Sidebar. Multiplicity in star systems.
Great numbers of observations show that about 2/3 of the stars in our Milky Way Galaxy are single stars like our Sun. The other 1/3 of star systems are binaries, ternaries, or higher multiplicity. That makes about half of all individual stars as loners and the other half as having one or more companions. More massive stars are more likely to have companions than are smaller stars. For stars like our Sun, the M-class dwarf stars, a bit less that half have one or more companions:
| Multiplicity likelihood for Population I main sequence stars[63] | |
| Mass Range | Multiplicity Frequency |
| ≤ 0.1 M☉ | 22%+6% −4% |
| 0.1–0.5 M☉ | 26%±3% |
| 0.7±1.3 M☉ | 44%±2% |
| 1.5±5 M☉ | ≥ 50% |
| 8±16 M☉ | ≥ 60% |
| ≥ 16 M☉ | ≥ 80% |
(From https://en.wikipedia.org/wiki/Binary_star)
Here, M☉ is the mass of the Sun.
Our Sun may well have started as a binary, with either it or its partner ejecting the other (maybe that’s quarantine so that other stellar systems aren’t infected with humans; find the cartoon by Eric Lewis in the 2002 New Yorker magazine). Some binaries orbit each other very closely, others at large distances. For M dwarf stars the most common separations appear to be about 10 astronomical units – about the distance from our Sun to Saturn. The orbital periods then cluster about 100 years. So, if you go through the analysis in the main text here you’d find that problematic for energy balance for a habitable planet unless the planet orbits tightly around only one of the stars, which is possible. Our own big planets may have ejected another planet so the prospect of a companion star seriously deranging the orbit of a habitable planet is not readily dismissed.
Binaries present some amusing speculations or even fantasies – a sky with two suns, two beautiful sunsets per day (or more), like Magrathea orbiting the twin suns Soulianis and Rahm in the Hitchhiker’s Guide to the Galaxy. Scientifically relevant to habitability are the effect of two suns on repeated variation of radiation received at a planet orbiting a binary system, the possible chaotic orbiting of the planet or even ejection, and the possible effect on the lifetime of the binary itself.