We have never seen anything like this

Paul Butler, Carnegie Institution of Washington

The scientists from the University of California, Berkeley, US, say a star 123 light-years from Earth is being circled by two objects, one of which may be the biggest planet ever found.

In the second system, two planets orbit a star in a gravitationally synchronised "dance".

The discoveries were announced at a meeting of the American Astronomical Society in San Diego. They are based on observations made at the Keck I telescope atop Mauna Kea in Hawaii and the Lick Observatory telescope in California.

Hybrid planet

The leader of the planet-searching team, Geoffrey Marcy, said a star called HD168443, in the constellation Serpens, was being circled by a planet 17 times more massive than Jupiter. It is the largest planetary-like object to be found beyond our own Solar System.

Star - HD168443

123 light-years away One planet is 44m km from star. It has 7.7 Jupiter masses and orbital period of 58.1 days Second planet is 430m km from star. It has 17.1 Jupiter masses and orbital period of 4.8 yrs

"This is more massive than a planet and it defies the conventional definition for a planet,'' said Marcy.

The new body is big enough to be called a brown dwarf, an object too large to be a conventional planet but too small to qualify as a proper star. Such objects are not massive enough to ignite the nuclear reactions in their cores that would make them shine like stars.

"We have never seen anything like this,'' said Paul Butler, a Carnegie Institution of Washington astronomer who works closely with Marcy. "To call it a brown dwarf sweeps the mystery under the rug. It is a mystery system."

Profound implications

Marcy added: "This defies explanation. We don't know if it is a brown dwarf or some type of hybrid.''

Geoff Marcy and colleagues find the discoveries perplexing

Most of the 50 or so new planets found circling stars other than our Sun are two to three times more massive than Jupiter. The detection technique does not allow astronomers to find Earth-sized planets.

"This is one of the most exciting discoveries yet,'' said Douglas Lin, of the University of California, Santa Cruz. "This discovery has profound theoretical implications.''

Marcy said a second planet in the HD168443 system had seven times the mass of Jupiter and orbited closer to its central star. He said both planets were probably huge gas balls, much like Jupiter and Saturn in our own Solar System.

Strong resonance

Another announcement at the American Astronomical Society's conference detailed two small planets orbiting a star called Gliese 876, a red-dwarf star 15 light-years from Earth in the constellation of Aquarius.

Star - Gliese 876

15 light-years away One planet is 31.5m km from star. It has 1.8 Jupiter masses and orbital period of 61 days Second planet is 19.5m km from star. It has 0.56 Jupiter masses and orbital period of 30 days

Debra Fischer, a member of the Marcy-Butler team, said one planet was about half the mass of Jupiter and the other was 1.9 times more massive. She added that the planets' orbits were gravitationally locked to each other in what is termed a resonate orbit. One circles the star every 30 days, and the other takes 60.

"These two resonate planets seem to be humming in harmony. They are like two harmonic notes on a stringed instrument," she said.

"We don't know how they could have gotten into that configuration,'' said Marcy. Jack Lissauer, from the American space agency Nasa, said: "The resonance between the two orbiting planets is among the most exciting planet detection discoveries to date.

"This is the first extrasolar planetary system to show a strong resonance. It is also the smallest star known to have any orbiting planets, much less two."