NASA / JPL-Caltech

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Photo:  This artist's conception shows four of the five planets that orbit 55 Cancri, a star much like our own. The most recently discovered planet looms large in the foreground. The colors of the planets were chosen to resemble those of our own solar system. Click on the image to watch a video from MSNBC's "Countdown."

Planet-hunters say they have detected a giant world that is nestled among four others in a planetary system 41 light-years from Earth. This newfound world is in the "Goldilocks zone" - a place that's not too hot, not too cold, but just right for the existence of liquid water and conceivably life.

The fresh discovery, announced Tuesday during a NASA teleconference, focuses on a star and planetary system called 55 Cancri, in the constellation Cancer. The system is already well-known to astronomers who search for the telltale signs of planets beyond our own solar system - but the newly detected planet has taken the search to a new level.

"We're announcing the discovery of the first quintuple-planet system," Debra Fischer, an astronomer at San Francisco State University and lead author of a paper due to appear in the Astrophysical Journal, told reporters.

Geoff Marcy, a pioneer planet-hunter from the University of California at Berkeley who contributed to the paper, said the planetary system is a "souped-up" version of our own. Like our own solar system, these planets make nearly circular orbits around the parent star - but they're super-sized.

The innermost planet is about the size of Neptune and whips around the parent star in less than three days, at a distance of about 3.5 million miles. The farthest-out planet is four times as massive as Jupiter and takes 14 Earth years to orbit, at a distance of about 539 million miles - or just a little farther out than our solar system's Jupiter.

The planets in between are in the range of Jupiter and Saturn, but the most interesting one is the fourth rock from its sun: a world 45 times the mass of Earth, perhaps a gas giant similar to Saturn or Neptune in composition and appearance. That planet is about 72.5 million miles out from the parent star, in an orbit that's similar to Venus' orbit.

55 Cancri is slightly fainter than our own sun - and that would put the newly detected planet in a habitable zone that should allow water to remain liquid on a rocky surface, astronomers say.

NASA / JPL-Caltech

Photo:  This diagram shows the 55 Cancri system at top and our own solar system at bottom.  In each view, the "habitable zone" is marked as a green band.

A gas giant isn't a likely suspect in the search for life - but any rocky moons around it would be. Just as Jupiter's moon Europa and Saturn's moon Enceladus hold promise for astrobiologists, a moon around the newly detected planet could conceivably be a prime suspect in the search.

"Such a moon would have to be fairly massive," Marcy cautioned. "In fact, it would have to be about as massive as the planet Mars … in order to retain its water."

Jonathan Lunine, a planetary scientist at the University of Arizona, said temperatures on this hypothetical moon might be just a little bit warmer than temperatures on Earth. But like Marcy, Lunine said a bit of caution was in order. "I would recommend not buying real estate on any of these planets" until more readings were available, he said.  

Marcy said the discovery of the fifth planet "has me jumping out of my socks" - not just because of the habitable-zone angle, but because it indicates that planetary systems like our own appear to be more common than astronomers thought just a few years ago.

The new planet, like the four other ones, was detected using the Doppler radial-velocity technique, in which a planet's gravitational tug is detected by the wobble it produces in the parent star. In a long-running project funded by NASA and the National Science Foundation, observations of 55 Cancri were collected using telescopes at the Lick Observatory in California and the Keck Observatory in Hawaii.

The method is one of the most time-honored tricks for finding extrasolar planets - but it takes a long time to gather enough information about the complex wobbles to identify multiple planets in a system. More than 350 velocity measurements were required to untangle the wobbly pattern created by the planets at 55 Cancri.

"Discovering these five planets took us 18 years of continuous observations at Lick Observatory, starting before any extrasolar planets were known anywhere in the universe," Marcy said in a NASA announcement. "But finding five extrasolar planets orbiting a star is only one small step. Earthlike planets are the next destination."

As currently used, the Doppler technique isn't sensitive enough to detect planets around the mass of our own - but Fischer said an intriguing orbital gap in the 55 Cancri system is big enough to harbor Earthlike planets that could be found in the future using more precise techniques.

"There could be 10 Earthlike planets there, but we've just not detected them yet," Fischer said.

Finding new Earths around sunlike stars would be a "holy grail" for planet-hunters, Lunine said. Such worlds could harbor alien life, or not. Either way, the quest could answer humanity's deepest questions about life, the universe and everything.

But like all quests, this one has its bumps in the road: Marcy and Fischer noted that one of the extrasolar planets that was once thought to be potentially hospitable to life, Gliese 581c, is now said to be too hot rather than just right. (There's a debate about that.)

To reach the true grail, scientists will have to develop new ground-based telescopes and launch new spacecraft such as NASA's Kepler probe and Europe's Darwin flotilla. Most importantly, they'll have to cast a wide, wide net.

"If you  asked me where the right place would be to look for Earthlike planets," Lunine said, "my answer would be anywhere, and everywhere."

Update for 9:35 p.m. ET: Be sure to check out this video segment from MSNBC's "Countdown." Host Keith Olbermann discusses the newfound planet and other space news with Derrick Pitts, chief astronomer at the Franklin Institute.

cosmiclog.msnbc.msn.com/archive/2007/11/06/451256.aspx