An international team of astronomers, including Saskia Hekker of the UvA, has discovered a rocky ‘super-Earth’ and a ‘hot Neptune’, which orbit the same star about 1200 light years from Earth. The result was published in Science Express on 21 June 2012.

The two planets move in orbit around the star Kepler-36a in the constellation Cygnus. The innermost planet, Kepler-36b is a rocky planet that is 1.5 times as large as Earth and 4.5 times as heavy. It orbits its star every 14 days at an average distance of about 17.5 million km. The outermost planet, Kepler-36c, is a gas or water planet that is 3.7 times larger than Earth and 8 times heavier. This 'hot Neptune' orbits over 19 million kilometers in 16 days. The two planets are 30 times more closely spaced than any adjacent pair of planets in the solar system ever found. The extrasolar planets are in practically the same area and their smallest distance between the two is 1.9 million kilometers, only five times the distance between Earth and the moon. Because Kepler-36c is much larger than the moon, this leads to a dramatic view from its smaller neighbour., Kepler-36b would be the size of our moon when observed from Kepler 36c.

Hekker’s role in the team was to conduct asteroseismic analysis of Kepler-36a , as a result of which the average density of the star was calculated. ‘Without the density of the star, it is not possible to determine the density of the planets,’ explains Hekker. NASA's Kepler telescope detects planets by measuring how much light a large group of stars emit. When a planet passes in front of its star, it causes a small dip in the amount of starlight. The large planet by Kepler-36a was already known. The small planet could be detected by applying a new algorithm to the data. The researchers deduce that the smaller planet is composed of 30% iron, 1% hydrogen and helium in the atmosphere, and not more than 15 percent water. The large planet probably has a rocky core, surrounded by a large amount of atmospheric hydrogen and helium. The density of the planets differs by a factor of 8, whereas their orbits only differ by 10%.

The composition of the two planets is difficult to explain, on the basis of the existing models for planet formation. In our own solar system, rocky planets are near the sun, while the gas giants are further away. The star itself is as heavy as the sun, but 25% closer, slightly warmer and contains less metals. The star is a few billion years older than the sun, and no longer burns hydrogen in its core. This means that it is almost approaching the giant star phase, and has a radius which is 60% larger than that of the sun. The team is now looking for more of these planetary systems in the Kepler data.