The Speculist

what's most fascinating (to me) about this story is that the first extrasolar planets were only discovered in 1995, and now there are at least 110 known planets outside our solar system. A good source of information is here:
http://exoplanets.org/

the other fascinating fact is that most of the discovered planets are very large and are in orbits very close to their stars. how large? how close? well, so large that the discoverers describe them in terms of Jupiter masses. for example, Upsilon Andromedae has three planets (B, C, and D) with masses of aproximately 0.69, 2.05 and 4.29 JM's, respectively, so they're big planets (Saturn is 0.3 JM). and they're really close: B has an orbit radius of 0.06 AU and orbits Upsilon Andromedae every 4.6 days! C has an orbit radius of 0.8 AU and orbits in 241 days, and D has an orbit radius of 2.6 AU and period of 1308 days. Jupiter has an orbit radius of 5.2 AU and an orbital period of 4333 days.

An AU by the way is an Astronomical Unit, the average orbit radius of the Earth's orbit about the Sun.

Chris,

Obviously the bigger a planet is, all else being equal, the easier it is to find. The current method is to observe perturbations in a star's position over a long period of time. But massive planets with very short orbital periods (which only occurs if they are close to the star) are the easiest to spot. Also, I believe an observation of at least one complete orbit is required.

I seem to recall that Jupiter would be difficult to detect because 1) it's so far away from the Sun, and causes a smaller perturbation in the Sun's position, and 2) it's twelve year period means that observations of the Sun would probably have to continue for around twelve years in order to properly detect Jupiter (and nail down it's orbital period).

Given that, I too find it interesting that there's a large number of big planets close by their suns. Another relevant fact is that many of these large planets have pretty elliptical orbits. One bit of speculation is that Earth might not exist (or be considerably altered) if Jupiter had a very elliptical orbit.

Still, having a giant planet in your midst isn't necessarily that bad, for example, if Jupiter and its moons were in our orbit (or perhaps a little further out), then the Gallileo satellites would all be capable of supporting life though radiation on Io would probably be very harsh, but balanced by a protecting atmosphere of sorts.

Interesting stuff, guys. So is it possible that planet C or D of Upsilon Andromedae (just to go with Chris's example) might have roughly earthlike moons orbiting them, capable of sustaining life?

"So is it possible that planet C or D of Upsilon Andromedae (just to go with Chris's example) might have roughly earthlike moons orbiting them, capable of sustaining life?"

I'd say yes. Jupiter's Europa is an example closer to home. One of the big questions, as Karl mentioned, is the long-term stability of the multi-planet (multi-body) orbital mechanics problem. Might make a good longish article I think.

For an interesting related sf example, the book Helliconia Spring by Brian Aldiss, involves a planet orbiting a smallish star which is itself in a highly elliptical orbit about a red giant. Thus the planet has two seasonal periods, one minor period associated with the orbit about the small star, and one major period associated with the motion about the large star. Two distinct groups of life forms exist, with one dominating during the Major Summer and the other dominating during the Major Winter. Spring and Fall are wartime while the rising species prevails over and enslaves the declining species. Fascinating example of orbital mechanics and sociology.

7680 Get your online poker fix at http://www.onlinepoker-dot.com