A Distant Solar System Has Five Planets
Wednesday, November 7, 2007
Astronomers have discovered a fifth planet orbiting a distant sun, marking the first time that another solar system with that many circling bodies has been found.
The central star, named 55 Cancri, has smaller and larger planets circling on paths similar to those in our solar system. Astronomers said that while the planets are unlike those in our solar system in terms of their size and distance from the sun, the fact that they are all circling in stable orbits is highly significant.
"Now we know our sun and its family is not unusual," said astronomer Geoff Marcy of the University of California at Berkeley. "Our Milky Way has 200 billion stars and billions of solar systems. We suspect that many harbor Earth-like planets."
While scores of extrasolar planets, or "exo-planets," have been located so far, they said, millions remain to be found.
The fifth planet, which eluded detection for years because it was tucked in between two other orbiting planets that had been detected previously, appears to be a gas giant like Jupiter or Saturn and so is unlikely to sustain life. But it orbits in what is termed the "habitable zone" of its solar system -- a band around the star where the temperature would permit liquid water to pool on solid surfaces -- meaning that a rocky moon orbiting the planet, if there is one, could theoretically support life.
"The gas-giant planets in our solar system all have large moons," said Debra Fischer, an astronomer at San Francisco State University and lead author of a paper that will appear in the Astrophysical Journal. "If there is a moon orbiting this new, massive planet, it might have pools of liquid water on a rocky surface."
The star 55 Cancri resides in the constellation Cancer, nearly 41 light-years away. It has about the same mass as our sun and is easily visible with binoculars. The planets that orbit it, however, are far too small to be seen with the most powerful telescopes and were located and identified by measuring the "wobble" their gravity creates in the motion of their sun.
Using the wobble method, as well as looking for distant specks crossing the face of stars, astronomers have identified about 260 exo-planets since the first was discovered in the early 1990s. Most are single planets circling their sun, with a few cases of three and four in a solar system.
The planet closest to 55 Cancri is believed to be about the size of Neptune and circles the star in less than three days. The second planet is a little smaller than Jupiter and completes an orbit every 14.7 days. The third planet, similar in mass to Saturn, completes one orbit every 44 days, and the newly found fourth planet is about the size of Saturn and orbits in 260 days. The farthest-out planet is huge -- four times the mass of Jupiter -- and orbits every 14 years.
The researchers said there may well be other, smaller planets in the vast space between the fourth and fifth, but no telescopes are powerful enough to detect them or measure their effects on the star.
Finding the five planets circling 55 Cancri took 18 years of continuous research using the Shane telescope at Lick Observatory in Northern California. While finding the fifth planet is an unprecedented achievement, astronomer Marcy said it marks a beginning rather than an end.
"Finding five extrasolar planets orbiting a star is only one small step," he said. "Earth-like planets are the next destination."
The research was funded by NASA and the National Science Foundation, which have both made finding exoplanets a high priority.