Exoplanet Excursions: June 2015

Jon heads to what may be the most Earth-like world we know of, Gliese 677 Cc.

There’s a fascinating trinary system in the constellation of Scorpius called Gliese 677, the three stars of which orbit together around a common gravitational centre. It sounds rather graceful.

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Image Credit: 
Mark Garlick
Credit: Illustration by Mark Garlick

I’ve always imagined that bodies in systems such as this would have elaborate, unusual orbits affected by many gravitational influences: orbits not just circular or elliptical, but curving, looping and doubling back along paths of beautiful complexity. It will be intriguing to get closer to the orbital clockwork of the Gliese 677 system – a country dance between celestial giants.

Steering the Perihelion to the heart of the system we’re given a closer view of its stars. Gliese 677 A is the largest, a main sequence K-type star. It is a red dwarf with a rosy hue, 76 per cent of the Sun’s diameter and 73 per cent of its mass. Second largest is Gliese 677 B at 69 per cent the Sun’s mass. These orbit each other in a cosmological ballet.

We’ll settle the Perihelion around Gliese 677 C, another red dwarf orbiting the A and B stars at a distance of 230 AU. It’s only 31 per cent the mass of the Sun and is quite cute to be honest. This red dwarf has two planets confirmed around it, both in stable orbits – surprisingly for a trinary system. The innermost appears to be a ‘hot Neptune’ racing round its star in just seven days.

Gliese 677 Cc, the second out of the two confirmed planets, lives up to its reputation as one of the most Earth-like worlds discovered. Positioned at the warmer end of Gliese 677 C’s Goldilocks zone, there’s a rich, warm lushness to this planet that most humans would find delightful. The fact the planet is 3.8 times the mass of Earth gives it a surface gravity 3.8 times what we’re used to; it feels oppressive at first but you do begin to adjust.

It’s an astonishing terrestrial landscape in this particular spot, resembling a supersized igneous Grand Canyon. What appears to be slow flowing liquid water runs along the bottom of a valley. And this alien vista is bathed in the fireglow shades of the parent red dwarf. It’s easy to imagine a partially terraformed Mars looking something like this thousands of years from now.

Seeing three stars in the daytime sky is quite marvellous. Day turns to night in a manner very different from our earthly dusks. As the parent star sets, the two more distant stars remain with a lower light intensity, like the Sun appears from the vicinity of Jupiter. This is like an odd-looking dusk and golden hour happening simultaneously; a light of such beautiful unfamiliarity. It’s impossible to stop gawping at the hybrid daylight and twilight cast on the mighty alien terrain.

There are tantalising signs of life too. At the bottom of the gorge, along the edge of the shallow water, are leathery growths of a seaweed-like vegetation. The fleshy protuberances are black in colour, perhaps to help photosynthesis or for protection against radiation from the parent star. All in all Gliese 677 Cc is a quite superb terrestrial exoplanet, adding an avalanche of greater meaning and fascination to that familiar term ‘Earth like’.


Jon Culshaw is a comedian, impressionist and guest on The Sky at Night
This column appeared in the June 2015 issue of BBC Sky at Night Magazine
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