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Planet faces
cataclysmic showdown

Posted: December 16, 2009

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A giant planet in the danger zone around a pair of sparring stars has been discovered by Chinese astronomers, whose observations are published in this week’s edition of the journal Monthly Notices of the Royal Astronomical Society.

A schematic of the planet (the outermost object) orbiting the two stars of QS Virginis. Image: Shengbang Qian (National Astronomical Observatories, Chinese Academy of Sciences).

The binary system QS Virginis, consisting of a cool, small red dwarf star and a hot, dense white dwarf star – the naked core of a dead Sun-like star – is located 157 light years away in the constellation Virgo. The two stars are extremely close to one another, just 840,000 kilometres apart (the average distance of the Moon from Earth is just 384,403 kilometres, which shows how close these stars really are) and are slowly but surely spiralling towards one another. If they get much closer, hydrogen gas will begin streaming from the red dwarf onto the white dwarf. We call such pairs of stars ‘cataclysmic variables’ because once enough matter has been gathered up on the surface of the white dwarf, nuclear fusion reactions will ignite, releasing a cataclysmic amount of energy that causes a huge explosion on the whit dwarf’s surface. We see this explosion in the sky as what we call a nova.

The light curve of QS Virginis as the white dwarf moves in front of the red dwarf. White dwarfs are typically only about the size of a planet like Earth. By finding variations in the timing of these eclipses, the Chinese astronomers were able to deduce the existence of an accompanying planet. Image: Shengbang Qian (National Astronomical Observatories, Chinese Academy of Sciences).

This would be bad news for anyone living on the planet, which orbits around the two stars at a distance of 4.2 astronomical units (4.2 times the distance from Earth to the Sun) and is 6.4 times more massive than Jupiter, the biggest planet in our Solar System. The two stars are so close that they can’t be separated even in a telescope (we can tell what kinds of stars they are by looking at their light through a spectrometer) but we can see them eclipsing one another as they move around every three hours and 37 minutes. The planet’s presence is only betrayed by its gravitational pull on the stars, which causes the timings of the eclipses to be periodically delayed, or speeded up, depending on where the planet is in its orbit around the stars.

The planet is the first to be found around a pair of stars this close together, and the system as a whole is a rare example of a binary system prior to entering the cataclysmic variable phase. Professor Shengbang Qian, who led the research from the Yunnan Observatory in China, says, “For once we have advance notice of the formation of a cataclysmic variable and the chance to study what will happen to the planet in orbit around it. The scientific community should watch this system over the decades ahead.”