planets

[annmmac]

How many planets in our galaxy?

[mibn2cweus]

Nine million million or so.

[Quizmonster]

As only a handful of planets - as opposed to stars - have so far been 'discovered' outside our own solar system, no-one has the slightest idea as to how many there might actually be in the entire Milky Way galaxy!

[jake-the-peg]

Well 177 outside the solar system - does that count as a handful? I guess it does in comparison to the number of stars.

http://exoplanets.org/

It's probably that a large number (if not the vast majority) of stars have planets

[Quizmonster]

Yes, Jake, I used 'handful' in the astronomical sense!

[vehelpfulguy]

annmmac, I may be wrong, but do you understand the difference between a star and a planet ?

In our small solar system the Sun is a star (gives out its own heat & light) but the earth, venus, jupiter etc are all planets.

While it is fairly easy to find stars it is very difficult to find planets (becuase they do not give out their own light).

In fact we are not even sure of the number of planets in our own solar system let alone galaxy.

Many people think Pluto is NOT a planet, just a lump of rock, while others think there may be another planet in our solar system we have not discovered yet.

[remesses]

lots and lots... there will probably be more gas giants like jupiter and saturn, as the smaller "rocky" planets like the first 4 in our solar system need very specific and stable formation criteria, as they are either engulfed or thrown out of orbit by the larger gas giants.
most of the few planets we have actually counted have very erratic and very fast orbits.

[jake-the-peg]

Why do you say that remesses?

Of those 177 the average orbital period is about 3 years with an average distance from their star of 1.3 AU - so it's not all that fast!

The average orbital eccentricity is 0.22 which is higher than our average but only about that of Mercury and 22 of the 177 have orbital eccentricities less than the Earth

[remesses]

Well the first question we have to ask is...
how do we spot / find other exoplanets which are very far away and which we can not see?... easy ,( well sort of), you have to find a star which fluctuates position, basically you are looking for a star with a wobble. this genrally happens when there are large masses at play. Thus far 115 out of the odd 177 planets have been gas giants all with some large degree of Orbital eccentricity, eg planet HD41004Bb has an orbit of 1.328 days that is pretty fliping fast! there are a whole lotta theories on how stellar orbital systems are fromed, but what does turn up in the stats is that rocky earth like planets are rare, and gas giants the norm. go to
http://exoplanets.org to find out more.

[jake-the-peg]

Er yes I think you'll find that was the webite I referred to at the top of the thread!

There are fast orbits and slow ones too, I think it's fair to generalise that most so far seen are more eccentric than in our solar system but that again may be due to the detection methods

[remesses]

sorry sorry sorry sorry just saw web site reference now I am a bit slow today,
yes i agree that there may be lots of "normal" planets out there. but... ... ... he he..
"The most common occurrence of stars appears to be as parts of binary systems.Although planets might still form in such binary systems by a similar mechanism as discussed before, it is an open question whether they would have stable orbits that would keep them bound in the system without running into the stars or each other. But the probability that there are a large number of solar systems with concentric orbiting bodies with rocky planets in the inner orbits are slim.