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Well now, this is an interesting discovery: astronomers have found what looks like a "super-Earth" a planet more massive than Earth but still smaller than a gas giant orbiting a nearby star at the right distance to have liquid water on it! Given that, it might might be Earthlike.

This is pretty cool news. Weve found planets like this before, but not very many! And it gets niftier: the planet has at least five siblings, all of which orbit its star closer than it does.

Now let me be clear: this is a planet candidate; it has not yet been confirmed. Reading the journal paper (PDF), though, the data look pretty good. It may yet turn out not to be real, but for the purpose of this blog post Ill just put this caveat here, call it a planet from here on out, and fairly warned be ye, says I.

The star is called HD 40307, and its a bit over 40 light years away (pretty close in galactic standards, but I wouldnt want to walk there). Its a K2.5 dwarf, which means its cooler, dimmer, and smaller than the Sun, but not by much. In other words, its reasonably Sun-like. By coincidence, it appears ot be about the age as the Sun, too: 4.5 billion years. It was observed using HARPS, the High Accuracy Radial Velocity Planet Searcher (I know, it should be HARVPS, but thats harvd to pronounce). This is an extremely sensitive instrument that looks for changes in the starlight as a planet (or planets) orbits a star. The gravity of the star causes the planet to orbit it, but the planet has gravity too. As it circles the star, the star makes a littler circle too (I like to think of it as two kids, one bigger than the other, clasping hands and swinging each other around; the lighter kid makes a big circle and the bigger kid makes a smaller circle). As the star makes its circle, half the time its approaching us and half the time its receding. This means its light is Doppler shifted, the same effect that makes a motorcycle engine drop in pitch as it passes you.

Massive planets tug on their star harder, so theyre easier to find this way. Also, a planet closer in has a shorter orbit, so you dont have to look as long to find it. But in the end, by measuring just how the star is Doppler shifted, you can get the mass and orbital period of the planet. Or planets.

In this case, HD 40307 was originally observed a little while back by HARPS, and three planets were found. But the data are public, so a team of astronomers grabbed it and used a more sensitive method to extract any planetary signatures from the data. They found the three previously-seen planets easily enough, but also found three more! One of them is from a planet that has (at least) seven times the mass of the Earth, and orbits with a 198 day period. Called HD 40307g (planets are named after their host star, with a lower case letter after starting with b), its in the "super-Earth" range: more massive than Earth, but less than, say Neptune (which is 17 times our mass).

We dont know how big the planet is, unfortunately. It might be dense and only a little bigger than Earth, or it could be big and puffy. But if its density and size are just so, it could easily have about the same surface gravity as Earth that is, if you stood on it, youd weight the same as you do now!

But the very interesting thing is that it orbits the star at a distance of about 90 million kilometers (55 million miles) closer to its star than is is to the Sun but thats good! The star is fainter and cooler than the Sun, remember. In fact, at this distance, the planet is right in the stars "habitable zone", where the temperature is about right for liquid water to exist!

Thats exciting because of the prospect for life. Now, whenever I mention this I hear from people who get all huffy and say that we dont know you need water for life. Thats true, but look around. Water is common on Earth, and here we are. We dont know that you need water for life, but we do know that water is abundant and we need it. We dont know for sure of any other ways for life to form, so it makes sense to look where we understand things best. And that means liquid water.

Heres a diagram of the system as compared to our own:

Note the scales are a bit different, so that the habitable zones of the Sun and of HD 40307 line up better (remember, HD 40307g is actually closer to its star than Earth is to the Sun an AU is the distance of the Earth to the Sun, so HD 40307 is about 0.6 AU from its star). What makes me smile is that the new planet is actually better situated in its "Goldilocks Zone" than Earth is! Thats good news, actually: the orbit may be elliptical (the shape cant be determined from the types of observations made) but still stay entirely in the stars habitable zone.

And take a look at the system: the other planets all orbit closer to the star! We only have two inside Earths orbit in our solar system but all five of HD 40307s planets would fit comfortably inside Mercurys orbit. Amazing.

So this planet if it checks out as being real is one of only a few weve found in the right location for life as we know it. And some of those weve found already are gas giants (though they could have big moons where life could arise). So what this shows us is that the Earth isnt as out of the ordinary as we may have once thought: nature has lots of ways of putting planets the right distances from their stars for life.

Were edging closer all the time to finding that big goal: an Earth-sized, Earth-like planet orbiting a Sun-like star at the right distance for life. This planet is a actually a pretty good fit, but we just dont know enough about it (primarily its size). So Im still waiting. And given the numbers of stars weve observed, and the number of planets we found, as always I have to ask: has Earth II already been observed, and the data just waiting to be uncovered?

Image credits: ESO/M. Kornmesser; Tuomi et al.

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