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A planet on a very oblique orbit?

The trajectory of a planet is generally located in the same plane as the equator of the star around which it orbits. This is the case for the planets of the Solar system as for the majority of known extra-solar planets. A recently detected planet, named XO-3b, could however be on a very oblique orbit, which would make it pass almost over the poles of its star. This large obliquity was measured thanks to recent observations at the Haute-Provence Observatory by a team of European astronomers. This result, to be confirmed by further observations, could be the signature of a particular past event during the life of this planet, such as a strong gravitational interaction with another star.

The Earth and the other planets of the Solar system turn around the Sun on orbits all located in about the same plane. This orbital plane is the same as the equatorial plane of the Sun, determined by the Sun's rotation. In other words, the rotation axis of the Sun is very close to the revolution axes of the planets; all of these axes are aligned, within a few degrees. It is thought that the planets kept the shape of the original disc of gas and dust in which it formed, nearly five billions years ago.

Approximately 300 planets are known today around stars other than our Sun. About fifty of these extra-solar planets have by chance an orbit aligned with the Earth, such that the planet can passes directly in front of its star when viewed from the Earth, causing a mini-eclipse called "transit". Transiting planets enable detailed studies, and in particular to measure the obliquity of the orbits, i.e. the angle between the rotation axis of a star and the revolution axis of a planet around its star. This measurement is done thanks to the Rossiter-McLaughlin effect, from the name of the two astronomers who predicted it in the Twenties.

The obliquity of the orbits has been measured for a dozen extra-solar planets within the last few years, utilizing this method. For each of them, the orbit of the planet has been measured to be coplanar with the equator of star. That confirms that, as for the Solar system, the other planetary systems were probably formed in flat proto-planetary discs and have retained their alignment to this day.

However, a team of European astronomers has just revealed a possibly very-oblique orbit for the first time, in the case of the transiting extra-solar planet XO-3b, discovered in 2007. Located 850 light-years away in the constellation of the Giraffe, XO-3b is a planet 12 times more massive than Jupiter, the largest planet in the Solar system. Being very close to its star, the XO-3b planet makes a complete orbit in just three days, causing frequent transits. One of these transits was observed at the beginning of this year with the SOPHIE instrument, installed on the 193-cm telescope of the of Haute-Provence Observatory. This observation showed the typical signature of an almost polar orbit.

SOPHIE is the new spectrograph that has replaced ELODIE two years ago, the famous instrument that allowed in 1995 the discovery of the first extra-solar planet, by astronomers of the Observatory of Geneva. SOPHIE allows accurate measurements of stellar radial velocities. It is in particular used by a team gathering of the researchers of Institut d'Astrophysique de Paris (CNRS, University Pierre & Marie Curie) and of the observatories of Marseilles-Provence, Geneva and Grenoble. The team has been carrying out a program of research and characterization of extra-solar planets around different types of stars.

The team measured the obliquity of the orbit of the XO-3b planet. This result has been published this month in the review Astronomy & Astrophysics. The result should be confirmed with further observations, as the end of the observation has been made under unfavorable conditions, with the observed star was low on the horizon. New transits of this planets are planning to be observed by this team with SOPHIE, and likely by other teams with other instruments around the world. If the large obliquity of the orbit of XO-3b were confirmed, it would be the first case of non-aligned orbit for a planet. This surprising configuration could be the signature of a particular past event in the life of this planet since its creation in a proto-planetary disc. For example, it could have undergone the gravitational interaction of another body (star or planet), which would have made it leave the original plane of the system. Many theorists use computer simuations to model such events. This observation put tight constraints on such models, allowing this type of phenemena to be better understood.

Note : This result, announced in mid-2008 (arXiv:0806.0719), was confirmed in early-2009 by an American team (arXiv:0902.3461).

 
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Contacts :

Guillaume Hébrard & François Bouchy, Institut d'Astrophysique de Paris, CNRS, Université Pierre et Marie Curie (Tel : (33-1) 44 32 80 78 / 80 79 ; Fax : (33-1) 44 32 80 01 ; E-mail : hebrard@iap.fr / bouchy@iap.fr).

Figures:


The 193-cm Telescope at the Haute-Provence Observatory

This telescope made possible to detect and characterize many extrasolar planets, with the instrument ELODIE, and now with SOPHIE.


The SOPHIE spectrograph.

Set up at the summer 2006 at the focus of the 193-cm telescope at the Haute-Provence Observatory, the SOPHIE spectrograph was used to measure the obliquity of the orbit of the extra-solar planet XO-3b.

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Schematic view of the orbit of the XO-3b planet, as seen from the Earth.

The XO-3b planet could have an oblique orbit, which makes it orbiting almost over the top of the poles of its star.


September 2008