Astronomy Question

[jimlintott]

My daughter asked this question and I didn't know the answer. It did spark some debate in the family with rational ideas for yes and no being offered. Truth is, we have no idea.

Could a star ever orbit a planet?

I am inclined to think not.

[Taffer]

Our sun does, in fact, orbit the earth (and every other planet). We tend to say that a planet orbits a star, but more accurately the planet and the star orbit each other, about the center point of their masses. In the case of the earth and the sun, this 'center point' is actually inside the sun (quite close to the sun's center, IIRC), so the most you would observe would be a very slight wobble in the sun. So to answer the question, each star orbits any planet that orbits it, just as the planet orbits the star; they orbit each other.

[jimlintott]

Thanks. That does make sense.

I did have more of a picture in my head of a planet with a star as a sattelite like the moon.

Any examples of this occuring?

[Taffer]

As I said, even the moon and the earth oribit each other. If there was a planet with a 'star sattelite', the star would hardly move (due to it's much larger mass) and you wouldn't really be able to tell.

There are stars with other stars orbiting them, like the Centauri system, in which Apha Centauri A and B orbit each other, and Proxima Centauri orbits those two at a distance about equal to two pluto's orbits.

[SpaceFluffer]

Our sun does, in fact, orbit the earth (and every other planet). We tend to say that a planet orbits a star, but more accurately the planet and the star orbit each other, about the center point of their masses.But even in the reference frame where we view the sun as orbiting the earth, the other planets still orbit the sun. Therefore it's only really correct to say that body 1 orbiting body 2 is the same as body 2 orbiting body 1 when there are no other body involved.

[Taffer]

But even in the reference frame where we view the sun as orbiting the earth, the other planets still orbit the sun. Therefore it's only really correct to say that body 1 orbiting body 2 is the same as body 2 orbiting body 1 when there are no other body involved.

Not true. All bodies orbit each other. The sun orbits about the earth, and saturn, and pluto, etc.

[StaticEngine]

If you had a massive planet (bigger than Jupiter) and a dead star (say, a tiny brown dwarf) that had somehow been captured by the gravitational pull of the planet, then yes, it techincally would be possible for a traditional orbit to occur where the star was orbiting a planet.

But in general, at any given scale, the less massive object tends to orbit the more massive object. If the objects are nearly equal mass, then they "orbit each other", which really means they orbit around the combined center of mass of the two objects (see Charon and Pluto for example).

[toddjh]

Thanks. That does make sense.

I did have more of a picture in my head of a planet with a star as a sattelite like the moon.

Any examples of this occuring?

In order for the star to orbit a planet in the way that you're picturing, the planet would have to be much more massive than the star. The problem there is, it's not really possible to have a planet that big; at least not a planet in the sense that we normally think of them.

Normal planets maintain their size and shape because electromagnetic repulsion between the atoms is enough to counteract the force of gravity pulling inward. However, this can only work up to a point. If you start with a planet that's as massive as a star, the force of gravity is just too strong. Depending on the planet's mass and composition, it'll either turn into a star itself, or it will become what's known as a "degenerate body": a superdense object like a white dwarf, a neutron star, or a black hole. No matter what happens, it'll probably be a very hot and violent event. :)

Jeremy

[goodgirlonhere]

There is an example of stars revolving around eachother in plain sight. It involves the "Little Dipper". The second to last star in the formation of the handle portion of the Little dipper are actually 2! They are 2 stars that are revolving around eachother. They are predicted to "separate" in the next few thousand years thus making the Little Dipper's shape change. The star maybe called "M13". Not sure.
Yes, I took Astronomy and this may have been the only detail that ever stuck in my head! Thanks for letting me share my piece of trivia with you!:cool:

[toddjh]

Actually, single stars are the exception, not the rule. If you pick a star at random, odds are it's going to be part of a binary system, or something even more elaborate.

Jeremy

[Mojo]

If you had a massive planet (bigger than Jupiter) and a dead star (say, a tiny brown dwarf) that had somehow been captured by the gravitational pull of the planet, then yes, it techincally would be possible for a traditional orbit to occur where the star was orbiting a planet.

But in general, at any given scale, the less massive object tends to orbit the more massive object. If the objects are nearly equal mass, then they "orbit each other", which really means they orbit around the combined center of mass of the two objects (see Charon and Pluto for example).For a two object system, even if one object is much less massive than the other, they will both orbit around their mutual centre of mass. It's just that the more massive one will not appear to move much. In the solar system, all the planets and the sun orbit around the centre of mass of the whole system. Go up to a big enough scale and you will find that the solar system and the next nearest star are also orbiting each other (albeit very slowly) and, of course, everything in our galaxy is orbiting around the galaxy's centre. And so on...

I also seem to remember (from long ago astronomy classes) that if Jupiter had been a bit more massive it could be considered to be a star.

[Taffer]

For a two object system, even if one object is much less massive than the other, they will both orbit around their mutual centre of mass. It's just that the more massive one will not appear to move much. In the solar system, all the planets and the sun orbit around the centre of mass of the whole system. Go up to a big enough scale and you will find that the solar system and the next nearest star are also orbiting each other (albeit very slowly) and, of course, everything in our galaxy is orbiting around the galaxy's centre. And so on...

This is what I was getting at, but as usual someone else managed to put it much more clearly. Thanks, Mojo. :)

[CaveDave]

For a sufficiently large planetary mass,
and for a sufficiently small stellar mass,
It could happen.

But the large planet would behave like a star,
and the small star would never light off,
so you would be back at square one, anyway.

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Others have already said as much, but I thought this sounded cool.:D

Dave