Tag: trinary

Exoplanet in a triple star system smack dab in the habitable zone

By Phil Plait | February 7, 2012 7:00 am

In the race to find the weirdest planet orbiting another star, we may have a front runner: GJ 667Cc, a super-Earth orbiting one star in a triple system that’s actually relatively closeby. And oh yeah: it just so happens to be in just the right spot to be potentially inhabitable!

Of course, I have some caveats, so don’t get too excited. But this is a weird and pretty cool one!

GJ 667 is a triple star system that’s right in our back yard as these things go: it’s only about 22 light years away, making it one of the closest star systems in the sky. It’s composed of two stars a bit smaller and cooler than the Sun which orbit each other closely, and a third, smaller star orbiting the pair about 35 billion km (20 billion miles) out. Stars in multiple systems get capital letters to distinguish them, so the two in the binary are GJ 667 A and B, and the third one is GJ 667C.

That third star is the interesting one. It’s a cool, red M dwarf with about a third the diameter of the Sun. Fainter, too: it only puts out about 1% of the light the Sun does. It’s been studied for years to look for planets around it, and while there have been some signs found, this new research is the first solid detection of planets that’s been published.

They used the Doppler method (sometimes called the Reflexive Velocity method): as planets orbit a star, their gravity tugs on it. We usually can’t see this motion directly, but a spectrum can reveal a Doppler shift, similar to the change in pitch you hear when a car or train goes by. If the spectrum has a high enough resolution, and the analysis very carefully done, there’s a lot you can tell by measuring it. You can get the planet’s mass, its period, and even the shape of its orbit.

In this case, the spectrum reveals GJ 667C may have four planets! Two very strong signals pop up with periods of 7 and 28 days, a third one at 75 days, and a possible trending shift in the spectrum that may point to a planet orbiting in a very roughly 20 year period.

It’s that second planet, GJ 667C with a 28 day orbit that’s so interesting. Its mass is at least 4.5 times that of the Earth, so it’s hefty. A 28 day orbit puts it pretty close to the parent star — about 7 million kilometers, or less than 5 million miles (Mercury is 57 million km from the Sun, by comparison). But remember, GJ 667C is a very dim bulb, so being that close means that the planet is actually right in the middle of the star’s habitable zone! The HZ is the distance where liquid water could exist on a planet — it depends on the size and temperature of a star, and also on the planet’s characteristics. A cloudy planet can hold heat better through the greenhouse effect, so it can be farther from the star and still be warm, for example.
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CATEGORIZED UNDER: Astronomy, Cool stuff

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