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Astronomy and Space articles by Martin George of the Launceston Planetarium
25 September 2011
A Planet With Two Suns
Many readers, no doubt, have watched one or more of the famous science fiction movies or series, with spectacular
planetary landscapes and weird atmospheric effects. Some of them have two moons in the sky - quite a possible
scenario for some planets, although we of course have only one.
Another type of object that is unique within our solar system is our Sun. It's a single star, whereas more than half of
all stars are double or multiple, with two, three or more stars in the system.
For a long time, a big question has been: do planets exist in such systems? We now know that they do, but up until
recently, we had established only the fact that there exist planets that orbit one of the two stars. That is, there are
systems in which two stars orbit each other (actually, orbiting their common centre of mass), with one of the stars
having one or more planetary companions in orbit around itself.
The other way in which the laws of physics can allow a planet to exist in a so-called binary system of stars is for the
planet to orbit the pair of stars, instead of just one star of the pair. It's possible for a planet to exist in such a stable
orbit, at some distance from the two stars, but up until recently we had not been able to confirm that such systems
exist.
Now, for the first time, NASA's Kepler spacecraft has confirmed the existence of one of these several suspected
'circumbinary' planets - that is, planets that orbit a star pair.
Kepler's mission is to carefully observe a large number of stars to try to detect the slight drop in light as planets
pass in front of their parent stars as seen from here. This particular observation is of a star system called Kepler 16,
which is about 200 light years away.
By comparison, the nearest star system to the Sun, called Alpha Centauri, is 4.3 light years away. One light year is
equal to about nine and a half million million kilometres.
Kepler 16 is a binary (two-star) system, with the stars orbiting each other every 41 days and averaging about 34
million kilometres apart. They eclipse each other as seen from here, so that twice during each 41 days there is a
slight drop in the combined light from the pair. However, Kepler has shown us that there is also occasionally a very
tiny drop in light as a third object - a planet - passes in front of each of the stars.
By measuring and timing these extra events, astronomers have concluded that the planet orbits both stars at an
average distance of about 106 million kilometres, with an orbital period of 229 days - which represents a 'year' for
the planet. It is a Jupiter-like planet, about 100,000 kilometres across with no solid surface, so it is very different
from our Earth.
However, if it were possible to stand on such a world, and if that world were rotating independently, there would be
some spectacular double sunsets!
Article by Martin George, Launceston Planetarium, QVMAG. Reproduced with permission of the Sunday Tasmanian newspaper