HST finds water vapor in atmosphere of habitable zone exoplanet

An artist’s impression of K2-18b, a super-Earth orbiting in its host star’s habitable zone where water can exist as a liquid. In a first, the Hubble Space Telescopes spotted the molecular signature of water vapor in the exoplanet’s atmosphere. Image: ESA/Hubble, M. Kornmesser

The Hubble Space Telescope has spotted the molecular signature of water vapour in the atmosphere of a super-Earth orbiting in the habitable zone of its host star where water could exist in liquid form – a key requirement for the development of life as it’s known on Earth.

Known as K2-18b, the exoplanet is located 110 light years from Earth, orbiting a red dwarf star in the constellation Leo. While it is located in the star’s habitable zone, it is likely exposed to much higher levels of radiation than Earth.

Even so, “finding water on a potentially habitable world other than Earth is incredibly exciting,” said Angelos Tsiaras of the University College of London’s Centre for Space Exochemistry Data and first author of a paper published in Nature Astronomy. “K2-18b is not ‘Earth 2.0’ as it is significantly heavier and has a different atmospheric composition. However, it brings us closer to answering the fundamental question: Is the Earth unique?”

Using archived data collected by Hubble in 2016 and 2017, the researchers developed algorithms to analyse light from the host star that passed through K2-18b’s atmosphere on the way to Earth, revealing the spectral signatures of water vapour, hydrogen and helium. The researchers believe other molecules, including nitrogen and methane, likely are present but beyond the reach of current technology.

Next generation space telescopes, however, including NASA’s James Webb Space Telescope and ESA’s Atmospheric Remote-sensing Infrared Exoplanet Large-survey – ARIEL – mission, will be equipped with more sensitive instruments, allowing astronomers to gather much more detailed data about exoplanet atmospheres.

“Our discovery makes K2-18b one of the most interesting targets for future study,” said co-author Giovanna Tinetti of UCL/CSED and principal investigator for ARIEL. “Over 4,000 exoplanets have been detected but we don’t know much about their composition and nature. By observing a large sample of planets, we hope to reveal secrets about their chemistry, formation and evolution.”