The intensive and exciting hunt for planets around other stars
(“exoplanets”) is continuing with great success in both
hemispheres.
Today, a team of astronomers of the Geneva Observatory [1] are announcing the discovery of no less than
eight new, very-low mass companions to solar-type stars. The masses of
these objects range from less than that of planet Saturn to about 15
times that of Jupiter.
The new results were obtained by means of high-precision
radial-velocity measurements with the CORALIE
spectrometer at the Swiss 1.2-m Leonhard Euler telescope at
the ESO La Silla
Observatory. An earlier account of this research programme is
available as ESO Press Release
18/98.
This observational method is based on the detection of changes
in the velocity of the central star, due to the changing direction
of the gravitational pull from an (unseen) exoplanet as it orbits the
star. The evaluation of the measured velocity variations allows to
deduce the planet’s orbit, in particular the period and the
distance from the star, as well as a minimum mass [2].
The characteristics of the new objects are quite diverse. While six
of them are most likely bona-fide exoplanets, two are
apparently very low-mass brown-dwarfs (objects of sub-stellar
mass without a nuclear energy source in their interior).
From the first discovery of an exoplanet around the star 51
Pegasi in 1995 (by Michel Mayor and Didier Queloz of the
present team), the exoplanet count is now already above 40.
“The present discoveries complete and enlarge our still
preliminary knowledge of extra-solar planetary systems, as well as the
transition between planets and “brown dwarfs”, say Mayor and
Queloz, on behalf of the Swiss team.
An overview of the new objects
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ESO [Preview – JPEG: 400 x 242 pix – |
Caption: A representation of the sizes and shapes of |
The sizes and shapes of the orbits of the eight new planets and
brown-dwarf candidates are illustrated in Photo 12/00. More
details about the individual objects are given below.
A sub-saturnian planet in orbit around HD 168746
HD 168746 is a quiescent solar-like star of type G5 in the
constellation Scutum (The Shield). It is slightly less massive than
the Sun (0.92 solar mass) and is located at a distance of about 140
light-years. The visual magnitude is 7.9, i.e. about six times too
faint to be seen with the unaided eye.
The Swiss team found a new planet that orbits this star every 6.4
days, a fairly short period. The orbit is circular and the deduced
minimum mass of the planet is only 80% of the mass of planet
Saturn. This is only the third exoplanet detected so far with a
possible sub-saturnian mass.
Two planets slightly more massive than Saturn around HD 83443 and
HD 108147
The planetary candidates detected around HD 83443 (visual
magnitude 8.2; in the constellation Vela – the Sail) and HD
108147 (7.0 mag; Crux – the Cross) also have very low minimum
masses, 0.35 and 0.34 times the mass of planet Jupiter, or 1.17 and
1.15 times that of Saturn, respectively.
The companion of HD 83443 is particularly remarkable, not only by
virtue of its low mass – it is also the exoplanet so far detected with
the shortest period (2.986 days) and the smallest distance to the
central star, only 5.7 million km (0.038 AU), i.e., 26 times smaller
than the Sun-Earth distance. HD 83443 is of type K0V, it is at a
distance of 141 light-years and is somewhat less massive than our Sun
(0.8 solar mass).
Most interestingly, a small change with time (a “drift”) of the
mean velocity variation of HD 83443 has been detected. This drift
suggests the possible existence of an additional low-mass companion;
earlier measurements show that it cannot be due to a more distant
stellar companion.
As for all other short-period exoplanets, this “Hot Saturn”
offers good chances for future observations of a planetary transit
across the disk of the central star, seen when the planetary orbit is
(nearly) perpendicular to the sky plane. Precise photometric
monitoring of the star has been conducted by a team of Danish
astronomers with their 50-cm telescope at La Silla, but has so far
failed to reveal any drop of the stellar luminosity.
The mass of HD 108147 (of type F9-G0V) is slightly above that of
the Sun (1.05 solar mass). The orbit of its low-mass companion is
surprisingly eccentric (e = 0.56), despite of its fairly short period
of 10.88 days. This star seems to be rather “young” (about 2,000
million years old); this is also corroborated by a comparatively high
rotational velocity and a moderate chromospheric activity level.
Three Jovian planets with longer periods around HD 52265 [3], HD 82943 and HD 169830
The deduced minimum masses, 1.07, 2.2 and 2.96 times the mass of
Jupiter, of the planetary companions to HD 52265 (6.3 mag; G0V;
Monoceros constellation – the Unicorn), HD 82943 (6.5 mag; G0;
Hydra – the Water-Snake), and HD 169830 (5.9 mag; F8V;
Sagittarius – the Archer), respectively, together with the orbital
eccentricities (0.38, 0.61 and 0.34) and periods (119, 443 and 230
days) for these systems are rather typical for exoplanets with
intermediate periods.
Whereas all giant planets in our own solar system (Jupiter, Saturn,
Neptune, Uranus) have nearly circular orbits, most of the extra-solar
planets that have been discovered with periods of months to years are
elongated. The origin of the elongated shape of those planetary orbits
is still under debate.
Two very low-mass brown-dwarf companions to HD 162020 and HD
202206
While about 40 giant exoplanet-candidates have so far been detected
with masses in the range from 0.22 to 8.13 times that of Jupiter, only
one companion object (in orbit around the star HD 114762) was known
until now with a minimum mass between 10 and 15 times that of Jupiter.
Such objects, referred to as “brown dwarfs”, are easier to
detect than giant planets with similar periods because their greater
mass induces larger velocity changes of the central star; they must
therefore be very rare. This strongly points towards different
formation/evolution processes for giant planets and stellar companions
in the brown-dwarf domain.
The brown-dwarf candidate around HD 162020 orbits this star
(in constellation Scorpius – the Scorpion; visual magnitude 9.1;
stellar type K2V) in 8.43 days on a moderately eccentric orbit. The
inferred minimum mass of the companion is 13.7 times that of
Jupiter.
The second brown-dwarf candidate has a comparable minimum mass of
14.7 Jupiter masses. It orbits HD 202206 (in constellation
Capricornus; visual magnitude 8.1; stellar type G6V) in 259 days and
the orbit is fairly eccentric.
The search for exoplanets: current status
Most of the stars around which giant planets have been found so far
show a significant excess of heavy elements in their atmosphere when
compared to the majority of stars of the solar vicinity. This is also
the case for most of the central stars of the eight new objects
described here. This additional indication of an abnormal chemical
composition of stars with giant gaseous planets provides a promising
line for a better understanding of the mechanism(s) that ultimately
lead to the formation of planetary systems.
The high-precision radial-velocity survey with CORALIE in
the southern hemisphere has the ambitious goal to make a complete
inventory of giant exoplanets orbiting about 1600 stars in our
galactic neighbourhood, all of which are relatively similar to our
Sun. To date, 11 such exoplanets have been detected by CORALIE
within this programme.
Up to now, a total of 43 low-mass companions to solar-type stars
have been detected by different research teams with minimum masses
less than 15 Jupiter masses. Of these, 34 have minimum masses smaller
than 5 Jupiter masses, 6 are between 5 and 10 Jupiter masses, and 3
are between 10 and 15 Jupiter masses.
This repartition of observed planetary masses (and low-mass brown
dwarfs) strongly suggests that the maximum mass for giant exoplanets
is less than 10 Jupiter masses.
Continuation of the programme
Significant progress within the current programme is expected soon,
when the Very Large
Telescope Interferometer (VLTI), now being constructed at
Paranal, will become available. This new instrument will have the
observational capability of very high-accuracy astrometry and thus to
detect even very small wobbles of stellar positions that are due to
orbiting planets. This will provide a crucial contribution to the
determination of the true repartition of exoplanetary masses, a hotly
debated question.
Important advancement in our understanding of the formation of
planetary systems is also expected with the advent of HARPS.
This new high-resolution spectrograph, capable of reaching a
radial-velocity precision of 1 m/sec, will be installed on the ESO
3.6-m telescope at La Silla. HARPS will extend the domain of
planets accessible with the radial-velocity technique towards
significantly lower masses – down to about ten Earth masses on
short-period orbits. It will also greatly improve our capability of
detecting planets with longer periods and multi-planet systems.
More information about this project
Further detailed information about these new planet candidates, as
well as the corresponding radial-velocity curves, are available on the
dedicated web page at the Geneva Observatory web site: http://obswww.unige.ch/~udry/planet/planet.html
Notes
[1] The team consists of Michel Mayor,
Dominique Naef, Francesco Pepe, Didier Queloz, Nuno Santos, Stephane
Udry and Michel Burnet (Geneva Observatory, Sauverny,
Switzerland).
[2] A fundamental limitation of the
radial-velocity method, currently used by all planet-hunting research
teams, is that because of the uncertainty of the inclination of the
planetary orbit, it only allows to determine a lower mass limit for
the planet. However, statistical considerations indicate that in most
cases, the true mass will not be much higher than this value. The mass
units for the exoplanets used in this text are 1 Jupiter mass = 3.35
Saturn masses = 318 Earth masses; 1 Saturn mass = 95 Earth masses.
[3] The exoplanet in orbit around HD 52265 was
independently announced last week by another group, cf.
(
http://www.physics.sfsu.edu/~gmarcy/planetsearch/planetsearch.html)
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