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In our Solar System, planets fall into two types. First, there are the rocky planets like Earth, Mars, and Venus, which are similar in size and support gaseous atmospheres. Then there are the gas giants, like Jupiter, Saturn, and Uranus. These huge puff balls are two or more orders of magnitude bigger than their rocky cousins.

Perhaps strangest of all, there are no planets in between; nothing that sits on the borderline between rocky minnow and gas giant.

This sharp distinction has driven much of astronomers’ thinking about planet formation. One of the main challenges they have faced is to come up with a theory that explains the formation of two entirely different types of planet, but no hybrids that share characteristics of both.

That thinking will have to change. It now looks as if we’ve been fooled by our own Solar System. When astronomers look elsewhere, this two-tiered planetary division disappears.

Astrophysicists have now spotted more than 500 planets orbiting other stars and all of these systems seem entirely different to our Solar System. They’ve seen entirely new class of planets such as the Super-Jupiters that are many times larger than our biggest planet with orbits closer than Mercury.

But the one we’re interested in here has a mass that spans the range from Earth to Uranus, exactly the range that is missing from our Solar System.

Astronomers are calling these new types of planet Super-Earths, and so far they have found more than 30 of them.

Today, Nader Haghighipour at the University of Hawaii in Honolulu reviews what we know about Super-Earths and shows they are changing the way astronomers think about planet formation. Their mere existence, for example, should allow astrophysicists to reject a large portion of current theories about planet formation.

Of course, the question about Super-Earths that generates the most interest is whether they can support life. To that end, Haghighipour discusses the possibility that these planets may be rocky with relatively thin atmospheres, that they have dynamic cores that generate a magnetic field and that they may support plate tectonics. Above all, there is the question of whether they can support liquid water.

It makes for fascinating reading. But when all this new information has been absorbed by the community, astronomers will still be left with an important puzzle. That is why our Solar System is so different from all the others we can see, why it has this sharp distinction in planet type and what relevance this has to the question of habitability.

This is a mystery that astronomers are only just getting their teeth into.

Ref: http://arxiv.org/abs/1108.0031: Super-Earths: A New Class of Planetary Bodies

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