European Southern Observatory: Six Extrasolar Planets Discovered
During the last 5 years, more than 40 exoplanets have been detected around
stars in the solar vicinity, but only one system with multiple planets has
been unambiguously identified — around the star Upsilon Andromedae. Today at the International Astronomical Union General Assembly in Manchester, UK, we report the discovery of a second extra-solar planetary system with two Saturnian planets around the star HD 83443.
The new system was detected by means of high-precision radial-velocity
measurements with the CORALIE high-resolution spectrograph at the 1.2-metre
Leonhard Euler Swiss telescope (ESO-La Silla) as part of the Geneva
planet-search project conducted by the CORALIE team.
HD 83443 is a fairly bright star of 8.2 visual magnitude, slightly less
massive than the Sun (0.8 solar masses). It lies the constellation Vela, and
is 141 light years away from the solar system.
The new planetary system around HD 83443 is unusual in more than one way,
because it has two very low mass gaseous giant planets, and both orbit very
close to the star.
Their main characteristics:
HD 83443 b. Period 2.9853 day; minimum mass (m2 sin i) 0.35 Jupiter masses;
eccentricity 0.08; semi-major axis (a) 0.038 astronomical units; K velocity
56 m/s.
HD 83443 c. Period 29.83 day; minimum mass (m2 sin i) 0.15 Jupiter masses;
eccentricity 0.42; semi-major axis (a) 0.17 astronomical units; K velocity
14 m/s.
This inner planet is the planet with the shortest period and with the
smallest separation from the parent star among the known candidates. Its
period expressed in sideral time is very close to an integer: 2.9935 sideral
days. As a consequence every 3 days almost the same radial velocity is
observed and several months are needed to fill in observations at different
phases of the orbit. After a whole season of measurements clumps of
observations are still present at three phases.
Early on, abnormal behaviour was detected in the residuals of the
short-period Keplerian solution showing an apparent periodicity of about one
month. New measurements added after the announcement of HD 83443 b have
fully confirmed the presence of the second planet. Interestingly, HD 83443 c
is the planet with the smallest projected mass discovered to date. The
system HD 83443 is thus composed of 2 planets close to the star and less
massive than Saturn:
- m2 sin i (HD 83443 b) 1.17 Saturn masses
- m2 sin i (HD 83443 c) 0.50 Saturn masses.
Looking at such a system with two planets on so close orbits immediately
raises concerns about the system’s dynamical stability. A preliminary
analysis of its dynamical behaviour clearly confirms that the system is
stable. Nevertheless, dynamical perturbations between the planets are far
from being negligible and in particular explain the non-circularity of the
inner orbit despite its very short period. This system once more emphasizes
the impressive diversity of extra-solar planets compared to our own solar
system.
New planets from both celestial hemispheres
In addition to the new planet HD83443c described above, we can also announce
new planetary candidates discovered in our planet-search programmes in both
hemispheres.
From the North: One additional planet has been detected from the
Haute-Provence Observatory around the star HD 190228. It has been
discovered by using the ELODIE spectrometer on the 1.93-metre telescope. In
addition to HD 190228b, the ELODIE programme has already revealed four
planetary companions including 51 Pegasi, the first extra-solar planet to be
detected, and one very low mass brown dwarf (m2 sini =19 Jupiter masses).
From the South: Four new planetary candidates have been detected around HD
6434, HD 19994, HD 92788 and HD 121504. As for the HD 83443 system, these
new planets have been discovered by the CORALIE team as a result of the
fruitful systematic radial-velocity survey carried out at La Silla
Observatory (ESO, Chile). In about two years, 19 planets and very low mass
brown dwarfs (m2 sin i less than or equal to 15 Jupiter masses} have already
been detected with the CORALIE spectrometer fed by the small 1.2-m Euler
Swiss telescope.
The main stellar characteristics as well as some orbital parameters and
inferred planetary properties for the 4 systems, rather similar to what was
observed for the previously detected planets, are summarized below:
HD 6434 – spectrum G3IV; distance 40.3 parsecs (131 light years); period
22.09 days; eccentricity 0.3; m2sin i (minimum mass) 0.48 Jupiter mases;
semimajor axis (a) 0.15 AU; site – ESO.
HD 19994 – spectrum F8V; distance 22.2 parsecs (72 light years); period
454.2 days; eccentricity 0.2; m2sin i (minimum mass) 1.8 Jupiter mases;
semimajor axis (a) 1.3 AU; site – ESO.
HD 92788 – spectrum G5; distance 32.3 parsecs (105 light years); period
340.8 days; eccentricity 0.36; m2sin i (minimum mass) 3.8 Jupiter mases;
semimajor axis (a) 0.94 AU; site – ESO.
HD 121504 – spectrum G2V; distance 44.4 parsecs (145 light years); period 64.62 days; eccentricity 0.13; m2sin i (minimum mass) 0.89 Jupiter mases; semimajor axis (a) 0.32 AU; site – ESO.
HD 190228 – spectrum G5IV; distance 62.1 parsecs (203 light years); period 1127 days; eccentricity 0.43; m2sin i (minimum mass) 5.0 Jupiter mases; semimajor axis (a) 2.3 AU; site – ESO.
To date, the ELODIE and CORALIE exoplanet searches have revealed 25
companions to solar-type stars with masses smaller than 20 Jupiter masses.
The quest for exoplanets is now revealing not only more and more candidates,
but also planets with smaller and smaller masses.
