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More planets found

Artist's impression of two Saturn-like planets orbiting very close to the star Kepler-9b.

Artist's impression of two Saturn-like planets orbiting very close to the star Kepler-9b.

Two planets, similar in size to Saturn, have been spotted by NASA’s Kepler space telescope moving in front of, or transiting, their star.

The transit technique has been used to detect many so-called exoplanets (ones that orbit stars beyond our Solar System), but this is the first confirmed detection via this method of two planets transiting the same star.

The star is known as Kepler-9, and the planets have been dubbed Kepler-9b and Kepler-9c.

Launched in early 2009, Kepler is focusing on over 150,000 candidate stars in the hunt for small, Earth-sized planets. In particular, the aim is to find Earth-sized planets in stars’ habitable zones…orbital slots that are neither too near nor too far from the stars, so that the planets are neither too hot nor too cold.

Kepler can’t see planets—they’re too small and too far away—but its sensitive instruments can see the small dip in starlight as a planet moves in front of its star. From the amount and duration of the dip, the size and orbit of each planet can be derived.

And if there are small deviations in the regularity of the dips, they could indicate the presence of other planets that aren’t on orbits that produce transits.

In June 2010, Kepler scientists published data on 700 potential planets seen in data collected in the first 43 days of the mission. A handful of the stars seemed to have more than one transiting planet.

“Kepler’s high quality data and round-the-clock coverage of transiting objects enable a whole host of unique measurements to be made of the parent stars and their planetary systems,” said Doug Hudgins, the Kepler program scientist at NASA Headquarters in Washington.

Artist's impression of NASA's Kepler space telescope

NASA's Kepler space telescope is dedicated to hunting for planets orbiting distant stars by spotting the dip in starlight as a planet moves in front, or transits, a star.

Scorching orbits

Follow up observations using the giant telescopes of the W.M. Keck Observatory in Hawaii have shown that both planets are slightly less massive than Saturn, and that Kepler-9b is the larger of the two.

The planets orbit very close to their star, which means their years are very short—just 19 Earth days in the case of Kepler-9b, and 38 days for Kepler-9c. Their close proximity to the star means the planets also would be very, very hot.

Seven months of observations have revealed slight variations in the timing of the two planet’s transits, which is exactly what would be expected as each planet gravitationally tugs on the other.

Their observations of the Kepler-9 system also have given the scientists tantalising hints that there could be another, much smaller planet in orbit around the star. The data suggest it could be about 1.5 times the size of Earth and orbiting scorchingly close to the star, taking just 1.6 Earth days to complete one orbit. This would make it a “hot Earth”.

The scientists still have work to do to confirm the presence of the third planet, though, as what can sometimes seem to be a transiting planet can turn out to be an unrelated phenomenon.

Story by Jonathan Nally, editor Images courtesy NASA.

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