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To us, the solar system seems normal. But new studies of the planets sprinkled around other stars suggest that our corner of the galaxy is actually a pretty weird place.

Scientists have discovered that a large proportion of the recently discovered planets outside our own solar system are nothing like the familiar planets orbiting our sun. Instead, many are "mini-Neptunes": roughly the size of Earth but, unlike Earth, composed of a thick layer of gases around a solid core, which is more like the composition of the planet Neptune. That implies that the recipe for making planets in other solar systems is far different than the process that led to Earth's formation.

The mini-Neptunes "dominate the inventory" of the 3,000-plus planets found by NASA's planet-hunting spacecraft Kepler, said University of California-Berkeley astronomer Geoff Marcy on Monday at an American Astronomical Society meeting just outside Washington, D.C. "These are planets we never expected based on our own solar system."

Marcy and his team examined dozens of "exoplanets" – planets that circle a star other than the sun – spotted by Kepler before it went bust last year. With the keen-eyed Keck telescope in Hawaii, Marcy's team looked for tiny perturbations that planets create in their stars. The bigger the perturbation, the more massive the planet.

The researchers found that exoplanets fall into two groups. Planets that are roughly twice the size of Earth or smaller have a rocky core plus some water, like Earth. But planets two to four times the size of Earth have a rocky core but are also built of lots of gas, like Neptune.

The findings of Marcy's team were validated by scientists who turned to a different method of sizing up planets. Northwestern University's Yoram Lithwick said Monday that his team measured the heft of a selection of exoplanets by examining how two planets orbiting the same star change each other's journey around their star. The bigger the change, the more massive the planet.

Lithwick and his colleagues examined some 60 exoplanets smaller than Neptune but bigger than Earth and found that roughly one in three are small, fluffy bodies containing high amounts of gas. That composition means that unlike Earth, which formed relatively late in the solar system's history, these fluffball planets formed "much earlier. So these planets must be dramatically different from the Earth, contrary to what we expected," Lithwick says.

Yet another lightweight planet called KOI-314c is almost exactly the mass of Earth but much larger across, meaning it, too, must have a thick atmosphere of gas, David Kipping of the Harvard-Smithsonian Center for Astrophysics said at the meeting Monday.

Despite its Earth-like measurements, "it's nothing like Earth," Kipping said. Until now, he would've assumed an exoplanet with Earth's mass would also have the same basic ingredient as Earth, namely rock. "You can't just draw a line in the sand … and say, everything below this point is rocky, everything above that point is gaseous."

So why don't we have a mini-Neptune in our own solar system?

Some 20% to 30% of stars have "these crazy planets," Lithwick said. "You could say that all these planets Kepler has found are strange. But another way of looking at it is it's really the solar system that may be strange."

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