Back in 1995, astronomers found the first exoplanet orbiting a star like our Sun. Now, using a novel technique, we’ve detected water in its atmosphere.

The world is called 51 Pegasi b, found 51 light-years away, and is known as a hot Jupiter – a gaseous planet that orbits very close to its star. Thus, it’s unlikely the water is liquid, but rather a vapor in the planet’s atmosphere. However, the detection is important all the same, because it highlights a new way that we can look at exoplanets.

Most that we have found and studied so far have been detected using the transit method, where the planet passes in front of its star relative to us. In this study, accepted for publication in The Astronomical Journal (available on arXiv), Jayne Birkby from the Harvard-Smithsonian Center for Astrophysics led a team that studied the star – 51 Pegasi – and its planet side by side.

Using the Very Large Telescope (VLT) in Chile, the team were able to observe the orbit of the planet for 4 hours (51 Pegasi b completes an orbit in just 101.5 hours, or 4.2 days). In so doing, they could see the shift in its light as the planet moved away and towards Earth.

From this shift in light, moving towards the redder and bluer ends of the spectrum due to the Doppler effect, the team was able to work out what the planet’s atmosphere was made of. And they found a clear detection of water, described as “pretty rock solid” by team member Matteo Brogi at the University of Colorado, Boulder, to New Scientist.

Although 51 Pegasi b was the first exoplanet to be discovered around a Sun-like star, it was not the first exoplanet to be found. That honor goes to HD 114762 b, a gas giant found in 1989, but not confirmed to be a planet until 2012, orbiting a binary star. Until it was confirmed, PSR B1257+12 B – a planet found orbiting a pulsar in 1992 – was often referred to as the first exoplanet discovery.

But this new study is important, because we’ve only really been able to study exoplanet atmospheres before using the transit method. If we can use a similar technique for other worlds, then we could greatly expand our knowledge of other exoplanets.

Plus, water is one of the most important ingredients for life as we know it. While a world like this is unlikely to be habitable, the more water we find in our galaxy, the better.