But simulations of how the planets moved in the system were difficult to resolve with data from just the four known worlds.
"This was one of the more annoying stars—it resisted mathematical modeling," said Debra Fischer, an astronomer at San Francisco State University and another member of the 55 Cancri team.
A 2004 upgrade to the W. M. Keck Observatory in Hawaii allowed astronomers to eventually confirm that a faint signal previously seen around the star is a fifth planet.
With the newest planet, the system "perfectly fits our models," Fischer said.
"As we were able to isolate this as a separate object, the orbital eccentricities for other planets settled down a bit," she added.
The new data suggest that "all five planets orbiting this star, 55 Cancri, reside in nearly circular orbits," UC Berkeley's Marcy said.
For example, 55 Cancri f keeps a roughly circular orbit about 72.5 million miles (116.7 million kilometers) away from its sun.
"So [the new planet's] temperature would vary … not unlike the seasons on Earth or Mars," Marcy said.
But San Francisco State's Fischer noted that even though 55 Cancri f is in the right place to potentially harbor life, it is probably an inhospitable gas giant that would most resemble a "beefy Neptune."
If the planet had a small, rocky moon, that would be a better candidate for finding life.
"This would have to be a fairly massive moon in order to retain its water," UC Berkeley's Marcy cautioned.
"You could imagine microbial life springing up … but all bets are off on what the evolutionary biology might be on such a moon."
Finding Life
The fact that the 55 Cancri system has so many large planets suggests that there could be Earthlike worlds lurking in the gaps.
Jonathan Lunine is a planetary scientist at the University of Arizona in Tucson and another member of the 55 Cancri team.
"For all we know there may be Earthlike planets around most solar-type stars—we simply can't see then yet," Lunine said.
Current technology allows astronomers to infer the presence of distant planets by looking for their gravitational pull on their host stars or for a transiting world periodically dimming the light from its star.
Such methods don't allow for direct observation of planets as small as Earth.
"This discovery will really help push for the next steps in planet detection," Lunine said.
"There should be a range of ground-based and space-based systems that should be brought to bear" to find Earthlike worlds in a star's habitable zone, he said.
Even if a planet is found in the zone, it's impossible to tell if it could support life without using spectrograph tests to determine what its atmospheric chemistry is like.
Earth, for example, would be a cold, lifeless place if we didn't have gases in our atmosphere contributing to greenhouse warming.
"I would advise not buying real estate on these other planets until we have the spectroscopy," Lunine said.
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