An exoplanet named K2-18b has become the frontrunner in the race to find a habitable planet beyond our own, as astronomers have found evidence of water existing in its atmosphere. Using data captured by the Hubble Space Telescope, scientists at UCL’s Centre for Space Exochemistry Data have developed algorithms to analyse starlight filtering through K2-18b’s atmosphere.


The results reveal both the molecular signature of water vapour and the presence of hydrogen and helium in the exoplanet’s atmosphere.

The science team hope further research will reveal how much water is there, as well as whether molecules such as nitrogen and methane are present.

The exoplanet, eight times the mass of Earth, is located around 110 lightyears away in the constellation of Leo, in a 33-day orbit – and crucially, in the ‘habitable’ zone – around the cool dwarf star K2-18.

“Finding water in a potentially habitable world other than Earth is incredibly exciting”, says Dr Angelos Tsiaras, one of the authors of the research.

“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?”

Video: Animation showing exoplanet K2-18b, its host star and an accompanying planet. Credit: ESA/Hubble, M. Kornmesser

Although it’s the only planet other than Earth known to have both water and life-sustaining temperatures so far, the research team believe K2-18b could be the first of many habitable, Earth-like exoplanets to soon be discovered.

For more on this, read our guide What makes a planet habitable?

NASA’s TESS mission, the NASA/ESA/CSA James Webb Space Telescope and ESA’s ARIEL mission are all expected to detect hundreds more super-Earths – exoplanets with a mass between Earth and Neptune – in the coming years.

“With so many new super-Earths expected to be found over the next couple of decades, it is likely that this is the first discovery of many potentially habitable planets,” says report co-author Dr Ingo Waldmann.


“This is not only because super-Earths like K2-18b are the most common planets in our Galaxy, but also because red dwarfs – stars smaller than our Sun – are the most common stars."


Jane Williamson science journalist and writer
Jane WilliamsonScience journalist

Jane Williamson is BBC Sky at Night Magazine's Production Editor.