New extra-solar planet transits near-by star

5 October 2005

An international team of astronomers led by François Bouchy has announced the discovery of a new transiting extra-solar planet. The planet HD189733b, in the constellation Vulpecula, was detected and studied by the combination of two different methods, radial velocities and photometric transits, using the telescopes at Haute-Provence Observatory (OHP). It is one of the few extra-solar planets for which scientists have been able to accurately determine both its radius (1.26 Jupiter radii) and mass (1.15 Jupiter masses). Thus, and also due to its nearness, about 60 light-years from Earth, the extra-solar planet HD 189733b offers exciting new possibilities for follow-up studies.

The star HD189733 is shown by the green arrow. It is located less that 0.15° (about half the diameter of the Moon) from the M27 "Dumbell" planetary nebulae (photo by Daniel Jaroschik)

A new extra-solar planet has been found at Haute-Provence Observatory on September 15, 2005 by an international team of astronomers from the Laboratoire d'Astrophysique de Marseille (LAM), the Observatoire de Haute-Provence (OHP) and Observatoire de Genève. This discovery was made, using the Doppler method, with the ELODIE spectrograph at the 1.93-m telescope, the same one used ten years earlier to find the very first extra-solar planet 51 Peg b. Simultaneously, the CCD camera at the 1m20 telescope was used to detect the transit of the extra-solar planet across the disk of the star. This phenomenon can be explained by the favorable orientation of the system as seen from the Earth: the orbital plane is seen edge-on and the extra-solar planet crosses the disk of the star, occulting it partially. The scientific team has thus been able to derive both the exact mass and radius of the planet, concluding that it is a large "hot Jupiter". The planetary system in HD189733 is particularly interesting for several reasons:

• This new extra-solar planet joins the very exclusive group of planets outside our solar system that scientists have been able to measure precisely. Even if 160 extra-solar planets are presently known, accurate values for masses and radii are known only for 9 systems. HD189733b has a mass 365 times that of the Earth and a radius 14 times larger. Its density is comparable to that of Saturn. This extra-solar planet is located nearby, only 63 light-years away. The star HD189733 can be seen using binoculars (visual magnitude 7.7) in the constellation Vulpecula (The Fox). It just happens to be located by in the vicinity of the famous M27 "Dumbell" planetary nebula, well known to amateur astronomers.

• Its orbital period is one of the shortest known (only 2.2 days), nearly 2000 times faster than Jupiter, which takes 12 years to make a trip around the Sun. Due to the favorable orientation of its orbital plane, the planet HD189733b occults the central star every 2.2 days, producing a photometric transit (analogous to the Venus transit of the Sun seen in June 2004), a small decrease of flux lasting 2 hours.

• The transit of HD 189733b produces a decrease in luminosity of 3% which makes this system the one with the deepest partial occultation of the nine known transiting systems. This is due to the small size of the star (3/4 the size of the Sun) and the large size of the planet (1.26 times the size of Jupiter).

• The central star of this new planetary system is bright (V=7.7); most ground-based and space-borne telescopes will soon try to measure other interesting parameters, in particular its atmosphere. The small distance separating the planet from its star (only 3/100 of the Earth-Sun distance) implies that its atmosphere must be very hot, several hundred degrees. Attemps will be made to measure the reflectivity of its atmosphere, its chemical composition and the rate of evaporation. The light emitted by the planet itself may even be within reach of interferometry.

Reference: ELODIE metallicity-biased search for transiting Hot Jupiters : II. A very hot Jupiter transiting the bright K star HD189733, Bouchy et al., submitted to Astronomy and Astrophysics.

Radial velocities of the star HD189733 obtained with the ELODIE echelle spectrograph on the 1.93-m OHP telescope showing the presence of a planet 1.15 times heavier than Jupiter located at 0.03 AU from the star. Radial-velocities measure the gravitational pull on the star by the planet.
Differential photometric measurements of the star HD189733 obtained with the CCD camera on the 1.2-m telescope showing the transit of a planet 1.25 times the size of Jupiter.

The 1.2-m OHP telescope which was used to observe the photometric transit of HD189733. The 1.93-m dome, where the radial velocities were measured, is seen in the background.

F. Bouchy, Laboratoire d’Astrophysique de Marseille, Observatoire de Haute Provence
C. Moutou, Laboratoire d’Astrophysique de Marseille
N. Iribarne, S. Ilovaisky, Observatoire de Haute Provence
S. Udry, M. Mayor, F. Pont, R. Da Silva, D. Queloz, D. Segransan, Observatoire de Genève
N.C. Santos, Lisbon Observatory
S. Zucker, Weizmann Institute of Science, Israel

Contact points :

• François Bouchy
  Observatoire Astronomique Marseille Provence, (33) 1 44 32 80 79
• Claire Moutou
  Laboratoire d'Astrophysique de Marseille, (33) 4 91 05 59 66
• Stéphane Udry
  Observatoire de Genève, (41) 22 37 92 467
• Michel Mayor
  Observatoire de Genève, (41) 22 37 92 460

2, place Le Verrier
13248 Marseille cedex 4
France
Tél. : (+33) 4 95 04 41 00
Fax : (+33) 4 91 62 11 90
www.oamp.fr