Herschel Finds Oceans of Water in Planet-Forming Disk
This artist’s concept illustrates an icy planet-forming disk around a young star called TW Hydrae, located about 175 light-years away in the Hydra, or Sea Serpent, constellation. Image Credit: NASA/JPL-Caltech
Using data from the Herschel Space Observatory, astronomers have detected for the first time cold water vapor enveloping a dusty disk around a young star. The findings suggest that this disk, which is poised to develop into a solar system, contains great quantities of water, suggesting that water-covered planets like Earth may be common in the universe. Herschel is a European Space Agency mission with important NASA contributions.
Scientists previously found warm water vapor in planet-forming disks close to a central star. Evidence for vast quantities of water extending out into the cooler, far reaches of disks where comets take shape had not been seen until now. The more water available in disks for icy comets to form, the greater the chances that large amounts eventually will reach new planets through impacts. Water is essential for life as we know it, and identifying systems where water might be delivered to forming planets is useful in determining where best to search for habitable, extrasolar worlds. Studying these systems can also help us understand how Earth itself formed, evolved and became habitable for life.
"Our observations of this cold vapor indicate enough water exists in the disk to fill thousands of Earth oceans," said astronomer Michiel Hogerheijde of Leiden Observatory in The Netherlands. Hogerheijde is the lead author of a paper describing these findings in the Oct. 21 issue of the journal Science.
The star with this water-logged disk, called TW Hydrae, is 10 million years old and located about 175 light-years away from Earth, in the constellation Hydra. The frigid watery haze detected by Hogerheijde and his team is thought to originate from ice-coated grains of dust near the disk’s surface. Ultraviolet light from the star causes some water molecules to break free of this ice, creating a thin layer of gas with a light signature detected by Herschel’s Heterodyne Instrument for the Far-Infrared, or HIFI.
This graph of data from Herschel shows how the cool water vapor was detected. Image Credit: ESA/NASA/JPL-Caltech/Leiden Observatory
"These are the most sensitive HIFI observations to-date," said Paul Goldsmith, NASA project scientist for the Herschel Space Observatory at the agency’s Jet Propulsion Laboratory in Pasadena, Calif. "It is a testament to the instrument-builders that such weak signals can be detected."
TW Hydrae is an orange dwarf star, somewhat smaller and cooler than our yellow-white sun. The giant disk of material that encircles the star has a size nearly 200 times the distance between Earth and the Sun. Over the next few million years, astronomers believe matter within the disk will collide and grow into planets, asteroids and other cosmic bodies. Dust and ice particles will assemble as comets.
As the new solar system evolves, icy comets are likely to deposit much of the water they contain on freshly created worlds through impacts, giving rise to oceans. Astronomers believe TW Hydrae and its icy disk may be representative of many other young star systems, providing new insights on how planets with abundant water could form throughout the universe.