They say there is no place like home, but it is beginning to look as if there is a place sort of like home 41 light-years from here in the constellation Cancer.
Astronomers reported Tuesday that there were at least five planets circling a star there known as 55 Cancri, where only four had been known before, making it the most extensive planetary system yet found outside our own. It is also the one that most resembles our solar system, with a giant planet orbiting far out from the star and four smaller ones circling closer in.
The new addition to the system circles 55 Cancri at roughly the distance of Venus in our own solar system, in the so-called habitable zone where it is warm enough for liquid water. But, with 45 times the mass of Earth, the planet is more apt to resemble Neptune or Saturn than Earth, and thus would be a deadly environment for any kind of life that we know.
"It's a system that appears to be packed with planets," Prof. Debra Fischer of San Francisco State University said of 55 Cancri. She is the leader of the team that reported its results in a paper to be published in The Astrophysical Journal and in a telephone news conference on Tuesday from NASA's Jet Propulsion Laboratory in Pasadena, Calif.
The scientists said the discovery augured well for the chance that with time and more data, astronomers would find places out there that look like home. They also said it marked the beginning of a transition between studying planets and studying planetary systems.
Another team member, Geoff Marcy, a professor at the University of California, Berkeley, said the discovery had him "jumping out of my socks." He said, "We now know our Sun and its family of planets is not unusual."
Jonathan I. Lunine, a professor at the University of Arizona who was not part of the work, said that astronomers were on the verge of beginning to answer a question posed by Albertus Magnus, the medieval German philosopher and priest who wondered whether there was but one world or many worlds. We now know, Dr. Lunine said, "how lonely the universe is, how far we live from distant stars."
In the last decade, about 250 planets have been discovered around other stars - the vast majority of them by the so-called wobble technique of monitoring a star's light for signs of the slight to-and-fro motion induced by the gravitational tugs of orbiting planets.
As technology and techniques have improved, the planet hunters have been able to move down the scale from Jupiter-size planets to ones only a few times as massive as Earth. But detecting rocky planets like Earth is probably beyond the current technology and must await future space-based missions, the astronomers admit.
One of the first of these "exoplanets" discovered, in 1996, was at 55 Cancri. Dr. Fischer and her colleagues have been observing that star for 18 years, adding more planets to the list of its retinue as they have made their presence known.The outermost and heaviest planet in the system, which is four times as massive as Jupiter, circles at a distance of 500 million miles, slightly farther than Jupiter in our own system, and takes 14 years to complete an orbit.
The star's three innermost planets all circle more tightly than Mercury at distances from 22 million to 3.5 million miles. The closest of three is also the smallest, only 18 as massive as Earth and surely permanently scorched.
The new planet, which Dr. Fischer called "one of the more annoying planets" because it resisted being folded into their mathematical models for such a long time, basks in the lukewarm light of its star from a distance of around 70 million miles, taking 260 days to complete one orbit. Although too massive for life itself, Dr. Marcy said, the planet could harbor rocky moons, just as Saturn and Neptune in our own solar system do, and these would be warmed to the same lukewarm temperatures as Earth.
The moons would have to be as massive as Mars, however, in order to keep their water from escaping into empty space. Dr. Marcy said, "All bets are off on what evolutionary biology would be like on one of these moons."The astronomers said they were also intrigued by the large gap - a band about 450 million miles across - between the new planet and the outermost one, in which they have detected nothing. There is a similar, but smaller, gap in our own solar system between Jupiter and Mars, caused by the disruptive effects of Jovian gravity on planetary formation. Dr. Lunine suggested that the more massive Cancri planet could have had a similar and deeper disruptive effect.
But the possibility remains that rocky planets could be lurking beneath detectability in that gap. Dr. Lunine said, "This gives us a name and an address to point out space telescopes at in the future."
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