You wait ages for one, then...

August 11, 2000

... nine new planets are announced in one week. Stuart Clark reports on the International Astronomical Union meeting.

It has been a week like no other for the study of exoplanets - the planets outside our solar system. No fewer than nine newly discovered worlds have been announced and debated during the "Planetary Systems in the Universe" symposium. All were found by teams looking for the delicate pirouettes of stars that betray the presence of planets, tugging on their parent stars through gravity.

Included in this week's catch was the smallest planet discovered so far. Michel Mayor, the Geneva Observatory astronomer, who found the first exoplanet in 1995, has identified another with half the mass of Saturn, in orbit with a second around a star known only by its catalogue number, HD 83443.

However, William Cochran, of the University of Texas, stole the show. His team had pored through 20 years of high-precision observations to discover the closest planet to our solar system.

The star, Epsilon Eridani, is 10.5 light years from the Sun, making it one of our nearest stellar neighbours.

The find is doubly exciting because this is the first time astronomers have found a world with so much in common with our own Jupiter. All previous detections of exoplanets have been similar to Jupiter in size but they have been much closer to their parent stars. As a result, most rush around their orbits in less than a year, some in just a few days.

The planet found around Epsilon Eridani, a red dwarf star only 80 per cent the size of the Sun, is different. It takes seven years to complete an orbit, comparable to Jupiter's 12-year journey around our Sun. Its distance from its star is not too different either - if the new discovery were located in the solar system, it would move back and forth between the orbits of Mars and Jupiter. On top of that, it weighs in at between 0.8 and 1.6 times the mass of Jupiter.

The only significant difference is that Jupiter orbits the Sun in a near circle whereas this new planet travels around Epsilon Eridani in a more elliptical path. Even so, Cochran remains optimistic that the system might be a good place to look for small planets like our own.

It is possible that an Earth-like planet might be found closer to Epsilon Eridani, conceivably within the "habitable zone", the region around a star in which a planet could possess liquid water due to stellar heating.

Finding Earth-like worlds is far from easy. A planet like our own would only be one hundredth the mass of Saturn, undetectable by current technology. Astronomers are working hard to rectify this problem.

Chas Beichman of American space agency Nasa's Jet Propulsion Laboratory said that "Earth-finder" spacecraft were being studied by four industrial and academic groups in the United States. Their designs called for a flotilla of between four and six telescopes to be placed into space where they would fly in tight formation. Each telescope would contain a 3-4m mirror, roughly one and a half times bigger than the Hubble Space Telescope. There would also be a central space probe that would combine the images with unprecedented resolution. Building is scheduled to start in 2007 with launch in 2012.

Meanwhile, astronomers have tantalising evidence that a second planet might exist in the Epsilon Eridani system. Measurements from the British James Clerk Maxwell Telescope on Mauna Kea, Hawaii, show that the star is encircled by a lop-sided ring of dust grains. Such an uneven distribution can be caused by the gravitational pull of a planet, but Cochran's does not fit the bill. His data does, however, hint at another planet in a very large 150-year orbit. Its gravity could be distorting the dust ring.

Deborah Fischer, of the University of California at Berkeley, found similar evidence for second planets around five of the 12 stars she studied. "It's important to know if there are other companions out there because anything else in the system will affect the dynamics and theories of how the planets moved in and parked in their current orbits," she said.

The sentiments felt by many at the conference were echoed by Geoff Marcy of San Francisco State University, the world's most successful planet hunter with 25 discoveries and another ten pending confirmation: "We are fortunate to live in such wonderful and intriguing times."

Stuart Clark is director of public astronomy education at the University of Hertfordshire and author of Life on Other Worlds and How to Find It.

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