SCIENTISTS have found evidence of hydrogen and helium in the atmosphere of a “super-Earth” planet which is orbiting an alien star 40 light years from our world.

However, they found no sign of water around the planet – 55 Cancri e – which is twice the size of Earth and eight times its mass.

It is the first time astronomers have managed a direct measurement of gases on such a planet.

The UK team’s findings, published in the Astrophysical Journal, revealed that 55 Cancri e has an 18-hour year and surface temperatures above 2,000C.

Angelos Tsiaras, a PhD student at University College London (UCL) and the lead author of the paper, said: “This is a very exciting result because it’s the first time that we have been able to find the spectral fingerprints that show the gases present in the atmosphere of a super-Earth.

“Our analysis of 55 Cancri e’s atmosphere suggests that the planet has managed to cling on to a significant amount of hydrogen and helium from the nebula from which it formed.”

Astronomers believe that super-Earths – any world heavier than Earth but smaller than gas giants such as Saturn or Jupiter – are the most abundant planets in our galaxy.

The host sun for 55 Cancri e is 55 Cancri – also known as Copernicus – which forms part of the constellation of Cancer. When a planet such as this moves in front of its sun it normally blocks out about 1 per cent of the star’s light.

“The entire planet has a signal that doesn’t depend on wavelength. It’s just a solid body that is blocking the light,” said Professor Giovanna Tinetti, a co-author of the study, who is also from UCL.

But if the planet has an atmosphere, its spectral “fingerprint” can be detected in how the transit affects different wavelengths, as the gases filter the star’s light. These signals are much smaller, and can dim the star by as little as 0.001 per cent.

A fingerprint like this has never before been found, and Tinetti added: “There are just two other observations of super-Earths. They found a signal that was very flat.”

One of those, known as GJ 1214b, has been studied in detail, and Tinetti said its flat signal could indicate that it is shrouded in clouds, or its atmosphere might contain heavier molecules like water.

“A flat spectrum can be interpreted in many ways. It’s hard to get more information out of it,” he said.

As a result, a direct measurement of a super-Earth’s atmosphere has never been made, and Tinetti said the team was “very excited”.

Their study was based on data from the Hubble Space Telescope. Using an examination “pipeline” developed by Tsiaras and two colleagues, they tested how repeated, rapid scans of the star from Hubble’s wide-field camera varied across different wavelengths during the planet’s transits.

This was characteristic of an atmosphere rich in light elements like hydrogen and helium.

There were also hints of hydrogen cyanide, a chemical used as a marker for atmospheres rich in carbon, relative to oxygen.

Professor Jonathan Tennyson, another UCL co-author, added: “If the presence of hydrogen cyanide and other molecules is confirmed in a few years time by the next generation of infrared telescopes, it would support the theory that this planet is indeed carbon rich and a very exotic place.

“Although, hydrogen cyanide or prussic acid is highly poisonous, so it is perhaps not a planet I would like to live on.”