How Comets In Distant Solar Systems Could Deliver Oceans to Exoplanets

Cool, Clear Water

The nearby star TW Hydrae has a burgeoning solar system with a sprawling cloud of cold water vapor. The vapor is cold enough to eventually form comets, which could deliver oceans to dry planets that are forming in the system.NASA/JPL-Caltech/T. Pyle

A pair of new comet studies from two space telescopes show how other planets might grow oceans. For the first time, astronomers have detected a ring of cold water vapor encircling a young star's dusty planetary disk. And a separate study in a different star system shows a hailstorm of icy bodies could be bombarding a young planet. Together, the studies bolster a theory about how comets may have delivered Earth's oceans — and they show this is not a unique occurrence in the universe.

Today's issue of the journal Science contains a paper describing cold water surrounding a young star called TW Hydrae, located in the Hydra constellation. Astronomers have previously found warm water around planet-forming regions, but not huge amounts of cold water. This cold reservoir demonstrates how comets form in a solar system's outer reaches.

"Our observations of this cold vapor indicate enough water exists in the disk to fill thousands of Earth oceans," said astronomer and lead author Michiel Hogerheijde of Leiden Observatory in The Netherlands. TW Hydrae is just 10 million years old, and astronomers believe the dusty disk surrounding it will eventually coalesce to form planets. The frosty water vapor will probably coalesce to form comets, some of which might rain down on those new planets.

This does indeed happen in other solar systems, as a separate study shows. On Wednesday, astronomers using the cold-observing Spitzer space telescope described finding a dusty band around a star called Eta Corvi, a billion-year-old star system in the constellation Corvus, the crow. The band's contents strongly match that of an obliterated comet, suggesting a massive collision, perhaps with a planet. This sounded like what happened during Earth's own Late Heavy Bombardment. The system has a second, bigger ring much farther out, just like our own Kuiper Belt. And the bombardment is even happening around the same time that ours did, about a billion years after the Earth formed.

During the Late Heavy Bombardment here, a cascade of frosty space rocks rained down on the planet, delivering water and other things. (Earth was too hot after its formation to hold on to any native H2O.) Earlier this month, researchers using the European Space Agency's Herschel Space Observatory said they found the chemical isotope signature of Earth water out in the distant reaches of our solar system, suggesting that Oort Cloud comets may have been one source of this bombardment, not necessarily just asteroids. This new Spitzer study suggests the same type of bombardment is happening at Eta Corvi.

So it sounds like comet clouds and water reservoirs are not uncommon at all. Perhaps around each star's a pool of water — which could mean very interesting things for the search for extraterrestrial life.

A Barrage of Comets

This artist's concept illustrates a storm of comets around a star near our own, called Eta Corvi. Evidence for this barrage comes from NASA's Spitzer Space Telescope, whose infrared detectors picked up indications that one or more comets was recently torn to shreds after colliding with a rocky body.NASA/JPL-Caltech