Scientists have discovered water vapour on a distant “super-Earth” known as K2-18b for the first time.

The exciting development, published in Nature Astronomy on Thursday, represents the first time water — an essential ingredient for life as we know it — has been detected in the atmosphere of a potentially habitable exoplanet, meaning a planet with Earth-like conditions outside our solar system.

It’s a promising step towards finding other life in space. But it’s unlikely to ever become a second home for humans, the study’s author, Dr Angelos Tsiaras, said.

“K2-18b is now the best candidate for habitability that we know. It’s the only planet outside our solar system that we know has the correct temperatures, atmosphere and water,” the University of London (UCL) research associate told reporters at a press conference in London.

“At the same time, K2-18b is not a second Earth. This is a planet that is much bigger. It has a different atmospheric composition and it’s orbiting a completely different star.

“The search for habitable planets is very exciting, but it’s here to always remind us that this is our only home. And it’s probably out of the question whether we’ll ever be able to travel to other planets,” he said.



WHERE IS EXOPLANET K2-18b?

K2-18b was discovered in 2015 by NASA’s Kepler Space Telescope. It’s about double the size of Earth with eight times the density, and located in the Leo constellation about 110 light years away.

It’s also what scientists refer to as a “super-Earth” – a planet with a higher mass than Earth but lighter than ice giants Neptune and Uranus.

Scientists believed it was a prime candidate for finding water, as it orbits within the “habitable zone” of a small, red dwarf star called K2-18. That means it’s not too hot that water will evaporate, but not too cold that it will freeze.

media_camera K2-18b orbits in the ‘habitable zone’ of the K2-18 star. Picture: Supplied

To detect the water, a team of researchers used data captured by the Hubble Space Telescope to analyse the starlight filtered through K2-18b’s atmosphere. Those results then revealed the “molecular signature” of water vapour.

But co-author and fellow UCL researcher, Dr Ingo Waldmann, stressed the team had not yet found any signs of life.

“We know there’s an atmosphere, we know that it’s the right temperature and we know there is water. But the current observations just don’t let us constrain whether there are any biomarkers,” he said.

He hopes the upcoming launch of NASA’s new James Webb Telescope in 2021, as well as a suite of other ambitious projects on the horizon, will help uncover more clues.

“In space, we have the six-metre James Webb telescope which will be launched in 2021. In 2028, we have the UCL Ariel space mission which will look at thousands of exoplanets. And also from the ground we have the Extremely Large Telescope (ELT) in Chile which is 42 metres in size.”

“With these instruments in the next decade, or even in the next few years we’ll have a very good chance of understanding what the chemistry is like (on exoplanets) and whether there are so-called biomarkers.”

media_camera The James Webb Space Telescope has been specially designed to capture infra-red light from the first galaxies that formed in the early universe. Picture: Laura Betz/NASA/AP

media_camera The James Web Space Telescope will have a primary mirror, sun shield, solar panels and spacecraft bus. Picture: Supplied.

WHY CAN’T HUMANS LIVE ON K2-18b?

There are several things that make a planet habitable. They include liquid water, temperate conditions, an atmosphere to protect life from harmful radiation, and a stable environment to allow life to develop over time.

K2-18b is considered “potentially habitable” by scientists because it ticks three of those boxes: it has temperate conditions, an atmosphere, and water.

But Professor Jonathan Tennyson, from UCL’s department of physics and astronomy, said the planet would likely be too harmful for humans.

“This planet will get a lot of ultraviolet radiation at the top of its atmosphere. We know that on Earth that’s not necessarily good news for us because it gives us cancer. But it doesn’t necessarily mean that other forms of life can’t evolve differently, for instance underwater,” he said.

Then there’s the issue of travel.

“We’re really far away from being able to travel to these planets — even the closest one is just too far away even with current technology,” co-author Professor Giovanna Tinetti said.

Dr Tsiaras said his research was instead an important step towards answering the fundamental question: is our Earth unique?

“This study contributes to our understanding of habitable worlds beyond our solar system and marks a new era in exoplanet research, crucial to ultimately place the Earth, our only home, into the greater picture of the cosmos,” he said.

Originally published as ‘Super-Earth’ offers real hope of alien life