Water in Another Solar System

23 October 2011
Posted by Chris Smith.

Our Solar System may not be alone in having an abundant supply of water. That's according to a paper published this week in the Journal Science. Michiel Hogerheijde and his colleagues have been using the Herschel Space Observatory to study the star TW Hydrae -- a star so young, only around 10 million years old, that it's arguably still forming, but which we expect will eventually end up at around 60% of the mass of our own Sun.  TW Hydrae is a very well-studied system because it's the closest example to us of such a very young star, and of particular interest is the disk of gas and dust which surrounds it, which we expect will eventually form into a planetary system not dissimilar our own Solar System.

It's interesting to ask how similar to our Solar System that planetary system might turn out to be. One of the really important characteristics of our Solar System, certainly in terms of making life possible in it, is that it has an abundant supply of water in the form of comets. We have models for where we think that water came from in the Sun's planet-forming disk. Very close in to the Sun, in the hottest part of the disk, it would have sublimed (boiled) and been in the form of water vapour or steam. But out beyond the asteroid belt, beyond what we call the ice line, it would have been frozen into solid lumps of ice, which explains why the Solar System has rocky bodies in the asteroid belt, but icy comet-like bodies further out. Those comets played a vital role in bringing the oceans to the Earth, because the Earth formed inside the Solar System's ice line, as a dry planet: only through later bombardment of comets onto its surface did water arrive and make Earth a wet planet.

Looking at TW Hydrae from the outside, we've had no trouble in the past inThe Hubble Space Telescope detecting water vapour in its inner central regions. But detecting whether ice is present further out has proven problematic. What Michiel Hogerheijde has done in this new study is to make use of the fact that, if ice is exposed to ultraviolet light, you'd expect a tiny fraction of the water molecules to break away from the solid lumps and form very cold gaseous water vapour. In TW Hydrae, there does indeed appear to be evidence for very cold water vapour extending a long way out in the disk, where it couldn't have formed by straightforward sublimation. So, this appears to be evidence that there is a vast amount of water ice -- several thousand times the total amount in the Earth's oceans -- around TW Hydrae, and suggests that this system has the potential to form both comets, and eventually wet habitable planets like our own Earth.

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