# Can a star orbit around a planet? [duplicate]

I have a story where, there's a planet much like our Earth, (only hundreds of times larger than our Earth) is in the middle of their Solar System, and around this planet there's a Sun, which orbits the planet.

When thinking about this I came up with a 2 problems -

1. Can this occur naturally?
2. If this can't occur naturally, is it possible to create this artificially?

The density of the planet is exactly like Earths, if this isn't dense / realistic enough weight could be added, extra iron etc to create a planet with more mass.

Edit - it's not a duplicate as my question isn't about having a planet in the center of the universe.

• Same question, on Astronomy.SE
– Mark
Commented May 30, 2016 at 18:05
• Note that technically, two bodies actually always orbit around each other. They both move. The fact that one appears to be fixed is due to the "fixed" one having so much more mass than the "moving" one. Our sun actually moves constantly as the various planets move around it. Commented May 30, 2016 at 18:46
• "it's not a duplicate as my question isn't about having a planet in the center of the universe" If you read the proposed duplicate beyond the question title, you will see that center of the universe specifically is not a requirement in the proposed duplicate. The question body specifically says asks about a planet "in the center of universe/galaxy/Milkyway/ solar system " (my emphasis).
– user
Commented May 31, 2016 at 7:26
• in that case it is a duplicate. Commented May 31, 2016 at 7:37
• Commented Jun 2, 2016 at 10:31

I would say this is not possible using actual physics.

Firstly - Planetary Mass

A planet a hundred times the size of the Earth will NOT be exactly like the Earth. Gravity and mass will increase so much life will simply not be possible.

Secondly - Orbiting star

A star is always much more massive than a planet. A planet about 90 times the mass of Jupiter would have had enough mass to begin fusing hydrogen and ignite, thus becoming a star. So since the lighter objects orbit around the heavier ones, you cannot have a star orbiting a planet...

You can try and make your planet as massive as possible, and your star as light as possible, but event with that I think the barycenter of your two-bodies system would still be inside the star, only giving it a slight wobble. Your planet, however, will still orbit the star.

• "Gravity and mass will increase so much life will simply not be possible." - That depends. If the mass of the hypothetical planet is 100 times as large (with identical density), surface gravity would only be 4.6g. Not something we humans can handle, but certainly no obstacle for life in general. If the radius is 100 times as large, surface gravity would be 100 times as large. I suspect in that case, waterbourne life might still be possible. (used formulas from wikipedia) Commented May 30, 2016 at 15:59
• @marcelm you won't get identical density! Jupiter is as large as gas planets get. Add more hydrogen and it keeps the same radius but gets denser. Commented May 30, 2016 at 21:03
• @JDługosz A gas giant, yes. But if you ignore mechanisms of planetary formation (as the OP does), there's other materials you can build your giant out of. And since stars can in fact stop fusing while also avoiding collapse, it is possible to have a planet that is more massive than a small star. The data we already have from other star systems suggests that rocky planets can get much bigger than what we see in the Solar system (Kepler 10c has about the mass of Neptune, while apparently being predominantly rocky) - we'll see what we'll see :)) Commented May 31, 2016 at 8:03
• That's worth asking in Physics or Astronomy or somewhere like that. Who handles Astrophysics? Commented May 31, 2016 at 9:19
• @JDługosz : here you go : astronomy.stackexchange.com/questions/8396/…
– Nico
Commented May 31, 2016 at 9:26

The only reasonable solution I can see would be if the sun appeared similar to our sun to people on the ground, but was in fact something with far less mass orbiting far closer. No natural entity or phenomenon really seems to fit these criteria, so it would have to be some kind of megastructure. It could be purely artificial, like a moon-sized heat lamp essentially, or you could posit that some advanced technology (magic) allowed the creation of a miniature star, fuelled by some means other than its own mass. Perhaps an extremely handwavey wormhole type arrangement siphoning off plasma from a real star elsewhere and containing it within a field of the appropriate size?

The Earth and Sun orbit around their combined center of mass, the Barycenter. That point happens to be inside the sun because it so much more massive.

If the sun and your planet were the same mass, they'd orbit around a point in the middle.

To reverse the situation and have the Barycenter be near or inside the "planet", your "planet" would need to be hundreds of solar masses and yet not have fusion going. Whatever it would be, it would collapse and turn into a black hole or neutron star.

• However, the barycenter for Sun/Jupiter is outside the sun. Among other entertaining implications, this means that there is no anti-Earth, since we would occasionally see it. Commented May 30, 2016 at 16:46
• @WhatRoughBeast or that there is an anti-Earth and we occasionally see it Commented May 30, 2016 at 19:55
• The least massive star we know of is around 13 Jupiter masses - noöne says the star is necessarily a Sun-massive star. Conversely, there are "afterstars" that are no longer fusing, yet still have their mass and do not collapse - the key is composition here. You could have a planet that is composed of materials that do not fuse easily, with enough mass to offset a small star, but not enough mass to collapse. Would this form naturally? Hard to say, but observing planets in other star systems is giving us loads of unexpected data (a rocky planet as massive as Neptune!), so who knows? :) Commented May 31, 2016 at 8:11

How about having two suns orbiting around their common center of mass? With the Earth placed right in it.

I imagine that would require some unbelievable balance and a lot of assumptions about how it comes to be, but, at least theoretically, that is something not violating too much physics. As a bonus you get to keep standard Earth mass.

It's probably not very comfortable to live on a planet with no nights, and it's certainly won't be Earth-like planet with that day-night cycle. As a solution you might consider replacing one of the suns with some kind of non-emitting counterweight. Maybe white or black dwarf, which, as far as I know, are pretty heavy and cool, so they may do the trick.

• While that could be stable in a purely 3-body system (so long as the planet remains exactly in the center of mass of the system at all times), you could run into trouble if you want to have other planets or really any other massive bodies in this universe at all.
– Neil
Commented May 30, 2016 at 20:12
• @Neil I totally agree, that this is very unstable. However at this point you can consider some artificial solutions to come into play. I can't name any though, other than some ridiculous jet engines or something :) Commented May 30, 2016 at 20:19

As Cyrus mentioned, the Earth does not orbit around the Sun, but around the centre of mass inside the system. However, all motion is relative the point of reference, so considering that the Earth is fixed, our own Sun revolves around the Earth.

The only problem with this is that the orbits of the other planets (and other space objects) become very eccentric if we consider the Earth to be fixed, so we can use a much simpler mathematical model if we consider the Sun to be fixed. Similarly, if we look at the bigger picture it might make more sense to consider the centre of the Galaxy to be fixed and the Sun to orbit around it.

If your solar system contains only one planet and one sun, it's very believable to consider the planet to be the point of reference and the sun to move around the planet, just like humans believed for a time regarding our Earth and Sun.

It is possible, but the planet needs to be massive enough to dwarf the star.

Now, if the planet was truly that massive, it would likely begin to fuse simply as a matter of density, and then it would become a star, which is kind of moot. However, if you had a Ringworld-esque world, that is, a spherical dome structure around a massive stellar body (star, small black hole, neutron star, et cetera), people could live on that as though it were a planet, and a star could orbit it. If you have a star or neutron star on the inside, you'd have to worry about the energy radiating into the ground of the planet.

That, however, leads to further issues. A dome around a massive stellar body would inevitably huge, to the point that the solar radiation from the star wouldn't amount to much proportionate to the planet's surface area.

Reaching the goal of moving the barycentre to within or near the planet might be possible if it's in a stable configuration with lots of other planets:

I've no idea if such a combination is possible, though.

You'll need a Dyson sphere that you live on the outside of. Unfortunately, the support structure has to be made of unobtainium. If you have Earth-normal gravity on the surface of something the size of Earth's orbit you can have a star going around it. Note, however, that it takes far more than a day to do so.