Swiss astronomers find new planets

17 Jun 2002

A team of astronomers from the Geneva Observatory in Switzerland have discovered eight new low-mass planets.

The astronomers were using the CORALIE spectrometer on the Swiss 1.2-meter Leonhard Euler telescope at the European Southern Observatory's La Silla Observatory in Chile.

"All of the objects have been discovered with a very small telescope," noted Michel Mayor, one of the Geneva Observatory researchers.

The CORALIE instrument uses the blue to red range in the visible portion of the electromagnetic spectrum. Since a planet will exert a small gravitational pull on the star it orbits, a star which has at least one planet will wobble with the motion amplitude depending upon the planet's orbital distance and the mass.

"The effect of the Doppler shift is so small that you should have a very stable spectrograph," Mayor noted.

The CORALIE spectrograph met such requirements. Light coming froma star which is moving towards the Earth will have a Doppler shift toward the blue (shorter) wavelengths, while a star moving away from the earth will emit light shifted to red (longer) wavelengths. With the proper precision to observe the variations, a periodic Doppler shift of the star light can reveal the presence of a planet.

"It's a gradual discovery," Mayor explained. "After a certain time you have a preliminary orbit."

The Geneva Observatory team found single low-mass planets orbiting eight different stars. "We had selected a big sample of 1,600 stars," Mayor noted. "We had not selected the stars. We are trying to look systematically to find planets on all stars."

The objects may not necessarily be planets. Two are suspected to be low-mass brown dwarfs, or objects of sub-stellar mass with no nuclear energy source in their interior.

"Three of them are very low mass objects. One of them is only 70 percent of the mass of Saturn," Mayor said.

One of the planets discovered has set a pair of records for discovered exoplanets, which now number approximately 40. The planet which orbits around HD 83443 rotates around that star every 2.986 days, the first known planet to have an orbit of under three earth days. That celestial object is also only 0.038 astronomical units, or 5.7 million kilometers, from its sun. "It's the closest planet ever discovered," Mayor noted.

CORALIE also detected a small change with time, or a drift, of the mean velocity variation of HD 83443, indicating the possible existence of at least one other low-mass object orbiting that star.