June 15, 2002: Earth I to Earth II: we can see you
Australian, British and American astronomers have announced the discovery of 15 planets around other stars - including, in a landmark find, a solar system roughly like Earth's.
One of the planets is the smallest discovered outside Earth's solar system, an encouraging sign in the hunt for Earth-like planets.
But it is the planet orbiting the star 55 Cancri, in the constellation of cancer, that has excited astronomers. It is between three and five times the size of Jupiter, the largest planet in our solar system, and orbits Cancri in 13 years. This means Cancri's solar system is roughly like Earth's, and would allow smaller, Earth-like planets, which can harbour life, to exist.
"This is the first near analogue to our Jupiter," said a member of the Anglo-Australian planet search team, Geoff Macey. "All other extrasolar planets discovered up to now orbit closer to the parent star, and most of them have had elongated eccentric orbits. This new planet orbits as far from its star as our own Jupiter orbits the sun."
More than 90 planets orbiting other stars have been discovered since 1995. All of them are large gas planets, like Jupiter. Their orbits are so eccentric that any Earth-sized planet nearby would get pushed aside.
But Cancri could support an Earth-like planet, said one of the American astronomers, Greg Laughlin.
"Just as the other planets in our solar system tug on the Earth and produce a chaotic but bounded orbit, so the planets around Cancri would push and pull an Earth-like planet in a manner that would not cause any collisions or wild orbital variations," he said.
None of the planets discovered since 1995 have been photographed or observed directly. Astronomers have only inferred their existence by the gravitational wobble they cause their parent sun.
However, NASA is planning a new space telescope called the Terrestrial Planet Finder, which will photograph them, and Cancri will be a priority.
"This planetary system will be the best candidate for direct pictures when the Terrestrial Planet Finder is launched later this decade," said University of California astronomer Debra Fischer.
But while the similarities to Earth's solar system are there, Cancri does differ in many ways. In 1996, a Jupiter-sized planet was discovered that was so close to Cancri it took only 14 days to orbit it.
By contrast, the planet closest to our sun, Mercury, is a small rocky planet with a year of 88 days. Venus, Earth and Mars, the next closest planets, are small and rocky also.
Cancri, 41 light years from Earth, may also harbour other planets, as the two known ones can't explain all of the "wobbling" found so far. One possibility, according to the planet search team, is a Saturn-sized planet also orbiting close to Cancri.
While Cancri was observed in the Lick observatory in California, four of the new planets were discovered at the Anglo-Australian Telescope in Siding Spring, New South Wales. They bring the tally of new planets discovered by the telescope to 10. The first was discovered in 2000.
The new planets discovered at Siding Spring range from two to eight times the size of Jupiter, which is 300 times bigger than Earth. Three of them are further from their parent star than the Earth is from our sun.
One of the planets discovered in the US, using the Keck telescope in Hawaii, is the smallest extrasolar planet discovered. It is only 40 times the mass of Earth, smaller than both Jupiter and Saturn. But it is so close to its sun it only takes five days to orbit it.
"Although this planet is probably a gas giant like Neptune and not a rocky planet, it really shows we are working our way down," said one of the British astronomers, Hugh Jones.
The search teams, using the Lick, Keck, and Siding Springs telescopes, are tracking more than 1000 stars in their search for planets. In a couple of years, said another member of the Anglo-Australian search team, Paul Butler, another telescope at Chile would allow the team to raise that number to 2000.
"This will cover all the good candidates out to 50 parsecs (150 light years) of Earth, so we will know where to look when we have the terrestrial planet Finder and the Space Interferometry Mission, which will do the first reconnaissance to identify Earth-like planets," he said.
By Stephen Cauchi
The potential to find life on another planet has inched closer, with the discovery of eight more planets - three of which closely mimic the orbit paths of Earth and Mars.
The discoveries, by an international team of astronomers using telescopes in Australia and the United States, make a total of 74 planets found orbiting nearby stars.
Most known extra-solar planets have elongated, non-circular orbits, but the trio of planets following a nearly circular path raise the prospect of finding Earth-like planets in other solar systems and possibly other life forms, according to Chris Tinney, an astronomer with the Anglo-Australian Observatory in Sydney.
"These discoveries suggest we could find a true counterpart of our solar system in only a few years," Dr Tinney said.
The three new circular-orbit planets lie near the stars HD23079, HD4208 and HD 114783. They range in mass from 80 per cent the size of our largest planet, Jupiter, to 10 times its size.
Dr Tinney said astronomers were just beginning to find planets with circular orbits because they existed far from their parent stars and took longer to orbit, requiring more precise measurements over a longer period. To find evidence of planets, the astronomers use a high-precision technique that measures how much a star "wobbles" in space as it's tugged on by a planet's gravity.
Steve Vogt, of the Lick Observatory, University of California at Santa Cruz, said most known planetary systems were nothing like our solar system but "now we're starting to see ... second cousins. In a few years we could be finding brothers and sisters".
The astronomers, from Australia, the United States, Belgium and Britain are searching the nearest 1200 sun-like stars for planets similar to those in our solar system.
Now that scientists know what to look for, extrasolar planets are being discovered at an incredible rate:
June 13, 2002 - 15 new planets, 3 New Multiple-Planet Systems; Smallest Exoplanet yet discovered.
February 7, 2002 - A New Planet 12 Times Jupiter's Mass.
January 8, 2002 - Planet Orbiting a Giant Star.
November 27, 2001 - Atmosphere detected on an extrasolar planet.
October 15, 2001 - Eight new planets.
August 15, 2001 - A new planetary system in the Big Dipper constellation.
April 20, 2001 - Multiple planet systems appear common.
January 10, 2001 - A planetary system in a 2:1 resonance orbit.
January 10, 2001 - Discovery of a planet that is 17 times the mass of our Jupiter.