Life as we know it requires liquid water, which is why those studying exoplanets search for traces of H₂O in the atmospheres. A team of astronomers, led by first author Alexandra Lockwood from Caltech, have detected water vapor on one of the alien planets nearest to Earth. The results were based on data obtained using the Near Infrared Echelle Spectograph (NIRSPEC) at the Keck Observatory in Hawaii and were published in The Astrophysical Journal Letters, with an open access version available on Arxiv. 

The planet, Tau Boötis b, is located about 51 light-years away and orbits the primary star of a binary system in the Boötes constellation. It is a hot Jupiter, which means that it is a gas giant, incredibly close to its parent star. Tau Boötis b is approximately 6-7 times more massive than Jupiter, but its distance from its star is only about one-seventh of the distance from Mercury to our Sun. It only takes about 3 days to complete an orbit.

This isn’t the first time that water vapor has been detected in an exoplanet’s atmosphere. Planets that cross in front of their parent star can have their atmospheres analyzed as the light shines through. Spectrographs works by teasing apart parts of the whole. Instead of viewing all of the light from the planet, the light is broken into its components like a prism and scientists are able to observe the wavelengths that indicate chemical composition. 

Tau Boötis b was first discovered in 1996, but researchers have been unable to use traditional spectrographic techniques to determine the chemical composition of the atmosphere, because Earth does not have a view of the planet transiting its star. The researchers developed a new technique using NIRSPEC, which can operate with a much weaker light signal. This was adapted from a previous technique from 2012 which determined that Tau Boötis b’s atmosphere also contains carbon monoxide.

The discovery of water on an exoplanet is always exciting, because it renews the conversation of potentially finding alien life someday. While there could be forms of life out there that do not require the presence of water, it would be impossible to widen the search without knowing exactly what to look for. 

Of course, tau Boötis b is much too close to its star to have habitable conditions, but the novel method of using NIRSPEC to analyze the atmosphere’s chemical composition could open huge doors for those hunting exoplanets in the future. The ability to detect water with a weaker signal only works on large planets near bright stars, but future refinement could allow analysis of other planets that do not transit the star from our perspective as well as exoplanets that are more distant from the parent star, potentially even out in the star’s habitable zone. The upcoming James Webb Space Telescope and Thirty Meter Telescope will be better equipped to observe cooler planets.