A planet going the wrong way

Jun 07, 2011
Image courtesy of NASA.

(PhysOrg.com) -- 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 a planet that breaks the mold.

Dr. Bayliss, from the Research School of Astronomy and Astrophysics, is one of 16 early-career scientists unveiling their research to the public at Fresh Science – a national program sponsored by the Australian Government.

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.

Planets form from the same disk of rotating material that gives birth to the star around which they move. So until now 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, Dr. Bayliss 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,” he said.

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.

At present, only a handful of distant planets are known, but Dr. Bayliss is part of a project, called HAT-South, which is monitoring millions of stars in the southern hemisphere to see if they have orbiting . 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.

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User comments : 7

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2.3 / 5 (3) Jun 07, 2011
"At present, only a handful of distant planets are known" ??? There are thousands of known planets outside our solar system. What year does the Australian National University think it is?
not rated yet Jun 07, 2011
Perhaps they mean 'in Southern skies' ??
not rated yet Jun 07, 2011
Maybe they are thinking that in the overall picture of the billions of stars in our galaxy there are only a handful of known planets. Or this thing was written a long time ago!
5 / 5 (2) Jun 07, 2011
"All planets move around their stars in the same direction as the star spinsat least thats what we thought."

This sounds like hyperbole to sell this story; at least they DO mention "It is not known what fraction of planets orbit their stars in this retrograde manner". Gravitational encounters between bodies in orbit around a star is an obvious mechanism (but a decidedly uncommon(?) affair). For a local example: http://en.wikiped...inclined
1 / 5 (2) Jun 07, 2011
Could a retrograde orbit be caused by stellar precession? That is, could the star occasionally flip over due to a highly eccentric axial precession (perhaps as a result of a long ago impact)?
not rated yet Jun 08, 2011
I'm thinking that an early invading meteor or comet trapped in an opposed orbit came in during the accretion phase, and at the end, a planet was born well out of line from the spin of the system.
5 / 5 (3) Jun 08, 2011
"At present, only a handful of distant planets are known" ??? There are thousands of known planets outside our solar system. What year does the Australian National University think it is?

I think the Australian National University are correct in their statement and, strictly speaking, it is you that are in error here.

There are to date 531 confirmed extra solar planets and over 1200 candidates located by the Kepler space telescope using the transit method of detection - notice the word candidates, these are not yet confirmed.

However, given the short time (i.e. less than 20 years) that astronomers have had the technology capable of regularly finding extra solar planets and the increasing numbers being found - especially those with masses approaching that of the earth - the odds are high that there are millions, if not billions, of planets just in our galaxy. So your assertion will probably in time be proven to be correct.

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