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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.

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  • $\begingroup$ 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? $\endgroup$ – LocalFluff Nov 19 '15 at 21:13
  • $\begingroup$ See worldbuilding.stackexchange.com/questions/9948/…. $\endgroup$ – HDE 226868 Nov 19 '15 at 21:35
  • $\begingroup$ 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? $\endgroup$ – Rob Jeffries Nov 20 '15 at 12:14
  • $\begingroup$ By terrestrial planet I mean a planet made principally of silicate rocks and metals (defn from wikipedia) $\endgroup$ – James K Nov 20 '15 at 17:21
  • $\begingroup$ 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. $\endgroup$ – userLTK Nov 21 '15 at 1:05
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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.

Part of this depends on the environment. For instance, you cite Kepler-10c as a possibly upper limit to terrestrial planets. The reason Kepler-10c can't be a gas planet, as Dumusque et al. (2014) explain, is that it's way too close to its parent star to retain a hydrogen envelope. It would likely lose a lot of such an atmosphere via Jeans escape, the same mechanism that leads to chthonian planets. It's possible, then, that Kepler-10c would have been an ice giant if it had formed farther away from Kepler-10. It has a similar mass and potentially similar composition; it just happens to be much closer to its parent star than Neptune.

This is something of an unsatisfying answer, but there simply isn't an exact line that separates terrestrial planets and gas planets, just like there isn't a good line between massive gas giants and, say, sub-brown dwarfs.


1 There are, of course, exceptions, like Kepler-138a, which has a mass similar to Earth yet is a good gas dwarf candidate.
2 For more information on the transition region, see e.g. Lopez & Fortney (2013).

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