One star, five planets

With the discovery of a fifth planet circling the nearby star 55 Cancri, astronomers have found the richest—and heaviest—planetary system beyond the sun’s.

PLANETARY MENAGERIE. Artist’s depiction of four of the five planets known to orbit the nearby star 55 Cancri. The recently discovered fifth planet looms largest. The star’s three innermost planets are seen in the background. JPL/NASA

The sunlike star 55 Cancri lies just 41 light-years from Earth. Exoplanet hunters found the fifth planet by recording the tiny wobble it induces in the motion of its parent star—the same way researchers found the four other orbs. Three of those earlier finds, ranging in mass from that of Neptune to that of Jupiter, have orbits smaller than about one-tenth Earth’s distance from the sun. The fourth is about four times as massive as Jupiter and circles 55 Cancri at about Jupiter’s distance from the sun.

Theorists predicted that the large gap between the inner and outer planets could accommodate an additional planet in a stable, circular orbit, provided it was not too large. The newfound planet fills the bill, residing at a distance of about three-quarters Earth’s separation from the sun and weighing at least 45 times as much as Earth, Debra Fischer of San Francisco State University and her colleagues announced at a Nov. 6 NASA briefing.

That orbit places the planet in the star’s habitable zone—the region where water can be liquid. The new planet is presumably a giant ball of gas, but if it has moons, “like all the giant planets in our solar system, then the moons could provide a rocky surface for liquid water to pool,” says Fischer.

The planetary entourage of 55 Cancri “exhibits the ‘fullness’ of our solar system,” notes Fischer. “It is hard to find a place in the inner part of that planetary system where another planet could be hiding.” If other planetary systems are similarly full, she adds, “we’d know exactly where to look for the currently invisible Earths: the empty, unperturbed habitable zones of nearby stars.”

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