There's cold water vapor orbiting the star TW Hydrae... and a lot of it. Enough to fill the Earth's oceans thousands of times over!

TW Hydrae is a star located pretty close by, about 175 light years away. It's lower mass than the Sun, so it glows an orange-red, but it's also very young, less than 10 million years old. Stars that age are still shaking off the remnants of their formation, and that's just the time you expect planets to get started. And in fact it's been known for years that TW Hydrae is surrounded by a giant disk, the leftover materials from its formation. Disks like that around other stars have been closely scrutinized, and we see lots of different materials in these disks, including various minerals, complex dust molecules, and even water. In general the water that's been found is usually close in to the star and warm (which makes it easier to see). Astronomers used the orbiting Herschel telescope to look at the disk of the star TW Hydrae in the infrared, and found water in the spectrum of the material there. And what they discovered is that it's cold vapor, not warm. That's the first time this has even been seen, and it's kinda neat how this was done. When you break light up into individual colors, you can learn a lot about the object emitting that light, including what it's made of and what temperature it is. Water emits light at a lot of specific colors in the IR, but there's one in particular that reveals its temperature. That's the one the astronomers used to see the water around TW Hydrae. Water is made up of one oxygen atom and two hydrogen atoms: H2O. Each hydrogen atom has a proton in it, and protons have a property scientists call spin. They can spin either one way or another; scientists call this spin up or spin down. This is important because the total amount of energy in a water molecule is different if the two hydrogen atoms spin the same way (say, both up) versus different ways (one up and the other down). The first case is called ortho, and the second para. Each emits a slightly different wavelength of light, which can be measured if you're careful. And have Herschel at your service.

In room temperature water, you get three times as many ortho water molecules as para, but as the temperature drops that ratio gets closer to 1. When they looked at TW Hydrae, the ratio was low enough to know that the water was cold, and must be coming from farther out in the disk than is usually seen. Not only that, but the strength of the line (how tall the feature in the spectrum is) tells you how much water is there. The amount they found? Enough to fill "thousands of Earth oceans" according to Michiel Hogerheijde, who led the team that made the discovery. That's pretty cool. It means that a lot of water is available to make planets when they form. The thing is, we kinda knew this already: there's a lot of water in our solar system. It's not just Earth: moons in the outer solar system (like Europa and Enceladus) are almost entirely made of water, and it's prevalent in the comet-like iceballs in the outer solar system called Kuiper Belt Objects, too. But our solar system is only one example of how stars and planets are formed. We still don't know how typical our solar system is. We do know that making planets is easy, so easy that nearly half the stars in the sky may have them. But we still haven't spotted another Earth-like planet, and even if we do, will it have water? We still don't know, but the odds of it being wet look better every day. And I can't leave this without noting that in this case, nature is imitating art: the TW Hydrae got its name for being in the constellation of Hydra, the water serpent... a beast that needs to live in water to survive! Coincidence, of course, but a funny one. Disk art credit: NASA/JPL-Caltech. Plot: ESA/NASA/JPL-Caltech/Leiden Observatory

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