Jan. 26, 2012: Is the Earth Normal?

Michael Jura

Knowing that extrasolar planetary systems are common, we would like to learn whether the Earth is normal or distinctive. Bulk Earth is 94% composed of O, Mg, Si and Fe and very deficient compared to the Sun in volatiles such as C and N. With our recent observations of white dwarf stars that have recently accreted tidally-disrupted minor planets that are about 300 km in diameter, we find a similar compositional pattern in extrasolar asteroids. While there must be individual exceptions, in aggregate, the studied extrasolar asteroids also are as “dry”; they probably formed interior to a snow line. Although the current sample is tiny, it appears that bulk Earth is compositionally normal for a rocky body. In the future, we may learn whether extrasolar planetesimals have undergone differentiation, a fundamentally important process in the history of our own planet.

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