Celestia Forums

They're down to 5.5 Earth masses or so now... Press release.

Though I wonder if a planet that massive at the kind of temperatures implied by its projected separation would be more of a mini-Neptune than a super-Pluto.

In several forums, people are saying that this world is still likely a gas giant, indeed, a mini-Neptune.

What is the cut-off point, theoretically, between a terrestrial world and a gas giant? Shouldn't you be able to have even a terrestrial world of this mass?

...John...

Good question. I did a few quick back-of-the-envelope calculations on this. The projected separation is 2.6 AU - however the orbital eccentricity is unknown - if the planet is in an eccentric orbit which brings it rather closer to its star then it is less likely to be a gas giant.

The star's mass is given as 0.22 solar masses, which is close to that of Gliese 783 B, so using the parameters of this star I get a temperature of 45 K, which fits with the -220 Celsius value being given in the various press releases.

At that temperature, and assuming Earthlike density, I get a minimum molecular weight retained figure of about 0.3, which would allow for a hydrogen atmosphere.

Note that these calculations do contain loads of approximations and quite a lot of assumptions, so they should NOT under any circumstances be treated as fact, though hopefully they are not total fantasy either.

I though the cut off limit was around 15-20 Earth masses, not 5. Either way no one will know for sure until we can see the size of the planet. However If it was a mini Neptune, then the density of the planet would only be ~1.6g per cm^3 verses the earths 5.6g per cm^3 and would also be ~266% larger in diameter as well.

At Neptune-density, I get a minimum mass retained of about 0.5... I'm still not ruling out a hydrogen atmosphere here.

As for cut-off mass, I think the cores of the ice giants (Uranus, Neptune) are comparable in size to Earth (at least that's about as much as I've been able to find on this matter), so as far as I know I wouldn't be able to say this isn't a mini-Neptune.