# If planet like Jupiter is massive enough to hold hydrogen with its gravity is it bound to become a star?

I have been reading about Jupiter as of late and reason I am asking is if Jupiter has strong enough gravity to pull hydrogen to itself then it should be only matter of time when it will acquire enough mass to ignite itself and become a star, same would apply to any other planet that is big enough to hold hydrogen (In other words if planet gains more matter than it looses it will become a star eventually). Or am I missing something here?

• May 22, 2015 at 12:35
• @Qmechanic don't think it's duplicate. My question is generic to any planet that has strong enough gravity to grow, Jupiter is just a specific example. May 22, 2015 at 12:37

Jupiter will never (not on any timescale like the lifetime of the Sun anyway) accrete enough mass to begin hydrogen fusion.

It would need to accrete 12 times its current mass to undergo a brief period of fusing its interior deuterium and to accrete more than 70 times its current mass to attain a central temperature high enough to sustain hydrogen (pp chain) fusion. There isn't enough accretable mass in the solar system to do this.

The problem is electron degeneracy pressure, which is able to support lower masses even if the gas is cooled completely. The planet cannot contract and heat up its core enough to begin fusion.

If the length of time over which any material is accreted is unlimited, then a more detailed consideration, discussed in https://physics.stackexchange.com/a/710967/43351 reveals there are two possibilities.

1. If the accretion is faster than the cooling timescale of the planet (which is tens of billions of years) then by accumulating enough mass it will eventually become a low mass star.

2. If the mass is accumulated very slowly, then the planet may never get hot enough to ignite and will become a low mass "hydrogen white dwarf" - or I suppose you could call it a high mass brown dwarf.

• Are gas giants really supported by degeneracy pressure, or just very large gas pressures? May 22, 2015 at 11:17
• Why do you make a distinction? The electrons in the core gas have ratios of Fermi energy to $kT$ of 10-20, they are thus partially to nearly degenerate and exert a large pressure. Perhaps a better answer is to say that without the Pauli exclusion principle Jupiter would be much smaller and hotter and maybe could become a star. May 22, 2015 at 11:23

I'm not entirely sure what you mean about 'pulling hydrogen', all bodies, whether they be planets or literally human bodies, will pull hydrogen via gravity. Earth can lose the H it attracts as H is so light that it can have speeds greater than the escape velocity (just due to random thermal motion). Perhaps Jupiter is sufficiently massive that this happens significantly less.

However having hydrogen isn't the only condition needed for a star to be born. That hydrogen needs to be hot enough that it is moving fast enough that it can collide with another hydrogen atom at sufficient speeds to overcome electromagnetic repulsion and get close enough for the two to fuse, releasing energy. These temperatures are only reached in incredibly dense high pressure cores of massive collapsing clouds (and the stars these subsequently form). There is a sharp cut-off between the temperature, and hence mass needed, for fusion to start, and Jupiter falls slightly short. (The cutoff is at about 0.08 solar masses)

If jupiter were a bit heavier deuterium fusion might occur, giving a brown dwarf "star", but, happily for us, it'll just while away it's life as a gas giant. Binary star systems, where two bodies in the system are massive enough to form stars, are fairly commonly seen in the galaxy (and more than two bodies is perfectly possible, but the system is unstable and all but two will be ejected pretty quickly) but it's probably a good thing for life on earth that we're not in one!

• Not only that, but is it true that having an iron core as Jupiter has, doesn't help matters, although the core of Jupiter is probably miniscule compared to the width of any required proto-star gas cloud and would have no significant effect?
– user81619
May 22, 2015 at 11:46

Can Jupiter become a star? Yes and no.

Jupiter can never self ignite because it lacks the necessary mass. Hundreds of Jupiters would have to be combined to produce enough gravity for self ignition.

However, Jupiter can be ignited by a high energy device such as a nuclear bomb. In a way that would make it a star for a very short time. A star like the sun has a careful balance of gravity vs expanding forces due to temperature. Jupiter lacks the gravity to oppose the expansion forces and hold it together. In all likelihood igniting Jupiter would be more like blowing it up. It would be the biggest hydrogen bomb ever observed. Parts of Jupiter and it's moons would rocket through the solar system combined with a giant flash or energy. The effects of such an explosion could be devastating to Earth. I would speculate that half the Earth facing Jupiter would be instantly scorched by the incoming energy. The temperature of the atmosphere would go up all over the globe. We would have giant atmospheric events, leveling ground structures and forests. Even the oceans and land would get warmer.

This could make for a great Hollywood disaster movie.

• Jupiter can be ignited by a high energy device such as a nuclear bomb. [citation needed] ;) I suppose that exploding a thermonuclear device deep in Jupiter's atmosphere might cause a tiny amount of the nearby hydrogen to fuse, but it certainly won't ignite the whole atmosphere. BTW, getting plain hydrogen to undergo fusion is really hard, see en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_reaction Fusing deuterium is a lot easier, but even if Jupiter's atmosphere were pure deuterium, dropping an H-bomb in it still wouldn't cause much of the atmosphere to fuse. Oct 29, 2019 at 7:14