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PLANETS OF THE DOUBLE SUN
It's a common staple in numerous science fiction stories and movies: a planet
with two suns (often of drastically different colors) producing double shadows,
multiple sunrises and sunsets, and other exotic phenomena that would appear
utterly alien to us Earthlings. While substantial liberties are often taken with
such stories, the basic underlying scenario is -- at face value, anyway -scientifically sound, and in fact reflects a basic reality. Approximately 2/3 of
the stars like the sun in our galaxy are members of double or multiple star
systems, and any planets that might be accompanying these stars would experience
at least some of the "double sun" phenomena. Indeed, the nearest star to our
solar system, Alpha Centauri, is a double-star system, comprising two stars
(both quite similar to our sun) orbiting around each other every 80 years. At
their closest they are slightly farther apart than the distance between Saturn
and our sun.
Science fiction stories and speculation aside, a major question becomes whether
or not planets could exist within double-star systems. The presence of a second
star, with its accompanying significant gravitational field, would be expected
to produce major effects upon a planet's orbit and its stability. Over the years
numerous astronomers have performed stability analyses with a variety of
mathematical techniques, and the general conclusion is that there are two
scenarios within double-star systems that can produce stable planetary orbits.
One of these scenarios involves two stars that are very close together with
planets that orbit well away from the stars; from a gravitational perspective,
the planets see the two stars as a single object. The other scenario involves
two widely separated stars, with planets orbiting around either of them, and the
companion star being too far away to affect their orbits gravitationally. (The
Alpha Centauri system falls within this latter category for planets orbiting out
to about the distance of Jupiter.) Double-star systems where the two stars are
separated from each other by about the same distance as Earth's distance from
the sun are out of luck as far as planets are concerned, except perhaps for
planets in orbits at around Pluto's distance (which would be orbiting the
combined center of gravity of both stars).
An even more basic question concerns whether or not planets could actually form
in such systems. Our knowledge of how the planets formed in our solar system is
far from complete, however we know that they formed through the gradual
accumulation of smaller objects colliding with each other over periods of
millions of years. A nearby massive object can disrupt this process; indeed, the
asteroid belt in our solar system is widely thought to be a planet that was
unable to form as a result of Jupiter's gravity. We don't have enough knowledge
of this process to determine just how much the presence of a companion star can
disrupt the formation of planets over an entire system; the best we can do right
now is try to see if we can find any planets that might exist within such
systems, and work from that point.
Among the more than 100 planets outside the solar system that have now been
identified, a handful are indeed within double-star systems. The first-known
such planet was independently discovered by two teams of astronomers (Geoff
Marcy and Paul Butler in California; Bill Cochran and Artie Hatzes in Texas) and
announced in 1996. It orbits around one of the components of 16 Cygni, a system
comprising two stars each of which is almost identical to our sun, and now
visible low in our northeastern sky during the pre-dawn hours. The two stars of
the 16 Cygni system are so far apart, however -- a thousand times the distance
between the Earth and the sun, with an orbital period of perhaps thirty thousand
years -- that from a gravitational perspective they can be treated as single
stars. Until recently, similar statements could be made about the handful of
other planets that have been found in double-star systems.
Last year, however, the Texas team announced their discovery of a planet
orbiting around Gamma Cephei, a moderately bright star located near Polaris and
currently visible in our northwestern sky during the evening hours. The Gamma
Cephei system comprises a primary star that is somewhat larger than our sun,
together with a smaller companion star that's about half the sun's size orbiting
the primary at the approximate distance of the planet Uranus in our system. The
planet appears to be about twice the size of Jupiter, and orbits the primary
star every 2 1/2 years in an orbit that corresponds to that of the asteroid belt
in our solar system.
The existence of this planet in the Gamma Cephei system -- which is not
dramatically different in terms of gravitational influences from what one would
encounter in the Alpha Centauri system -- suggests that planets can indeed form,
and exist in stable orbits, within double-star systems. As our techniques
improve over the coming years hopefully other planets in such systems can be
detected, which will aid us in understanding the process of planet formation as
well as give stronger indications of the number of planets in the galaxy.
Several years ago this author performed some calculations for hypothetical
Earthlike planets within the Alpha Centauri system, and found that changes in
the additional radiation received by the planets due to the companion stars was
minimal, never amounting to more than a few percent over the 80-year period of
the system. Any intelligent race that might live on such a planet would probably
have their myths and cultures strongly influenced by the sight of the companion
star (and any accompanying large planets it might have) in their sky, and of
course astronomical observations would be profoundly affected. If the recent
discovery around Gamma Cephei is any indication, such phenomena might be fairly
commonplace in our galaxy, and the nearest view of the "double sun" might be
right next door.