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Orbits Of Other Distant Planets Oval - Not Circular
Public Affairs Office, San Francisco State University, January 9, 1999
Ligeia Polidora

Unlike the nine planets that make circular orbits around our Sun, all nine of the 17 extrasolar planets which are in distant orbits around their host stars travel in oval-shaped paths. This surprising pattern suggests that our heliocentric perspective skews expectations of worlds elsewhere. The circular paths of planets in our solar system may require special conditions for them to acquire and maintain their more stable circular orbits. "For the first time we have enough extrasolar planets out there to do some comparative study. We have the statistical basis for starting to consider how our solar system compares with other planetary systems," said astronomer Geoffrey Marcy, University Distinguished Professor of Science at San Francisco State University. "We are realizing that most of the Jupiter-like planets far from their stars tool around in elliptical orbits, not circular orbits, which are the rule in our solar system." Marcy, a member of the most prolific team of extrasolar planet discoverers, presented this emerging trend in a press conference today at a meeting of the American Astronomical Society in Austin, Texas. He is the lead author of a paper describing the conclusions that has been submitted for publication to the Astrophysical Journal.

Among the nine planets with elliptical orbits is a new discovery. This planet orbits the star HD168443 in the constellation Serpens, the Snake. It was detected during observations with the world's largest telescope at the W.M. Keck Observatory atop Mauna Kea on the island of Hawaii. The new planet has an orbital period of 58 days and orbits at an average distance of 0.28 Astronomical Units from its star (one A.U. equals the mean distance from the Earth to the Sun). The eccentricity of its orbit - the degree that it deviates from a circular path - is 0.54, about ten times the eccentricity in the orbit of Saturn. The other eight extrasolar planets that orbit farther than 0.2 A.U. from their host stars all have orbital eccentricities greater than 0.1.

Debra Fischer, a post-doctoral researcher at San Francisco State and part of the planet-finding team, said, "With 10 or 20 of these [planets around other] stars you can start looking at what's normal and what's not normal." And elliptical orbits appear to be the norm for planets as massive as Jupiter. As these Jupiter-mass planets careen close to their host stars and swoop back out again, their powerful gravitational attraction might sweep aside any smaller worlds around the same star. Scientists engaged in the Search for Extra-Terrestrial Intelligence (SETI) have begun to ponder the implications of the orbital trend for their endeavor. "This does represent a bit of a wet blanket on the search for intelligence in the galaxy," said Marcy. "The big bullies may wipe clean the terrestrial planets in those planetary systems, rendering them void of any Earth analogues."

However, Marcy pointed out that about five percent of stars in the Galaxy are found to have Jupiter-mass planets within 3 A.U. That leaves 95 percent of stars that may lack such world-wrecking planets; any of those stars could conceivably harbor life-bearing planets. "What we're learning is that 95 percent of Sun-like stars don't have these wildly wacky Jupiters in such close-in orbits, and I would say that's quite good news for the SETI efforts," said Marcy. Researchers seeking other worlds with intelligent life can also take heart in a second trend exhibited by extrasolar planets. Nearly half of those found so far are smaller than or equal to two Jupiter masses, and above four Jupiter masses the number of planets drops off sharply. There are more smaller planets out there, just as in our solar system. "This mass distribution is pointing toward a plentitude of planets smaller than Saturn and Jupiter," said Marcy.

Theorists still have their work cut out for themselves in explaining how the nine extrasolar Jupiter-like planets developed their eccentric orbits. Four plausible scenarios have been proposed. One posits that when enough large planets orbit a star in close proximity, they generate a gravitational slingshot that projects the planets into elongated orbits. Another idea is that a passing star rips through the heart of a solar system and perturbs planetary orbits. The perpetrator could also be an orbiting companion star. Sixty-four percent of Sun-like stars have a companion star, and HD168443 appears to be part of one such binary pair. (Another recently announced extrasolar planet orbits the solitary star HD210277, so that planet's elliptical orbit demands a different explanation.)

The fourth scenario points to the protoplanetary disk, out of which a planet first takes form from dust and gas. The disk may somehow permanently perturb the planet's orbit. Theorists who could comment further on these various ideas include Fred Adams (University of Michigan), Greg Laughlin (University of California at Berkeley), Doug Lin (University of California at Santa Cruz), Fred Rasio (MIT) and Stu Weidenschilling (Planetary Science Institute in Tucson, Arizona). Research colleagues of Geoffrey Marcy's include Debra Fischer (San Francisco State University), R. Paul Butler (Anglo-Australian Observatory), Steven Vogt (U.C. Santa Cruz), Kevin Apps (University of Sussex, England), and Michael Liu (U.C. Berkeley).