Planetary system discovered around double stars

Two massive planets, which are perhaps the youngest ever discovered, have been found orbiting a binary star system, thanks to research by an international consortium including astronomers at the Universities of Sheffield and Warwick.

Observations taken over two decades have found that the binary system consists of two stars in a very close orbit, and contains two Jupiter-like planets. Due to the disturbing gravitational effects of a binary star system, researchers do not normally expect to find any planets in orbit around these binary systems.

The most massive star at the centre of the system is a very small but very hot white dwarf. The lighter star, which it jointly orbits, is a larger and cooler star with a mass only one tenth that of our Sun.

Whilst the new research has detected the planets, scientists are unsure exactly how the planets have formed. It is thought that the planets may have been born along with the original stars, but would have needed to survive a dramatic event one million years ago, when the primary star bulged into a red giant, causing the secondary star to be pulled into a very tight orbit and forcing the larger star to lose much of its material.

Alternatively, the planets may have been formed from the cast off material, making them the first planets discovered which formed from the remnants of the death of one of their parents, and also make them the youngest planets known.

When the white dwarf was a normal star, twice as massive as the Sun, it was further away from its companion. However upon expanding into a red giant, it increased its radius by more than 3,000 times, engulfing the other star and forcing it to plunge into the red giant, a process known as 'common envelope phase'. The resulting release of energy caused three-quarters of the red giant's mass to be lost, leaving only the hot core of the original star and a relatively unscathed companion.

For the planets, if they existed during this time, the dramatic loss of mass from the red giant would have also meant less gravity, which could have resulted in them being lost from orbit.

The planets were detected thanks to the fact that the Earth lies on the same plane as the binary system, allowing astronomers to see an 'eclipse' when, every three hours and seven minutes, the larger star moves in front of the smaller star, blocking its light. This acts as a highly precise clock, and by measuring changes in these timings, the researchers were able to detect the presence of the two extrasolar planets.

Astronomers from the Universities of Sheffield and Warwick and the UK Astronomy Technology Centre in Edinburgh, built ULTRACAM, an ultra-fast camera and one of the instruments used in the study. The camera, which is capable of taking and storing up to 500 red, green and blue images per second, was used to monitor eclipsing white dwarf binary stars, which resulted in many of the data points used in the study.

Astronomers are now routinely finding planets around a wide variety of stars, thanks to advances in the technological power of instruments and software. There are currently at least 490 known exoplanets. Very few of these planets, however, are in binary systems.

Professor Vikram Dhillon, from the University of Sheffield's Department of Physics and Astronomy, said: "If these planets were born along with their parent stars they would have had to survive a dramatic event a million years ago: when the original primary star bloated itself into a red giant, causing the secondary star to plunge down into the present very tight orbit, thereby casting off most of the original mass of the primary.

Planetary orbits would have seen vast disturbances. Alternatively, the planets may have formed very recently from the cast off material. Either way, in relatively recent times in astronomical terms this system will have seen a vast shock to the orbits of the stars and planets, all initiated by what is now the white dwarf at the heart of the system."

Dr Stuart Littlefair, from the University of Sheffield's Department of Physics and Astronomy, said: "More than half of all stars are in binaries, but we still have much to learn about how that affects the presence of planets around them. Since these planets may be very young, they could still be quite bright, which means we might be able to look at the light from them directly. It's a very exciting possibility."

Notes for Editors: The research was conducted by a consortium of researchers from the University of Sheffield and the University of Warwick, Georg-August-Universitat and Eberhard-Karls-Universitat in Germany, Universidad de Valparaiso in Chile, and the University of Texas in Austin in the USA.

ULTRACAM is used on the 8.2-m Very Large Telescope and 3.5-m New Technology Telescope in Chile, and the 4.2-m William Herschel Telescope on La Palma.

Observations for the study were also made remotely via the internet on the 1.2m MONET telescope in Texas. This instrument is operated by a consortium including the Georg-August-Universität Göttingen and the University of Texas at Austin and was funded by the Alfried Krupp von Bohlen und Halbach Foundation primarily in order to enable access to research-grade telescopes to high schools classrooms.

The figure of 490 exoplanets is correct to 16 September 2010, and is based on those listed in the 'Extrasolar Planets Encyclopaedia' . For more information, please see http://exoplanet.eu.

The study is published in an article titled 'Two planets orbiting the recently formed post-common envelope binary NN Serpentis': K. Beuermann et al., in Astronomy and Astrophysics

For further information please contact: Kyle Christie, Media Relations Assistant, on 0114 2229852 or email k.christie@sheffield.ac.uk