To become a world bathed in oceans of water and habitable, Earth first had to take a beating. A popular hypothesis holds that icy comets and asteroids pummeling early Earth delivered the planet's water from the icy outer reaches of the solar system.
Rocky, terrestrial worlds in other planetary systems might become watery by the same process, but assessing just how much ice is available to distant, newborn planets has been challenging. With the help of the European Space Agency's Herschel Space Observatory, however, astronomers have gotten a good look at the seeds of a planetary system around a young star 175 light-years away, and there seems to be plenty of water to go around.
The researchers used Herschel to scan the protoplanetary disk around the 10-million-year-old star TW Hydrae, one of the nearest such disks available for study. (Protoplanetary disks are the swirling pancakes of dust and gas surrounding young stars that can coalesce over millions of years, as the sun's disk did, into terrestrial planets and gas giants.) The astronomers reported in the October 21 issue of Science that they picked up a faint signature of water vapor from TW Hydrae's disk, which they presume emanates from a much larger reservoir. The icy outer portion of the disk probably contains enough water to fill Earth's oceans thousands of times over, the researchers estimate.
Locating faraway reservoirs of water is generally hampered by the abundant water vapor in Earth's own atmosphere, which clouds the view of ground-based telescopes. That is not a problem for Herschel, a large far-infrared/sub-millimeter–spectrum telescope stationed 1.5 million kilometers from Earth, well beyond the orbit of the moon. From its unobstructed vantage point, Herschel's spectrometer located a small amount of water vapor coming off of TW Hydrae's dusty protoplanetary disk at a distance of roughly 100 astronomical units, or 100 times the Earth–sun distance, from the star.
The very presence of vapor so far from TW Hydrae points to an interaction between the star's radiation and the ice in the disk. "We know that at these distances the temperature is so low that it should be frozen," says lead study author Michiel Hogerheijde, an astronomer at Leiden Observatory in the Netherlands. But ultraviolet (UV) radiation emanating from TW Hydrae should liberate some water molecules from icy dust grains in the protoplanetary disk, in a process known as photodesorption.
Having measured how much water vapor is produced by photodesorption, Hogerheijde and his colleagues were able to fashion a rough estimate of the amount of ice present in the disk. "We know the efficiency of this photodesorption process very well," he says. "We know that we need to have an underlying reservoir of several thousands times Earth's oceans to produce this small amount of vapor."
TW Hydrae joins a small number of nascent planetary systems that have been found to contain water in some form, either as hot vapor close to the star or as cold ice in the outer protoplanetary disk. In those past instances, though, it has been "relatively hard to actually quantify how much water ice was present," Hogerheijde says. "We've always thought that there should be a large amount, but we just didn't know, because there's no data."
With a relatively clear look at one planetary system to compare with our own, astronomers can now more confidently ponder the existence of distant, watery worlds. For if TW Hydrae develops rocky worlds, there should be plenty of icy leftovers on the outskirts of the planetary system to supply those worlds with water, in much the same way that comets and asteroids may have provided for Earth billions of years ago. "If the mechanism that delivered Earth's oceans happened here, it may be happening in other solar systems as well," Hogerheijde says.