So you’ve probably heard the exciting news, that a small, earth-mass planet has been identified around one of the components of Alpha Centauri. It’s the nearest neighbouring system to our Sun. Despite that it isn’t the brightest star in the sky (that’s Sirius), also it’s a southern star so is not very familiar to those of us who live in the Northern hemisphere. In-fact I didn’t see it until I was 28 and had moved to Hawai`i. Here’s a picture I took of it rising above Kilauea.
Because it was so far south, Alpha Cent. wasn’t well studied by early European astronomers. In the 1830s Thomas Henderson, the first Astronomer Royal for Scotland measured its distance using trigonometric parallax. This was the first distance measurement to a star other than the Sun. However he hesitated in publishing the result and Bessell scooped him to the first published distance to a star (61 Cygni).
The system itself actually consists of three stars. The two brightest components appear as a single source to the naked eye. The two stars orbit each other at a distance about 17-18 times the Earth-Sun distance. There’s third star in the system, Proxima Centauri. It’s 100 times too faint to see with the naked eye and while the two brighter components are of similar mass to the Sun, Proxima is less than an eighth of that. That said for people like me who study low mass stars, it’s still not that low mass. It was discovered by another Scot, Robert Innes who worked as a wine merchant in Australia before taking up astronomy fulltime. Simply, he found a star with a similar motion across the sky to Alpha Centauri and given their close positions (yet it’s still 17,000 times the Earth-Sun distance from Alpha Centauri AB), deduced they were a pair. This is what I do a lot in my research, but I use large catalogues produced by data pipelines, he used many painstaking measurements by hand. Sometimes I feel like modern astronomy is cheating. One interesting side-note, this double star with a wide companion set-up seems to be more common than a single star with a wide low mass companion. A rather nice recent paper by Peter Allen and others quantified this and indicated that this may say something about how these systems form.
A quick note about the planet. It appears to be too close to the star to sustain liquid water and hence life. This system has a fairly complicated Habitable Zone. The planet is orbiting the smaller of the two stars but it will also be heated by Alpha Centauri A. Duncan Forgan wrote a paper about this earlier this year saying the difference will be small but will induce oscillations of a few degrees. Also sometimes on the surface of the planet Alpha Centauri B will have set but the planet will still be lit by Alpha Centauri A. This will of course change as the two stars orbit each other. Duncan has a nice blog post about the paper where he also talks about sleeping rhythms on a hypothetical planet around Alpha Cent B. When Alpha Centauri A is on the other side of Alpha Centauri A from the planet I guess sunset will look a bit like that on Tatooine. Although with a different brightness ratio between the two stars.
So it’s Alpha Centauri has a planet. It isn’t able to support life, but maybe there’s another one in the system that can. What this discovery makes me think of is Civilisation. No, not the noted BBC TV series, but the classic Sid Meier strategy game. One of the victory conditions was to send a spacecraft to Alpha Centauri. I never got that far, on more difficult levels my civilisation would die in the Bronze Age and in the harder levels I’d get bored of nuking phalanxes in about 1900 and give up. However if I hadn’t given up perhaps I could have built a ship to head for what the Nature press release calls a “scorched barren rock”.