Thousands of planets have been found outside the Solar System, ranging from Earth-sized worlds to massive gas giants. An intermediate type of planet called a “super-Earth” has garnered significant interest, though, as it may be the most common type in the universe – and now astronomers have detected gases in the atmosphere of one of these worlds for the first time.

The planet in question is 55 Cancri e, also known as "Janssen," which is more than eight times the mass of Earth and located 40 light-years from us. The planet was already interesting because it had been dubbed a “diamond planet,” with some models suggesting it had a carbon-rich interior similar to diamond. Later research, however, dampened those theories. Now it seems the initial line of thinking may have been correct.

“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 Ph.D. student at the University College London, where the research was led, in a statement. The results are to be published in the Astrophysical Journal, with a pre-print available on Arxiv. 

The findings, made using data from the Hubble Space Telescope, show the atmosphere contains both hydrogen and helium, but no water vapor. This latter point is to be expected, considering the planet orbits extremely close to its host star, with a year lasting just 18 hours and temperatures reaching 2,000K (1,727°C). Its surface is likely a hellish place, inhospitable to life as we know it.

^{Above is an animation of the planet in orbit around its parent star. ESA/Hubble/M. Kornmesser}

But perhaps most surprising was that hydrogen cyanide was also found within the atmosphere, at an abundance that suggests a very high ratio of carbon to oxygen. This, in turn, supports the theory that the planet is rich in carbon, and thus maybe it is a "diamond planet" after all.

“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 Professor Jonathan Tennyson, also from UCL, in the statement. “Although, hydrogen cyanide or prussic acid [as it’s sometimes called] is highly poisonous, so it is perhaps not a planet I would like to live on!”

In their paper, the researchers note that the discovery of an atmosphere is a significant finding in itself, especially considering the planet’s position around its star. They postulate that its hydrogen and helium may have been retained from the protoplanetary disk from which it formed.

55 Cancri e is unlikely to be the norm for super-Earths, but the techniques used here could support analysis of other such worlds.