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             Information from the European Southern Observatory

ESO Press Release 09/03

22 April 2003                                              [ESO Logo]

For immediate release

NB! Five photos with extensive captions are omitted in this version -
they are available at:
http://www.eso.org/outreach/press-rel/pr-2003/pr-09-03.html
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Glowing Hot Transiting Exoplanet Discovered

Part 1 of 2

VLT Spectra Indicate Shortest-Known-Period Planet Orbiting OGLE-TR-3

Summary

More than 100 exoplanets in orbit around stars other than the Sun
have been found so far. But while their orbital periods and distances
from their central stars are well known, their true masses cannot be
determined with certainty, only lower limits.

This fundamental limitation is inherent in the common observational
method to discover exoplanets - the measurements of small and regular
changes in the central star's velocity, caused by the planet's
gravitational pull as it orbits the star.

However, in two cases so far, it has been found that the exoplanet's
orbit happens to be positioned in such a way that the planet moves in
front of the stellar disk, as seen from the Earth. This "transit"
event causes a small and temporary dip in the star's brightness, as
the planet covers a small part of its surface, which can be observed.
The additional knowledge of the spatial orientation of the planetary
orbit then permits a direct determination of the planet's true mass.

Now, a group of German astronomers [1] have found a third star in
which a planet, somewhat larger than Jupiter, but only half as
massive, moves in front of the central star every 28.5 hours. The
crucial observation of this solar-type star, designated OGLE-TR-3 [2]
was made with the high-dispersion UVES spectrograph on the Very Large
Telescope (VLT) at the ESO Paranal Observatory (Chile).

It is the exoplanet with the shortest period found so far and it is
very close to the star, only 3.5 million km away. The hemisphere that
faces the star must be extremely hot, about 2000 degrees and the
planet is obviously losing its atmosphere at high rate.

PR Photo 10a/03: The star OGLE-TR-3.
PR Photo 10b/03: VLT UVES spectrum of OGLE-TR-3.
PR Photo 10c/03: Relation between stellar brightness and velocity
(diagram).
PR Photo 10d/03: Observed velocity variation of OGLE-TR-3.
PR Photo 10e/03: Observed brightness variation of OGLE-TR-3.
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The search for exoplanets

More than 100 planets in orbit around stars other than the Sun have
been found so far. These "exoplanets" come in many different sizes
and they move in a great variety of orbits at different distances
from their central star, some nearly round and others quite
elongated. Some planets are five to ten times more massive than the
largest one in the solar system, Jupiter - the lightest exoplanets
known at this moment are about half as massive as Saturn, i.e. about
50 times more massive than the Earth. 

Astronomers are hunting exoplanets not just to discover more such
objects, but also to learn more about the apparent diversity of
planetary systems. The current main research goal is to eventually
discover an Earth-like exoplanet, but the available telescopes and
instrumentation are still not "sensitive" enough for this daunting
task. 

However, also in this context, it is highly desirable to know not
only the orbits of the observable exoplanets, but also their true
masses. But this is not an easy task.

Masses of exoplanets

Virtually all exoplanets detected so far have been found by an
indirect method - the measurement of stellar velocity variations. It
is based on the gravitational pull of the orbiting planet that causes
the central star to move a little back and forth; the heavier the
planet, the greater is the associated change in the star's velocity.

This technique is rapidly improving: the new HARPS spectrograph (High
Accuracy Radial Velocity Planet Searcher), now being tested on the
3.6-m telescope at the ESO La Silla Observatory, can measure such
stellar motions with an unrivalled accuracy of about 1 metre per
second (m/s), cf. ESO PR 06/03. It will shortly be able to search for
exoplanets only a few times more massive than the Earth.

However, velocity measurements alone do not allow to determine the
true mass of the orbiting planet. Because of the unknown inclination
of the planetary orbit (to the line-of-sight), they only provide a
lower limit to this mass. Additional information about this orbital
inclination is therefore needed to derive the true mass of an
exoplanet. 

The transit method

Fortunately, this information becomes available if the exoplanet is
known to move across ("transit") the star's disk, as seen from the
Earth; the orbital plane must then necessarily be very near the
line-of-sight. This phenomenon is exactly the same that happens in
our own solar system, when the inner planets Mercury and Venus pass
in front of the solar disk, as seen from the Earth [3]. A solar
eclipse (caused by the Moon moving in front of the Sun) is a more
extreme case of the same type of event. 

During such an exoplanet transit, the observed brightness of the star
will decrease slightly because the planet blocks a part of the
stellar light. The larger the planet, the more of the light is
blocked and the more the brightness of the star will decrease. A
study of the way this brightness changes with time (astronomers refer
to the "light curve"), when combined with radial velocity
measurements, allows a complete determination of the planetary orbit,
including the exact inclination. It also provides accurate
information about the planet's size, true mass and hence, density. 

The chances that a particular exoplanet passes in front of the disk
of its central star as seen from the Earth are small. However,
because of the crucial importance of such events in order to
characterize exoplanets fully, astronomers have for some time been
actively searching for stars that experience small regularly
occurring "brightness dips" that might possibly be caused by
exoplanetary transits. 

The OGLE list

Last year, a first list of 59 such possible cases of stars with
transiting planets was announced by the Optical Gravitational Lensing
Experiment (OGLE) [2]. These stars were found - within a sample of
about 5 million stars observed during a 32-day period - to exhibit
small and regular brightness dips that might possibly be caused by
transits of an exoplanet. 

For one of these stars, OGLE-TR-56, a team of American astronomers
soon thereafter observed slight variations of the velocity, strongly
indicating the presence of an exoplanet around that star.

 - Continued -

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