# Is there an upper limit to the mass of terrestrial planets?

The exoplanet Kepler-10c has a mass between 15 and 19 times the mass of the Earth (making it comparable in mass to Neptune), and yet is thought to have a density of about 7g/cm3 and to be a terrestrial planet, with a substantial proportion of "hot ice"

Is there an upper limit to the mass of terrestrial planets, or can rocky planets form that are larger than Kepler-10c?

This, older, article in Universe Today suggests that terrestrial planets can't form more than 5-10 Earth Masses, substantially smaller than Kepler 10c.

• Doesn't the size limit for terrestrial planets simply come from them collecting gas and becoming gas giants if they get too massive? That near to a G-star, 0.24 AU, there should be no gas. But there shouldn't be that much material in such a small orbit either. And a migrated gas giant should've kept its atmosphere, even hot Jupiters do. It's a 10 bn years old star, is it maybe in a red giant phase, blowing away a migrated gas giant's atmosphere? – LocalFluff Nov 19 '15 at 21:13
• – HDE 226868 Nov 19 '15 at 21:35
• What is meant by a "terrestrial planet"? Do you mean is there an upper limit to the mass of a planet that is made out of certain elemental constituents or an upper limit to the mass for certain phases of those elemental constituents to exist? – Rob Jeffries Nov 20 '15 at 12:14
• By terrestrial planet I mean a planet made principally of silicate rocks and metals (defn from wikipedia) – James K Nov 20 '15 at 17:21
• The upper level of mass that a rocky planet could get to should be quite a bit larger than the observed 15-19 earth masses but the problem is, that mass is likely retains too much hydrogen. it does raise an interesting question about what do you call a planet that has a 20 or more earth mass rocky interior but a gas giant exterior. Based on the chart, en.wikipedia.org/wiki/Atmospheric_escape you could get a very hot earth with maybe 30,000 km/s escape velocity and still lose hydrogen & helium. That would correspond to roughly about 20 or so Earth masses. – userLTK Nov 21 '15 at 1:05

I'll shamelessly reference an answer I wrote on Worldbuilding to an almost identical question. Lammer et al. (2014) suggested that "super-Earths" with masses of $2$-$5M_\oplus$1 could retain massive hydrogen/helium envelopes, up to $\sim10^{25}$ kilograms. Above this, up to about $10M_\odot$ or more, "mini-Neptunes" exist, possibly composed of volatiles and having significant gas envelopes2. In other words, there's a transition region between terrestrial planets and gaseous planets (gas dwarfs included) that depends significantly on whether the planet can hold onto an envelope for a significant amount of time.