WASP-17 (pictured) was another planet found to have a weird reverse orbit in a study published in mid-2009.
Credit: ESA
SYDNEY: Astronomers have found an extrasolar planet with an "outlandish orbit" that circles its star either backwards, or at an angle of around 90º to the orientation of the star's rotation.
Planets in our own Solar System orbit in the same plane and direction as the Sun's own rotation. This led astronomers to propose the 'nebula hypothesis' - whereby planets form from a flat, swirling disk of gas around a proto-Sun.
Now two teams of astronomers - one in Japan and the other in the U.S. - have independently discovered a planet about 1,000 light-years away, which orbits its star either in reverse or at an angle of more than 86º.
Predicted but never seen
Such objects have been predicted in models of Solar System formation, whereby a companion star or gravity from another planet knocks it out of orbit. However this strange phenomenon has never been observed until now.
The exoplanet, HAT-P-7b, is 1.8 times the mass of Jupiter and orbits a star about 1.5 times the mass of the Sun. Out of the more than 400 exoplanets discovered so far, only three are known to have misaligned orbits, but none as widely divergent from their Sun's orbital plane as this one.
Details of the discovery (made using Hawaii's Subaru Telescope) were published in October by both astronomers led by Norio Narita, from the National Astronomical Observatory of Japan in Tokyo, and a second team led by Joshua Winn from the Massachusetts Institute of Technology in Boston, USA.
Blue-shift
The teams calculated the distant planet's orbit by looking at how its transit affected the spectrum of light from its rotating star. As the surface of the star spins towards us, its light is blue-shifted (the light spectrum is shifted towards the blue end of the spectrum) due to the Doppler effect (where light is squashed or stretched depending on its motion towards or away from us).
The other side of the star spinning away from us is red-shifted, so astronomers expected to see a blue- then red-shift pattern. But because of the interference of the dark body of the transiting planet, this pattern is reversed in HAT-P-7's case.
"The extraordinary orbit of HAT-P-7b presents an extreme case for theories of planet formation and subsequent orbital evolution," write Winn and colleagues in their paper.
