For the first time, astronomers have managed a direct measurement of the gases present on a "super-Earth" planet orbiting an alien star.
They found evidence for hydrogen and helium in its atmosphere, but no water.
Called 55 Cancri e, the world is twice the size of Earth and eight times the mass - but orbits unusually close to its host star, with an 18-hour year and surface temperatures above 2,000C.
The UK team published their findings in the Astrophysical Journal.
"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," said Angelos Tsiaras, a PhD student at University College London and the first author of the paper.
"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 super-Earths are the most abundant planets in our galaxy. The term describes any world heavier than Earth but not as massive as gas giants like Saturn and Jupiter.
With its very close orbit, 55 Cancri e is a particularly exotic example.
As with all extrasolar planets, or "exoplanets", the evidence for its existence comes from tell-tale, regular dips in the brightness of its host sun, 55 Cancri. This star is 40 light-years away and extremely bright; it is also known as Copernicus and forms part of the Cancer constellation.
When a planet like 55 Cancri e makes a transit in front of its star, it typically blocks about 1% of the star's light - and this happens across all the colours of light.
"The entire planet has a signal that doesn't depend on wavelength; it's just a solid body that is blocking the light," explained Prof Giovanna Tinetti, a study co-author also from UCL.
But if the planet has an atmosphere, its "fingerprint" can be detected in how the transit affects different wavelengths, as the gases filter the star's light. These signals are much smaller, dimming the star by as little as 0.001%.
Researchers studying super-Earths have never detected such a fingerprint before.
"There are just two other observations of super-Earths," Prof Tinetti told the BBC. "They found a signal that was very flat."
One of those other examples, GJ 1214b, has been studied in some detail. Its "flat" signal could indicate that it is shrouded in clouds, Prof Tinetti said, or its atmosphere might contain heavier molecules like water - as a previous study has suggested.
"A flat spectrum can be interpreted in many ways… It's hard to get more information out of it."
Consequently, a direct measurement of a super-Earth's atmosphere has never been made, and Prof Tinetti said the team is "very excited".
Hot and poisonous
They based their study on publicly available data from the Hubble Space Telescope. Using a special analysis "pipeline" developed by Mr 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.
The pattern was characteristic of an atmosphere rich in light elements like hydrogen and helium.
"It's the first time that we have a direct measurement of these components," said Prof Tinetti.
There were also hints in the data of hydrogen cyanide, a chemical used as a marker for atmospheres that are rich in carbon, relative to oxygen.
This fits with previous research, based on the 55 Cancri e's mass and radius, that suggested an abundance of carbon in its interior - earning it the label of "diamond planet".
"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," said Prof Jonathan Tennyson, another co-author from UCL.
"Although, hydrogen cyanide or prussic acid is highly poisonous, so it is perhaps not a planet I would like to live on!"
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