Using data from the Herschel Space Observatory, astronomers have detected for the first time cold water vapour enveloping a dusty disc around a young star. The findings suggest that this disc, 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 vapour in planet-forming discs close to a central star. Evidence for vast quantities of water extending out into the cooler, far reaches of discs where comets take shape had not been seen until now. The more water available in discs for icy comets to form, the greater the chances that large amounts eventually will reach new planets through impacts.
"Our observations of this cold vapour indicate enough water exists in the disc 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 21 October issue of the journal Science.
The star with this waterlogged disc, 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 disc'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.
"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 disc 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 disc 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 disc may be representative of many other young star systems, providing new insights on how planets with abundant water could form throughout the Universe.