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SFSU Public Affairs Press Release

Published by the Communications / Public Affairs Office
at San Francisco State University, Diag Center.

#24 October 22, 1996
Contact: Ligeia Polidora 415/338-1665


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


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.


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.


P=    804.4 days   s.e. =  12.4
T (JD)= 48941.508 J.D.  s.e. =  10.523
K =   46.592 m/sec   s.e. =   8.219
e =    0.666   s.e. =   0.091
w (omega)   =   86.807 degrees     s.e. =  12.908

a*sin(i)   =    3.84328E+08 meters    s.e. =  6.21915E+07
f(m) =    3.49068E-09 solar masses    s.e. =  1.69798E-09

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.

+                                                                             +
+  Weave a circle 'round him thrice, and close your eyes with holy dread...   +
+                                                                             +