Scientists Detect "Wrong-Way" Planet

The Planetary Society
Article By Amir Alexander
August 12, 2009

An international team of scientists has detected the first extrasolar planet orbiting in the "wrong" direction. This means that the planet, designated WASP-17, is circling its star in a direction opposite to the rotation of the star itself. Such a motion, known as a "retrograde orbit," is very unusual since the motions of both star and planet were acquired from the swirling cloud of gas and dust that formed them both. As a result, the planets orbiting the same star almost always move in the same direction, which is the same as the rotation of the star itself.

A retrograde orbit is almost certainly a legacy of a planet's violent past, most likely dating to the planetary system's early days. "Newly formed solar systems can be violent places" explained graduate student David Anderson of Keele University, who is a member of the team that made the discovery. "A near-collision during the early, violent stage of this planetary system could well have caused a gravitational slingshot, flinging WASP-17 into its backwards orbit."

WASP-17 was first detected through the transit photometry technique by the Wide Area Search for Planets (WASP) consortium of British universities, using the WASP-South camera array in South Africa. But in order to detect its retrograde motion the WASP team needed an assist from planet hunters at the Geneva Observatory, who specialize in radial velocity measurements.

According to Darin Ragozzine of the Harvard-Smithsonian Center for Astrophysics astronomers can identify the direction of a planet's orbit because of slight discrepancies in the radial velocity data when a planet transits a star. Because a star is rotating, one side of it is moving towards (or away) from Earth faster than the other side. During a transit, the planet covers first one side of the star and then the other, causing a slight but measurable shift in the radial velocity readings. If during the transit the star first appears to be moving relatively slowly towards the Earth, but then faster as the transit progresses, then the planet is orbiting in the same direction as the star's rotation. But if the reverse is the case – as it is for WASP-17 – then the planet is in a retrograde orbit.

WASP-17 is located about 1000 light years from Earth, and is unusual not only because of the direction of its orbit but also because of its size and low density. Although its mass is only half that of Jupiter, its diameter is nearly twice that of our giant neighbor, which makes WASP-17 the largest known planet. The reason, according to Coel Hellier of Keele University, is related to the planet's unusual orbit. Retrograde motion coupled with a highly eccentric orbit subject the planet to intense tidal forces. Such tidal compression and stretching would have the effect of heating up the planet, causing it to expand to its current bloated dimensions. As a result, Hellier noted, the density of WASP-17 is only one seventieth (1/70) of the density of Earth.

Just as there are moons in retrograde orbits in our solar system, it stands to reason that there are also planets in retrograde orbits, and the discovery of WASP-17 did not therefore come as a complete surprise to planetary scientists. Nevertheless, this highly unusual planet does contribute to our understanding of the birth and life of planets, and adds one more member to the menagerie of strange and wonderful worlds astronomers are uncovering in the depths of space.