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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 2 of 2

UVES spectra of OGLE-TR-3

Now, a team of German and ESO astronomers [1] have used the UVES
High-Dispersion Spectrograph on the 8.2-m VLT KUEYEN telescope at
the Paranal Observatory (Chile) to obtain very detailed spectra of
another star on that list, OGLE-TR-3, cf. PR Photos 10a-b/03.

Over a period of one month, a total of ten high-resolution spectra -
each with an exposure time of about one hour - were obtained of the
16.5-mag object, i.e. its brightness is about 16,000 fainter that
what can be perceived with the unaided eye. A careful evaluation
shows that OGLE-TR-3 is very similar to the Sun, with a temperature
of about 5800 degC (6100 K). And most interestingly, it undergoes
velocity variations of the order of 120 m/s.

The exoplanet at OGLE-TR-3

The 2 per cent dip in the brightness of OGLE-TR-3, as observed during
the OGLE programme, occurs every 28 hours 33 minutes (1.1899 days),
cf. PR Photo 10e/03. The UVES velocity measurements (PR Photo 10d/03)
fit this period well and reveal, with high probability, the presence
of an exoplanet orbiting OGLE-TR-3 with this period. In any case, the
observations firmly exclude that the well observed brightness
variations could be due to a small stellar companion. A red dwarf
star would have caused velocity variations of 15 km/s and a brown
dwarf star 2.5 km/s; both would have been easy to observe with UVES,
and it is clear that such variations can be excluded. 

Although the available observations are still insufficient to allow
an accurate determination of the planetary properties, the
astronomers provisionally deduce a true mass of the planet of the
order of one half of that of Jupiter. The density is found to be
about 250 kg/m3, only one-quarter of that of water or one-fifth of
that of Jupiter, so the planet is quite big for this mass - a bit
"blown up". It is obviously a planet of the gaseous type.

A very hot planet

The orbital period, 28 hours 33 minutes (1.1899 days), is the
shortest known for any exoplanet and the distance between the star
and the planet is correspondingly small, only 3.5 million kilometres.
The temperature of the side of the planet facing the star must
therefore be very high, of the order of 2000 =B0C. Clearly, the
planet must be losing its atmosphere by evaporation. The astronomers
also conclude that it might in fact be possible to observe this
exoplanet directly because of its comparatively strong infrared
radiation. An attempt to do so will soon be made. 

As only the third exoplanet found this way (after those at the stars
HD209458 and OGLE-TR-56), the new object confirms the current
impression that a considerable number of stars may possess giant
planets in close orbits. Since such planets cannot form so close to
their parent star, they must have migrated inwards to the current
orbit from a much larger, initial distance. It is not known at this
time with certainty how this might happen. 

Future prospects

It is expected that more observational campaigns will be made to
search for transiting planets around other stars. There is good hope
that OGLE-TR-3 and OGLE-TR-56 are just the first two of a substantial
number of exoplanets to be discovered this way.

Some years from now, searches will also begin from dedicated space
observatories, e.g. ESA's Eddington and Darwin, and NASA's Kepler.

More information

The information contained in this press release is based on a
research article which has just been published in the European
research journal "Astronomy & Astrophysics" ("OGLE-TR-3:
A Possible
New Transiting Planet" by Stefan Dreizler and collaborators; Vol.
402, page 791; astro-ph/0303183). 

Notes

[1]: The team consists of Stefan Dreizler, Sonja L. Schuh, Wilhelm
Kley, Thomas Rauch and Klaus Werner (Institut fuer Astronomie und
Astrophysik, Tuebingen, Germany), Peter H. Hauschildt (Hamburger
Sternwarte, Germany), and Burkhard Wolff (ESO). Thomas Rauch is also
associated with the Dr.-Remeis-Sternwarte (Bamberg, Germany).

[2]: OGLE-TR-3 and 58 other stars on the OGLE-list were discovered
during an extensive photometric search for planetary and
low-luminosity object transits in the galactic disk stars within the
third phase of the Optical Gravitational Lensing Experiment (OGLE
III, cf. the research paper by Udalski and collaborators in the
Polish research journal "Acta Astronomica", Vol. 52, page 1).

[3]: With the Mercury Transit on May 7, 2003 as a fine prelude to
next year's Venus Transit on June 8, 2004, ESO and the European
Association for Astronomy Education (EAAE), together with the
Institut de Mecanique Celeste et de Calcul des Ephemerides (IMCCE)
and the Observatoire de Paris in France, are launching a major public
programme that allows all interested persons to participate actively.
The common web-address is:

http://www.eso.org/outreach/eduoff/vt-2004/.

Contact

Stefan Dreizler
Institut fuer Astronomie und Astrophysik
Tuebingen, Germany
Phone: +49 7071 29 78612
email: dreizler{at}astro.uni-tuebingen.de

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