A team of Chinese astronomers have discovered a giant planet close to the exotic binary star system QS Virginis.

Although dormant now, in the future the two stars will one day erupt in a violent nova outburst.

QS Virginis lies in the direction of the constellation of Virgo and is about 157 light years from the Sun. The system is made up of a cool red dwarf and a hot dense white dwarf (an object similar to the remnant that will be left behind by the Sun at the end of its life).

The two stars are just 840000 km (525000 miles) apart or about twice the distance from the Earth to the Moon. Being so close together they race around each other, taking just 3 hours and 37 minutes to complete each orbit. No Earth-based telescope can see them as separate stars, but as they move around the two stars successively eclipse one another, leading to a characteristic periodic dip in the brightness of the system.

In many close binaries, described as 'cataclysmic variables (CVs)', material flows from one star to its denser companion. The stars of QS Virginis are only slightly too far apart for this to happen and so are a 'hibernating' CV and relatively quiet at the moment.

Professor Qian's team looked closely at the way the light from QS Virginis fluctuated throughout each orbit. By timing the eclipses, they found that the duration of the orbit changed with the time of mid-eclipse periodically advanced or delayed.

This shift is explained by an unseen third object exerting a gravitational pull on the two stars, so that sometimes the light has to travel a little further and sometimes a little less to reach us. From their measurements, the Chinese team deduced that there is a giant planet, with a minimum of 6.4 times the mass of Jupiter, at an average distance from the stars of 4.2 times that from the Earth to the Sun (about 630 million km or 394 million miles).

The new world is the first to be found in orbit around a close pair of stars of this type. Although almost certainly uninhabited, a hypothetical observer there would see a pair of rather small 'Suns' - one red and one white appearing very close together in the sky.

But things will not look this way forever. The red dwarf star is being braked by the interaction between its stellar wind and magnetic field and is decelerating. As it loses energy it is moving closer to the white dwarf and sometime in the next few thousand years it will be near enough that hydrogen will start to be dragged off the red dwarf by its companion star.

"A schematic plot of the change in brightness of the QS Virginis system over a period of 80 minutes, where each measurement is represented by a white point. As the white dwarf moves behind the larger red dwarf star, its eclipse leads to a substantial dip in the observed brightness of the system. By timing these eclipses which take place every 3.4 hours, astronomers were able to deduce the presence of the unseen giant planet. (Credit: Shengbang Qian, National Astronomical Observatories, Chinese Academy of Sciences)"

Source: Royal Astronomical Society