It must have been one bodacious crunch -- akin to the moon and Mercury racing to occupy the same spot at more than 22,300 miles an hour.
The story of a collision between nascent planets is written in a disk of dust around a star 100 light-years away, according to an international team of astronomers formally reporting the results in an upcoming issue of the Astrophysical Journal. A plain-English version appears here.
The dust surrounds a 12-million-year-old star labeled HD172555. The dust disk shouldn't be there. It should long since have clumped into larger objects that form the building blocks of planets.
But the dust is there. Its make-up, revealed via NASA's Spitzer Space Telescope, suggests that it formed in a collision between two so-called planetesimals. The smash-up took place within the last 100,000 years, and perhaps as recently as 1,000 years ago.Among the minerals the team identified in the disk's spectra: obsidian and tektites, glassy rocks that forms at very high temperatures. Tektites in particular form during a collision. Rock melted by the heat of impact hurtles from the impact site, cooling as it travels.
The observations recorded vast amounts of silicon-oxide gas, which forms when silicate-bearing rocks vaporize. The scientists also detected large amounts of cold, sizable dust. But the smoking gun was the presence of unusually large amounts of warm, fine dust, opening a unique window on the dust's chemical make-up.
The relative abundance of each led them to conclude that an object at least as large as the moon was the loser plano-a-plano with an object at least as large as Mercury.
"In any system that's older than 10 million years, when we see warm dust" returning remarkably detailed information about its chemical composition, "it tells us that something extraordinary has happened," says Carey Lisse, a senior research scientist at the Johns Hopkins University's Applied Physics Laboratory in Laurel, Md.