The present discoveries complete and enlarge our still preliminary knowledge of extra-solar planetary systems, as well as the transition between planets and brown dwarfs

Michael Mayor and Didier Queloz

The objects, which are circling stars similar to our own Sun, have masses that range from less than that of Saturn to about 15 times that of Jupiter.

The discoveries were made using a 1.2-metre telescope at the European Southern Observatory's La Silla complex in South America.

The planets were picked up by the now standard radial velocity technique that has discovered most of the known so-called exoplanets.

Brown dwarfs

This hinges on detecting changes in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. In other words, the star appears to wobble under the influence of its smaller companion.

La Silla is on the southern extremity of the Atacama desert in Chile

The new objects are quite diverse. Six of them are most likely bona fide exoplanets. The other two are apparently very low-mass brown dwarf stars. These are objects that do not have sufficient mass to trigger the type of nuclear reactions that make other stars shine.

"The present discoveries complete and enlarge our still preliminary knowledge of extra-solar planetary systems, as well as the transition between planets and brown dwarfs", said planet hunters Michael Mayor and Didier Queloz from the Geneva Observatory.

One of the new planets has just 80% of the mass of Saturn. This is only the third exoplanet detected so far with a possible sub-saturnian mass. It was picked up orbiting a Sun-like star called HD 168746.

Positioned in the constellation of Scutum (the Shield), HD 168746 is located at a distance of about 140 light-years (1,300 million, million kilometres). The planet goes around the star every 6.4 days, a fairly short period.

Hot planet

Another very interesting world was detected around a star called HD 83443 in the constellation Vela - the Sail. It is 141 light years away.

Astronomers look for a "wobble" in the star's motion

It has a mass just greater than that of Saturn and the shortest orbital period (2.986 days) and the smallest distance to the central star (5.7 million km) so far discovered.

Being so close to its star, the exoplanet must be heated to a temperature of many hundreds of degrees.

There are also indications that there could be an even smaller body in this system.

Jupiter is about 317 times more massive than the Earth and Saturn is about 95 times more massive.

The San Francisco State University Planet Search Team independently announced one of the new planets last week.