A team of scientists has found a new planet which orbits the wrong way around its host star. The planet, named WASP-17, and orbiting a star 1000 light years away, was found by the UK’s WASP project in collaboration with Geneva Observatory. The discovery, which casts new light on how planetary systems form and evolve, is being announced today (12th August) in a paper submitted to Astrophysical Journal.

Since planets form out of the same swirling gas cloud that creates a star, they are expected to orbit in the same direction that the star spins. Graduate students David Anderson, of Keele University, and Amaury Triaud, of Geneva Observatory, were surprised to find that WASP-17 is orbiting the wrong way, making it the first planet known to have a “retrograde” orbit. The likely explanation is that WASP-17 was involved in a near collision with another planet early in its history.

WASP-17 appears to have been the victim of a game of planetary billiards, flung into its unusual orbit by a close encounter with a “big brother” planet. Professor Coel Hellier, of Keele University, remarks: “Shakespeare said that two planets could no more occupy the same orbit than two kings could rule England; WASP-17 shows that he was right.”

David Anderson added “Newly formed solar systems can be violent places. Our own moon is thought to have been created when a Mars-sized planet collided with the recently formed Earth and threw up a cloud of debris that turned into the moon. 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.”

The first sign that WASP-17 was unusual was its large size. Though it is only half the mass of Jupiter it is bloated to nearly twice Jupiter’s size, making it the largest planet known.

Astronomers have long wondered why some extra-solar planets are far bigger than expected, and WASP-17 points to the explanation. Scattered into a highly elliptical, retrograde orbit, it would have been subjected to intense tides. Tidal compression and stretching would have heated the gas-giant planet to its current, hugely bloated extent. “This planet is only as dense as expanded polystyrene, seventy times less dense than the planet we’re standing on”, notes Prof. Hellier.

Professor Keith Mason, Chief Executive of the Science and Technology Facilities Council, which funded the research, said, “This is a fascinating new find and another triumph for the WASP team. Not only are they locating these far flung and mysterious planets but revealing more about how planetary systems, such as our own Solar System, formed and evolved. The WASP team has proved once again why this project is currently the World’s most successful project searching for transiting exoplanets.”

WASP-17 is the 17th new exoplanet (planet outside our solar system) found by the Wide Area Search for Planets (WASP) consortium of UK universities. The WASP team detected the planet using an array of cameras that monitor hundreds of thousands of stars, searching for small dips in their light when a planet transits in front of them. Geneva Observatory then measured the mass of WASP-17, showing that it was the right mass to be a planet. The WASP-South camera array that led to the discovery of WASP-17 is hosted by the South African Astronomical Observatory.

Notes for editors

Contacts
Prof. Coel Hellier
Keele University
ch@astro.keele.ac.uk
+44 (0)1782 734243
+44 (0)7817 182867

Julia Short
Press Officer
STFC
Tel: + 44 (0)1793 442 012
Email: Julia.short@stfc.ac.uk

Prof. Didier Queloz
Geneva Observatory
didier.queloz@unige.ch
+41 22 379 2477

Prof. Andrew Collier-Cameron
University of St. Andrews
Andrew.Cameron@st-and.ac.uk
+44-1334-463147

Prof. Don Pollacco
Queens University Belfast
d.pollacco@qub.ac.uk
+44 (0)2890 973512

Dr. Richard West
University of Leicester
rgw@astro.le.ac.uk
+44 (0)116 252 5206

Images:
Artist’s impression of two close extra-solar planets
http://www.scitech.ac.uk/resources/image/Lonely1.jpg
Credit: KASI/CBNU/ARCSEC

An artist’s impression of a transiting exoplanet
http://photojournal.jpl.nasa.gov/catalog/PIA10364
Credit: ESA C Carreau

An artist’s impression of a transiting exoplanet
http://www.scitech.ac.uk/PMC/PRel/images/ExoPlanet02.jpg
Credit: ESA C Carreau

http://imgsrc.hubblesite.org/hu/db/2008/41/videos/a/stills/3/image.jpg
Credit NASA/Hubble

Paper: The discovery paper for WASP-17 is available at http://arxiv.org/abs/0908.1553 and has been submitted to Astrophysical Journal. A forthcoming paper led by Amaury Triaud confirms the retrograde orbit and studies the orbital dynamics in greater depth.

The WASP Project (Wide Area Search for Planets) is the UK’s leading extra-solar planet detection program and includes Keele University, St. Andrews University, Queen’s University Belfast, Leicester University, the Open University, and the Isaac Newton Group. WASP collaborates with the Geneva Observatory, Switzerland, who have a world-leading programme of planet discovery using the Coralie and HARPS spectrographs.

Shakespeare’s Henry IV, Part 1, Act 5, Scene 4:
“Two stars keep not their motion in one sphere,
Nor can one England brooke a double reign,
Of Harry Percy, and the Prince of Wales.”

(The word “stars” referred to planets in the cosmology of Shakespeare’s time.)
Glossary:

“Transit”: Term for a planet passing in front of its star, blocking a tiny fraction of its light.

“Retrograde”: Term for an orbit in the opposite direction to the rotation of the star being orbited. All Solar System planets are in “prograde” orbits around our sun, orbiting in the same direction that the sun is spinning.

“Extra-solar planet”: A planet not in our Solar System, instead orbiting another star.

“Elliptical orbit”: While the planets in our solar system have orbits that are close to circular, planet orbits can also trace out long, thin ellipses.

“Tidal interactions”: Just as our moon pulls the earth’s oceans, causing tides, a star’s gravity would cause tidal bulges on any planet nearby.

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