Three Super-Earths

Ken Tapping, July 3, 2013

In the sky this week…

  • Look for Mars and Jupiter low in the dawn twilight.
  • Venus lies in the sunset glow. Saturn is high in the south overnight.
  • The Moon will be New on the 7th.

So far, the score sheet of planets orbiting stars other than the Sun is well over a thousand. It is apparent that most stars have planets of some kind, so the search is now focussing more on finding planets like ours. The instruments we are now using can tell us the size of a planet, and from the distance at which it orbits its star, what its temperature is likely to be. In some cases we can tell whether the planet has an atmosphere and we can even get an idea of what it consists of.

Most of the creatures in our world live in the oceans, and we believe that all life started there. Since the chemicals forming the basis of life as we know it on Earth are common in the dark gas and dust clouds filling large areas of our and other galaxies, we can reasonably assume that those ingredients must have come together on other worlds as well as ours to produce life. Even though our alien friends may look very different from us, their basic chemistry may well be similar to ours. In which case searching for planets we might be able to live on could be good places to look for living creatures. However, our search criteria could be: the planets should not be too big or too small; they should have surface temperatures such that rivers, lakes and oceans of water can exist on their surfaces, and they should have atmospheres.

Gliese 667C is a red dwarf star lying some 22 light years away (a light year is a little less than 10 billion kilometres). It is about a third of the mass of the Sun, which means it is far dimmer. This star lies in the constellation of Scorpius, “The Scorpion”, which is visible low in the southern sky these evenings. You won’t be able to see it, but by looking at Antares, a giant red star, the brightest star in that constellation, you’ll be looking in the right direction.

Astronomers used a device called HARPS (High Accuracy Radial Velocity Planet Searcher) to measure the wobbling of the star due to the gravitational pull of its planets, which is like watching a dancer with an invisible partner. They found three planets, all about three times the size of the Earth, and taking respectively 27, 29 and 62 days to orbit their star. The Earth takes a year (365.25 days) to complete one trip around the Sun. This places the three planets very close to their star. However, since their star is so dim, this puts them at the right distance for liquid water to exist on their surfaces. One advantage of dim, dwarf stars is that they can smoulder for many billions of years, easily long enough for life to start and to evolve. On Earth, life appeared about 2.5 billion years ago, but stayed very primitive for the first 2 billion.

By cosmic standards, Gliese 667C is a close neighbour. However compared with the distances in our Solar System, it lies far away; it is amazing to think that we can detect planets orbiting such stars and to get an idea what those planets are like. It could be a while before we get a chance to find out much more about these worlds, and before we get a close look at them. Nevertheless, since what we are doing today would have been undreamable a couple of decades ago, it would be very short-sighted of us to rule out the possibility.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355

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