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Is it theoretically possible for a star to orbit a planet?

  1. May 19, 2015 #1
    I was doing some research on the findings of the Kepler space telescope this morning. I noticed that some of the stars it has found are quite small, and some of the planets in other systems are quite large. Is it theoretically possible for there to be a situation somewhere in the universe where one of the small stars came near one of the large planets, and the planet became the center of the star's orbit. Could geocentrism really be possible after all?
  2. jcsd
  3. May 19, 2015 #2
    That still would not satisfy the requirement that "all" celestial bodies revolve around said planet.
  4. May 19, 2015 #3


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    The two body interaction in general has both bodies revolving around the center of mass. When one body is much larger, the center of mass will be inside it. When one is a star and the other a planet, the motion depends only on the relative masses.
  5. May 19, 2015 #4


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    And it should be noted that there can be no planets larger (in terms of mass) than stars. What happens when a planet gets too large (i.e., too massive), is that the pressure and temperature in its core becomes so great that it ignites fusion - it becomes a star.
  6. May 19, 2015 #5
    According to this Wikipedia article on OGLE-TR-122, the lowest possible mass for a star is estimated to be around 0.07 or 0.08 solar masses. So, if a star had a mass of 0.07 suns, and the gas giant had a mass of 0.06 suns, it sounds possible that they would be in a binary orbit with each other. That's not a geocentric system, but it's not heliocentric either.
  7. May 19, 2015 #6
    And I'd like to add—how would stellar wind affect this possiblity?
  8. May 20, 2015 #7


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    The point is, no system is either. Dynamically, all gravitating systems are barycentric.

    There's a minor confusion on what geo- and helio-centric system means today and what it meant historically. Back in Copernicus' day, these were dogmatic claims about the ontology of the universe. One assumed the Earth to be the centre of the universe, the other took the Sun to be the central point of all existence. Both were incorrect in this respect.

    In modern usage, these two mean merely the choice of coordinate systems, and both are in use when convenient. You would still use the geocentric system to describe motion of Earth's satellites or send a resupply ship to ISS, or to figure out 'where does the sun rise'. You can use heliocentric coordinates e.g. if high accuracy in planetary motion is unnecessary. Both are equally valid coordinate choices as the barycentric, and you can in principle describe all motion in whichever one you choose. None is 'true' in the historical, ontological sense, but some are better suited for some tasks than others.

    So the question of whether a system can be heliocentric or not doesn't make all that much sense. It can be either, if you mean coordinate choice. It can be neither if you mean actual centre of the universe, or even in the weak formulation - if you only mean the barycentre to coincide with the centre of either body.
    Last edited: May 20, 2015
  9. May 20, 2015 #8
    In short, the answer is no. A planet will always have less mass than a star.
  10. May 20, 2015 #9
    Theoretically, somewhere in the universe, I suppose it's possible that there are two massive planets in close orbit of each other and a very small star far away from it in an orbital resonance. The combined mass of the planets would be greater than the star so the center of gravity would be closer to the double planet system. They probably wouldn't have formed that way though.
  11. Jun 1, 2015 #10
    If the planet mass is larger than the star, then why not the planet itself turn to a star?

    large planet, like Jupiter, Saturn, will turn to a star if they get enough mass, since they contain the same elements like the sun.
  12. Jun 1, 2015 #11
    I suppose it's possible to have a low mass red dwarf star in a binary configuration with a brown dwarf type object, a truly massive planet that is almost but not quite massive to trigger fusion.
    In this case the barycenter would lie between the objects, so at a pinch one could say that both objects are orbiting each other.
  13. Jun 1, 2015 #12
    Imagine two objects on a lever - one large, and one small. The balance point of the lever will be closer to the larger one. The quantities called "moments" must be equal. The "moment" is the mass times it's distance from the balance point:
    M1 * d1 = m2 * D2
    It is the same for stars and planets. The really revolve around a balance point called the 'barycenter'. This point is always closer to the heavier body. If the difference is masses is great enough, the barycenter will be *inside* the larger body.
    The Wiki article on "Barycentric_coordinates_(astronomy)" has some very good graphics on this.
  14. Jun 2, 2015 #13
    According to Dr Raymond at the University of Colorado, once a gas giant reaches about 15 times the mass of Jupiter, it's going to ignite a fusion reaction and become a brown dwarf star. Rocky planets like the Earth can get to be about 5 times the size of the Earth before they collect too much of an atmosphere to be considered terrestrial panet and become gaseous ones. I suppose it is just possible that you could get twin gas giants with somewhat differing composition that would allow the slightly lighter one to ignite while the bigger one does not. In fact, considering the number of star systems in the universe, I'd just about guarrantee that there is at least one that matches that description. But I don't know if you'd consider the non-ignited gaseous giant a planet or a "failed" star.
  15. Jun 2, 2015 #14
    The planet should be much larger than the star so at least have a hundred times Jupiter's mass. It is argued above that a very massive planet become a star itself. However, fusion requires a suitable composition. Wikipedia: "The fusion of two nuclei with lower masses than iron ... generally releases energy, while the fusion of nuclei heavier than iron absorbs energy."
    So a very large planet with insufficient concentration of light elements would not become a star!
    Last edited: Jun 2, 2015
  16. Jun 2, 2015 #15
    Rocky planets are the size that they are because they are too small to hold onto lighter elements. Hydrogen and helium are almost non-existent on Earth because they rise in the atmosphere and then float away into space. If the rocky planet gets too large, gravity starts being able to hold hydrogen and it's not longer at an equilibrium. The amount of hydrogen sticking to the planet from interstellar space becomes greater than it's ability to leak it away, so it enters a runaway mass gathering phase.
    Last edited: Jun 2, 2015
  17. Jun 2, 2015 #16
    Unless there is no hydrogen around for some reason.
    Such a planet is unlikely to happen, I agree, but it is in principle possible.
  18. Jun 3, 2015 #17
    I was playing around with a gravity simulator and I decided to try and tackle this question. If I made the limit of a star in this simulation 110, you can have two failed stars orbiting very close to each other and a real star's trajectory completely engulfing their own.
  19. Jun 3, 2015 #18
    Even at 0.06 solar masses the planet would have more than 62 Jupiter masses. Planets with more than ~14 Jupiter masses are called brown dwarf stars, because at that mass the deuterium fusion process begins.

    Therefore, to be a planet it must be smaller than ~14 Jupiter masses, and large enough to achieve hydrostatic equilibrium. Since all stars of every type, even dead ones, are more massive than ~14 Jupiter masses, and all objects orbit their center of mass, it is not theoretically possible for star to orbit a planet.
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