Earth-sized exoplanet with an atmosphere detected

06 April 2017
Posted by Chris Smith.

An atmosphere around an Earth-sized planet has been detected for the first time by UK scientists. 

The planet in question is a rocky world called GJ1132b, which is about 1.6 times heavier than the Earth and orbits a small star called GJ1132 about 40 light years away. The distance between the planet and the host star is only about 15% of the distance that separates the Earth from the Sun.

The mass of the star is only about 20% of the mass of our own Sun, meaning that it is relatively cool and dim, which makes it easier for astronomers to observe orbiting planets without being dazzled. 

Using a 2.2 metre telescope in Chile, Keele University scientist John Southworth observed planet GJ1132b making 9 passes - or transits - in front of the star. Such transits briefly block out some of the starlight, producing an apparent dip in the star's intensity. The scale of the dip gives away the size of the planet. In this case, the intensity dropped by 0.4%, suggesting that the planet has a radius about 1.4 times larger than the Earth.

Southworth and his colleagues looked at the planet using 7 different colours or wavelengths of light. Some wavelengths produce a smaller dip during the transit than others. This difference corresponds to the atmosphere around the planet, which is opaque for some wavelengths but transparent to others. Subtracting the largest dip from the smallest suggests that GJ1132b has an atmosphere about 10% of the diameter of the planet, so potentially hundreds of kilometres deep.

The discovery is a first on many levels. Apart from being the lowest-mass planet with an atmosphere yet detected (the previous candidate was 55 Cancri e, a massive "super-Earth" 8 times the size of our planet), the small stature of star GJ1132 means that it burns its hydrogen much more slowly that stars like the Sun. "That means it can survive for billions of years, it's slow rotation says it's old, and that means that rocky worlds like GJ1132b can hang on to an atmosphere for a long time. That we didn't know before," says Southworth.

Stars like GJ1132 account for more than half of the 100 billion or so stellar objects in the Milky Way, so this discovery means that the odds of finding other planets like the Earth, with an atmosphere, are now much greater.

As yet the team have not got a clear picture of the likely composition of the atmosphere, but their measurements, which are published in the April edition of the Astronomical Journal, fit best with either a dense methane atmosphere or, more tantalising, a steamy, water-rich atmosphere. "But, some colleagues have since looked at this star with Hubble," says Southworth. "And they do have the resolution we need to diagnose what's in that atmosphere, so watch this space!"

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