THE FIFTH PLANET discovered around the sunlike star 55 Cancri lies in the projected habitable zone (green), where water would neither boil nor freeze, in between the system's outermost planet (widest blue ring) and its inner members. Image: NASA/JPL-CALTECH
Astronomers have spotted a record-setting fifth planet orbiting the sunlike star 55 Cancri, 41 light-years away in the constellation Cancer. Researchers say the planet, a "mini-Saturn" of about 46 Earth masses, lies fourth out from the star in a large gap between the third and fifth planets, placing it squarely in the estimated habitable zone around the star where water might remain liquid, according to the group's report, accepted for publication in The Astrophysical Journal.
Although the planet's size implies that it is a ball of hydrogen and helium gas incapable of supporting pools of liquid water, the finding raises the possibility that additional, earthlike planets might be discovered around it.
"This discovery of the first ever quintuple planetary system has me jumping out of my socks," says group member and veteran planet hunter Geoffrey Marcy, an astronomer at the University of California at Berkeley. "We now know that our sun and its family of planets is not unusual."
One of the first stars discovered to harbor an extrasolar planet or exoplanet, the 55 Cancri system has come to resemble a jumbo version of our own solar system. Its five planets all seem to orbit along relatively circular paths, and the farthest planet out, a gaseous behemoth the size of four Jupiters, revolves at roughly the same distance that separates Jupiter from the sun.
55 Cancri's innermost planet, weighing in at more than 10 earth masses—meaning it could have a rocky or icy core—lies closer to its star than Mercury does to our own. The new planet sits at 0.8 earth-sun distances (astronomical units) from the star, or roughly the distance between Venus and the sun. Before this discovery, researchers knew of only one other four-planet system, Mu Arae, and several three-planet systems.
Astronomers have uncovered 55 Cancri's planets one by one during 18 years of repetitive measurements at the Lick and Keck observatories in California and Hawaii, respectively. Researchers were looking at the star's Doppler shift, the change in the wavelength, or color, of its light as it moved toward and away from Earth. A star tugged by an orbiting planet will wobble slightly, which can be detected as a regular shift in the star's color corresponding to the time the planet requires to complete an orbit.
Multiple planets imprint multiple overlapping shifts, which require time—and mathematical modeling of possible planetary arrangements—to tease apart. 55 Cancri's outer planet, for example, has an orbital period of 14 years, and was therefore only discovered in 2004. The latest planet was even trickier to identify. "For me personally," says astronomer Debra Fischer of San Francisco State University, the study's lead author, "this was one of the more annoying stars. It resisted mathematical modeling because of this extra planet we finally have extracted."
The study authors are scanning several thousand other stars for exoplanets, but most of them haven't been scrutinized for as long as 55 Cancri, suggesting that more systems with five or more planets are lurking in plain sight of telescopes, says David Charbonneau, professor of astronomy at Harvard University, who was not involved in the study. "The excitement is, yes, there may be gaggles of planets around other stars in their survey as well."
Fischer says she expects 55 Cancri to harbor additional, smaller planets in the large remaining gaps around the new find, given that our own solar system is so densely packed with planets. However, its large outermost planet could have long ago swept that vicinity clean of planetary material, notes planetary scientist Jonathan Lunine of the University of Arizona at Tucson.
Future ground- and space-based experiments should have the power to discover Earth-size planets, which may be anywhere, Lunine says, but current technology is still too limited to spot them.