Star/Planet size question.

This probably seems like a dumb question, but i'm no physicist and it popped into my head, so I thought where a better place to ask than ATS, with all the great minds here? So here goes.

There are many stars, like hyper-giants, that make our sun seem like a grain of sand in comparison, like Majornis (I think that is correct) the biggest known star. Well it got me wondering, if that star is so huge, couldnt it have planets in proportion with it somewhat? Like our planets to the sun, and if these planets are so huge, and had something alive on them, could they grow in proportion with that?

Well i kinda know the answer is no, but why cant they?

Appreciate your answers.

I imagine the upper limit of size/mass for a planet would be when it would be able to actually become a star on its own. Many solar systems have 2 or even more suns orbiting around each other.

So planets can only get so big before then turning into a star themselves?

I no expert either but I think generally the star forms at the same time as the planets that end up orbiting it.......and the formation of planetary systems around stars is at best a 'theory'.

It is not known with certainty how planets are formed. The prevailing theory is that they are formed from those remnants of a nebula that do not condense under gravity to form a protostar. Instead, these remnants become a thin, protoplanetary disk of dust and gas revolving around the protostar and begin to condense about local concentrations of mass within the disc known as planetesimals. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets.[13] After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals.[14]

Wikipedia as Source

So what we are really speculating on is how much possible 'stuff' there could be around a protostar.....really I have no idea....I would surmise however that if a situation occurred that would allow a really massive planet to form, that life would still flourish in some form or another.....it just has a tendency to do that if you accrue the evidence on life surviving in extreme environments. I doubt a large gravity planet would be the reason for it not existing

If they were to live on a massive planet however, I imagine that there mass would be very small.....not sure why I think that...just something about elephants and jumping I think. :/



[edit on 11/4/07 by Strodyn]

Originally posted by StrangeVision
So planets can only get so big before then turning into a star themselves?

Well there is that but I was presuming that you were assuming that the big planets were proportional to ours in make up.......

Jupiter is not far off a failed dwarf....

Originally posted by StrangeVision
So planets can only get so big before then turning into a star themselves?

Gas giants like Jupiter could, but Jupiter is something like 1/80th of the mass it would need to ignite, that's from memory so it might be off somewhat.

Rocky planets like Earth, Mars, Venus etc could NOT turn into a star - there's not enough hydrogen.

My uninformed guess is that you get gas giant formation up until the point the protostar ignites, then all you'll form will be rocky planets. The hydrogen left over will be blown out of the system by the new star's radiation pressure.

So is it completely possible for a planet like earths composition to get massive? Like Jupiter for example, if there was enough 'stuff', without it being a gas giant.

i don't really see why it wouldn't be but keep in mind i know little about astronomy and that sort of thing

I would suppose so. I'm not sure if there are typical values for how much rocky debris you get in a system.

I think most of the extrasolar planets they're finding are gas giants though.

All of the exo-planets save for one, have been Gas Giants. It's not because that's all there is too see, it's due to the fact we don't have the technology to detect rocky planets in any great quantity yet. In a decade, expect an explosion of discovery as new telescopes come online.

My point being, if there were gas giant sized rocky planets, I'd expect they'd have seen at least one by now just by chance. I think the one rocky planet they saw was big but not gas giant sized.