A planet going the wrong way

By travelling backwards it’s pushing knowledge forwards

All planets move around their stars in the same direction as the star spins—at least that’s what we thought.

But now Australian National University astronomer Dr Daniel Bayliss and his colleagues have found that some planets break the mould.

Using one of the world’s largest telescopes in Chile, Daniel and his collaborators discovered that a distant planet, WASP-17b, is moving in the opposite direction to the spin of the star around which it orbits. The discovery throws traditional theories about how planets form around stars into doubt.

Daniel’s work is being presented for the first time to the public through Fresh Science, a communication boot camp for early career scientists held at the Melbourne Museum. Daniel is one of 16 winners from across Australia.

Planets form from the same disk of rotating material that gives birth to the star around which they move. So it has been assumed that any planets orbiting a star would be moving in the same direction as the star’s spin. This is certainly true in our own Solar System.

WASP-17b is quite different, Daniel says, and its backwards motion is somewhat of a mystery to scientists. “It is possible that the planet underwent a close encounter with another giant planet billions of years ago, which altered its orbit so much that it began orbiting backwards.”

It is not known what fraction of planets orbit their stars in this retrograde manner, but astronomers are now actively trying to monitor other distant planets to see how common it is.  If it were common, this would not bode well for the chances of life around other stars.  Close encounters between giant planets would most probably destroy any small Earth-like planet in that system, and wipe out any chance of life arising.

To date some 500 extra-solar planets have been discovered but we can only detect the orbital direction of a handful of them. Daniel is part of a project, called HAT-South, which is monitoring millions of stars in the southern hemisphere to see if they have orbiting planets. As part of this program, he runs a set of telescopes in Australia, the data from which is combined with those of identical sets of telescopes operated in Chile and Namibia.

This global enterprise should uncover dozens of new planets and reveal how common backwards movement is in our galaxy.

Daniel’s original paper is Confirmation of a Retrograde Orbit for Exoplanet WASP-17b, Daniel D. R. Bayliss, Joshua N. Winn, Rosemary A. Mardling, Penny D. Sackett,  Astrophysical Journal Letters 722:L224-L227, 2010. It is available online at http://adsabs.harvard.edu/abs/2010ApJ…722L.224B

Daniel Bayliss is one of 16 early-career scientists unveiling their research to the public for the first time thanks to Fresh Science, a national program sponsored by the Australian Government. The research was supported by several Australian government programs, by NASA and by the MIT Class of 1942.

Over the course of Fresh Science he is presenting his work

• At a Melbourne pub
• over dinner with Australia’s Chief Scientist in Melbourne
• to school students in Melbourne and Geelong.

For interviews contact Daniel Bayliss on daniel@mso.anu.edu.au

For Fresh Science contact Sarah Brooker on 0413 332 489 or Niall Byrne on 0417 131 977 or niall@scienceinpublic.com.au

For ANU contact James Giggacher on james.giggacher@anu.edu.au