Celestial treasure hunt: a very special planet in the Northern sky

The star HD 80606 in the Big Dipper is orbited by a planet on an extremely eccentric orbit, so eccentric that the distance to its host star varies from 20% of the Earth-Sun distance at periastron to 80% at apastron. The planet is a gas giant four times more massive than Jupiter, on a 111-day period orbit. It was discovered in 2001 at the Haute-Provence Observatory in France.

Last year, an American team has announced the detection of the occultation of the planet behind its host star. The discovery of this rare configuration caused a big stir in the exoplanet community. It was calculated that the planet also had about 1 chance in 10 to transit in front of its host star, which would allow astronomers to measure its size. At the predicted time, 14 February 2009, several teams worldwide turned their telescopes towards that star.

A European team of exoplanet specialists (1) has not missed this stellar Valentine's date. The planetary transit was detected both as a dip in the brightness of the star with an imaging telescope, and as a spectroscopic anomaly using the planet-hunting spectrograph, both at Haute-Provence Observatory. The next transit will happen on June 5, and may be observed by space telescopes.

The team realizes how lucky they have been: the probability for the orbit of that planet to be so perfectly aligned that it would transit across its star as seen from Earth was lower than 1%!

Planets that transit in front of their host star have a special importance to astronomers, because they can be studied in much more depth than the others. Their mass and size can be measured independantly, so that their composition can be inferred. Most transiting planets known to date are extremely close to their host stars, with periods generally smaller than 5 days. Bathed in intense stellar light, they are untypical planets. The planet of HD 80606, with its period of 111 days, is less affected by its star. Knowing the size of this planet is important to understand the influence of the stellar flux on the structure of giant planets. Because of its eccentric orbit, this planet has short white-hot summers, followed by long frozen winters. Thanks to the occultations and transits, some glimpses of its climate may soon be known.

Useful links:

The team members are:

Claire Moutou, LAM/OAMP, UMR 6110, CNRS, INSU & Université de Provence, [ ]

Guillaume Hébrard, Isabelle Boisse, Alfred Vidal-Madjar, IAP, [ ]

François Bouchy, IAP/OHP, [ ]

Anne Eggenberger, Xavier Bonfils, Xavier Delfosse, Morgan Desort, David Ehrenreich, Thierry Forveille, Anne-Marie Lagrange, Christian Perrier, LAOG, [ ]

Didier Gravallon, OHP

Frédéric Pont, Université d'Exeter, Grande-Bretagne, [ ]

Nuno Santos, Porto, Portugal [ ]

Christophe Lovis, Michel Mayor, Francesco Pepe, Didier Queloz, Stéphane Udry, Damien Ségransan, Obs de Genève, Suisse [ ]


(Up): Both domes of 120-cm (foreground) and 193-cm (background) telescopes at Observatoire de Haute Provence, with stars trailing in the western sky, during a long exposure.

(Bottom): both stars HD 80606 (right) and HD 80607 (left) observed together during the transit night, with the 120-cm telescope and camera.

The peculiar configuration of the extrasolar system HD80606; the transit observed on 14 February corresponds to the planet crossing the stellar disk, at a distance of 0.29 times the Earth-Sun distance.


Both measurements done at Observatoire de Haute Provence on 14 February: top, the lightcurve of the star, showing the dip due to the planet passing across the stellar disk; bottom, an equivalent measurement of this transit, obtained with the SOPHIE spectrograph.

February 2009