Researchers have discovered the first planet that orbits its star in the direction opposite to the star’s own spin. The planet, dubbed WASP-17b, is also the lowest density exoplanet known so far: it is only 6-14% as dense as the gas giant Jupiter, and with twice the volume of Jupiter it might also be the largest planet found to day.(ArXiv

Most planets orbit their star in the same direction as the star’s own spin – all are thought to have formed from the same initial cloud of gas and dust. But orbits can be tilted by the gravitational pull of passing objects – some asteroids’ and comets’ orbits are tilted so much that they end up orbiting the sun in the opposite direction, for example. Although planets with tilted orbits have been detected before, this is the first one that is titled so much (by 150%) that the planet actually orbits its star in the opposite direction.

“All the others have been going in more or less the right direction, just tilted at crazy angles,” says Andrew Collier Cameron of the University of St. Andrews in Scotland and co-author of the paper submitted to Astrophysical Journal, adding that the WASP-17b, which lies 1000 light years away from Earth “really throws the cat among the pigeons.” (New Scientist)

It remains unclear what tilted this planet’s orbit so much. Two contending theories are that a gravitational pull from either a passing object or a still-undetected companion star have caused the tilt (BBC).

Posted for Mico Tatalovic

Image: ESA/C. Carreau