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. Astronomers using the Herschel Space Observatory detected copious amounts of cool water vapor, illustrated in blue, emanating from the star's planet-forming disk of dust and gas. The water vapor, which probably comes from icy grains in the disk, is located in the frigid outer regions of the star system, where comets will take shape.
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 -- 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.
The graph of data (Figure 1) from Herschel shows how the cool water vapor was detected. Water molecules come in two "spin" forms, called ortho and para, in which the two spins of the hydrogen nuclei have different orientations. In this case, the team compared the ratio of ortho to para water seen in the TW Hydrae disk to that in comets, and found very low values. Lower ratios indicate cooler temperatures, though in practice the analysis is much more complicated. This is the first demonstration that water exists in large quantities in the frigid, outer regions of solar systems, where comets take shape.
Illustration credit: ESA/NASA/JPL-Caltech/Leiden Observatory
Note: For more information, see Herschel Discovers Tip of Cosmic Iceberg Around Nearby Young Star.