AUFORA News Update
Sunday, October 27th, 1996
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SFSU RESEARCHERS DISCOVER NEW PLANET WITH OBLONG ORBIT
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.
ORBITAL ELEMENTS
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
PHYSICAL ELEMENTS
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.
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