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Five Earth-sized planets, in orbit in the habitable zone around five different stars, smaller and cooler than the Sun, and a planetary system with 6 transiting planets. These are the most ground-breaking results from the Kepler space telescope.
The five "Earths" are actually awaiting confirmation, but the 6 planet system, around the solar type star Kepler-11 (2000 light years distant) can already provide some surprising results.
The 5 innermost planets are on virtually coplanar orbits very close to the star, with orbital radii all smaller than that of Mercury, so that they all orbit in less than 50 days. By using the slight deviations in the timing of transits, caused by the gravitational interaction between the planets, it has been possible to measure their masses, which lie in the range 2.3 to 13.5 times that of Earth.
Combined with measurements of their diameters, deduced by the amount of the stellar disk they obscure, their densities have also been determined, and found to be much lower than the Earth.
These planets are therefore most likely to be gaseous, possibly with water (especially the inner two) and probably also hydrogen and helium.
The presence of gaseous planets so close to a solar type star implies that they formed quickly (as hydrogen and helium in a protoplanetary disk disappears after about 5 million years) and that they are in the process of losing their atmospheres, given the hot environment.
The proximity of the 5 planets almost certainly indicates that they formed further out, and later migrated to smaller orbits.
The mass of the sixth and outermost planet in the Kepler-11 system, with an orbital period of 118 days, is actually unknown, but it is thought to have dimensions similar to Uranus or Neptune.
Apart from individual cases such as this system, the most important contribution of Kepler is the number of extra-solar planets that it's discovering: 1235 planet candidates, 68 with dimensions similar to the Earth, with 5 of these having orbits that place them in the "habitable zone", where liquid water could be present.
If we consider for a moment that up until now this satellite has observed "only" 156,000 relatively close stars, confined to an area in the constellations of Cygnus and Lyra that covers only 1/400th of the entire sky, then we can conclude that our galaxy alone must contain millions of planets similar to our own.
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