It's full of planets...

All the signposts were there but as always in science the proof is in the pudding. Today the Kepler mission made its second major data release and served up a massive dose of sugar and carbohydrates. With 1,235 good planet candidates, 90% or more of which are likely to pan out as the real deal then things are looking awfully rich in the hunt for those small rocky worlds that could resemble the Earth. Indeed, 68 of the new Kepler finds are less than about 1.25 times the radius of the Earth and 54 are orbiting in their stellar habitable zones. Of those latter worlds then one is 0.9 times the size of Earth, and four are less than twice the size, the rest are rather bigger - all the way up to gas giants.

There's a good chance that if you look down the street you'll see an astronomer running along chanting 'here we go, here we go'. It's such an outpouring of data that its impact is likely to continue well into the years to come. As long, that is, as societies continue to support the scientific effort. An excellent series of discussions on the state of play for exoplanetary science can be found in the recent posts by Lee Billings on boingboing, and he nails the pros and cons of these exhibitions of success. Hype can be good, but it's a tricky business.

The big Kepler list is fabulous but some of the details are even more fascinating. In this week's Nature the Kepler team also report on a remarkable system Kepler-11 in which no less than 6 planets appear to be transiting the parent star. The analysis by Lissauer et al.  shows what you can do with this kind of data. Single transiting planets yield no direct handle on planet mass, only radius. Multiple transiting planets can provide a wealth of information on the orbital dynamics of a system and constraints on masses as the planets tug at each other, together with estimates of orbital ellipticity. Kepler-11 is a highly, even ridiculously 'packed' system. The 5 inner planets all have orbital periods between 10 and 47 days. Since Kepler-11 is a G-dwarf star like the Sun this is as if 5 planets orbited within Mercury's territory. Yet in dynamical terms the system appears to be quite stable, and should remain so for at least the next few hundred million years.

At least four of the planets seem to be less than 10 times the mass of the Earth, with the smallest at around 4 Earth masses. They all also appear to have substantial atmospheric envelopes on the basis of their densities. The likely components of this gas range from hydrogen to water vapor or 'steam' dominated. Kepler-11 presents a fascinating test case for models of planet formation. Most are probably lacking. The arrangement of these worlds, their apparent compositions, and their uncomplicated orbits suggests that as they formed they were coaxed and settled by a significant amount of gas or small rocky objects that helped smooth things out - a bit like a dynamical muffler. This is not simple though, and almost certainly not a universal rule.

It's all enough to give one heartburn. The great news is that Kepler is confirming that planets are extremely numerous. Time will tell exactly how many 'Earth-type' worlds there are, that is still open for bets. The bad news, a bit like getting the bill after such extravagance, is that we are really, really going to have to work hard on our models of planet formation.