SFSU Researchers Discover New Planet With Oblong Orbit

October 22, 1996

"Eccentric" orbit shatters long-held theory of circular orbits

SAN FRANCISCO--A remarkable new planet around a Solar-like star (16 Cygni B) has been discovered by Drs. Geoff Marcy and Paul Butler of SFSU, and Drs. Bill Cochran and Artie Hatzes of the University of Texas - two teams working independently. This planet orbits its star with the most extreme ``eccentricity'' (i.e., oblong shape) ever found for any planet, e = 0.6, on a scale of 0 to 1. All of the planets in our Solar System reside in nearly circular orbits, having eccentricities less than 0.2. This new planet dismantles the long-held theory that other planets in the universe would all have nearly circular orbits.

The discovery was made by measurements of the Doppler shift of the light from the Solar-type Star, 16 Cyg B, (spectral type = G2.5) which is 85 light years from Earth. The star exhibits a periodic Doppler variation, with a period of 804 days (= 2.2 years). The star changes its velocity by +- 46.5 meters/sec every 2.2 years, in a pattern that is NOT a perfect sine wave.

This wobble implies that a planet orbits the star with an orbital period of 2.2 years and has a mass of at least 1.5 Jupiter masses. The actual mass of the planet may be slightly greater than 1.5 Jupiter masses, the uncertainty being due to the unknown tilt of the orbit plane which enters into the orbital physics (as the trigonometric sine of inclination).

Of extreme importance is the unprecedented eccentricity of the orbit, unlike that for any other planet. Its orbit carries the planet from a closest distance of 0.6 Earth-Sun distances to 2.7 Earth-Sun distances at its farthest from its host star, 16 Cyg B. The planet would experience extreme variations in the heat energy it receives from its star, as it varies from Venus-like distances to Mars-like distances.

The oblong shape of the orbit is easily determined from the graph of Doppler-shift versus time. This graph is not a sine wave, which occurs for circular orbits. The departure from a sine wave is due to the speeding-up of the planet as it rounds the star at closest approach, much as the sound of a car engine changes pitch (also by the Doppler effect) as it rounds a sharp curve.

This planet adds to the mystery of a previously discovered planet around the star, 70 Virginis (discovered by the SFSU Marcy and Butler team). Its planet also has a large eccentricity of 0.4 , the previous record holder. But that non-circular orbit was so discordant with the expected circular orbits from theory, that some theorists hoped it could be dismissed as a failed star (i.e., a "brown dwarf"), thereby ignoring the problem of how a planet (or any object) might become so eccentric. Now, new theories must be found to explain these two eccentric planets. Proposed theories involve collisions of two planets that scatter them into wacky orbits (Doug Lin, UC Santa Cruz and Fred Rasio MIT) , or gravitational perturbations from the disk of gas and dust out of which the planets formed (Pat Cassen of NASA Ames Research Center, and Pawel rtymovicz of Stockholm Observatory).

This new planet was discovered completely independently by two teams: Drs. Bill Cochran and Artie Hatzes from the University of Texas and Drs. Paul Butler and Geoff Marcy of San Francisco State University and U.C. Berkeley. Each team has an ongoing, extremely sensitive technique for measuring the Doppler shifts of stars, designed explicitly to detect the perturbations imposed on the stars due to the gravitational force exerted on it by orbiting planets. This planet represents the sixth planet discovered by the team of Butler and Marcy, and brings the total of known planets outside our Solar System to eight.
TECHNICAL SOLUTION: Formally, this is the solution for 16 Cyg B from the COMBINED measurements of both teams. The San Francisco State team provides Doppler measurements that have better precision (8 m/s compared with 27 m/s). But both teams detect virtually the same orbit. One deduces that 16 Cyg B is about 1.0 solar mass, as it's spectrum (G2.5 V) is nearly the same as the Sun's (including age and metalicity). Indeed, it is often deemed a ``Solar Twin''.

This gives a companion mass of :

M_comp = 1.52/sin i Jupiter masses.

The semimajor axis of the planet about the star is:

a = 1.7 AU (1.7 earth-sun distances) coming directly from Kepler's 3rd Law.

San Francisco State University

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