# What would happen with a supermassive planet?

What would happen to a planet if it was really massive, like something around the size of the sun (likely made of asteroid materials)? What kind of environment might it have? What would happen to an atmosphere (if it actually had one, and it didn't just turn solid from the immense gravity)? Could it turn into something like a star that could burn really heavy materials? etc...

Also, I would preferably like to know both how it being rogue and in a solar system might also effect it. The solar system wouldn't really need to be ours.

It would collapse into a neutron star, possibly a black hole.

The volume of the sun is about $1.412\times 10^{27}\ \text{m}^3$. The density of normal rock is about $2.65\times 10^6\ \text{g}/\text{m}^{3}$.

So something the volume of the sun made of asteroid material would weigh about $3.7418 × 10^{27}\ \text{t}$.

Unfortunately the Chandrasekhar limit is about $2.765\times 10^{27}\ \text{t}$. Masses under this limit remain stable as white dwarfs, masses above that limit collapse into neutron stars producing an explosion that briefly outshines the galaxy.

At this point you either get a neutron star or a black hole and this depends on how big the explosion was.

The Tolman–Oppenheimer–Volkoff limit, the limit on the mass of a neutron star is $1.98\times 10^{27}\ \text{t}$, (yes lighter than the limit for turning into a neutron star) so this stage depends on whether enough mass was ejected by the explosion. If not you get a black hole, if enough is ejected you get a neutron star.

If you want a really massive rocky world that's not just a ball of flame or a neutron star I'd suggest staying bellow about $2\times 10^{24}\ \text{t}$ (a bit heavier than Jupiter) and sticking it somewhere short on hydrogen and helium, your planet would have a radius of about $44238\ \text{km}$, about $334$ times the mass of the earth and a surface area $42$ times that of the earth.

• G x (2×10^27 kg) / (1.957×10^15 m^2) = 68.2 m/s^2 is the gravitational acceleration felt on the surface of your planet. It is almost 8 times Earth's gravitational attraction. It would be impossible for anyone to walk normally with regular muscles. – Ephasme Jul 22 '15 at 13:33
• @Ephasme Thanks! I was having trouble getting wolfram alpha to calculate the surface gravity for me. – Murphy Jul 22 '15 at 14:56
• Murphy - It's not hard, and you can do it yourself. For a given uniform density the surface gravity scales as the radius (or diameter, if you prefer). So if you double the size of the earth but keep the overall composition the same, you double the surface gravity. Well, it's a bit more complicated than that, since the earth has a heavier iron core and a lighter mantle, so simple size changes don't quite scale simply, but if you assume uniform density the relationship holds. – WhatRoughBeast Jul 22 '15 at 17:17

If you took a bunch of asteroid materials (say carbonaceous because otherwise boring) and cloned up a sun-scale mass (probably below sun size) of them, they'd turn into a star. Depending on the kind of asteroid and its elemental makeup all sorts of weird stuff might happen. There's a colossal amount of energy produced by putting that much mass nearby. Even if it starts out cold and not moving, gravitational energy released as it compacts into a sphere will make it very hot.

• Lots of light elements: mass collapses -> ignite fusion -> A regular star, for a little while at least.

Deuterium fusion begins at $13 M_J$ and lighter bodies are normal planets. Hydrogen fusion begins at $75−80 M_J$ (bodies that burn deuterium but not hydrogen are brown dwarfs), but I (editor) have heard that with bigger amount of elements heavier than helium even lighter stars could ignite.

• Mixture of light and heavy elements -> ???? (but probably explosions as you skip straight to the supernova stage very quickly)
• Many heavy elements: Ignition is impossible, so we will end with degenerate matter.

At 1 sun mas, white dwarfs are probable. According to Wikipedia, helium fusion is possible already above $0.5 M_\odot$, but fusion of carbon, oxygen, neon and silicon happens only in very heavy stars. Iron is most stable and one cannot get energy from it with nuclear reactions.

Chandrasekhar limit for white dwarf mass is $1.39 M_\odot$ and than we may end with a neutron star. Tolman–Oppenheimer–Volkoff limit for neutron star mass is $1.5−3.0 M_\odot$, and heavier bodies without fusion become black holes (unless some exotic stars can exist).

Elements heavier then iron would be a case without precedence. They could undergo some strange nuclear reactions between fusion and fission.

Finally, among others, the last common element, uranium, undergoes well known nuclear fission and explodes above critical mass (mere kilograms). It could shine similarly to typical fusing star even for only planetary mass if only the mass was big enough to prevent shattering.

Unless you have very weird asteroid materials your giant planet is going to burn deuterium for at least a while. It's not the most luminous (but it is magenta) but adding a second solar mass to a solar system like ours is going to wreck some stuff. Expect to lose planets to interstellar space, see others crash into each other or swap orbits or disintegrate. If Jupiter hit this new gas giant/rocky weird thing at any speed expect, uh, fiery cataclysm.

In short: A lot of really weird stuff but you probably can't live on it. Either it'll be too hot or it'll collapse into a white dwarf, a neutron star or a black hole.

• Generally I agree, but I would like to add three details (numbers from Wikipedia): 1) White dwarfs are also possible and probable at 1 sun mas. Chandrasekhar limit for white dwarf mass is $1.39 M_\odot$ and Tolman–Oppenheimer–Volkoff limit for neutron star mass is $1.5-3.0 M_\odot$. Heavier bodies without fusion become black holes (unless some exotic stars can exist). 2) Limit for brown dwarfs: Hydrogen fusion begins at $75-80 M_J$ and I have heard that with bigger amount of elements heavier than helium even lighter stars could ignite. – BartekChom Jul 22 '15 at 10:43
• 3) According to Wikipedia, fusion of carbon, oxygen, neon and silicon happens only in very heavy stars, and this also suggests that mega-planets made of them should be white dwarfs. Iron is most stable and heavier elements could undergo some nuclear reactions. Uranium undergoes nuclear fission and it could explode or shine similarly to typical fusing star. – BartekChom Jul 22 '15 at 10:43
• White dwarves are post-star things, but given their composition (carbon-oxygen) we should look to them and perhaps to how they explode for hints on this. @BartekChom I encourage you to edit in all those details to the answer. – Resonating Jul 22 '15 at 13:22