# Could a planetary system have a planet in its center? [duplicate]

Is it known, or at least known that it is feasible, to have a planetary system with a planet in its center, and not a star? Would it be too surprising to find a planet as barycenter and some stars as its satellites? That is, is it possible at all to have a planet that is more massive than a star and build a stable planetary system?

• You could have a rogue planet with moons, which is possible, but I'm not sure that fits within the scope of your question. Jun 3 at 20:38
• Only if you stretch your definition of "planet" to include things like cooled-down white dwarfs & neutron stars. It's simply a matter of mass: the "planet" has to be heavier than the star for the star to orbit it. (Actually they always orbit a common barycenter: the Sun and Jupiter orbit around a point just outside the Sun's surface, so an outside observer would see the Sun wobble a bit. That's one way we detect planets around other stars.) Jun 4 at 17:38

The maximum mass of a planet is about 13 times the mass of Jupiter, above that limit they are considered to be "brown dwarfs" and have at least some deuterium fusion in their cores at some stage of their life, and would probably glow a little.

The minimum mass of a star is 80 times the mass of Jupiter. Stars have hydrogen fusion and require this mass to form a core that is hot and dense enough for hydrogen fusion to oppose further gravitational collapse. So it would not be possible for a star to be less massive than a planet, and so it is not possible for a star to orbit a planet.

• But what if the planet was very massive and the star was very low mass? Like, what's the maximum mass of a planet? Jun 3 at 21:12
• @DaddyKropotkin Well, stars, like James K said, are at least 80 Jupiters, and the largest planets are ~13 Jupiters. In between are the brown dwarfs, neither planet nor star. Jun 3 at 21:21
• @DaddyKropotkin I've edited to make it a little clearer for you. Jun 3 at 21:32
• @DaddyKropotkin Here’s a paper from 2011 on the lower limit for brown dwarfs, based on the traditional definition of “able to fuse (at least some) deuterium but not hydrogen”: ui.adsabs.harvard.edu/abs/2011ApJ...727...57S/abstract Jun 4 at 7:08
• OP may be thinking about rocky or metallic planets - obviously a ball of iron is not susceptible to deuterium fusion, but I think what's missing is that once a planet gains enough mass it will attract interstellar hydrogen and helium to eventually become a gas giant, brown dwarf, etc.
– J...
Jun 4 at 12:02

What it misses is that it is simply way too early for "planets" composed of non-fusible substances (mainly iron, like Earth) to exist with mass higher than the minimum for a star.

Our universe is simply too hydrogen-rich and iron-poor for now. Whenever we have a mass sufficient for a star to collapse, it always contains a great percent of hydrogen, so we get a star (see "metallicity" of a star).

If we wait long enough, there will be probably places where the interstellar substance is mostly iron and if it collapses, it will go from its Hayashi track directly into the white dwarf phase without stopping at the main sequence. It is up to you to decide if this counts as a "planet". Next, you can catch a red dwarf in orbit. Red dwarfs are very long-lived so hopefully you will still have plenty of them fooling around.

Be aware that such a "planet" will be very dense and thus very small - much like the other white dwarfs - and the common barycenter with the red dwarf may still happen to be outside of it. (On the other hand, our Sun and Jupiter also have barycenter that is outside the Sun and no one says the Sun is not in the center)

• "Early earth-centric models of the universe" had the earth motionless at the center of the universe. This fails to explain Coriolis effects, the orbits of artificial satellites, stellar parallax, the aberration of starlight, etc., etc. So, no, not "technically correct". Jun 4 at 8:29
• "techncally correct" in the sense that one is free to choose whatever frame of reference they want, as long as they know enough math to do the calculations. At the times when Earth-centric model was used, the world's math knowledge was limited, too - to the point where Kepler and Newton laws would not be discovered even with a Sun-centric model. So the Earth-centric model was "good enough". Jun 4 at 8:40
• I think you're using "technically right" in some sense like "mathematically consistent", while ignoring the sense of "scientifically right". Which, sure. The early geocentric models were mathematically consistent, even if they ended up failing to predict all sorts of observed phenomena. But I think "technically right" is a misleading way to describe this. Jun 4 at 9:16
• Also, the problem that e.g. Hellenistic Greeks had with respect to Kepler (though not Newton) wasn't really the math -- they knew about conic sections and equations with powers of 2 and 3. What they lacked was instruments and data -- e.g., the telescopes necessary to see the phases of Venus, or the accurate (non-telescopic) instruments of Tycho Brahe, whose measurements Kepler used to show that elliptical orbits following his laws fit the data better than circles. Jun 4 at 9:19
• You are right. OK, removing the part about earth-centric models. Jun 4 at 9:22

Certainly, a stellar system could have a planet at its center instead of a star. If a binary system has both stars with the same mass, then

1. The barycenter of the system will be equidistant from the two stars.
2. The gravitational pull from each of the stars for an object at the barycenter will be equal.

(Image from wikipedia) So, a planet at the barycenter of such a star system would theoretically remain there.

Edit: Any mass deviations between the two stars or location deviations from the planet will cause destabilization of this orbit. See:

.

• This can't happen. Jun 4 at 6:22
• This probably CAN happen but will not be stable (at least, in regard to the planet staying in the center). Jun 4 at 6:54
• "But will not be stable" is another way of saying "This can't happen" when it comes to orbits. When we talk about an orbit, we mean something that is stable over the longer term. This can't happen without magic. Jun 4 at 7:16
• You’ve described the L$_{1}$ Lagrange point, which is (as pointed out) unstable. Jun 4 at 7:20
• That would make it technology, not astronomy! Jun 4 at 7:44

Depends on your point of reference. If it is the planet, then the star revolves around it, if it is the star, the planet revolves around it.

And if it is the barycenter, then both are revolving around that.

There is no universal point of reference.  The entire system is moving rapidly relative to any other distant star or planet.  "Everything is relative in its own way." (apologies to Ray Stevens)

• But since the mass difference between stars and planets is huge, the barycenter will usually be very close to (often inside) the star, so we would consider that the planet is orbiting the star. Jun 4 at 14:24

The previous answers seem to answer the question more principally. But there are two, sort of finely tuned, scenarios where the product of some process could result in a:

a planetary system with a planet in its center, and not a star.

By "center" I assume you mean "barycenter."

1. Consider a star system with multiple stars and possibly multiple planets (though only one is required, I think, for this example). The stars can be ejected due to dynamical scattering with its companions or perturbing nearby companions if in a cluster. IT seems possible that scattering could provide the planet an orbit that remains near the barycenter, similar to the Sun in our solar system, but this would require fine tuning of the motions of the remaining stars in the system. AS shown in @fraxinus' answer, such a system composed of two stars is likely to be unstable, but as pointed out in the comments this ultimately depends on various factors, and one might be able to find a configuration that is quasi-stable on some timescale. Our own solar system is stable on timescales comparable to the life of the Sun, but there are nuances.

2. Consider a planet of 13 Jupiter masses in a disk around a star of 80 Jupiter masses. Let's assume, for reasons perhaps unknown to us, the planet is accreting at a higher rate than the star. If there's enough hydrogen/helium in the disk around the planet, then, as @J pointed out in a comment to James K.'s answer, the planet would become a brown dwarf - something more massive than a planet that does not fuse hydrogen in its core, but it radiates thermally. Planets can also radiate thermally (I think), so there's a gray area here. Such a nearly equal mass ratio system could have a complicated orbit, and if there are other planets or stars with similar masses then one could conjure an orbit where the planet of interest passes through (or perhaps remains nearby) the barycenter.

It's interesting to ponder, but to really hammer down an exact answer would require running some simulations.

I believe so. Let's start with our solar system as a model. Then mentally gradually decrease the size of the sun, and evaluate what would change. The orbits would certainly start changing. The outer stars may also escape orbit altogether.

As you decrease the size of the sun, there comes a point where the sun ceases to be a star, and eventually becomes a planet. As long as at least Mercury stuck around, you would have a planetary system with two planets (although whether the miniaturized sun would really still be considered to be the center may be an open question).

The other thought experiment you could try is what would happen if you suddenly removed the sun altogether. Would the remaining planets all escape, or start orbiting Jupiter and each other? If so, you'd again have a planetary system, although whether any of the planets could be considered to be in the "center" is again an open question.

In the end, the calculations required are beyond me. My hunch is that the first possibility would lead to a planetary system without a sun, while the second one probably would lead to all planets escaping.

And my other hunch is that we wouldn't really care either way; we'd be dead from the lack of sun. Fascinating question from a scientific standpoint, though.

• Even under the most favorable positioning, the relative velocities of each pair of planets exceeds their mutual escape velocities for the distances between them. If the Sun miraculously disappears, all the planets are going their separate ways. Jun 4 at 19:55

Such a situtation is totally impossible, except for certain cases in which vary strange interpretations of the meanings of the words "planet" and "star" are used.

Part One: Why Not.

All stars emit light because they are very, very massive and thus have very dense and hot cores, cores dense and hot enough for fusion of hydrogen to happen, producing the energy which the stars radiat into space. And all objects that massive will have collected a lot of hydrogen (the most common element in the universe) as they formed.

As many sources will say, the maximum mass for a planet is about 13 times the mass of jupiter. If an object is more massive than 13 times the mass of Jupiter, deuterium will be fused in its core during at least part of its lifetime, and it will be classified as a brown dwarf. Brown dwarfs can be up to about 80 times as massive as jupiter.

Objects more than about 80 times as massive as Jupiter will be massive enough to fuse hyrogen in their cores. Thus they will be stars.

If the most massive planet is no more than 13 times the mass of jupiter, and the least massive star is at least 80 times the mass of jupiter, every star will be at least six times as massive as any planet in its star system. And of course it is more usual for stars to be thousands of times - sometimes even millions of times - as massive as planets in their star systems.

So if a system consists of the most massive possible planet and the least massive possible star, the barycenter between them which both object orbit around will be six times farther from the planet than it is from the stars, if I estimated correcty. And in most star systems the barycenter between a planet and a star will be thousands of imes closer ot the star thanto the planet.

Part Two: Changing the Definitions of Stars and Planets.

As I wrote, planets, brown dwarfs, and stars are defined as three different types of objects.

Suppose that someone defines a brown dwarf as a type of star. In that case a system with a planet of about 12.5 Jupiter masses and a brown dwarf of about 13.5 Jupiter masses could be considered a planet and star with the "star" only 1.08 times as massive as the planet. Thus the barycenter would be almost halfway between them, being 1.08 times as far from the planet as it is from the "star".

Suppose that someone defines a brown dwarf as a type of planet. In that cases a system could have a brown dwarf "planet" with a mass of 79.5 Jupeter masses and a star with a mass of 80.5 Jupiter masses. And in that case the star would be only about 1.0125 times as massive as the "planet' and thus the barycenter would be almost halfway between them, being only 1.0125 times from the "planet" than from the star.

Since the chemical composition of objects can affect the mass limits between planets, brown dwarfs, and stars, it is possible that out of gazillions of systems in the universe, there may be a few where supermassive planets are slightly more massive that super low mass brown dwarf "stars", or where supermassive brown dwarf "planets" are slightly more massive than super low mass stars.

Even in those cases, the barycenter should almost exactly halfway between the two objects, only slightly closer to the "planet" than to the "Star". And of course that depends on arbitrarily defining brown dwarfs as "stars" or "planets" when astronomers consider brown dwarfs to be a third class of objects.

Part Three: Artificial Suns For Planets Without Stars.

But there is a slight possibility of a star system where a planet could be orbited by an object which could be considered a "star" - from a certain point of view.

An advanced civilization could find a rogue planet the right size for them to inhabit wandering in interstellar space light years from the nearest stars. Such a rogue planet would usually be very, very, cold. The advanced civilization could modify the planet to make it habitable for them.

To do so they would have to heat it up, and provide an artificial "sun" since there was no star close enough. So they would put a gigantic artificial satellite in orbit around the rogue planet. That gianet artifical satellite would orbit the planet at a distance calculated to combine with the rotational period of the plnaet to produce a "day" about equal to the day of their home planet.

The satellite would have gigantic fusion reactors that fuse hydrogen- the most common element in the universe - to provide energy to power countless gigrantic lamps aimed at the planet below, to give the planet enough light and heat to be confortable for members of their species.

So the gigantic artifical satellige would function as a sort of "sun" or "star" for the rogue planet, and it would orbit around the planet.

And that is the only situation I can think of where a sort of a "star" - from a certain point of view - could orbit around a planet.

• Some stars might not have much hydrogen: arxiv.org/abs/1406.5509
– sno
Jun 4 at 19:52
• BTW, plain hydrogen is a really poor fusion fuel, so you can't use it in a practical artificial fusion reactor, as I mentioned here. So your artificial star needs deuterium (or maybe some kind of neutron generator) or some other fusion fuel, eg boron. Jun 4 at 21:34