The field of extrasolar planet studies is undergoing a seismic shift. To date, 4,940 exoplanets have been confirmed in 3,711 planetary systems, with another 8,709 candidates awaiting confirmation. With so many planets available for study and improvements in telescope sensitivity and data analysis, the focus is transitioning from discovery to characterization. Instead of simply looking for more planets, astrobiologists will examine “potentially-habitable” worlds for potential “biosignatures.”

This refers to the chemical signatures associated with life and biological processes, one of the most important of which is water. As the only known solvent that life (as we know it) cannot exist, water is considered the divining rod for finding life. In a recent study, astrophysicists Dang Pham and Lisa Kaltenegger explain how future surveys (when combined with machine learning) could discern the presence of water, snow, and clouds on distant exoplanets.

Dang Pham is a graduate student with the David A. Dunlap Department of Astronomy & Astrophysics at the University of Toronto, where he specializes in planetary dynamics research. Lisa Kaltenegger is an Associate Professor in Astronomy at Cornell University, the Director of the Carl Sagan Institute, and a world-leading expert in modeling potentially habitable worlds and characterizing their atmospheres.

Artist’s impression of a multi-planet system where three are making a transit. Credit: NASA

Water is something that all life on Earth depends on, hence its importance for exoplanet and astrobiological surveys. As Lisa Kaltenneger told Universe Today via email, this importance is reflected in NASA’s slogan – “just follow the water” – which also inspired the title of their paper:

“Liquid water on a planet’s surface is one of the smoking guns for potential life – I say potential here because we don’t know what else we need to get life started. But liquid water is a great start. So we used NASA’s slogan of “Just follow the water” and asked, how can we find water on the surface of rocky exoplanets in the Habitable Zone? Doing spectroscopy is time intensive, thus we are searching for a faster way to initially identify promising planets – those with liquid water on it.”

Currently, astronomers have been limited to looking for Lyman-alpha line absorption, which indicates the presence of hydrogen gas in an exoplanet’s atmosphere. This is a byproduct of atmospheric water vapor that’s been exposed to solar ultraviolet radiation, causing it to become chemically disassociated into hydrogen and molecular oxygen (O2) – the former of which is lost to space while the latter is retained.

This is about to change, thanks to next-generation telescopes like the James Webb (JWST), Nancy Grace Roman (RST), and Origins Space Telescope, as well as next-next-generation observatories like the Habitable Exoplanet Observatory (HabEx) and Large UV/Optical/IR Surveyor (LUVOIR). There are also ground-based telescopes that will become operational in the coming years, like the Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT).

This artist’s impression shows the planet orbiting the Sun-like star HD 85512 in the southern constellation of Vela (The Sail). Credit: ESO

Thanks to their large primary mirrors and advanced suite of spectrographs, chronographs, adaptive optics, these instruments will be able to conduct Direct Imaging studies of exoplanets. This consists of