium in its atmosphere. However, a planet with a mass of less than 10 times the mass of Earth does not have sufficient gravity to hang onto hydrogen unless the planet is as cold as Neptune. Super-Earth mass planets with a significant amount of hydrogen and helium in their atmospheres are called "Mini-Neptunes", such worlds are cryogenic cold with temperatures of less than 40 Kelvin.


Planets with a global ocean of water will have a moderate amount of oxygen (O2) and water vapor in its atmosphere. A realistic atmospheric composition would be:

Nitrogen (N2) ~80%
Oxygen (O2) ~5%
Water Vapor (H2O) ~4%
Helium (He) ~4%
Carbon Dioxide (CO2) ~2%

Also, Beta Hydri is a yellow subgiant. The star started out as a main sequence dwarf of spectral type F8V, but the star is now dying. The planet having a global ocean of water would be a plausible scenario, as the planet would have had an ice-cap climate
(with an Earth-like atmosphere) when Beta Hydri was on the main sequence, but the ice melted as the star brightened, forming a global ocean.

Might want to check out some of the literature on the subject of super-Earths and ocean planets.

L?ger et al. (2003) "A New Family of Planets? "Ocean Planets"
Selsis et al. (2007) "Could we identify hot Ocean-Planets with CoRoT, Kepler and Doppler velocimetry?"
Seager et al. (2007) "Mass-Radius Relationships for Solid Exoplanets"

As for hydrogen-rich atmospheres on super-Earths, this may affect hot super-Earths too. The critical mass for nucleated instability accretion of hydrogen/helium envelopes may be less than 10 Earth masses under certain conditions, and there is also the possibility for accretion of some nebular gas before such runaway processes kick in. There is the case of the recently-discovered GJ 1214b, which has a radius larger than the expected value for an ocean planet and has a high equilibrium temperature. This implies it has a substantial atmosphere which may be hydrogen-rich, though there is still the possibility this may be due to an inflated steam atmosphere, or possibly outgassing of hydrogen from a primarily rocky planet:

Charbonneau et al. (2009) "A super-Earth transiting a nearby low-mass star"
Rogers and Seager (2009) "Three Possible Origins for the Gas Layer on GJ 1214b"
Miller-Ricci and Fortney (2010) "The Nature of the Atmosphere of the Transiting Super-Earth GJ 1214b"

AVBursch-
I think that this planet lies just outside Beta Hydri’s habitable zone at 2.2 AU, however this might now be different since it's a yellow subgiant. I've read about the star's age and current properties in my initial research into the star itself, but I didn't think it would be of major consequence just yet, as it still seemed to be a relatively "stable" place for a habitable planet.
Thanks for taking the time to come up with a more realistic composition for the atmosphere. IT seems that the one you came up with would have a much stronger greenhouse effect as well, which is what I was trying to go for in the first place.
As for the planet's history, that was my thought process too. Originally it would have been like a much larger version of Europa or a similar body, but when the star began to brighten and expand the ice melted and formed the global ocean, so it is a fairly recent change. But, further in (~1.3) AU there is a habitable world, slightly larger than Earth. Does this mean that that planet would not be able to support life anymore, or would it be far enough away that there would still be a chance for life to survive? (Just for the complete picture, the innermost planet is a "hot Neptune" (although it's mass puts it kind of between a massive super Earth and a stunted Neptune...))

ajtribick-
Thanks a lot for those links to actual scientific literature! They're incredibly helpful. It's always these kinds of papers I never seem to find. Perhaps I should use Google Scholars more often...

I've come across GJ 1214b many times in my research, and I have been using it as sort of a rough template (although it is more massive, I think, than my "Beta Hydri d"). Although, here again, scientific papers would have been much more helpful than press releases and wikipedia articles. I need to improve my internet search skills.

UPDATE: I've decided to not use Beta Hydri as the parent star (and also I have since changed the entire planetary system). They now orbit around 82 Eridani.