Johannes Kepler succeeded in establishing a formula for relating a planet's orbital period to its mean distance from the sun, but he failed in his ardent attempt to discern a pattern in the spacings or periods among the planets. Such a pattern, enforced by resonant gravity effects, was subsequently observed in the commensurate periods of some of Jupiter's moons.
Now the reality of synchronous planetary orbits has turned up in a solar system unknown to Kepler -- or to anyone else -- until recently.
Geoffrey Marcy (UC Berkeley) and his associates, discoverers of tens of extrasolar planets, now report that the star Gliese 876, 15 light years from Earth, is orbited by one planet every 60 days and by a second every 30 days.
(The presence of the planets around the star and their orbital properties are deduced from the subtle wobble of the star's position as it is tugged by its satellites.)
The almost exact 2:1 (octave) ratio in the orbital periods should help theorists model the formation of planetary systems.
Marcy, speaking at last week's meeting of the American Astronomical Society (AAS) in San Diego, also reported a second two-planet extrasolar system no less novel.
The star HD168443, 123 light years from Earth, is circled every 58 days by one heavy planet (7.7 Jupiter masses) at a distance of only 0.3 astronomical units (1 AU is the distance from Earth to sun) and by another every 4.8 years.
The second planet's mass, estimated to be at least 17 Jupiter masses, is the most massive exoplanet ever found, and calls into question the notion that planets could not reach much above 13 Jupiter masses without igniting as stars.
(Editor's Note: This story reprints, with only minor editing, an item from PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News Number 520 January 12, 2001 by Phillip F. Schewe, James Riordon, and Ben Stein.)
Related website:
The Search for Extrasolar Planets
16-Jan-2001
