In the fall of 2014, a “Manx” comet hurtled past Earth on an 860-year journey around the solar system. Its trajectory told astronomers that the object came from the far-distant Oort cloud, a vast shell of icy objects that orbit the Sun up to thousands of astronomical units away (1 astronomical unit is the distance between the Earth and the Sun). But its severely diminished tail—hence its nickname after the Manx breed of tailless cat—told astronomers it differed from the usual ice-rich visitors from those parts, which typically sport long, bright tails.

In a recent paper in Science Advances, the comet’s discoverers take a close look at the object, which revealed that it’s made up mainly of minerals found on Earth and contains little ice—the source of a comet’s tail. The findings indicate that scientists have confirmed for the first time the existence of a rocky, asteroid-like body from the Oort cloud. Moreover, the passing chunk of rock likely constitutes a long-lost and largely pristine sample of what made up the inner solar system as it formed billions of years ago.

If astronomers can find more rocky Oort cloud visitors, they may be able to calculate the ratio of icy to rocky denizens of that vast cloud—a missing but essential clue to how our Solar System took shape long ago.


Misplaced Asteroid?

How did the newfound comet, labeled C/2014 S3, get tossed into the Oort cloud when it originated in our very own neighborhood, possibly from the very material that built Earth? According to Karen Meech, an astronomer at the University of Hawai’i at Mānoa in Honolulu and lead author on the 29 April paper, the object started off as a chunk of rock drifting around the inner Solar System while Earth and the other interior planets were forming. Then, gravitational forces, perhaps from migrations of giant gas planets such as Jupiter, might have flung it from its place of birth.

Learn more at the EOS site.