Press Release

Two Extremely Hot Exoplanets Caught in Transit – VLT Measures Properties of New Jupiter-Size Objects in Very Close Orbits

By SpaceRef Editor
May 11, 2004
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Two Extremely Hot Exoplanets Caught in Transit – VLT Measures Properties of New Jupiter-Size Objects in Very Close Orbits

A European team of astronomers [1] are announcing the discovery and study
of two new extra-solar planets (exoplanets). They belong to the OGLE
transit candidate objects and could be characterized in detail. This
trebles the number of exoplanets discovered by the transit method; three
such objects are now known.

The observations were performed in March 2004 with the FLAMES multi-fiber
spectrograph on the 8.2-m VLT Kueyen telescope at the ESO Paranal
Observatory (Chile). They enabled the astronomers to measure accurate
radial velocities for forty-one stars for which a temporary brightness
“dip” had been detected by the OGLE survey. This effect might be the
signature of the transit in front of the star of an orbiting planet, but
may also be caused by a small stellar companion.

For two of the stars (OGLE-TR-113 and OGLE-TR-132), the measured
velocity changes revealed the presence of planetary-mass companions in
extremely short-period orbits.

This result confirms the existence of a new class of giant planets,
designated “very hot Jupiters” because of their size and very high
surface temperature. They are extremely close to their host stars,
orbiting them in less than 2 (Earth) days.

The transit method for detecting exoplanets will be “demonstrated” for
a wide public on June 8, 2004, when planet Venus passes in front of
the solar disc, cf. the VT-2004 programme.

The full text of this press release, with five photos and all weblinks,
is available at:

Discovering other Worlds

During the past decade, astronomers have learned that our Solar System
is not unique, as more than 120 giant planets orbiting other stars
were discovered by radial-velocity surveys (cf. ESO PR 13/00, ESO
PR 07/01, and ESO PR 03/03).

However, the radial-velocity technique is not the only tool for the
detection of exoplanets. When a planet happens to pass in front of its
parent star (as seen from the Earth), it blocks a small fraction of
the star’s light from our view. The larger the planet is, relative to
the star, the larger is the fraction of the light that is blocked.

It is exactly the same effect when Venus transits the Solar disc on
June 8, 2004, cf. ESO PR 03/04 and the VT-2004 programme website. In
the past centuries such events were used to estimate the Sun-Earth
distance, with extremely useful implications for astrophysics and
celestial mechanics.

Nowadays, planetary transits are gaining renewed importance. Several
surveys are attempting to find the faint signatures of other worlds,
by means of stellar photometric measurements, searching for the
periodic dimming of a star as a planet passes in front of its disc.

One of these, the OGLE survey, was originally devised to detect
microlensing events by monitoring the brightness of a very large
number of stars at regular intervals. For the past four years, it has
also included a search for periodical shallow “dips” of the
brightness of stars, caused by the regular transit of small orbiting
objects (small stars, brown dwarfs or Jupiter-size planets). The OGLE
team has since announced 137 “planetary transit candidates” from
their survey of about 155,000 stars in two southern sky fields, one
in the direction of the Galactic Centre, the other within the Carina

Resolving the nature of the OGLE transits

The OGLE transit candidates were detected by the presence of a
periodic decrease of a few percent in brightness of the observed
stars. The radius of a Jupiter-size planet is about 10 times
smaller than that of a solar-type star [2], i.e. it covers about
1/100 of the surface of that star and hence it blocks about 1 % of
the stellar light during the transit.

The presence of a transit event alone, however, does not reveal the
nature of the transiting body. This is because a low-mass star or a
brown dwarf, as well as the variable brightness of a background
eclipsing binary system seen in the same direction, may result in
brightness variations that simulate the ones produced by an orbiting
giant planet.

However, the nature of the transiting object may be established by
radial-velocity observations of the parent star. The size of the
velocity variations (the amplitude) are directly related to the mass
of the companion object and therefore allow to discriminate between
stars and planets as the cause of the observed brightness “dip”.

In this way, photometric transit searches and radial-velocity
measurements combine to become a very powerful technique to detect
new exoplanets. Moreover, it is particularly useful for elucidating
their characteristics. While the detection of a planet by the radial
velocity method only yields a lower estimate of its mass, the
measurement of the transit makes it possible to determine the exact
mass, radius, and density of the planet.

The follow-up radial-velocity observations of the 137 OGLE transit
candidates is not an easy task as the stars are comparatively faint
(visual magnitudes around 16). This can only be done by using a
telescope in the 8-10m class with a high-resolution spectrograph.

The nature of the two new exoplanets

A European team of astronomers [1] therefore made use of the 8.2-m
VLT Kueyen telescope. In March 2004, they followed 41 OGLE “top
transit candidate stars” during 8 half-nights. They profited from
the multiplex capacity of the FLAMES/UVES fiber link facility that
permits to obtain high-resolution spectra of 8 objects
simultaneously and measures stellar velocities with an accuracy of
about 50 m/s.

While the vast majority of OGLE transit candidates turned out to
be binary stars (mostly small, cool stars transiting in front of
solar-type stars), two of the objects, known as OGLE-TR-113 and
OGLE-TR-132, were found to exhibit small velocity variations. When
all available observations – light variations, the stellar
spectrum and radial-velocity changes – were combined, the
astronomers were able to determine that for these two stars, the
transiting objects have masses compatible with those of a giant
planet like Jupiter.

Interestingly, both new planets were detected around rather remote
stars in the Milky Way galaxy, in the direction of the southern
constellation Carina. For OGLE-TR-113, the parent star is of F-type
(slightly hotter and more massive than the Sun) and is located at a
distance of about 6000 light-years. The orbiting planet is about 35%
heavier and its diameter is 10% larger than that of Jupiter, the
largest planet in the solar system. It orbits the star once every
1.43 days at a distance of only 3.4 million km (0.0228 AU). In the
solar system, Mercury is 17 times farther away from the Sun. The
surface temperature of that planet, which like Jupiter is a gaseous
giant, is correspondingly higher, probably above 1800 degrees C.

The distance to the OGLE-TR-132 system is about 1200 light-years.
This planet is about as heavy as Jupiter and about 15% larger (its
size is still somewhat uncertain). It orbits a K-dwarf star (cooler
and less massive than the Sun) once every 1.69 days at a distance of
4.6 million km (0.0306 AU). Also this planet must be very hot.

A new class of exoplanets

With the previously found planetary transit object OGLE-TR-56 [3],
the two new OGLE objects define a new class of exoplanets, still
not detected by current radial velocity surveys: planets with
extremely short periods and correspondingly small orbits. The
distribution of orbital periods for “hot Jupiters” detected from
radial velocity surveys seems to drop off below 3 days, and no
planet had previously been found with an orbital period shorter
than about 2.5 days.

The existence of the three OGLE planets now shows that “very hot
Jupiters” do exist, even though they may be quite rare; probably
about one such object for every 2500 to 7000 stars. Astronomers
are truly puzzled how planetary objects manage to end up in such
small orbits, so near their central stars.

Contrary to the radial velocity method which is responsible for
the large majority of planet detections around normal stars, the
combination of transit and radial-velocity observations makes it
possible to determine the true mass, radius and thus the mean
density of these planets.

Great expectations

The two new objects double the number of exoplanets with known
mass and radius (the three OGLE objects plus HD209458b, which
was detected by the radial velocity surveys but for which a
photometric transit was later observed). The new information
about the exact masses and radii is essential for understanding
the internal physics of these planets.

The complementarity of the transit and radial velocity
techniques now opens the door towards a detailed study of the
true characteristics of exoplanets. Space-based searches for
planetary transits – like the COROT and KEPLER missions –
together with ground-based radial velocity follow-up
observations will in the future lead to the characterization of
other worlds as small as our Earth.

More information

The information contained in this press release is based on a
research article which is being published by the European
research journal “Astronomy & Astrophysics” (“Two new ‘very hot
Jupiters’ among OGLE transiting candidates”, by Francois Bouchy
and collaborators, astro-ph/0404264). OGLE-TR-113b has recently
been confirmed by another team of astronomers (astro-ph/0404541).
Note also that the discovery last year of the ostensibly
shortest-known-period planet orbiting OGLE-TR-3 (outlined in
ESO PR 09/03) was later proved to be wrong (cf. Astrophysical
Journal Volume 497, page 1076) – it was based on much less
accurate radial velocities than for the present results.


[1] The team consists of Francois Bouchy and Frederic Pont at
Laboratoire d’Astrophysique de Marseille (LAM) in France, Nuno
Santos of the Lisbon Astronomical Observatory, Portugal, Claudio
Melo of ESO-Chile, Michel Mayor, Didier Queloz and Stephane Udry
of the Geneva Observatory in Switzerland. Frederic Pont is now
associated with the Geneva Observatory.

[2] The diameter of Jupiter is about 11 times larger than that
of the Earth.

[3] See CFA Press Release 03-01.

[4] The Digitized Sky Survey was produced at the Space
Telescope Science Institute under U.S. Government grant NAG
W-2166. The images of these surveys are based on photographic
data obtained using the Oschin Schmidt Telescope on Palomar
Mountain and the UK Schmidt Telescope. The plates were processed
into the present compressed digital form with the permission of
these institutions.


Francois Bouchy

Laboratoire d’Astrophysique de Marseille


Phone: +33 4 91 05 59 00


Frederic Pont

Observatoire de Geneve


Phone: +41 22 755 26 11


SpaceRef staff editor.