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They actually do, but let's set Relativity aside for a moment. And the whole barycenter thing too. Here's the scenario:

Somewhere in the backwaters of a galaxy there is this huge planet. It is NOT a failed star. It could be considered a gas giant, except most of the gas is nitrogen, with traces of oxygen. The planet has a nucleus with a rocky surface covered in part by water. There's life on the planet, maybe even intelligent life. Orbiting this planet is a dwarf star. It is sufficiently close to the planet to provide energy and warmth, and not close enough to incinerate it. Its orbital speed is sufficient to warm the equatorial parts of the planet without singing them when it passes over them and to come back to them before they freeze over. Or maybe there are two stars of slightly different sizes, traveling along different orbits.

Is this at all possible?

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  • $\begingroup$ Or is your question actually - can a rocky planet be more massive than a star? $\endgroup$ – ProfRob Nov 19 '15 at 15:27
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    $\begingroup$ @RobJeffries: No, that's already been answered in the comments, and I agreed with the answer. My question is about the scenario I described in my post. When you labeled my post "a possible duplicate," I hastened to accept Nico's answer. Causa finita est. $\endgroup$ – Ricky Nov 19 '15 at 15:43
  • $\begingroup$ A sub-brown dwarf around the lower limit for mass (1 Jupiter mass) could orbit around a large gas giant, but that doesn't really fit the constraints of your question. $\endgroup$ – called2voyage♦ Nov 20 '15 at 18:25
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I think the answer is no.

A star is much more massive than the most massive planet you could imagine.

Take Jupiter as an example : it is a massive planet, with a gravitational pull so intense it makes the Sun move. Not that much, but it actually does. However, the Sun is not in the slightest orbiting any planet.

For a star to orbit a planet, it would require a wandering star to be caught into a planet's orbit. But it's pretty sure that only the opposite can work.

So... No, no star orbiting around planets.

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    $\begingroup$ I've also read somewhere that Jupiter was actually quite close to being a brown dwarf, but it didn't, due to a whole lot of parameters. I don't know enough about the subject, unfortunately, but concerning stars orbitting planets, I'm fairly certain it is impossible. Also, thanks for my english. $\endgroup$ – Nico Nov 19 '15 at 11:22
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    $\begingroup$ I don't think Alexandre Dumas was an astro-physician, though. But the universe is big (REALLY big) and maybe we can find something like that. $\endgroup$ – Nico Nov 19 '15 at 11:30
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    $\begingroup$ Still, the laws of physics have to apply. They require us to have roughly 13 Jupiter-masses (sun is ~1000 $M_J$) to start hydrogen-burning in the gas-blobs core (the line between planet and star becomes blurry, thus gas-blob). Deuterium burning starts at lower masses. No one speaks about radius in an astrophysical context, as mass is the determining factor in stellar physics. As you require the star to orbit the planet (and given the fact that we can't ignore the barycenter stuff) its mass must be several factors higher than the star's. $\endgroup$ – AtmosphericPrisonEscape Nov 19 '15 at 14:00
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    $\begingroup$ Thus what you require is a purely rock-planet of several tens $M_J$. I wouldn't call this an impossibility. We just wouldn't know how to form such an object, as it would rather accrete gas and turn itself into a star / gas giant. $\endgroup$ – AtmosphericPrisonEscape Nov 19 '15 at 14:02
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    $\begingroup$ @AtmosphericPrisonEscape I think your point is spot on but your numbers are off. 13 Jupiter masses is around where the Deuterium fusion begins. Hydrogen fusion is closer to 75 Jupiter Masses. en.wikipedia.org/wiki/Brown_dwarf $\endgroup$ – userLTK Nov 19 '15 at 18:35
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And the whole barycenter thing too.

You're throwing out the definition of orbit at this point, and moving into a different realm. If you throw out barycenter, you have to leave me with my frames of reference.

A star can definitely orbit a planet, if one such as this did in fact exist. You are assuming it is simply a planet-star system, which makes things easier. If you're on the planet, the star is orbiting you, if you're on the star (ouch), the planet is orbiting you.

Once they are of comparable size, the barycenter thing has to be taken into account, you can't just throw out this crucial part of the physics, even in a thought experiment. If you want that part out, the planet must be much bigger than the star.

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  • $\begingroup$ Okay, stick the barycenter somewhere underneath the surface of the planet. Not too deep. Didn't I ask not to involve Relativity? Yes, thanks to Einstein et al we now can go all the way back to the geocentric model. Yes, the Sun does in fact orbit the Earth. Conventional (or Newtonian, anyway) pseudo-wisdom, however, compels us to put the larger body in the middle, and have the smaller one orbit it. This may not be the right thing to do (since our civilization is, in fact, geocentric), and I only insist on it for the sake of argument. $\endgroup$ – Ricky Nov 19 '15 at 23:27
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    $\begingroup$ If the barycenter is somewhere in the planet then you are basically describing the Earth-Sun system. I feel like you can't ask the question without the barycenter, and with it the answer is yes. $\endgroup$ – Doug Nov 19 '15 at 23:53
  • $\begingroup$ Last I checked, the barycenter of the Earth-Sun system was somewhere inside the Sun, not Earth. What am I missing? $\endgroup$ – Ricky Nov 20 '15 at 0:04
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    $\begingroup$ Your planet has to be bigger than the star for the barycenter to be inside of it, we are thinking of point particles, their composition doesn't matter in this limit. $\endgroup$ – Doug Nov 20 '15 at 0:24
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    $\begingroup$ The answer is yes. $\endgroup$ – Doug Nov 20 '15 at 0:38

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