Imagine watching a glowing red sunset – with not one but two red discs dipping below the horizon. Or perhaps with another sun shining high in the sky. Impossible to visualise? It sounds surreal – but binary star systems are actually pretty common (estimates range from a third to 85% of all stars in the Milky Way) and some of them are sure to have planets. Several binary systems with planets have already been found, with the 10th discovery (known as the Kepler 453 system) announced just earlier this month.
So what is a binary star system exactly? Many stars when viewed with a telescope are seen to be in fact two points of light close together. Sometimes this is just a chance alignment of two totally unrelated stas. But in a binary system two stars are physically closely linked by gravity, orbiting each other (in fact orbiting around a common centre of mass somewhere in between the two stars).
Binary systems come in all shapes and sizes. For example, the time taken to complete an orbit varies enormously, from less than an hour, up to thousands of years. An example of the latter is Albireo, one of my favourite objects in the summer sky - through a small telescope you can clearly see that it is actually two stars, in beautifully contrasting colours – one blueish green, one amber.)
Sometimes one of the stars in a binary system crosses in front of the other and eclipses it, reducing the brightness significantly – a well known example is the star known as Algol, which dims every 3 days or so for about 10 hours.
Often one star is much smaller than the other (like Sirius A and B in this image – B is the tiny dot to the bottom left!)
And the universe doesn't limit itself to two-star systems - multi star systems with three or even more stars have also been observed!
Inportance in astronomy
Astronomers take a particular interest in binary systems because their movements obey well-understood physical laws that relate gravity to mass, and so careful observation and measurement of the relative movement of the two stars enables calculation of the mass of the stars.
So how do planets fit into a binary system? As you can imagine, if there are any planets, their logistics can get rather complicated. For example, in 2014 researchers at the ALMA observatory in Chile detected two rings of planet-forming material in a binary system named GG Tauri A, with one large ring around the whole system, and a much smaller inner ring around just one of the stars (as in the image above). So it seems that a planet can take a wide orbit around both its parent stars, or a tight orbit around just one of the stars, and if there is more than one planet in a system, the situation can get rather complicated.
The climate on a planet with two suns could be very interesting indeed. Perpetual daylight? Violent temperature swings? It all depends on the size of the parent stars and the distance between them. So could life exist on a planet in a binary system? Some astro-biologists say yes, if the parent stars are relatively close together and the planet orbits both in the so-called 'habitable zone', and they even claim that certain binary systems could have a better chance of hosting life than a single star system.
http://www.eso.org/public/news/eso1434/ Planet-forming Lifeline Discovered in a Binary Star System
http://www.ifa.hawaii.edu/info/press-releases/Kepler453b/ Discovery of 10th Tatooine-like Circumbinary Planet
http://www.astrobio.net/news-exclusive/habitable-binary-star-systems/ Habitable binary star systems
http://www.atnf.csiro.au/outreach/education/senior/astrophysics/binary_intro.html Introduction to binary stars