ESO HARPS Instrument Discovers Smallest Ever Extra-Solar Planet (Forwarded)

ESO Education and Public Relations Dept.

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Contacts:

Nuno Santos
Centro de Astronomia e Astrofisica da Universidade de Lisboa
Portugal
Phone: +351 21 361 67 43
Email:

François Bouchy
Laboratoire d'Astrophysique de Marseille
Marseille, France
Phone: +33 4 91 05 59 00
Email:

Sylvie Vauclair
Laboratoire d'Astrophysique, Observatoire Midi-Pyrénées
Toulouse, France
Phone: +33 5 61 33 29 50
Email:

Didier Queloz
Observatoire Astronomique de l'Université de Genève
Sauverny, Switzerland
Phone: +41 22 755 26 11
Email:

Michel Mayor
Observatoire Astronomique de l'Université de Genève
Sauverny, Switzerland
Phone: +41 22 755 26 11
Email:

For immediate release: 25 August 2004

ESO Press Release 22/04

Fourteen Times the Earth

ESO HARPS Instrument Discovers Smallest Ever Extra-Solar Planet

Summary

A European team of astronomers [1] has discovered the
lightest known planet orbiting a star other than the sun
(an "exoplanet").

The new exoplanet orbits the bright star mu Arae located in
the southern constellation of the Altar. It is the second
planet discovered around this star and completes a full
revolution in 9.5 days.

With a mass of only 14 times the mass of the Earth, the new
planet lies at the threshold of the largest possible rocky
planets, making it a possible super Earth-like object.
Uranus, the smallest of the giant planets of the Solar System
has a similar mass. However Uranus and the new exoplanet
differ so much by their distance from the host star that
their formation and structure are likely to be very
different.

This discovery was made possible by the unprecedented
accuracy of the HARPS spectrograph on ESO's 3.6-m telescope
at La Silla, which allows radial velocities to be measured
with a precision better than 1 m/s. It is another clear
demonstration of the European leadership in the field of
exoplanet research.

PR Photo 25a/04: The HARPS Spectrograph and the 3.6m
Telescope
PR Photo 25b/04: Observed Velocity Variation of mu Arae
(3.6m/HARPS, 1.2m Swiss/CORALIE, AAT/UCLES)
PR Photo 25c/04: Velocity Variation of mu Arae Observed by
HARPS (3.6m/HARPS)
PR Photo 25d/04: "Velocity Curve" of mu Arae

A unique planet hunting machine

ESO PR Photo 25a/04

The HARPS Spectrograph and the 3.6m Telescope

Caption: ESO PR Photo 25a/04 represents a montage of the
HARPS spectrograph and the 3.6m telescope at La Silla. The
upper left shows the dome of the telescope, while the upper
right illustrates the telescope itself. The HARPS
spectrograph is shown in the lower image during laboratory
tests. The vacuum tank is open so that some of the high-
precision components inside can be seen.

Since the first detection in 1995 of a planet around the star
51 Peg by Michel Mayor and Didier Queloz from the Geneva
Observatory (Switzerland), astronomers have learned that our
Solar System is not unique, as more than 120 giant planets
orbiting other stars were discovered mostly by radial-velocity
surveys (cf. ESO PR 13/00, ESO PR 07/01, and ESO PR 03/03).

This fundamental observational method is based on the detection
of variations 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 continued quest for exoplanets requires better and better
instrumentation. In this context, ESO undoubtedly took the
leadership with the new HARPS spectrograph (High Accuracy
Radial Velocity Planet Searcher) of the 3.6-m telescope at the
ESO La Silla Observatory (see ESO PR 06/03). Offered in October
2003 to the research community in the ESO member countries,
this unique instrument is optimized to detect planets in orbit
around other stars ("exoplanets") by means of accurate (radial)
velocity measurements with an unequalled precision of 1 metre
per second.

HARPS was built by a European Consortium [3] in collaboration
with ESO. Already from the beginning of its operation, it has
demonstrated its very high efficiency. By comparison with
CORALIE, another well known planet-hunting optimized
spectrograph installed on the Swiss-Euler 1.2-m telescope at La
Silla (cf ESO PR 18/98, 12/99, 13/00), the typical observation
times have been reduced by a factor one hundred and the
accuracy of the measurements has been increased by a factor
ten.

These improvements have opened new perspectives in the search
for extra-solar planets and have set new standards in terms of
instrumental precision.

The planetary system around mu Arae

The star mu Arae is about 50 light years away. This solar-like
star is located in the southern constellation Ara (the Altar)
and is bright enough (5th magnitude) to be observed with the
unaided eye.

Mu Arae was already known to harbour a Jupiter-sized planet
with a 650 days orbital period. Previous observations also
hinted at the presence of another companion (a planet or a
star) much further away.

The new measurements obtained by the astronomers on this
object, combined with data from other teams confirm this
picture. But as François Bouchy, member of the team, states:
"Not only did the new HARPS measurements confirm what we
previously believed to know about this star but they also
showed that an additional planet on short orbit was present.
And this new planet appears to be the smallest yet discovered
around a star other than the sun. This makes mu Arae a very
exciting planetary system."

"Listening" to the star

ESO PR Photo 25b/04 ESO PR Photo 25c/04


Observed Velocity Variation Velocity Variation of mu Arae
of mu Arae Observed by HARPS

Captions: ESO PR Photo 25b/04 shows the measurements of the
radial velocity of the star mu Arae obtained by HARPS on the
ESO 3.6m telescope at La Silla (green triangles), CORALIE on
the Swiss Leonhard Euler 1.2m telescope also on La Silla (red
dots) and UCLES on the Anglo-Australian Telescope (blue
circles). The solid line shows the best fit to the
measurements, assuming the existence of two planets and an
additional long-period companion. The fact that the line
happens to have a given width is related to the existence of
the newly found short period planet. The data shown span the
interval from July 1998 to August 2004. ESO PR Photo 25c/04
illustrates the high-quality radial velocity measurements
obtained with HARPS. Here also, the solid line shows the best
fit to the measurements, assuming the existence of two
planets. The data were obtained over a time span of 80 days
and the first points shown are the data from the 8 nights in
June. Note that the full span of the vertical axis is only
40 m/s! Error bars indicate the accuracy of the measurements.
The lower part of the diagram displays the deviation of the
measurements from the best fit.

ESO PR Photo 25d/04

Observed Velocity Variation of mu Arae

Caption: ESO PR Photo 25d/04 displays the HARPS radial
velocity measurements phase-folded with the orbital period
of the newly found exoplanet (9.5 days). The measurements
have been corrected from the effect of the two longer period
companions. The semi-amplitude of the curve is less than
5 m/s! Coupled with the 9.5 days orbital period, this implies
a minimum mass for the newly discovered planet of 14 times
the mass of the Earth.

During 8 nights in June 2004, mu Arae was repeatedly observed
and its radial velocity measured by HARPS to obtain information
on the interior of the star. This so-called astero-seismology
technique (see ESO PR 15/01) studies the small acoustic waves
which make the surface of the star periodically pulsate in and
out. By knowing the internal structure of the star, the
astronomers aimed at understanding the origin of the unusual
amount of heavy elements observed in its stellar atmosphere.
This unusual chemical composition could provide unique
information to the planet formation history.

Says Nuno Santos, another member of the team: "To our surprise,
the analysis of the new measurements revealed a radial velocity
variation with a period of 9.5 days on top of the acoustic
oscillation signal!"

This discovery has been made possible thanks to the large
number of measurements obtained during the astero-seimology
campaign.

From this date, the star, that was also part of the HARPS
consortium survey programme, was regularly monitored with a
careful observation strategy to reduce the "seismic noise"
of the star.

These new data confirmed both the amplitude and the periodicity
of the radial velocity variations found during the 8 nights in
June. The astronomers were left with only one convincing
explanation to this periodic signal: a second planet orbits
mu Arae and accomplishes a full revolution in 9.5 days.

But this was not the only surprise: from the radial velocity
amplitude, that is the size of the wobble induced by the
gravitational pull of the planet on the star, the astronomers
derived a mass for the planet of only 14 times the mass of the
Earth! This is about the mass of Uranus, the smallest of the
giant planets in the solar system.

The newly found exoplanet therefore sets a new record in the
smallest planet discovered around a solar type star.

At the boundary

The mass of this planet places it at the boundary between the
very large earth-like (rocky) planets and giant planets.

As current planetary formation models are still far from being
able to account for all the amazing diversity observed amongst
the extrasolar planets discovered, astronomers can only
speculate on the true nature of the present object. In the
current paradigm of giant planet formation, a core is formed
first through the accretion of solid "planetesimals". Once this
core reaches a critical mass, gas accumulates in a "runaway"
fashion and the mass of the planet increases rapidly. In the
present case, this later phase is unlikely to have happened for
otherwise the planet would have become much more massive.
Furthermore, recent models having shown that migration shortens
the formation time, it is unlikely that the present object has
migrated over large distances and remained of such small mass.

This object is therefore likely to be a planet with a rocky
(not an icy) core surrounded by a small (of the order of a
tenth of the total mass) gaseous envelope and would therefore
qualify as a "super-Earth".

Further Prospects

The HARPS consortium, led by Michel Mayor (Geneva Observatory,
Switzerland), has been granted 100 observing nights per year
during a 5-year period at the ESO 3.6-m telescope to perform
one of the most ambitious systematic searches for exoplanets
so far implemented worldwide. To this aim, the consortium
repeatedly measures velocities of hundreds of stars that may
harbour planetary systems.

The detection of this new light planet after less than 1 year
of operation demonstrates the outstanding potential of HARPS
for detecting rocky planets on short orbits. Further analysis
shows that performances achieved with HARPS make possible the
detection of big "telluric" planets with only a few times the
mass of the Earth. Such a capability is a major improvement
compared to past planet surveys. Detection of such rocky
objects strengthens the interest of future transit detections
from space with missions like COROT, Eddington and KEPLER that
shall be able to measure their radius.

More information

The research described in this Press release has been submitted
for publication to the leading astrophysical journal "Astronomy
and Astrophysics". A preprint is available as a postscript file
at
http://www.oal.ul.pt/~nuno/

Notes

[1]: The team is composed of Nuno Santos (Centro de Astronomia
e Astrofisica da Universidade de Lisboa, Portugal), François
Bouchy and Jean-Pierre Sivan (Laboratoire d'astrophysique de
Marseille, France), Michel Mayor, Francesco Pepe, Didier
Queloz, Stéphane Udry, and Christophe Lovis (Observatoire de
l'Université de Genève, Switzerland), Sylvie Vauclair, Michael
Bazot (Toulouse, France), Gaspare Lo Curto and Dominique Naef
(ESO), Xavier Delfosse (LAOG, Grenoble, France), Willy Benz and
Christoph Mordasini (Physikalisches Institut der Universität
Bern, Switzerland), and Jean-Louis Bertaux (Service d'Aéronomie
de Verrière-le-Buisson, Paris, France).

[2] A fundamental limitation of the radial-velocity method is
the unknown of the inclination of the planetary orbit that only
allows the determination of 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 = 22 Uranus masses = 318 Earth masses; 1 Uranus
mass = 14.5 Earth masses.

[3] HARPS has been designed and built by an international
consortium of research institutes, led by the Observatoire de
Genève (Switzerland) and including Observatoire de Haute-
Provence (France), Physikalisches Institut der Universität Bern
(Switzerland), the Service d'Aeronomie (CNRS, France), as well
as ESO La Silla and ESO Garching.

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