Super-Earth planets detected orbiting nearby star

26 June 2020

A system of super-Earth planets – possibly rocky worlds, somewhat heavier than Earth, has been detected orbiting the nearby star Gliese 887.

An international team of astronomers, including Hugh Jones from the University of Hertfordshire, Carole Haswell and John Barnes from The Open University and Richard Nelson from Queen Mary University of London made the discovery as part of Red Dots, a project to detect terrestrial planets close to the Sun.

Gliese 887 is the 12th closest star to the Sun with a distance of 11 light years. It is the brightest so-called red dwarf star in our sky, about half as big as the Sun. Gliese 887 is much dimmer than the Sun. This means the habitable zone, where water can exist in liquid form, is closer to Gliese 887 than Earth’s distance from the Sun.

The two new planets, Gliese 887 b and Gliese 887 c, were found using a number of high resolution spectrographs. These are sensitive to the tiny back and forth wobbles of the star caused by the gravitational pull of the planets. In particular, the purpose built, high-precision planet finding spectrograph known as the High Accuracy Radial Velocity Planet Searcher (HARPS), on the European Southern Observatory’s 3.6m telescope at La Silla in Chile was used.

“We first found these signals back in 2012. However, when you find multiple weak signals you first suspect that they might be caused by spots on the star rotating in and out of view.” Hugh Jones from the University of Hertfordshire commented. “Usually with a few hundred precise measurements we can confidently measure the rotation of a star. But with this one, even a month of data from NASA’s Transiting Exoplanet Survey Satellite did not manage to find it. This means that any spots on the star are very weak and along with the addition of new data gained in recent years we are confident that we have found at least two planetary signals”

The two planets orbit their star every 9.3 days and 21.8 days respectively, much faster than Mercury's orbit around the Sun.  If these planets reflect a similar amount of starlight to Earth, Gliese 887 c, with an orbit of 21.8 days, has an estimated temperature of 70oC, slightly hotter than Earth.

We know that the Sun has strong stellar winds – outflowing material which can erode a planet's atmosphere. The atmospheres of planets as close to their star as Gliese 887 b and Gliese 887 c would be swept away if the star was active. But because Gliese 887 is much less active than the Sun, these planets could retain their atmospheres and potentially host life. Though Gliese 887 c receives more starlight than the Earth receives from the Sun, it could have a thicker atmosphere due to the inactivity of the central star.

Due to the inactivity of the star, it will be relatively easy to detect the atmospheres of Gliese 887 b and Gliese 887 c.  So, Gliese 887 will be a prime target for the James Webb Space Telescope, a successor to the Hubble Space Telescope due to be launched next year.

Sandra Jeffers, of the Institute for Astrophysics, Goettingen University, said: “I am very excited to be leading the team that is detecting the exoplanets orbiting our closest stellar neighbours.  These planets will provide the best possibilities for detailed studies including searching for evidence of life outside the Solar System.”

In addition to the two new planets published in the journal Science, a total of seven planets orbiting four of the nearest stars to the Sun have been discovered by the international team.

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