Small Star

Can a planet be so massive and star of so little mass that a solar system would have the star orbit the planet?

And would you still call it a solar system?

In my scenario I’m interested in the star would still be of a type that produces light and heat for the planet.

Thank you.

Hmm… I would tend to say no, not for planets as we know them, or stars as we know them that generate light and heat by gravity-induced fusion. If you want to imagine a small, light “star” that creates light and heat by some other mechanism, then that’s at least conceivable.

I’m actually noodling on a setup like this for some stories I’m writing, but they’re more magical-realism than hard SF.

Okay, the smallest possible star is around a very minimum of 13 times the mass of Jupiter. So the planet would have to be considerably more massive than that.

The collapsing clouds that form solar systems are mostly hydrogen and helium because the universe is mostly hydrogen and helium. And after a certain point, a stony/ironic planet gets large enough to hold on to it’s hydrogen and helium and thus becomes mostly hydrogen and helium. So there would have to be some really funky circumstance where a dust cloud was somehow mostly depleted of hydrogen and helium, yet still had enough heavier elements to form a much more massive rocky planet than anything we’ve ever detected as one object, and yet still have enough material to form a minimal star as the second object.

I could imagine some sort of two-step scenario–a very hot star ignites in a very dusty proto-planetary disc in which a large planet is just beginning to form, blows away the volatiles. Later, a slow moving passing brown dwarf is captured by this freak massive planet.

But if we ever found a system like this, I think the chances of it forming naturally are so very, very low that we would have to guess we’ve found our first alien astro-engineering example.

(And if you don’t want to conciser a brown dwarf a star, and if the “13 Jupiter masses” is too low, then it goes from very, very, very, very unlikely to very, very, very, very, very, very, very, very, very, very, very, very, very unlikely.)

Generally no …

To be a star, an object must be of sufficient mass so that gravity compresses the core enough to start the nuclear reaction … and a planet must be below this mass … at best we could have a tiny star and massive planet of approximately equal mass co-orbiting their barycenter …

Yes, this would be considered a solar system … however how the energy transfers in this case would depend on how far apart the two objects orbit each other … if they were close, they would be warming each other … far apart and the planet would be emitting much more energy that it receives from the star …

With ordinary stars and planets I don’t think that it is possible.

It is possible to have a star that is smaller in diameter than a planet going around it. We know this because we’ve discovered stars that aren’t much bigger than Jupiter and we’ve discovered planets that are bigger (in diameter) than that. The thing is, these stars, even though they are smaller in diameter, have a lot more mass than the larger diameter planets. Once you get that much mass, the compression due to gravity alone creates fusion in the core, and then you have a star.

Big planets tend to be gas giants, and gas giants compress and make fusion when you get enough mass to them. If your planet isn’t a gas giant, then I think you might have a shot at this.

I don’t know how it would form naturally (if it’s even possible), but if you artificially created a planet out of something like iron, I think it might be massive enough to have more mass than one of the smallest possible stars, but wouldn’t undergo fusion chain reactions like the lighter elements do. Elements lighter than iron release energy during fusion. Iron and heavier elements absorb energy during fusion. This is why stellar fusion reactions stop at iron, at least until you get up to super-nova type conditions, which you aren’t going to have here.

Admittedly, my understanding of the physics involved here is a bit iffy. Someone who understands it better than I do may have a different answer.

It is not quite true that the earth circle around the sun, but (if we removed the rest of the planets) they each circle around center of gravity of the combined system. The same is true of the earth and the moon. What is unusual in that case is that the center of gravity lies outside the earth. I believe the earth/moon system is unique in the solar system with that property.

As for the OP, as noted above, a planet more massive than a star would start to fuse too under the heat and pressure.

Actually, gas giants don’t get a whole lot bigger in diameter than Jupiter (unless they are puffy.) And get bigger than around 1.6 times the mass of Jupiter and they actually get smaller thanks to electron degeneracy. Brown dwarf stars also have around the same diameter as Jupiter.

The Earth/Moon barycenter is inside the Earth.

Gas giants can’t get much bigger than a hot, puffy Jupiter, which is just about the same size as a small red giant or large brown dwarf. Note that hot,puffy Jupiters are extremely inhospitable, so you wouldn’t want to live there.

How about rocky planets? Sorry, but the largest rocky planet would be much smaller. The largest solid planet, made of solid water ice (the lest dense of all the solid planet-forming materials) would only be about 4.5 times as wide as the Earth. If you add more mass the radius actually starts getting smaller, just like in gas giants, because of compression.

I thought Pluto and Charon orbit around a point above Pluto’s surface?

Correct – and they are tidally locked to each other (each is always in the same position in the sky relative to a location on the surface of the other: there is never a Charon moonrise or moonset on Pluto). That system has the lowest planet-to-primary-moon ratio (heaviest moon relative to the planetoid); the next lowest ratio is Earth/Moon, which has a barycenter about a thousand miles from the earth’s surface (of a ~3550 mile radius).

On the other hand, the barycenter between Jupiter and the Sun is in fact outside the sun, by over 30 thousand miles (more than 4 earths would “stack” in that distance). I am not sure what the exact definition of a orbits b is, but I suspect if the barycenter between two bodies is less than the sum of both diameters from the midpoint of the distance between the two, the bodies would be described as co-orbital.

I do know how it would form naturally: That’s a white dwarf. Yeah, all the ones we know of glow, but that’s just from residual heat: Not much changes once they cool down.

If you had a high-mass body with fairly high metalicity, it would naturally stratify, sending the heavier elements to the center. With iron other heavy stuff at the core, fusion would not occur, and material pressure would diminish toward the surface. The planet would not have to be solid iron, only just enough that there is not enough pressure for fusion where the lighter elements live. And, of course, its mass would have to be below the Chandrasekhar limit.

A small red dwarf and a massive white dwarf binary pair would eventually fit the description given in the OP. Take SS Cygni for example. The white dwarf has a mass of 0.6 solar and the red dwarf has a mass of 0.4 solar. In a few hundred billiion years the white dwarf will cool down to planetary temperatures and become a black dwarf, while the red dwarf star will continue to shine for another trillion years.

You could, in theory, walk on the solid surface of the black dwarf and look up at the red dwarf in the sky. If you could lift your head up against a gravitational field five orders of magnitude stronger than Earth’s.

Mercury is a real world example of an “iron planet”; they form when the outer layers of a terrestrial planet are blasted away leaving mostly the iron core behind.

I came across an interesting pop-sci article relevant to this question: Could a planet be as large as a star.

Just to clarify, is that because we only know about the ones that still glow, since those are the only ones we can see? How long does it take a white dwarf to cool down?

White dwarfs are primarily tightly-packed degenerate matter, which has a high residual heat from the original star. Because of their dense state, which is not really a solid but may be a sort of negative plasma, they do not conduct or convect heat very efficiently, so heat loss is very, very slow. Consider how hot a star is, then condense about half that into an earth-sized object that is pretty good at retaining most of its heat (especially since it has quite a bit less than half the surface area that the star did). There are no known or expected black dwarfs, because the universe is believed to be a lot less than a couple trillion years old.

The coolest known white dwarf has a temperature of about 3000K, similar to a small red dwarf in colour, but much smaller in size,

This is also one of the oldest white dwarfs known.

Note that white dwarfs don’t all cool at the same rate, because some of them are more massive, and denser, than others; the massive and denser objects cool down more slowly because they contain more heat, but they still have tiny surfaces to radiate with.

So it’s not just that we can’t see all the cold ex-white-dwarfs out there, it’s that they actually haven’t had time to cool down past the point of visible light?

Yes. It’s hard to calculate the exact time for a white dwarf to cool down completely because there are a lot of unknowns and possible interactions with the environment, but the estimates are generally that it would take around ten thousand to a hundred thousand times the age of the universe for a white dwarf to cool down enough to not be visible.