Surely any astronomer has spent at least some time wondering about Alpha Centauri. Being a tantalisingly close 4.37 light years from us, it really is our next door neighbour. The third brightest star in the sky (excluding the Sun), visible easily from the southern hemisphere and often talked about in science fiction. Better still, many believe that there’s a good chance the system may host planets. And we may be able to detect them using all the same techniques we’ve already been using!
I figured I could do with practicing my celestial mechanics a little, so I put this together. I’m ignoring Proxima Centauri in this, incidentally. On this scale, Proxima would be about 4 metres away from your computer screen. This is also all two dimensional — assuming all objects share the same orbital plane, as stellar systems often do.
Alpha Cen B orbits Alpha Cen A in an elliptical orbit. At periapsis it’s around 11.2AU away, and at apoapsis it’s about 35.6AU away. according to my back-of-the-envelope calculations, at periapsis the Lagrange L1 point is about 5.04AU from Alpha Cen B. So roughly speaking, any planets orbiting Alpha Cen A will have an orbital radius under 6AU and any around Alpha Cen B will have an orbital radius under 5AU. To give that some perspective, Jupiter orbits Sol at around 5AU. While a massive jovian planet may have trouble being stable in a similar orbit around either star (its hill sphere would be big enough that the other star could perturb it gravitationally), terrestrial planets may manage with little difficulty. Indeed, simulations made by some astronomers suggest that terrestrial planets would form readily in such a system.
Both stars are comparable to Sol in size, luminosity and metallicity, so it’s a definite possibility that planets could form. If Proxima is part of the system, it may even have one or two of it’s own. A is a class G2 just like Sol, and B is a class K2. If you were trying to recreate Earth, 1.25AU from A or 0.7AU from B would be about the right distance. The gravitational influence from the two stars may even shepherd any large asteroids or comets away from the habitable zones, the same way Jupiter does in our system. Doing some quick and dirty lagrange L2 point calculations imply that a circular orbit more than about 51AU from Alpha Cen A could also be stable. To compare here, Pluto is around 49AU from the Sun at Aphelion. Some Centaurian version of the Kuiper belt could conceivably exist past this point. No oort cloud has been detected yet around Alpha Cen, but if Proxima is genuinely part of the system, it would probably disperse any oort cloud objects. It’s currently around 1300AU from Alpha Cen A, which would put it in the midst of any hypothetical inner oort cloud.
Wouldn’t it be interesting if they discovered Earth-like planets around these stars? If there are any to be found there, I think we might find them someday really soon!