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Imagine a dwarf star and a gas giant orbiting each other around their barycenter. The gas giant does not have enough mass to ignite (but may be close to igniting).

Over time, the sun loses mass due to radiation, flares and so on.

Could the gas giant end up having a higher mass than the star?

(If necessary, for some reason, the star and the gas giant don't have to be in the same stellar system)

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I don't know if heavier is the right word in the title, feel free to edit-propose a better wording. – J_F_B_M Jan 20 at 18:13
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I believe "more massive" would be the proper term. I've edited as such. If you disagree, feel free to roll it back. – Frostfyre Jan 20 at 18:26
    
I don't think this is possible, because stars use their own weight and friction to fuse light elements into heavier ones. – XandarTheZenon Jan 20 at 19:05
    
Well if you had an all helium or some such gas giant; but how? – Joshua Jan 20 at 21:37
    
Some interesting stuff in the answers here - but imagine having them in the same solar system. The masses couldn't be that different - they'd be in some form of mutual orbit, like a binary star system. This fairly well precludes them having such vastly different compositions in such a configuration, without outside help or a heavily handwaved encounter with a rouge planet. – Sean Boddy Jan 21 at 3:14
up vote 15 down vote accepted

I don't really think so... The largest a gas giant can get without being a star itself is a L class brown dwarf, which is just under the size needed for fusion to start.

The least massive star known is 2MASS J0523-1403, which is just over the fusion threshold.

You can get smaller stars (in radius), like white dwarfs, or neutron stars, but they are going to have a lot more mass.

A brown dwarf just under the limit is going to be larger than a red dwarf just over the limit, but that's because as they get more massive the density increases.

Edit:
Adam Wood's answer got me thinking: if the gas giant had a massively oversized iron core, that might be able to stop the fusion process from starting up, as iron doesn't fuse easily because the process is endothermic.
I don't know how big of an iron core you'd need to keep something with one solar mass from beginning fusion, or how much mass it could have before it started to do weird things...

Edit 2: Weird things
Apparently, according to this answer, a iron sphere can be up to 1.44 solar masses before the gravitational pressure would cause it to collapse into a neutron star.
This gives you a lot of room for a small start to orbit it. So, if you had an iron core that was around .5 or so solar masses and then surrounded by thinnish layer of hydrogen gas so that it stayed under the fusion limit, it could be orbited by a red dwarf like 2MASS J0523-1403 (0.08 solar mass range) easily.

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If it had an iron core, would it still be a "gas giant", or would it just be a big solid planet with an atmosphere? – Adam Wood Jan 21 at 2:12
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Yea, especially if that iron core is most of the planet, with just a thin film of hydrogen over the surface, that's stretching the term "gas giant" rather too far. This does, however, bring to light another issue, that apparently the largest size possible for a rocky planet is actually bigger than the largest size possible for a gas giant, which is surprising. – Matthew Najmon Jan 21 at 3:56
    
@AdamWood I think the difference has to do with how much gas there is. Jupiter has a solid, partly iron core, but is 95% gas. I don't know what the official rules are concerning percentages of gas to core. – AndyD273 Jan 21 at 4:12
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@AndyD273 That's what makes it accurate, but that doesn't stop it being surprising to someone accustomed to thinking of gas giants as being the really big planets, and rocky planets as much smaller. Also, the entirely-too-small sample size of all the people I've pointed this out to (sample size 3 people so far) has shown a 100% rate of being surprised among people not going by the name AndyD273. – Matthew Najmon Jan 21 at 4:27
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If it was dense and massive enough to hold onto free hydrogen and helium, it would be a "gas giant". But really, I think such things would create brand new categories! – JDługosz Jan 22 at 13:54

The other answers are all correct, but no one stated succinctly why this would not occur:

Once a giant ball of gas is massive enough, it will ignite (fusion) into a star.

That doesn't mean you couldn't come up with some reason this wouldn't happen --- some kind of fusion-killling element mixed into the gas giant's composition, for example.

Note, too --- if you were to do this, the star would be "orbiting around" the planet, as opposed to the other way around. (Or, if they are roughly the same mass, they would orbit each other.)

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I kind of covered the mass thing in my answer, but your answer got me wondering, If the gas giant had a massive iron core, and iron does not fuse easily, as the process is endothermic (absorbs energy), then that could stop fusion while allowing the gas giant to get really really big. – AndyD273 Jan 20 at 21:23
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"Once a giant ball of gas is massive enough, it will ignite (fusion) into a star." Is "massive enough" the same for a ball of Hydrogen as it is for a ball of Neon, etc? The more massive the nucleus, the higher the heat required to fuse it, and the less mass is converted to energy. This process ends with Iron, because fusing more massive nuclei consumes, rather than produces, energy. If the gas giant is mostly made of heavier gases, it ought to get much larger before the outer layer of Hydrogen, or an inner layer of a more massive element, is under enough pressure to fuse. – Monty Harder Jan 20 at 23:07

If my understanding is correct, there is a continuum from gas giants, to brown dwarfs to red dwarfs. Physical size stays close to the same from large Jupiter sized planets to mid-massed brown dwarfs, the mass just increases. So It would be (almost) impossible for a planet to out mass it's star.

However, a neutron star will be physically smaller than most planets, but it is also super massive, just not enough to collapse into a black hole.

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While this is true, Brown Dwarfs do not do fusion, so they are an odd type of star. – Dan Jan 20 at 22:49

Sort of?

There is such a thing as a Sub-brown dwarf star, but the lower limit on their mass is 1 Jupiter Mass (or there isn't enough gravitational attraction to hold it together). Whereas the upper limit on gas giants is about 1.6 Jupter Masses (or it collapses and turns into a brown dwarf). So there's a narrow range there where what you describe could be possible, but brown dwarfs (and sub-brown dwarfs) aren't really "stars."

Also, this setup would likely lead to the system to resemble a binary star system, with the "parent star" and the "gas giant" coorbiting a point of empty space.

If you're interested in size and not mass then absolutely: neutron stars.

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Yes but none can exist for many, many times the present age of the universe.

Your "planet" needs to start life as an extremely low mass star. Its fires will burn for many trillions of years. Eventually the star is converted entirely to helium (such tiny stars have convection throughout their volume, there are no unburned outer layers and no red giant phase) and then the fires go out.

Eventually it captures an even smaller but younger (and thus still burning) star.

Note that the object will be classed as a black dwarf but you can't really draw a dividing line between a black dwarf and a large gas giant planet. Both have a gaseous outer envelope (assuming it's warm enough) and a degenerate core.

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