Scientists at the University of Birmingham are among an international team of astronomers which has today announced the discovery of an unusual planetary system circling a solar-type star 1,200 light years from Earth. The results, which are published in Science, come from observations made by the NASA Kepler space telescope.

The newly-found system contains two planets: Kepler 36b, a rocky planet 1.5-times the size of Earth which orbits the star every 14 days at an average distance of around 18 million kilometers; and Kepler-36c, a Neptune-like gaseous planet 3.7-times the size of the Earth which orbits once every 16 days at a distance of around 19 million kilometers.

What is so unusual about the system is that a rocky planet and a gaseous planet have been found so close together. In our own solar system, rocky planets (like Earth) lie much closer to the Sun than the larger gaseous planets (like Jupiter, Saturn and Neptune). Every 97 days the planets in the Kepler-36 system pass to within five Earth-Moon distances, meaning each would be clearly visible in its neighbour’s sky (Kepler-36c appearing several times larger from Kepler-36b than our own Moon appears from Earth). At closest approach immense gravitational tides are raised that distort both planets: Kepler-36b should experience violent quakes and widespread volcanism.

Accurate measurements of the host-star properties were crucial to determining the properties of the planets. This was made possible by asteroseismology, the study of the star by observation of its natural oscillations. The oscillations are due to sound waves trapped in the interior of the host star.

Birmingham scientist Professor Bill Chaplin, from the University of Birmingham's School of Physics and Astronomy, who led the analysis of the star, says: ‘We used the oscillations to measure the size, mass and age of the star to exquisite precision.’. He adds: ‘Without asteroseismology it would not have been possible to place such tight constraints on the properties of the discovered planets and on the dynamics of the system.’

Researchers are struggling to understand how these two very different worlds ended up in such close orbits. Dr. Josh Carter, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics and the lead author of the paper, said: ‘We found this one on a first quick look,’ adding:‘we’re now combing through the Kepler data to try to locate more.’

Kepler-36c was discovered by detectable transits of the host star. As the planet passes in front of the star, it blocks some of its light giving a characteristic dip, or “transit” signal. The dip repeats every 16 days, as the planet orbits the star. The transit signals from the smaller Kepler-36b were at first much harder to find, because the gravitational tugging of Kepler-36c meant the exact timing of the transits varied from one orbit to the next.

The transit times of the larger planet are also affected by the mutual interactions of the planets. Professor Eric Agol, from the University of Washington, who co-led the work, comments: "If you look at the transit time pattern for the large planet and the transit time pattern for the smaller planet, they are mirror images of one another.’

From the transit-time variations of the Neptune-like planet the team was able to infer the presence of the rocky planet before its smaller (and hence harder-to-uncover) transit signals were eventually found.


Notes to editors

* Kepler-36: A pair of planets with neighboring orbits and dissimilar densities. Science Express.

An image is available, please contact the press office to arrange. Please credit image to: David Aguilar, CfA/Harvard".

For further information
Kate Chapple, Press Office, University of Birmingham, tel 0121 414 2772 or 07789 921164.