SETI bioastro: Exoplanets Galore! (ESO PR 13/00)

From: Larry Klaes (lklaes@bbn.com)
Date: Thu May 04 2000 - 09:14:29 PDT


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Subject: Exoplanets Galore! (ESO PR 13/00)
Date: Thu, 04 May 2000 17:43:06 +0200
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Dear subscribers,

the Swiss 1.2-m Leonhard Euler Telescope at La Silla continues to
produce exciting discoveries of planets in orbits around other stars.
The latest news, together with an overview of the current situation and
future prospects in this dynamic research field are described in a new
Press Release, reproduced below. The original URL is:

http://www.eso.org/outreach/press-rel/pr-2000/pr-13-00.html

Kind regards,

The ESO EPR Dept.

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  ------------------------------------------------------------------------

             Information from the European Southern Observatory

 ESO Press Release 13/00

 4 May 2000 [ESO Logo]

 For immediate release
  ------------------------------------------------------------------------

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] ESO PR Photo Caption: A representation of the
                        12/00 sizes and shapes of the orbits of
                                         the eight new planetary and
 [Preview - JPEG: 400 x 242 pix - 76k] brown-dwarf candidates. The
 [Normal - JPEG: 800 x 483 pix - 184k] 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)

  ------------------------------------------------------------------------

Recent views of the Swiss 1.2-m Leonhard Euler Telescope at La Silla

  [ESO PR Photo 13a/00] ESO PR Photo Caption: Evening view of La
                         13a/00 Silla at the moment of
                                           "telescope start-up". The dome
 [Preview - JPEG: 400 x 335 pix - 30k] of the Swiss 1.2-m Leonhard
 [Normal - JPEG: 800 x 669 pix - 281k] Euler Telescope and the adjacent
 [High-Res - JPEG: 3000 x 2507 pix - building are seen in the
 2.8M] 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] ESO PR Photo Caption: Close-up of the dome of
                         13b/00 the Swiss 1.2-m Leonhard Euler
                                           Telescope at La Silla.
 [Preview - JPEG: 351 x 400 pix - 22k]
 [Normal - JPEG: 701 x 800 pix - 50k]
 [High-Res - JPEG: 2630 x 3000 pix -
 2.6M]

  [ESO PR Photo 13c/00] ESO PR Photo Caption: The Swiss 1.2-m
                         13c/00 Leonhard Euler Telescope in its
                                           dome at La Silla.
 [Preview - JPEG: 400 x 436 pix - 53k]
 [Normal - JPEG: 800 x 871 pix - 118k]
 [High-Res - JPEG: 3000 x 3265 pix -
 4.5M]

This telescope is named after the Swiss mathematician Leonhard Euler (1707 -
1783). An extensive biography is available on the web.

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