When talking about other planetary systems, or even our own, the word that tends to get us most excited is "water." And for good reason—it’s necessary for life on our own planet, and its presence in liquid form likely increases the odds of life on other worlds. Using the Herschel space telescope, astronomers have spotted evidence of water vapor at the edges of a planet-forming disk surrounding a nearby star. The presence of the vapor hints at enormous volumes of ice present in the disk.

In order to understand how water fits in the chemical dynamics of solar system formation, astronomers have been inspecting young solar systems elsewhere in our galaxy. As with all astronomical observations of chemistry, this involves analyzing the absorption spectra of the light emitted by the system in question. Different elements and compounds leave an electromagnetic fingerprint on the spectrum by absorbing radiation at very specific wavelengths.

Looking for water in this way has presented some unique challenges. The presence of water vapor in our atmosphere has made it difficult for ground-based telescopes to obtain precision data from distant objects. Enter the Herschel Space Observatory—a satellite launched in 2009 to study (among other things) the formation of solar systems.

Even space telescopes face difficulties, though. Near the center of new-born solar disks—close to the star—temperatures are high enough to keep water in the vapor phase. Beyond a certain distance, called the snow line, water is frozen onto dust particles. Absorption spectra of the inner water vapor are easily detected by space telescopes like Herschel, but the outer water ice is hard to spot. We’ve gotten lucky and observed a bit here and there when conditions were just right, but for the most part, we’ve been relying on models and other information to infer its existence.

Research has suggested that UV radiation from the host star (or other nearby stars) could drive some of the water ice into the gas phase, thus making it observable again. A group of astronomers decided to use the Herschel Space Observatory to go looking for that signal, and their results were published this week in Science. They decided to point the telescope at the nearest planetary disk, which surrounds a star named TW Hydrae. This 10 million-year-old star is about 60 percent the mass of our Sun and is "only" 175 light years away.

When the data came in, the cold water vapor beyond the snow line was clearly detected. The amount of water observed was calculated to be about 0.005 times the mass of Earth’s global ocean, but only a small fraction of the total water present beyond the snow line will be present as vapor. Estimates from model simulations show that the amount of cold water vapor we've detected indicates a reservoir of water ice several thousand times the mass of Earth’s oceans.

The researchers were also able to measure the ratio of the two spin isomers of water, which was much lower than the ratio measured in comets from our own solar system. They say this supports the idea that there is chaotic mixing of material in stellar disks.

One of the interesting observations from NASA’s recent Stardust mission, which captured material from the tail of a comet and brought it back to Earth for analysis, was that the comet contained particles of silicate minerals that form at high temperatures. Those mineral grains must have formed in the rocky interior of the stellar disk and then migrated out beyond the snow line to become incorporated into an icy comet.

There has long been a great deal of debate about whether Earth’s water was delivered by comets from beyond the snow line. The primary challenge has been the fact that the comets we’ve observed have an isotopic ratio that differs from ocean water. Considering another recent discovery made using the Herschel Space Observatory of a comet with an isotopic signature matching Earth’s oceans, it’s a good time for those curious about the water out there in space.

Science, 2011. DOI: 10.1126/science.1208931 (About DOIs).