Astronomers find triple-super-Earths

Contributed by
Jun 16, 2008

Astronomers from the European Southern Observatory have announced quite the find: a solar system 42 light years from us that harbors three super-Earths!

Edited to add: As noted in the comments, this artist's illustration is pretty far off the mark! All three planets are so hot there is no way they could have water, so making them blue is scientifically inaccurate. I can't believe I missed this when I first posted this entry and the picture!

The planets orbit the star HD 40307, a K-type star... meaning it's slightly less massive, cooler, and a bit more orange than the Sun. Still, the planets are cooked; they orbit the star with periods of 4.3, 9.6, and 20.4 days, so they're very close to the star itself. The surface temperatures of all three must be well over 1000 degrees Celsius. The planets are big, too, with masses of 4.2, 6.7, and 9.4 times that of the Earth. That makes them too small, as far as we know, to be gas giants like Jupiter and Saturn (which have masses of 100 or more times the Earth's), though it doesn't rule out these being small giants like Uranus and Neptune (roughly 15 and 17 times the Earth). Given their location so close to their star, it seems likely (though not for sure) that these are more like terrestrial planets, just a lot bigger -- the biggest would have about twice Earth's diameter.

So make no mistake. They're kinda sorta like Earth, but they are not Earth-like. They're not a nice place to visit, and you most certainly don't want to live there.

The planets were found using the usual technique of looking for a wobble in the star as the planets tug on it gravitationally. That can't be seen directly by looking at the star's position; the motion is way too small for that. Instead, astronomers look at the spectrum of the star and measure the Doppler shift. As the planets pull on the star this way and that, the spectrum shifts, and that can be measured... though it's very hard to do. That's why the first planets detected in this way weren't found until 1995*; coincidentally one of the astronomers on the team that found these planets, Michel Mayor, was on the original team to find the first planet orbiting another star like the Sun.

These findings were reported at a meeting in France for astronomers looking for such super-Earths. Two other extrasolar systems were announced there, too; one with a planet 9.5 times the Earth's mass and the other with much more massive planets.

And so this raises the question: just how many stars are out there harboring planets? Even as we look at more and more stars, the fraction with planets seems to go up with every discovery, mostly because we're getting pretty good at finding planets, including ones with smaller mass. And that takes us to the more important question: how many of these planets are like Earth? And that of course leads to the ultimate question: how many of them have begun receiving our transmissions of Doctor Who?

I mean, how many of them have life?

It'll be a while yet before we know, but I think it's a very, very cool thing that we're looking.

*The first extrasolar planets were found orbiting a pulsar using a similar technique, but they are very very very very very unlikely to look anything like planets we've ever seen before!

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