Press Release

Turning planetary theory upside down

13-Apr-2010, Plain html version.

Royal Astronomical Society Press Release (jointly with the European Southern Observatory)

RAS PN 10/25 (NAM 10)
April 12th 2010
eso1016 – Science Release

Turning planetary theory upside down

Scientists have announced the discovery of nine new transiting exoplanets. When these new results were combined with earlier observations of transiting exoplanets astronomers were surprised to find that six out of a larger sample of 27 were found to be orbiting in the opposite direction to the rotation of their host star — the exact reverse of what is seen in our own Solar System. The new discoveries provide an unexpected and serious challenge to current theories of planet formation. They also suggest that systems with exoplanets of the type known as hot Jupiters are unlikely to contain Earth-like planets. The new research will be presented in a paper by Professor Andrew Collier Cameron of the University of St Andrews at the RAS National Astronomy Meeting (NAM 2010) in Glasgow on Tuesday 13th April and at a press briefing at 1100 BST on the same day.

“This is a real bomb we are dropping into the field of exoplanets,” says Amaury Triaud, a PhD student at the Geneva Observatory who, with Andrew Cameron and Didier Queloz, leads a major part of the observational campaign.

Planets are thought to form in the disc of gas and dust encircling a young star. This proto-planetary disc rotates in the same direction as the star itself, and up to now it was expected that planets that form from the disc would all orbit in more or less the same plane and that they would move along their orbits in the same direction as the star’s rotation. This is the case for the planets in the Solar System.

After the initial detection [1] of the transiting objects with the Wide Angle Search for Planets (WASP, [2]), the team of astronomers established their planetary nature and characterised the nine new exoplanets [3] using the HARPS and CORALIE spectrographs on the 3.6-metre ESO telescope and the Swiss Euler telescope at La Silla in Chile, the SOPHIE spectrograph at the Observatoire de Haute-Provence, and the FIES spectrograph on the Nordic Optical Telescope on the Canary Island of La Palma.

Surprisingly, when the team combined the new data with older observations they found that more than half of all the hot Jupiters [4] studied have orbits that are actually misaligned with the rotation axis of their parent stars. They even found that six exoplanets in this extended study (of which two are new discoveries) have retrograde motion: they orbit their star in the “wrong” direction.

“The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their star's spin,” says Andrew Cameron.

In the 15 years since the first hot Jupiters were discovered, their origin has been a puzzle. These are planets with masses similar to or greater than that of Jupiter, but that orbit very close to their suns. The cores of giant planets are thought to form from a mix of rock and ice particles found only in the cold outer reaches of planetary systems. Hot Jupiters must therefore form far from their star and subsequently migrate inwards to orbits much closer to the parent star. Many astronomers believed this was due to gravitational interactions with the disc of dust from which they formed. This scenario takes place over a few million years and results in an orbit aligned with the rotation axis of the parent star. It would also allow Earth-like rocky planets to form subsequently, but unfortunately it cannot account for the new observations.

To account for the new retrograde exoplanets an alternative migration theory suggests that the proximity of hot Jupiters to their stars is not due to interactions with the dust disc at all, but to a slower evolution process involving a gravitational tug-of-war with more distant planetary or stellar companions over hundreds of millions of years. After these disturbances have bounced a giant exoplanet into a tilted and elongated orbit it would suffer tidal friction, losing energy every time it swung close to the star. It would eventually become parked in a near circular, but randomly tilted, orbit close to the star. “A dramatic side-effect of this process is that it would wipe out any other smaller Earth-like planet in these systems,” says Didier Queloz of Geneva Observatory.

Two of the newly discovered retrograde planets have already been found to have more distant, massive companions that could potentially be the cause of the upset. These new results will trigger an intensive search for additional bodies in other planetary systems.

Nine publications on this research are being submitted to international journals this week, four of them using data from ESO facilities. And on Thursday 15th April the WASP consortium will receive the 2010 Royal Astronomical Society Group Achievement Award, also at NAM 2010.


Andrew Collier Cameron
University of St Andrews

Didier Queloz
Geneva Observatory, University of Geneva

Henri Boffin
ESO La Silla-Paranal/E-ELT Press Officer
Garching, Germany

NAM 2010 Press Office (12th – 16th April only)
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Dr Robert Massey*
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Royal Astronomical Society
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Anita Heward*
Press Officer
Royal Astronomical Society
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*contact for details of press briefing


[1] The current count of known exoplanets is 452.

[2] The nine newly found exoplanets were discovered by the Wide Angle Search for Planets (WASP). WASP comprises two robotic observatories, each consisting of eight wide-angle cameras that simultaneously monitor the sky continuously for planetary transit events. A transit occurs when a planet passes in front of its parent star, temporarily blocking some of the light from it. The eight wide-angle cameras allow millions of stars to be monitored simultaneously to detect these rare transit events. The WASP cameras are operated by a consortium including Queen’s University Belfast, the Universities of Keele, Leicester and St Andrews, the Open University, the Isaac Newton Group on La Palma and the Instituto Astrofisica Canarias.

[3] To confirm the discovery and characterise a new transiting planet, it is necessary to do radial velocity follow-up to detect the wobble of the host star around its common centre of mass with the planet. This is done with a worldwide network of telescopes equipped with sensitive spectrometers. In the northern hemisphere, the Nordic Optical Telescope in the Canary Islands and the SOPHIE instrument on the 1.93-metre telescope at Haute-Provence in France lead the search. In the south, the HARPS exoplanet hunter attached to the 3.6-metre ESO telescope and the CORALIE spectrometer on the Euler Swiss telescope, both at La Silla, were used to confirm the new planets and measure the angle through which each planet's orbit is tilted relative to its star's equator. The robotic Faulkes Telescopes of the Las Cumbres Observatory, located in Hawaii and Australia, provided the brightness measurements that determined the sizes of the planets. Follow-up observations of WASP exoplanet candidates are obtained at the Swiss Euler Telescope at La Silla, Chile (in collaboration with colleagues at Geneva Observatory), at the Nordic Optical Telescope on La Palma, and at the 1.93-metre telescope of the Observatoire de Haute-Provence in France (in collaboration with colleagues at the Institut d'Astrophysique de Paris and the Laboratoire d'Astrophysique de Marseille).

The studies of the orbital tilt angles of the WASP planets were made with the HARPS instrument on the ESO 3.6-metre telescope and with the CORALIE instrument on the Euler Swiss telescope, both at La Silla in the southern hemisphere, and at Tautenburg Observatory, McDonald Observatory and the Nordic Optical Telescope in the northern hemisphere.

[4] Hot Jupiters are planets orbiting other stars that have masses similar to, or greater than, that of Jupiter, but that orbit their parent stars much more closely than any of the planets in our own Solar System. Because they are both large and close they are easier to detect from their gravitational effect on their stars and also more likely to transit the disc of the star. Most of the first exoplanets to be found were of this class.


ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope; the world’s most advanced visible-light astronomical observatory and VISTA, the world’s largest survey telescope. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Exoplanet media kit:

The WASP page is at


These are available from:

A – Artist’s impression of an exoplanet in a retrograde orbit.

Up to now it was expected that exoplanets would all orbit in more or less the same plane, and that they would move along their orbits in the same direction as the star’s rotation — as they do in our Solar System. However, new results unexpectedly show that many exoplanets actually orbit at a large angle to their star’s spin axis. In the case shown here (WASP 8b) the orbit is completely reversed, or retrograde. Full sized version.

Credit: ESO/L. Calçada

B – Gallery of exoplanets with retrograde orbits.

Exoplanets, discovered by WASP together with ESO telescopes that unexpectedly have been found to have retrograde orbits are shown here. In all cases the star is shown to scale, with its rotation axis pointing up and with realistic colours. The exoplanets are shown during the transit of their parent star, just before mid-transit. The last object at the lower right is for comparison and has a “normal” orbital direction. The size of each image is three solar diameters. Full sized version.

Credit: ESO/A. C. Cameron


Link to HD movie

A – Artist’s impression of an exoplanet WASP 8b in a retrograde orbit.

Up to now it was expected that exoplanets would all orbit in more or less the same plane, and that they would move along their orbits in the same direction as the star’s rotation — as they do in our Solar System. However, new results unexpectedly show that many exoplanets actually orbit at a large angle to their star’s spin axis. In the case shown here (WASP 8b) the orbit is completely reversed, or retrograde. The star spins in one direction (towards the right as seen here) and the planet orbits the opposite way (towards the left). The speed of motion is greatly increased in this animation.

Credit: ESO/L. Calçada



The RAS National Astronomy Meeting 2010 will take place from 12-16th April at the University of Glasgow. The conference is held in conjunction with the UK Solar Physics (UKSP) and Magnetosphere Ionosphere and Solar-Terrestrial Physics (MIST) meetings. NAM2010 ( is principally sponsored by the Royal Astronomical Society (RAS) and the University of Glasgow.


The Royal Astronomical Society (RAS:, founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognises outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.


The University of Glasgow (founded 1451) is one of the world’s top 100 research universities with more than 70 per cent of its research rated as world-leading or internationally excellent. The Physics and Astronomy Department is one of the top four in the UK’s major research-intensive universities, the Russell Group.

The conference comes to Glasgow during the 250th anniversary year of the founding of the Regius Chair of Astronomy at the University of Glasgow, first held by astronomer and meteorologist Alexander Wilson in 1760. The present incumbent is Prof. John Brown, 10th Astronomer Royal for Scotland.