Elliptical orbits may be planetary norm

Science News, Dec 11, 1999

Astronomers have found indirect evidence for another six planets orbiting nearby, sunlike stars. That brings to 28 the number of known planets outside the solar system.

All six have highly elongated orbits--far more elliptical than the paths of Earth, Jupiter, Saturn, and Uranus. That pattern follows a trend: All of the 18 previously discovered planets that lie at least one-fifth of the Earth-sun distance from their parent stars also have elongated orbits (SN: 1/30/99, p. 79).

"The rather frightening prospect looms that our solar system's nested, circular orbits may be a rarity among planetary systems," says Geoffrey W. Marcy of the University of California, Berkeley, codiscoverer of the new planets.

He speculates that the elliptical orbits may arise from the mutual gravitational tug among the members of a system of planets. The pull of gravity disturbs the planets, elongating their initially circular orbits, astronomers theorize.

For some reason, that disturbance didn't occur in our planetary system. Life evolved here, Marcy suggests, precisely because circular orbits keep planets at a constant distance from their parent star and therefore foster a stable climate. "We wouldn't be here if the orbits were highly elliptical," he says.

As with previous planet discoveries, Marcy and his colleagues inferred the presence of the six new planets by tracking the motion of stars. The orbiting planets cause a small but measurable wobble in the motion of their parent stars. The team detected the wobbles using a spectrometer on the Keck I Telescope atop Hawaii's Mauna Kea

Marcy, Steven S. Vogt of UC, Santa Cruz, R. Paul Butler of the Carnegie Institution of Washington (D.C.), and Kevin Apps of the University of Sussex at Brighton in England announced the findings Nov. 29. Details appear in an article the team posted on the Internet (http://xxx.lanl.gov/abs/astro-ph/9911506).

Three of the six planets lie, on average, the same distance from their parent star that Earth does from the sun. The six range in mass from slightly less than that of Jupiter to several times as heavy.