Planet discovered to go against the flow

By Natalie Muller 14 June 2011
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Astronomers have discovered a planet that moves in the opposite direction to the spin of its star.

ALL PLANETS MOVE in the same direction as the star around which they orbit, but Australian astronomer Dr Daniel Bayliss and his colleagues have found an exception.

Using one of the world’s largest telescopes, the HAT-South telescope at Las Campanas Observatory in northern Chile, the team found that a distant planet, Wasp-17b, moves in the opposite direction to the spin of its star.

The discovery throws into question accepted scientific theories about planetary movement.

“It was quite an exciting discovery,” says Daniel, from the ANU Research School of Astronomy and Astrophysics. “Here we have a very clear example of a planet moving backwards.”

Planets are formed from the protoplanetary disk – a mass of rotating dust, gas and matter surrounding a newly formed star. Prior to the discovery of Wasp-17b’s retrograde orbit, it was assumed that all the planets in a solar system orbited their star in a direction that corresponded to the star’s spin, as is the case in our own solar system

But Wasp-17b is different, and astronomers are at a loss to explain its unusual orbit.

Collision course

There are several theories that could shed light on the reason for Wasp-17b’s reverse movement, but Daniel says the complex measurements needed to study planetary rotations make it hard to ascertain exactly why this planet follows such a path.

“Perhaps there was a major collision, or a close encounter with another giant planet when they were forming billions of years ago, which threw it off balance and made it start orbiting backwards,” he explains.

Astronomers are unsure about how many planets are moving in the opposite direction to their host star. Although more than 500 extrasolar planets (planets outside our solar system) have been discovered, so far it has only been possible to measure the orbital direction of a handful of these.

Daniel estimates as many as 30 per cent of these planets could be orbiting their stars in a reverse direction, but he hopes to learn more about the phenomenon. Daniel is part of an international research team working on a project known as HAT-South.

The team uses a network of wide-field telescopes to search for – and monitor – extrasolar planets, by examining their host stars and looking for the characteristic dip in light that occurs when a planet passes in front of its star.

The HAT-South telescopes are located at Siding Spring Observatory in Australia, Las Campanas Observatory in Chile, and the Hess Site in Namibia. The researchers also aim to shed light on the atmospheric composition, density and temperature of faraway planets.

Wasp-17b is a “hot Jupiter planet” with a surface temperature of about 2000˚C, a mass about half that of Jupiter, and a year that is equivalent to about 3.5 Earth days.