Astronomers discover another extrasolar Jupiter
BY JEFF FOUST
SPACEFLIGHT NOW

Posted: June 19, 2002

For the second time in less than a week astronomers have announced the discovery of a planet around another star that closely resembles the planet Jupiter in our own solar system.

The discovery of an extrasolar planet, or exoplanet, with a mass and an orbit similar to Jupiter was one of eight new exoplanets recently discovered by a team of Geneva Observatory astronomers that brings the total number of exoplanets to around 100. The announcement of the discovery was made Tuesday by one member of the team, Stephane Udry, at an extrasolar planets conference in Washington, DC.

The Jupiter-like exoplanet was found around the star HD190360A, also known as Gliese 777A, a Sun-like star 52 light-years from the Earth. The planet has an estimated minimum mass the same as Jupiter, and is in a circular orbit 3.65 astronomical units (548 million kilometers, 339 million miles) from the star. Jupiter, by comparison, orbits 5.2 AU (780 million km, 483 million mi.) from the Sun.

The planet is the second extrasolar analog to Jupiter announced in less than a week. On Thursday another team of astronomers, led by veteran planet hunters Geoffrey Marcy and Paul Butler, announced the discovery of a similar planet orbiting the star 55 Cancri. This other extrasolar Jupiter is more massive -- four times the mass of Jupiter -- but orbits the star in a slightly elliptical orbit at an average distance of 5.5 AU. That planet is part of a solar system that includes two inner planets; no other planets have been found around HD190360A.

Udry announced Tuesday a total of 12 exoplanets found by the Geneva Observatory team. Eight of the 12, including the one around HD190360A, were previously unknown, while three others were announced last week by Marcy and Butlerís team. A twelfth planet was discovered in 2001, but the new observations significantly refined its mass and orbit.

This latest batch of exoplanets, like the ones announced last week as well as most other known exoplanets, was discovered using a technique known as radial velocity, or Doppler detection. Astronomers look for periodic shifts in wavelength of spectral lines in spectra of Sun-like stars. These shifts are caused by a slight wobble in the star created by the gravitational tug of one or more planets as they orbit the star. Measuring the amplitude and period of the variations allows astronomers to determine the orbit of the planet and estimate its minimum mass without directly observing the planet itself.

Five of the eight planets were discovered using data from ELODIE, a spectrograph mounted on a 1.93-meter (76-inch) telescope at Observatoire Haute-Provence in southeastern France. The other three were found using CORALIE, a spectrograph on the 1.2-meter (47-inch) Euler telescope in La Silla, Chile. The Geneva team is led by Michel Mayor, who, with Didier Queloz, discovered the first exoplanet around a Sun-like star, 51 Pegasi, in 1995.

The eight new planets bring the total number of exoplanets to approximately 100: the exact number depends on varying definitions used for planets and the status of a few borderline discoveries. Although many of the initial discoveries were of "Hot Jupiters" -- gas giant planets with orbits just a few million kilometers across -- or planets with highly elliptical orbits, many of the more recent discoveries have been of planets in more circular, distant orbits. Of the eight exoplanets announced Tuesday, six of the eight have orbits with an average distance of greater than 1 AU, the Earth-Sun distance.

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