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An international team
co-led by a French researcher has just measured the mass of an extrasolar
planet orbiting around the star Gliese 876 (Gl 876). The team used the
Hubble Space Telescope (NASA-ESA) for the astrometric measurements, and
the 1.93 m telescope equipped with the Elodie spectrograph of the Observatoire
de Haute-Provence for the radial velocity measurements. The planet has
a mass in the range of 1.9 to 2.4 times the mass of Jupiter. It is the
second extrasolar planet to have its mass measured, the first one being
the planet orbiting around the star HD 209458.
It was in 1998 that a team of Franco-Swiss astronomers discovered the
presence of a planet around the star GI 876 (CNRS press release of June
28, 1998). The team was led by Thierry Forveille from the Laboratoire
d'Astrophysique de Grenoble (a joint research laboratory run by CNRS,
Observatoire de Grenoble, and Université Joseph Fourier) and included
Xavier Delfosse and Christian Perrier from the same laboratory, and Michel
Mayor from the Geneva Observatory. The observations were made using the
1.93 m telescope of the Observatoire de Haute - Provence (a service and
research unit run by CNRS and the Observatoire Astronomique de Marseille-Provence)
equipped with the Elodie spectrograph, and using the Swiss telescope from
the La Silla Observatory in Chile (ESO), equipped with the Coravel spectrograph.
This star has the particularity of being a red dwarf, three times less
massive than the Sun, and it is very close to us, at about fifteen light-years
away. The detected planet goes round the star in about 60 days, with a
mean radius of 0.2 times the distance between the Earth and the Sun, but
on an eccentric orbit. At that time, as with all planets detected by the
radial velocity method, only a minimum value for its mass was determined:
more than 1.5 times the mass of Jupiter. Since the star was a red dwarf
(the only red dwarf currently known that possesses planets), the ratio
between the mass of the star and its planet had to lie around 100. This
discovery was also made in parallel by a team of American astronomers
who, a year later, discovered a second planet that was smaller and that
to the star.
The team from Grenoble very quickly understood that, since this star was
very close to us and the ratio of the star mass to the planet mass was
very small, it was possible to use astrometry to compute the movement
of the star induced by the planet, and thus to determine the mass of the
planet. A team was then set up including American researchers specialized
in astrometric measurements using the Hubble Space Telescope, and two
other teams were set up for analyzing precisely the radial velocities
so as to improve the precision on the orbit, one of the teams being Franco-Swiss
and working at the Observatoire de Haute-Provence, and the other being
American and working with the Lick and Keck telescopes.
This collaboration both at team level and at observation resources level
has thus made it possible to deduce with precision the mass of the planet
orbiting around GI 876: in the range of 1.9 to 2.4 times the mass of Jupiter.
The hundred or so extrasolar planets discovered to date have been discovered
by using the radial velocity method which consists in measuring changes
in the velocity of the star due to the presence of a planet orbiting around
it. This method is extremely accurate, but it makes it possible to give
only the minimum threshold for planet mass. Measuring the first extrasolar
planet mass (for a planet orbiting around HD 209458) was possible only
because that planet has the particularity of passing in front of its star
and thereby causing eclipses. This is not the case with the planet orbiting
around GI 876, and measuring its mass relies on a combination of measuring
the radial velocity of GI 876 to the very high precision of 40 km per
hour, and of astronomic measurements of its movement in the sky of 1/10,000,000
of a degree (the angle at which a 2 euro coin could be seen on the Moon).
These precisions are the limits of what it is possible to obtain currently
with the Hubble Space Telescopes. Measuring the masses of only a very
few other planets will be possible with this instrument. However, within
three years from now, precisions that are 30 times better will be possible
with the Very Large Interferometer of the ESO, opening up huge prospects
for measuring the masses of a very large number of planets. That will,
in particular, make it possible to answer very many questions about the
way in which planets are formed.
For further information:
Server of the Hubble Space Telescope: http://oposite.stsci.edu/pubinfo/latest.html
Server of the Observatoire des Sciences de l’Univers de Grenoble:
http://osug.obs.ujf-grenoble.fr/actualites.shtml
To be published in Astrophysical
Journal Letters of December 20, 2002
Researcher
contact:
Xavier Delfosse
Observatoire de Grenoble.
Tel: +33 4 76 63 55 10.
e-mail: delfosse@obs.ujf-grenoble.fr
Thierry Forveille. Canada-France-Hawaii Telescope.
Tel. (in Hawaii): 00 1 808 885 31 60
e-mail: thierry.forveille@cfht.hawaii.edu
CNRS-INSU
contact:
Philippe Chauvin
Tel: +33 1 44 96 43 36
e-mail: Philippe.Chauvin@cnrs-dir.fr
Press contact :
Carine Noël
Tel : +33 1 44 96 49 88
Fax: +33 1 44 96 49 93
e-mail: carine.noel@cnrs-dir.fr
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