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.

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.

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 deg C. 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@astro.uni-tuebingen.de