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Gliese 667C

October 6, 2015

Imagine for a moment that you wake up on a strange planet, just with a breathing apparatus and a backpack with the necessary supplies to survive. Your entire body feels heavier than usual, and you find it difficult to stand up. The landscape around you is barren and impassable, with red-brown sand and rocks. The air is cold, but on the ground you see no sign of frost. Something about the light is unfamiliar. Looking at the sky you see a red sun, several times as large as the Sun on Earth. On the hazy horizon you see mountains with snow on their tops. The snow appears reddish. In the other direction you see water. The edge of the river shimmers in the sun as if it were covered with ice or salt.

With great effort you will find a shelter between the rocks. In your backpack you will find a blanket. Huddled within the blanket, you close your eyes to think concentrate. Hours later you wake up from a restless sleep. But you are still there. Nothing has changed. Even the oppressive red sun is still in the same place in the sky. Doubts come to your mind whether you will ever leave this place again.

A human being might feel like this on the second planet of Gliese 667C, a star from the neighborhood of our Sun, about 23.2 light years away from us. The star is the third component of a triple system. The two larger stars, Gliese 667A and B, form a central pair, which orbits the common center of gravity every 42 years at an average distance of 12.6 astronomical units. Even these two stars are only slightly more than half as massive as the Sun, and only 12 or 5% of the solar brightness. The third star, Gliese 667C, orbits about 230 astronomical units from the central binary. It is a small red dwarf of spectral type M1.5. With only 31% of the solar mass and 42% of the solar radius, its brightness is only 1.4% of the solar brightness.

At Gliese-667C, two planets are safely confirmed, planet b and c. Both were detected by the radial velocity method, and their mass and rotation time is known. Gliese-667Cb is about 6-12 times as massive as Earth and orbits the star in 0.05 astronomical units distance once every 7.2 days. With 5.5 times the earth's solar radiation, it is outside the inner edge of the habitable zone and would be too hot for the known forms of life. On the basis of its mass and the exact position of its orbit, it is assumed that it could be a gas planet.

In 2011, Gliese-667Cc was discovered. This planet is located within the habitable zone and receives about 87% of the Earth's solar radiation. Its mass of 3.8 Earth masses suggests a terrestrial planet. The planet orbits its star once in 28 days at an average distance of 0.12 astronomical units, about a third of the distance between Mercury and the sun.

The age of the system Gliese-667 as a whole is estimated at between 2 and 10 billion years. It is unusually poor in metals, with only 26% of the value of our sun. For the planet Gliese-667Cc this can mean a whole range of scenarios. In the representation here, it is reproduced as a rocky planet. Its originally dense atmosphere and its water supply have been eroded by the frequent eruptions of the star over the ages. Even if the temperatures on the surface of the planet are likely to be within the life-friendly range, the sunlight, which is strongly shifted towards the red and infrared parts of the spectrum and the rotation, which has long been halted by the tidal forces of the star, impose severe limitations for possible life.

Artistic representation of the planet Gliese 667Cc. Most of the surface is desert wiht some pools of water.

Schematic overview of the Gliese 667 triple system.

G. Anglada-Escudé, M. Tuomi, E. Gerlach, R. Barnes, R. Heller, J. S. Jenkins, S. Wende, S. S. Vogt, R. P. Butler, A. Reiners, H. R. A. Jones (2013): A dynamically-packed planetary system around GJ667C with three super-Earths in its habitable zone. Astronomy & Astrophysics, vol. 556, A126
The high multiplicity systems Gliese 667C and KOI 3158. Second Kepler Science Conference, 5 th November 2013
European Southern Observatory. Press information: The HARPS search for southern extra-solar planets. 24 th Novemer 2011

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