A Moon is Born?
An artist's concept shows a planet-smashing collision in the star system HD 172555. The collision, which involved one body the size of Mercury and another the size of the Moon, probably took place a few thousand years ago. The impact vaporized much of the material in both bodies, spewing hot gas into space. Some of this material may coalesce to form a moon. A similar collision 4.5 billion years ago between Earth and a Mars-sized planet may have led to the birth of our own Moon. Spitzer Space Telescope discovered the infrared glow of collision's aftermath. [NASA/JPL]