A Nearby Faint Star With 6 or 7 Planets

Big news today from astronomers studying "exoplanets" -- planets that orbit other stars in the sky. Groups of astronomers in Europe and America, working together, have found a faint nearby star which has 6 or maybe even 7 planets orbiting it, three of which are in the "habitable zone" -- where water can be a liquid.

The name of the star is Gliese 667C (part of a system of three stars that orbit around each other). Only the faintest of the three stars has been found to have planets, but that faint one has quite a family of them. You can see the planets and the star on our diagram.

The way astronomers name these things is by giving each object a letter in order of discovery. So in this Gliese 667 system, capital A, B, and C are the letters for the three stars. Then the planets around star C are given lower-case letters, starting with b. (Planet "h" is not fully confirmed, so it has a question mark next to it.) The green zone is where the planets are that have the right temperature for life as we know it.

The three planets in the green zone of this star are what we call "super-Earths" -- they are bigger than Earth, but smaller than Uranus and Neptune. The method we use to discover these planets only gives us an estimate of their mass, but given how crowded the habitable zone seems to be, scientists are feeling reasonably sure that these planets are no bigger than about 10 Earth masses.

This is the largest number of planets ever found in the habitable zone of another star. (Planet h, the one that is not yet confirmed, is just tantalizingly at the edge of that zone.)

All the planets and the three stars are about 22 light years away in the constellation of Scorpius. (The closest star is 4 light years away, so, in the cosmic scheme of things, Gliese 667 is one of our closest neighbors!)

What's especially interesting about this discovery is that the faint star that has all the planets is what astronomers call an M-type star -- it has only 1/3 the mass of our Sun and shines with only about 2% of our Sun's light output. So the habitable zone is much closer to the cool star than our Sun's is. The planets in the habitable zone take between 28 and 62 days to orbit the star. (Recall that the Earth takes 365 days to go around our much hotter Sun!)

The interesting part is that M type stars are much more common in the universe than stars like our Sun. So if an M type star like Gliese 667C can have 6 or 7 planets crowded around it, that means that perhaps other such M type stars also have families of planets and the number of sites we can look for life in the universe has just gone up.

For more technical details, more pictures, and even short videos, see: http://www.eso.org/public/news/eso1328/

(By the way, Gliese comes from the name Wilhelm Gliese, a German astronomer, who constructed one of the most important catalogs of nearby stars. Many faint stars near us have Gliese numbers from his catalog.)