Fwd; Eight New Very Low-Mass Companions to Solar-Type Stars Discovered at La Silla
tony...@myinternetuk.com
04 May 2000 16:56:52
sci.astro
From: ay...@nova.astro.utoronto.ca
Subject: Eight New Very Low-Mass Companions to Solar-Type Stars
Discovered at La Silla (Fo
To: sci.astro
Eight New Very Low-Mass Companions to Solar-Type Stars Discovered at
La
Silla (Forwarded)
ESO Education and Public Relations Dept.
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Text with all links and the photo is available on the ESO Website at
URL:
http://www.eso.org/outreach/press-rel/pr-2000/pr-13-00.html
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For immediate release: 4 May 2000
ESO Press Release 13/00
EXOPLANETS GALORE!
Eight New Very Low-Mass Companions to Solar-Type Stars Discovered at
La
Silla
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.
Recent
views of this telescope and its dome are available below as PR Photos
13a-c/00.
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
ESO PR Photo 12/00
Caption: A representation of the sizes and shapes of the orbits of
the
eight new planetary and brown-dwarf candidates. The colours indicate
the deduced minimum masses: about one Saturn mass or less (red);
between
1 and 3 Jupiter masses (green); above 10 Jupiter masses (blue). The
dashed line indicates the size of the Earth's orbit (radius 150
million
km).
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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Recent views of the Swiss 1.2-m Leonhard Euler Telescope at La Silla
ESO PR Photo 13a/00
Caption: Evening view of La Silla at the moment of "telescope start-
up".
The dome of the Swiss 1.2-m Leonhard Euler Telescope and the
adjacent
building are seen in the foreground, immediately to the right of the
ramp leading to the ESO 3.6-m New Technology Telescope (NTT) in its
octogonal enclosure.
ESO PR Photo 13b/00
Caption: Close-up of the dome of the Swiss 1.2-m Leonhard Euler
Telescope
at La Silla.
ESO PR Photo 13c/00
Caption: The Swiss 1.2-m Leonhard Euler Telescope in its dome at La
Silla.
This telescope is named after the Swiss mathematician Leonhard Euler
(1707 - 1783). An extensive biography is available on the web.
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