# Can a planet fifteen times heavier than Jupiter not turn into a brown dwarf if it rotates fast enough and at the same time has a solid surface?

The book Mission of Gravity by Hal Clement describes a planet whose mass is 16 times the mass of Jupiter, rotating at a speed of one revolution every 17.75 minutes. This leads to the fact that it stretches into an oblate spheroid and has an effective gravity of ~ 3 g at the equator, while the gravity at the poles is 275 g (later Hal Clement checked the calculations and came to the conclusion that the gravity at the poles of this planet should be about 275g, not 700g as it was described in his book, a group from MIT also found out. Unfortunately, I have not found any links to these MIT calculations, so if anyone knows where to read these calculations, then I will be glad if someone leaves a link to them in the comments). However, how plausible is the existence of such an unusual massive planet with a very high rotation speed around its axis and the presence of a solid surface in reality, or in other words, can a planet fifteen times heavier than Jupiter not turn into a brown dwarf if it rotates fast enough and at the same time has a solid surface?

Note: in this case, what is meant is not the process of formation of such a massive planet with a solid surface like that of Earth-like planets, but the fact that it can rotate so fast around its axis, have the same flattened (almost flat) shape as Mesklin and whether there can be a different force of attraction on its surface (at the equator 3 or slightly more g, and at the poles up to 200 or more g)?

• It looks like you're asking about an an existing fictional world, rather than attempting to build a world of your own. Speculating about the realism of works of fiction is off topic for this site. Aug 15, 2022 at 17:46
• If you were to edit your question to something like "could a planet fifteen times as heavy as Jupiter avoid becoming a brown dwarf if it spins fast enough?" it would be less prone to be closed -- assuming that's actually what you're trying to ask, of course... Aug 15, 2022 at 18:04
• In an infinite universe, the highly improbable becomes the inevitable. As a science fiction writer, your job is to tell US how it could come into existence. Aug 15, 2022 at 18:17
• fixed, but I haven't received any answers to my question yet Aug 15, 2022 at 18:28
• We still don't know for certain if gas giants like Jupiter and Saturn have any solid surface at any depth, never mind what they'd look like if they spin several revolutions an hour vs. every 10-12 hours. Now go to 15 Jupiter masses, and we're pretty far into the unknown. Aug 15, 2022 at 18:44

Given we don't know with great precision where the border is between gas giants like Jupiter and brown dwarves, shining in infrared by the heat of their assembly and contraction, this is hard to answer.

It is pretty strongly surmised, however, that planets with even a single Jupiter mass (and quite a bit smaller) likely have no solid surface as we'd think of it; in Jupiter's case, there's likely a gradual transition from gas, to supercritical (hybrid of gas and liquid) to solid near the core, but if there ever was a rocky core that served to start capturing ices to start the process of building such a world, it's so deep and under so much pressure and likely so hot we might be unable to say with confidence whether it's liquid or solid.

As I understand it, rocky planets like Earth, Mars, and Venus can't get anything like that big, at least around a star similar to our Sun. There isn't enough mass available in the inner system, and in the outer system there's enough ice that anything that gets much heavier than Earth will become a gas giant (or an ice giant like Uranus and Neptune). The biggest "super-Earths" we know of are around three times the mass of Earth -- still only a tiny fraction of a Jupiter.

Bottom line is, we don't know for certain, but it seems doubtful that a world with a solid surface could reach even one Jupiter in mass, never mind fifteen or so, no matter how fast it spins. Or if it does manage it, we don't know if the surface can stay solid under the heat and pressure at the bottom of an atmosphere thousands of times as dense as Earth's.

In short, if you want to write a story or build a game set on a world like this, go ahead. Everything about it will be hand-waved; no on can say for certain that such a world can't exist, any more than they can tell you what the conditions are on the putative surface. Don't blatantly disrespect things like gravity, supercriticality, Coriolis effect, and so forth, and no one can tell you you're wrong -- at least not with a straight face.

• Such a world could plausibly form as the result of a large gas giant or small star having its gaseous envelope blown off / sucked off by a companion star. Aug 15, 2022 at 19:45
• And then a larger third star passing very close rips the two other stars apart leaving a rocky super world. Very unlikely but perhaps. This would help explain the problem of lack of materials, but all the other points remain. Aug 16, 2022 at 9:08
• @LoganR.Kearsley Current theory, however, suggests that Jupiter's rocky core (if it even exists) is no more massive than Earth. Jupiter formed where the volatiles didn't blow off, so kept them, and 99% of its mass would be gas at STP. Aug 16, 2022 at 11:04