Text with all links and the photo is available on the ESO Website at:


The intensive and exciting hunt for planets around other stars
(“exoplanets”) is continuing with great success in both hemispheres.

Today, an international team of astronomers from the Geneva Observatory
and other research institutes [1] is announcing the discovery of no less
than eleven new, planetary companions to solar-type stars, HD 8574, HD
28185, HD 50554, HD 74156, HD 80606, HD 82943, HD 106252, HD 141937, HD
178911B, HD 141937, among which two new multi-planet systems. The masses
of these new objects range from slightly less than to about 10 times the
mass of the planet Jupiter [2].

The new detections are based on measured velocity changes of the stars
[3], performed with the CORALIE spectrometer on the Swiss 1.2-m Leonard
Euler telescope at the ESO La Silla Observatory, as well as with
instruments on telescopes at the Haute-Provence Observatory and on the
Keck telescopes on Mauna Kea (Hawaii, USA).

Some of the new planets are unusual:

* a two-planet system (around the star HD 82943) in which one orbital
period is nearly exactly twice as long as the other — cases like
this (refered to as “orbital resonance”) are well known in our own
solar system;

* another two-planet system (HD 74156), with a Jupiter-like planet and
a more massive planet further out;

* a planet with the most elongated orbit detected so far (HD 80606),
moving between 5 and 127 million kilometers from the central star;

* a giant planet moving in an orbit around its Sun-like central star
that is very similar to the one of the Earth and whose potential
satellites (in theory, at least) might be “habitable”.

At this moment, there are 63 know exoplanet candidates with minimum masses
below 10 Jupiter masses, and 67 known objects with minimum masses below 17
Jupiter masses. The present team of astronomers has detected about half of

PR Photo 13a/01: Radial-velocity measurements of HD 82943, a two-planet

PR Photo 13b/01: Radial-velocity measurements of HD 80606, a star with a
planet in a very elongated orbit.

A major international effort

The discovery of eleven new exoplanets has resulted from three
high-precision radial-velocity surveys now searching for such objects:

* The CORALIE planet-search programme on La Silla [4], conducted by
astronomers of the Geneva Observatory [1]

* The ELODIE high-precision radial-velocity survey of solar-type stars at
the Haute-Provence Observatory (OHP/France) conducted by a Swiss-French
team, including the Geneva, Grenoble and Haute-Provence Observatories

* The G-dwarf project, an ELODIE-HIRES/Keck planet-search programme set
up by a team of astronomers from the Geneva Observatory, the Center
for Astrophysics (Cambridge, Mass., USA) and the Tel Aviv University
(Israel) [1]

The new results are the outcome of high-precision radial-velocity
measurements. This fundamental 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 [3].

Four of the new planets were detected from La Silla and three ELODIE
candidates were secured with CORALIE measurements. With the eleven new
discoveries, the CORALIE/ELODIE programmes have contributed to the detection
of about half (32) of the known (63) planetary candidates with minimum
masses below 10 Jupiter masses, or 36 out of 67 known objects with minimum
masses below 17 Jupiter masses [2].

Several unusual systems

Among the present detections, there are two new planetary systems (HD 82943
and HD 74156), each with two planets. They bring to six the number of known
multi-planet systems, four of which owe their detection to CORALIE/ELODIE
measurements. This demonstrates the outstanding role that comparatively
small telescopes can still play in modern astrophysics.

Detailed information about all of the new planets are available on the
dedicated web page at the Geneva Observatory web site:

Of the systems discovered at La Silla, two are quite unusual:

HD 82943: a “resonant” system

ESO PR Photo 13a/01

Caption: PR Photo 13a/01 shows the radial-velocity measurements of the
central star, 82493, in a two-planet system, as observed with the CORALIE
instrument at La Silla. The best-fit curve corresponds to expected
variations, caused by the planets described in the text. The abscissa
shows the date; the ordinate the velocity

The detection of the outer planet that orbits the star HD 82943 was
announced earlier (ESO Press Release 13/00), together with seven CORALIE
planet candidates at other stars. The follow-up observations at La Silla
soon revealed a departure from the previously determined orbit. The
accumulated measurements (PR Photo 13a/01) now allow the detection of a
second, inner planet in this system. Its orbital period (221 days) is about
half of that of the outer one (444 days). Future observations should confirm
the 1:2 ratio between the periods; this indicates a “resonance” that may
result from the gravitational interaction between the two planets. Similar
orbital resonances are known in the solar system, especially in case of the
minor planets (asteroids).

HD 28185: a giant planet in the “habitable” zone

With the exception of the planet iota Hor b (cf. ESO PR 12/99), circular
orbits among exoplanets have only been found for short-period systems,
contrary to what is the case for the giant planets in our own Solar System.
However, the orbit of the newly found planet near the sun-like star HD 28185
is very nearly circular and with a period of 385 days (close to 1 Earth
year), its distance from the star, 150.6 million km, is almost equal to the
distance betwen the Sun and the Earth (149.6 million km).

This new planet is therefore located in the “habitable zone” where
temperatures like those on the Earth are possible. Still, it is a giant,
gaseous planet (with a minimum mass of 3.5 times that of Jupiter, or about
1000 times that of the Earth) and thus an unlikely place for the development
of life. Nevertheless, it may be orbited by one or more moons on which a
more bio-friendly environment has evolved. The presence of natural
satellites (“moons”) around giant extra-solar planets is not a far-fetched
idea, just look at our own Solar System.

HD 80606: a giant planet in an extremely elongated orbit

ESO PR Photo 13b/01

Caption: PR Photo 13b/01 shows the radial-velocity measurements of the
star HD 80606 that hosts a planet in a very eccentric orbit.

A planet in an extremely elongated orbit around the star HD 80606 was found
in the frame of the ELODIE/Keck collaboration. The measured, very large
eccentricity (e = 0.93; PR Photo 13b/01) implies of factor of no less than
26 between the smallest and largest distance to the star. When the planet is
closest to the star, it is only a few stellar radii away (about 5 million

Continuation of the programme

Further progress within the current programme is expected soon, when the
Very Large Telescope Interferometer (VLTI) at Paranal becomes available, cf.
ESO PR 06/01. This new instrument will have the observational capability of
very high-accuracy positional measurements (astrometry) and thus be able to
detect even very small wobbles of stellar positions in the sky that are due
to the pull of 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 the extremely high radial-velocity
precision of 1 m/sec, will be installed on the ESO 3.6-m telescope at La
Silla at the end of 2002. 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

More information on these discoveries may be found in a Press Release from
the Tel Aviv University [http://wise-obs.tau.ac.il/news/hd80606.html] and
on the Geneva planet-search web page.


[1] The team consists of:

Geneva Observatory (Switzerland): Michel Mayor, Dominique Naef, Francesco
Pepe, Didier Queloz, Nuno C. Santos, Stephane Udry, Michel Burnet

Grenoble Observatory (France): Christian Perrier, Jean-Luc Beuzit
Haute-Provence Observatory (France): Jean-Pierre Sivan

Center for Astrophysics (Cambridge, Mass., USA): David Latham, Guillermo

Tel Aviv University (Israel): Tsevi Mazeh, Shay Zucker, G. Drukier

[2] The mass units for the exoplanets used in this text are 1 Jupiter mass =
318 Earth masses.

[3] 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.

[4] Earlier accounts of this research programme have been published as ESO
Press Release 18/98 and ESO Press Release 13/00. Views of the 1.2-m Leonard
Euler telescope and its dome at La Silla are available as PR Photos