Homing in on solar systems that could support life

Astronomers hunting for new planets say they have uncovered a solar system that is the closest analogue yet to the system Earth inhabits.

The search, which yielded 15 new planets and 10 new solar systems, is part of a project to select targets for a new generation of space telescopes that will look for planets from orbit.

Computer simulations suggest that the system around 55 Cancri, a star in the constellation Cancer, would be capable of holding an Earth-sized planet in a stable orbit within the narrow range of distances that would allow liquid water on the surface. Water is a necessary ingredient for organic life.

"We haven't found an exact solar-system analogue," says Paul Butler, an astronomer at the Carnegie Institution of Washington and one of the leaders of the planet-hunting team. "But this one shows we are getting close."

The Jupiter-class object is one of 13 new planets the team announced. Ten of these represent newly discovered solar systems.

The finds bring the total number of "extrasolar" planets to more than 90. They are the latest results from a broad ground-based effort to hunt for planets orbiting 2,000 stars within about 150 light-years of Earth.

Led by University of California at Berkeley astronomer Geoffrey Marcy and Dr. Butler, the project has 1,200 stars under surveillance. That number is expected to grow as new telescopes join the search. In addition to the telescopes in Hawaii, California, and Australia it currently uses, the group hopes to enlist the 6.5-meter Magellan telescope in Chile to help cover the Southern Hemisphere's skies.

Moreover, the group has recently received full funding to build a $5 million, 2-meter class telescope at the Lick Observatory in California, which could begin operation in a little more than a year.

"This telescope will be optimized for planet hunting. It will be a lean, mean planet-hunting machine," says Debra Fischer, another team member from UC Berkeley.

When astronomers train these telescopes on a candidate star, they don't see the planet itself. Instead, they detect its gravitational effects on the parent star.

Dr. Marcy and Butler's team, which announced its results yesterday at a NASA briefing, uses spectrographs to measure the Doppler shift of light that appears as a planet tugs on its parent star. The star, 55 Cancri, is 41 light-years away. It is between 4 billion and 7 billion years old, researchers estimate, and it is rich in carbon, iron, silicon, and sulfur.

While its planetary system is similar to ours in some ways, it also shows striking differences.

For example, it boasts a previously discovered planet of roughly Saturn's mass orbiting so close to the star that its "year" is 14.7 days long – a bit like replacing Mercury with Saturn. The Marcy-Butler team hints that the system may yet include a third planet, but the data at the moment are ambiguous.

Still, having a planet close to Jupiter's mass (3.5 to 5 times as massive) at close to Jupiter's distance (0.3 astronomical units farther out) with an orbital period about a year longer than Jupiter's is intriguing, Butler says.

He explains that in the early years of our solar system, Jupiter "gravitationally vacuumed most of the debris out of the inner solar system. This made it possible for life to evolve" with only occasional collisions from asteroids compared with the outer planets.

Moreover, he continues, Jupiter would eject any planet with the highly elliptical, or eccentric, orbits typical of many extrasolar planets found so far. Thus the giant planet could have helped stabilize Earth in its life-favorable location.

The planet's distance from the glare of its parent star also makes it a strong candidate for direct detection by space-based telescopes planned for the next decade and beyond, says Dr. Fischer. Indeed, during the past two or three years, she says, the team's focus has shifted to picking targets for these telescopes to examine.

The first, NASA's Kepler mission, is scheduled for launch in 2006. It would look for Earth-like planets. This would be followed by the Space Interferometry Mission, scheduled for launch in 2009, and the Terrestrial Planet Finder. The latter is still under consideration, but planners would like to launch in 2012.

When astronomers talk about searching for solar systems "like ours," they don't mean a Xerox copy, but one with similar gravitational effects that could render it hospitable to life.

"The discovery of a Jupiter-like planet ... really does begin to show a system that shares attributes" with our solar system, Butler says.