Read all the articles in our Unknown Solar System special
Since the first discovery of a planet orbiting another star in 1992, some have been identified. Most look quite unlike ours, and for good reason. Planets are mainly spied by the way their gravity makes their host star wobble as they orbit. The smaller the planet, the smaller the wobble: lightweight planets like Earth produce effects too small to detect with current technology.
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Most known extrasolar planets, or exoplanets, are gas giants similar in size to Jupiter or Neptune, but orbiting close in, within a few AU (Earth-sun distances) of their stars 鈥� 6 or 7 per cent of sun-like stars seem to have such satellites. The prevalence of giant planets orbiting at greater, Jupiter-like distances from their parent star is unknown. They would take a decade or more to complete an orbit, and few gravity-wobble surveys have been watching long enough to detect them.
According to the standard picture of solar system formation (see 鈥淗ow was the solar system built?鈥�), gas giants should not form that close to their host stars, as the heat means not enough solid material is present to make a sufficiently large rocky core. However, while the orbits of the planets in our solar system are almost circular, those of many of these giant exoplanets are often highly elliptical. This might provide a solution to the mystery: most solar systems seem to have had more chequered histories than our own, with initially distant giant planets competing for living space and bouncing each other into strange orbits closer in.
Definitive conclusions are difficult until we know the limitations of our observations. 鈥淚t might be that we see solar systems with very violent histories because those are the only ones we can see,鈥� says Phil Armitage of the University of Colorado, Boulder. Results from two sensitive space-based planet hunters should help reduce the uncertainty: the French-led mission, which launched in December 2006, and NASA鈥檚 , scheduled to blast off in March this year.
A foretaste of what they might find is given by the 10 or so known 鈥渟uper-Earths鈥� 鈥� planets with just a few times Earth鈥檚 mass. If the picture of planet formation gleaned from our solar system is correct, these are rocky worlds like our own. Two of them, Gliese 581 c and d, are at the sort of distance from their parent star at which liquid water might exist on their surfaces, depending on the warming effects of greenhouse gases and cooling effects of clouds in any atmospheres they might possess.
There are other hints that rocky planets are more common than our first observations suggest. Dust close in to young stars, reported in 2008 by NASA鈥檚 Spitzer Space Telescope, points to collisions connected to planet formation, and suggests that rocky worlds form around 20 to 60 per cent of stars.
But other evidence from Spitzer of dust circling much older stars dampens the prospects for tranquil rocky worlds that could harbour life. Nine in 10 solar systems seem to be more dusty than our own, in some cases by a factor of 20 or more. As planet formation is expected to be a relatively cursory process within the first 100 million years or so of a star鈥檚 existence, that dust is probably the remnant of catastrophic comet collisions later in the life of these solar systems.
Fortunately, our inner solar system is an exclusive club with heavyweight bouncers on the door. The powerful gravity of the more distant giant planets 鈥� Jupiter in particular 鈥� often ejects comets before they have a chance to penetrate the solar system鈥檚 inner sanctum.
That鈥檚 another reason to be glad our solar system is how it is. Ultimately, whether it is uniquely so will remain a mystery until we get down to seeing Earth-sized exoplanets, as well as giant planets farther out from their stars, says Jonathan Lunine at the University of Arizona in Tucson. 鈥淭he simple, honest answer is that we still don鈥檛 know.鈥�
