More than 1,200 planet candidates were identified in Kepler's first four months of data collection, 408 of which reside in systems with two or more planetary siblings, but their configuration is much different to our own Solar System, say scientists working on the data.

Kepler seeks out planets by searching for the tiny dips in light from the parent star as the planet transits in front of it. The extent of the dimming effect can be translated into the planet's size, and the time between each successive dip reveals how long the planet takes to orbit its star. Since this sort of observation relies on Kepler being in the line of sight of the planet, the results are biased to "flat" solar systems, that is, with planets travelling around their sun on orbits tilted by less than one degree to the plane of the system.

For comparison, planets in our Solar System are tilted by up to seven degrees, meaning an alien astronomer looking for transits would not spot all of our planetary neighbours, likely missing out Mercury (orbit tilted by seven degrees) and Venus (orbit tilted by 3.4 degrees).

The co-habiting planets all share one thing in common – they are smaller than Neptune, and the systems lack any Jupiter-sized denizens, likely resulting in a much calmer gravitational environment.

“Jupiters are the 800-pound gorillas stirring things up during the early history of these systems,” says David Latham of the Harvard-Smithsonian Center for Astrophysics. “Other studies have found plenty of systems with big planets, but they’re not flat.”

The detection of these multi-planet systems offers a helping hand to uncovering Earth-sized worlds, too. If the time between successive transits of the planets varies from orbit to orbit, it is likely that gravitational interactions with other planets are at play. Intuitively, the size of the effect depends on the masses of the planets involved in the tug-of-war.

“These planets are pulling and pushing on each other, and we can measure that,” says Smithsonian astronomer Matthew Holman. “Dozens of the systems Kepler found show signs of transit timing variations.”

Transit timing variations may also play a key role in confirming the first rocky planets in habitable zones – the region around a star which offers stable conditions for water to be liquid on a planet's surface – and hence the first true-Earth twin.

The results were presented today at the 218th meeting of the American Astronomical Society.

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