By analysing the spectrum, astronomers have detected the emission from cold water vapour in the planet-forming disc. The vapour arises when highly energetic radiation from the central star interacts with icy grains in the disc. The detection thus hints at a copious and otherwise undetectable supply of water ice hidden in the disc's deeper and colder layers.
The graph in the lower panel shows the spectral signature of water vapour in the disc. Water molecules come in two "spin" forms, called ortho and para, in which the two spins of the hydrogen nuclei have different orientations. By comparing the relative amounts of ortho and para water, astronomers can determine the temperatures under which the water formed. Lower ratios indicate cooler temperatures, though in practice the analysis is much more complicated. The ratio of ortho to para water observed in TW Hydrae's protoplanetary disc is low enough to point to the presence of cold water vapour.Credits: ESA/NASA/JPL-Caltech/M. Hogerheijde (Leiden Observatory) HI-RES JPEG (Size: 1196 kb)
Astronomers using the HIFI spectrometer on ESA's Herschel Space Observatory detected copious amounts of cold water vapour, illustrated in blue, emanating from the star's planet-forming disc of dust and gas. The water vapour, corresponding to temperatures lower than 100 K, is distributed across the entire extent of the disc and is likely confined to a thin layer at an intermediate depth in the disc. The vapour arises when highly energetic radiation from the central star interacts with icy grains in the disc, the very same grains that should ultimately coalesce into icy planetesimals, such as comets. The detection thus hints at a copious and otherwise undetectable supply of water ice hidden in the disc's deeper and colder layers.
In our own Solar System, comets are thought to have carried water to Earth, creating our oceans. A similar process might be taking place around TW Hydrae, where comets could, over the next several millions of years, transport water to young worlds. The Herschel results demonstrate that vast reservoirs of water are available around stars for creating these hypothetical water worlds. Credits: ESA/NASA/JPL-Caltech. HI-RES JPEG (Size: 1934 kb)
ESA’s Herschel space observatory has found evidence of water vapour emanating from ice on dust grains in the disc around a young star, revealing a hidden ice reservoir the size of thousands of oceans.
TW Hydrae, a star between 5-10 million years old, and only 176 light-years away, is in the final stage of formation, and is surrounded by a disc of dust and gas that may condense to form a complete set of planets.
It is believed that a large proportion of Earth’s water may have come from ice-laden comets that bombarded our world during and after its formation. Recent studies of comet 103P/Hartley 2 with Herschel shed new light on how water may have come to Earth, with its findings of the first Earth-like water in a comet. Until now, however, almost nothing was known about reservoirs in planet-forming discs around other stars.
This new detection is the first of its kind and has been made possible by Herschel’s HIFI instrument.
The tell-tale water vapour signature, believed to be produced when the ice coated dust grains are warmed by interstellar UV radiation, has been detected throughout the disc around TW Hydrae, and, though weaker than expected, it hints at a substantial reservoir of ice. This could be a rich source of water for any planets that form around this young star.
"The detection of water sticking to dust grains throughout the disc would be similar to events in our own Solar System's evolution, where over millions of years, similar dust grains then coalesced to form comets," says Michiel Hogerheijde of Leiden University in the Netherlands, who led the study.
"These comets we believe became a contributing source of water for the planets."
The scientists ran detailed simulations, combining the new data with previous ground-based observations and some from NASA’s Spitzer telescope. From this they calculated the size of the ice reservoirs in the planet-forming regions.
Their results show that the total amount of water in the disc around TW Hydrae would fill several thousand Earth oceans.
"We already have approved time on Herschel to study more planet-forming regions around three other stars," says Dr Hogerheijde.
"We believe that will show similar results in terms of the water detections, but as our next observations will be of objects up to three times further in distance away, we'll need many more hours of observation time."
This research breaks new ground in understanding water’s role in planet-forming discs and gives scientists a new testing ground for looking at how water came to our own planet.
"With Herschel we can follow the trail of water through all the steps of star and planet formation," comments Göran Pilbratt, Herschel Project Scientist at ESA.
"Here we are studying the 'raw material' for planet formation, which is fundamental to an understanding of how planetary systems such as our own Solar System once formed."
Notes to editors
HIFI is the Heterodyne Instrument for the Far-Infrared spectrometer on the Herschel Space Observatory. It was designed to observe water in a wide variety of objects, and aims to study not only planet-forming discs and star formation, but also galactic evolution. Its capability for highly detailed chemical identification of individual atoms and molecules makes it the instrument of choice for studying chemistry throughout space, particularly around embryonic and dying stars.
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