Two New Very Hot Jupiters

The European Southern Observatory in Chile. The Atacama Desert is in the north of Chile, about 1300km (800mi) from Santiago.[There are parts of Atacama where rain has never been recorded and the precious little precipitation (1cm/0.3in per year) that does fall comes from fog.]
Credit: ESO

A European team of astronomers has discovered two new extra-solar planets. These are the second and third planets to be discovered by the transit method, where a planet orbits in front of a star and blocks some of the starlight reaching Earth.

The larger a planet is relative to a star, the more light it blocks. An Optical Gravitational Lensing Experiment (OGLE) survey detected a temporary brightness "dip" for 41 stars. In order to determine if these dips were caused by other stars or by orbiting planets, the astronomers measured radial velocities for the stars.

Radial-velocity observations measure the Doppler shift of starlight as a star shifts toward and away from Earth. These shifts occur as a planet and star orbit around a common center of mass. The light shifts indicate the nature of the orbiting object, because the size of the light variations is directly related to the mass of the companion object.

Most of the 41 objects targeted by OGLE turned out to be binary star systems. But for two of the objects, OGLE-TR-113 and OGLE-TR-132 [banner image], the measured velocity changes indicated planetary-mass companions in extremely short-period orbits.

Scene from a moon orbiting the extra-solar planet in orbit around the star HD70642.
Credit:David A. Hardy, (c)

The planets orbit their stars in less than 2 Earth days. Such planets are called "very hot Jupiters" because of their mass and very high surface temperatures.

The OGLE-TR-132 planet orbits once every 1.69 Earth days, at a distance of 4.6 million kilometers (0.0306 AU).

The OGLE-TR-113 planet orbits its star once every 1.43 days at a distance of only 3.4 million kilometers (0.0228 AU). The planet Mercury is 17 times farther away from the Sun than this planet is from its star. The surface temperature of the OGLE-TR-113 planet probably is above 1,800 °C.

Both of the new planets orbit remote stars in the Milky Way galaxy, in the direction of the southern constellation Carina. For OGLE-TR-113, the parent star is an F-type, meaning it is slightly hotter and more massive than the Sun. This star is located about 6,000 light-years away. The orbiting planet is about 35 percent heavier than Jupiter, and 10 percent larger in diameter.

The OGLE-TR-132 system is about 1,200 light-years away. This planet is about as heavy as Jupiter and about 15 percent larger in diameter, although its size is still uncertain. It orbits a K-dwarf, a star that is cooler and less massive than the Sun.

"Very hot Jupiters" may be quite rare; there is probably one for every 2,500 to 7,000 stars. The distribution of orbital periods for "hot Jupiters" detected from radial velocity surveys seems to drop off below 3 days, and no planet had been found previously with an orbital period shorter than about 2.5 days. Astronomers are puzzled how such massive planetary objects end up so close to their stars.

Dip in brightness as prospective planet transits in front of parent star Credit: ESO

So far, more than 120 planets orbiting other stars have been discovered by radial-velocity surveys. Other surveys are attempting to find the faint signatures of other worlds through photometric transit measurements. While the detection of a planet by the radial velocity method yields a lower estimate of planetary mass, transit measurements can determine the exact mass, radius, and density of a planet.

The OGLE survey was originally devised to detect microlensing events, by monitoring the brightness of a very large number of stars at regular intervals. Over the past four years, OGLE also has included a search for periodical shallow "dips" of the brightness of stars caused by the transit of small orbiting objects, such as small stars, brown dwarfs or Jupiter-size planets. The OGLE team has announced 137 planetary transit candidates from their survey of about 155,000 stars in two southern sky fields.

The observations of the two new planets were performed in March 2004 with the FLAMES multi-fiber spectrograph on the 8.2-m VLT Kueyen telescope at the ESO Paranal Observatory in Chile.

The discovery team consists of François Bouchy and Frédéric 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 Stéphane Udry of the Geneva Observatory in Switzerland. Frédéric Pont is now associated with the Geneva Observatory. They are publishing a research article about their discovery in the European research journal "Astronomy & Astrophysics" ("Two new ‘very hot Jupiters’ among OGLE transiting candidates").

Future space-based searches for planetary transits, like the COROT and KEPLER missions, together with ground-based radial velocity follow-up observations, should be able to detect planets as small as Earth.

Related Web Pages

The University of California Planet Search Project
Astrobiology Magazine New Planets
Transit Search
Extrasolar Planets Encyclopedia
Planet Quest (JPL)
Kepler Mission
Darwin Mission
Herschel Mission
Space Interferometry Mission