Hubble makes first analysis of super-Earth’s atmosphere

An international team of astronomers have used the Hubble space telescope to analyse, for the first time, the air around a rocky, Earthlike planet orbiting another star.

An artist’s impression of Janssen, or 55 Cancri e, in front of its parent star. Image credit: ESA/Hubble, M. Kornmesser

The planet, recently named Janssen, is known as a super-Earth because it is about twice Earth’s diameter and eight times more massive than our own world. It lies about 40 light-years away in the constellation of Cancer.

Astronomers from University College London used the space telescope operated jointly by NASA and the European Space Agency to detect the gases in the planet’s atmosphere. They found it to be completely dry, with no sign of any water vapour, and to contain hydrogen and helium with a hint of poisonous hydrogen cyanide.

The super-Earth is not likely to be home to life as it orbits very close to its star, and very rapidly, with a year lasting only 18 hours. Surface temperatures soar to 2,000 degrees Celsius.

It was recently named Janssen by the International Astronomical Union, though it is more commonly known as 55 Cancri e. It is one of five exoplanets detected around the star 55 Cancri and was discovered in 2004.

The London astronomers used a new technique invented at the university to tell what the planet’s atmosphere was made of. They got Hubble’s Wide Field Camera 3 to make several quick scans of the starlight, then used analytical software to find the “fingerprints” of the planet within that.

Other recent observations of the changing brightness of 55 Cancri e by UK astronomers at Cambridge suggest they are seeing a huge volcano erupting on its surface.

Super-Earths like 55 Cancri e are thought to be common within the galaxy. Giant new telescopes being built, including NASA’s James Webb space telescope which will succeed Hubble, will be powerful enough to detect the weather on planets around nearby stars.

Angelos Tsiaras, a PhD student at UCL, who developed the analysis technique along with his colleagues Ingo Waldmann and Marco Rocchetto, 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.”

He said it suggested that the planet had managed to hold on to a significant amount of the hydrogen and helium that was in the cloud of gas and dust from which it was born.

Professor Giovanna Tinetti, also from UCL, said: “This result gives a first insight into the atmosphere of a super-Earth. We now have clues as to what the planet is currently like and how it might have formed and evolved, and this has important implications for 55 Cancri e and other super-Earths.”

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