The new planets–a dozen or so have been found so far–have come as something of a shock to theoreticians like Boss. With only the arrangement of planets in our own solar system to serve as a starting point, they predicted things would be similar elsewhere. There would be little rocky planets like Earth and Mars near the star, and big gassy planets like Jupiter farther out. The systems now discovered, though, all have big planets close to the star–sometimes extraordinarily close, with orbits measured in days.
How come? The theoreticians now suggest that the large planets actually move from the outskirts of these systems–where they are born–to the centers, where they lead their adult lives. The good news is that such astronomical commuting is not impossible. The bad news is that it bodes badly for rocky little planets close enough to their stars to have liquid water on their surfaces–that is to say, planets like Earth. The gravity of a Jupiter passing nearby would either stop any planets forming in such orbits or push them out of their orbits after they have formed. All the systems so far discovered, says Boss, are inhospitable to planets in orbits like Earth’s around the sun.
This is not completely disheartening. For one thing, the techniques currently being used to look for planets find the task easiest when there are large planets close to a star. Not surprising, then, that the planet hunters find big planets close to some stars. Other stars may well have planetary systems more like the sun’s, as yet undetected. And then there are the moons. Jupiter has four nice large moons, two of them as big as proper planets. Giant planets in other systems might be similarly endowed, and take their moons with them as they wander the spaceways. If their final destination is at the right distance from a star, such moons could have climates much like Earth’s.
Moons cannot be detected yet, and none of the big planets found so far look as though they would give their moons ideal climates. But some get close. Take the star Gliese 876, which is quite nearby–15 light-years away–and quite small–a third the mass of the sun. Judging by the way this star wobbles, there is a planet bigger than Jupiter orbiting it every 61 days. That makes it five times closer to Gliese 876 than Earth is to the sun, which sounds uncomfortably hot. But Gliese 876 is very dim. In fact, it is so dim that any moons around its giant planet might still be a bit too cold for liquid water. But they would be at least as interesting as Mars or Europa. And with new planets found all the time, you can be almost sure that better candidates will soon come along.
