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Young solar systems never seem to have enough elbow room for all their newborn planets. In our own solar system, for example, a collision between Earth and another planet created the Moon, while other impacts tipped Uranus on its side and stripped Mercury of its crust.

Artist's concept shows the violent impact. [NASA/JPL/Caltech]

A collision in a recently discovered solar system appears to have pulverized two worlds, while a near-collision in another system sent another world careening backwards.

In the first planetary system, which orbits the star HD 172555, Spitzer Space Telescope discovered glass, rock, dust, and other debris from a violent collision that took place just a few thousand years ago.

From the debris field and other measurements, a team led by Carey Lisse of Johns Hopkins University deduced that the two bodies — one the size of Mercury, the other the size of the Moon — hit each other at about 22,500 mph. The collision probably blasted the smaller world apart and stripped the larger one of most of its outer layers, spewing dust and molten rock into space. Much of the rock has cooled to form a type of glass. The Moon probably formed from just such a cloud of debris after Earth was hit by a Mars-sized body soon after our planet formed. There is no indication whether a moon is forming around the surviving planet in the HD 172555 system, though.

A British project, Wide Area Search for Planets (WASP), discovered the odd world in the second system. The planet, named WASP-17, orbits in the opposite direction from its parent star's rotation. The planet is about half as massive as Jupiter, the largest planet in our own solar system, but twice as large.

WASP team members surmised that the planet was thrown into its backward orbit by a close encounter with a more massive planet. Its odd, highly stretched orbit heats the planet, causing it to expand. It is so puffy that its average density is equal to that of Styrofoam, the team reports. DB