Scientists have found water vapour on a “super-Earth” for the first time – in a major step forward in the search for alien life.

The exoplanet, known as K2-18b, is perhaps the best candidate seen yet for supporting life on another world.

“This represents the biggest step yet taken towards our ultimate goal of finding life on other planets, of proving that we are not alone,” said University of Montreal astronomer Bjorn Benneke, who worked on a study of the planet.

“Thanks to our observations and our climate model of this planet, we have shown that its water vapour can condense into liquid water. This is a first.”

Scientists suspect that further study of K2-18b could reveal even more molecules that are key to life on Earth, such as nitrogen and methane. They also say that further examination of more than 4,000 other exoplanets could reveal other similarly promising worlds.

Best Nasa pictures of the month - August 2019 Show all 10 1 /10 Best Nasa pictures of the month - August 2019 Best Nasa pictures of the month - August 2019 "When stars like the Sun grow advanced in age, they expand and glow red. These so-called red giants then begin to lose their outer layers of material into space. More than half of such a star's mass can be shed in this manner, forming a shell of surrounding gas. At the same time, the star's core shrinks and grows hotter, emitting ultraviolet light that causes the expelled gases to glow. This type of object is called, somewhat confusingly, a planetary nebula, though it has nothing to do with planets. The name derives from the rounded, planet-like appearance of these objects in early telescopes" - European Space Agency Nasa Best Nasa pictures of the month - August 2019 Two galaxies are seen interacting with one another. This pair of galaxies is known as UGC2369 and they are being drawn together by mutual gravitational attraction. Our own Milky Way galaxy is next expected to interact with another galaxy, the Andromeda, in four billion years. The two galaxies will eventually merge together to form a galaxy that scientists are already calling the Milkomeda Nasa Best Nasa pictures of the month - August 2019 Mount Shasta in California as seen from 260 miles above earth onboard the International Space Station Nasa Best Nasa pictures of the month - August 2019 The galaxy NGC 5866 is seen at a side angle. We're used to seeing galaxies face-on but NGC 5866 is at such an angle to earth that we can only see it edge-on Nasa Best Nasa pictures of the month - August 2019 The most recent portrait of Jupiter taken by the Hubble Telescope shows a more intense colour palette swirling in Jupiter's atmosphere than in recent years. The different colours of the bands help scientists to understand what is happening in Jupiter's atmosphere Nasa Best Nasa pictures of the month - August 2019 This artist's illustration depicts the exoplanet LHS 3844b, which is 1.3 times the mass of Earth and 49 lightyears away. The illustration is based on research by Nasa into this recently discovered planet Nasa Best Nasa pictures of the month - August 2019 A Soyuz spacecraft carrying carg and supplies comes in to dock in the International Space Station Best Nasa pictures of the month - August 2019 A planetary nebula (radiation emitted by a dying star) is seen in the Gemini constellation by the Hubble Telescope Nasa Best Nasa pictures of the month - August 2019 North coast of Africa by Libya and Tunisia as seen from 260 miles above earth onboard the International Space Station Nasa Best Nasa pictures of the month - August 2019 This image from Nasa's Solar Dynamic Observatory shows our Sun in a different light. In colour are active regions of the atmosphere of the Sun e.g. solar flares and coronal mass ejections Nasa

Though the exoplanet was first spotted in 2015, new research has finally allowed scientists to reveal the existence of water.

Current equipment is only able to determine basic factors such as distance, mass and temperature but sophisticated tools developed at University College London helped translate existing data from the Hubble Space Telescope to make sense of the unique molecule signatures of water vapour.

K2-18b is classed as a super-Earth because it is bigger than our Earth – twice as big, as well as eight times heavier.

An artist’s impression shows the K2-18b super-Earth (M Kornmesser/ESA/Hubble/AFP) (M Kornmesser/ESA/Hubble/AFP)

K2-18b is too far away for astronomers to see, but they can look at how starlight is filtered through the planet’s atmosphere as it passes around its own sun, called K2-18.

The planet is a lot closer to its star than Earth is to the Sun, meaning it only takes around 33 days to transit.

“It’s the only planet outside our solar system that we know has the correct temperature, an atmosphere and water,” said lead author Dr Angelos Tsiaras.

“Of course, K2-18b is not a second Earth, because it is a planet that is much bigger, has a different atmospheric composition. It’s orbiting a completely different star, so it doesn’t look like Earth.

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

Although the planet sits in the habitable zone, scientists say that instruments available at present are not able to determine any signs of life.

Many more observations are needed to be sure that it is habitable.

Scientists, who have published their findings in the Nature Astronomy journal after more than a year of work, believe that the water content could be as low as 0.01 per cent or as high as 50 per cent.

“We don’t know how much water there is; however, what these models clearly say is that there is an atmosphere and that there is water there,” said co-author Dr Ingo Waldmann.

Researchers do not think K2-18b is the only potentially habitable super-Earth and expect to find others with similar characteristics among more than 4,000 exoplanets that have already been discovered.

They hope that newer technology, such as the James Webb Space Telescope due to launch in March 2021, will unlock more secrets beyond our solar system.

“We are looking forward to the next generation of telescopes and going even further,” Dr Tsiaras added.