The Kepler space mission has discovered two new Saturn-sized planets and a possible third planet one and a half times bigger than Earth orbiting a star over 2,200 light years away in the constellation Lyra. The discovery also heralds the first time that it has been possible to measure the masses of planets using transit observations.

Launched in 2009, NASA’s Kepler spacecraft is designed to hunt for extra-solar planets around stars other than our Sun by watching for planets orbiting in front of their stars, and hence blocking some of the starlight. This is called a transit. One of the mission’s first discoveries to be announced is a system of two worlds orbiting the star designated ‘Kepler 9’. One of the planets orbits its star every 19.2 days at a distance of 20.9 million kilometres, and the other every 38.9 days at a distance of 33.8 million kilometres. If placed in our Solar System, both would fall within the orbit of our innermost planet, Mercury, which orbits the Sun every 88 days at a distance of 57.9 million kilometres.

Based on the magnitude of the transits, scientists have been able to judge the size of these worlds as being just slightly smaller than Saturn, but in this case they have been able to measure their masses. Ordinarily the only way to measure the mass of an extra-solar planet is to measure its radial velocity – this is the wobble its gravity imparts on its host star. However, planets can also have a gravitational influence over each other, and this becomes apparent in timing variations in the transits that Kepler is sensitive enough to detect. In other words, the two confirmed planets of the Kepler 9 system, Kepler 9b and 9c, are in a 2:1 orbital resonance, so that for every two orbits that the inner planet 9b makes, the outer plant 9c makes one. When the planets swing past each other, this gravitational resonance tugs on the planets, slightly speeding up or slowing down their transits. Based on the degree of these variations, it is possible to calculate their masses, and from their mass and radius it is possible to work out their density and hence their composition. Kepler 9b and 9c have masses of 0.25 Jupiter masses and 0.17 Jupiter masses respectively, and densities that match gas giant planets made primarily from hydrogen and helium.

The planets wound up in this 2:1 orbital resonance after they migrated from their birthplaces further out from the star, before settling into these stable orbits. Alycia Weinberger of the Carnegie Institution in Washington DC, suggests that this could provide insight into the history of this planetary system. “We can work back and figure out what the initial conditions were that led to how they are today, how long their migration lasted, and where they originally formed,” she says.

There are also signs that there is a third planet in the system. Kepler discovered what appears to be a small, rocky planet 1.5 times the diameter of Earth, orbiting even closer to the star with a period of 1.6 days at a distance of four million kilometres. At this stage the third planet has yet to be confirmed, but if it proves to be real then it shows that smaller planets can survive the migration of larger planets around them, says Weinberger. If confirmed, the third planet would be the smallest yet seen by the transit method.

Ultimately, the discovery is a milestone in exoplanet research. “This is the first discovery of multiple planets transiting the same star,” says Kepler’s Principal Investigator, William Borucki of the NASA Ames Research Center. It should be noted that it is not the first discovery of a multiple planet system, only the first by the transit method – radial velocity measurements have found several examples, including the recent five planets discovered around a star by the HARPS radial velocity spectrograph at the European Southern Observatory (see here for more). However, it is impossible to measure the diameter of a planet using the radial velocity method. Now that Kepler has proven it is possible to measure planetary diameters and masses with transits, it will allow many more worlds to be fully characterised.

Intriguingly, the transit timing variations may also enable Kepler to discover additional planets that cannot be seen transiting (because the angle of their orbit is too great to cross the face of their star’s disc), but which still impart a gravitational effect on the planets that do transit. This is similar to how the existence of Neptune was predicted from its gravitational interaction with Uranus. Indeed, alien observers on a distant star watching the transits of the planets in our own Solar System would also see timing variations.

The existence of Kepler 9b and 9c was confirmed by the Keck I telescope on Mauna Kea in Hawaii, which helped to rule out false positives such as variations in the luminosity of the star. In total, ten ground-based telescopes are working overtime to try and confirm the 700 candidate planets discovered by Kepler during the first 43 days of its mission. Regarding the potential third planet in the Kepler 9 system, says Borucki: “Our hope is that in the coming days or weeks we will be able to be more definitive.”