The Wobbly Habitable Zone of Alpha Centauri B
It’s the nearest star system to our own. It has two Sunlike stars (and one dwarf star). We’ve been measuring the orbits of the binary accurately for over a century. And on top of all that, there are good indications from theory and observations that they are capable of hosting planets. So it’s not a surprise that the Alpha Centauri system has been in the minds of exoplanet scientists for some time.
Alpha Centauri A and B compose a binary system (Proxima Centauri may be closer to Earth, but it’s very far away from the other two Centauri stars). The two stars A and B orbit each other in an ellipse, with a period of about 70 years. We’ve been reasonably convinced that either Alpha Centauri A or Alpha Centauri B are suitable stars to host extraterrestrial exoplanets, mostly because they are quite similar to our own Sun, and the orbit of the binary system is sufficiently large (at closest approach, the stars’ separation is similar to that of Saturn from the Sun) that we’ve been able to more or less ignore Alpha Cen A when looking at Alpha Cen B’s habitable zone (and vice versa).
But as we’ve learned over the last few decades, we shouldn’t be too complacent about ignoring factors that could influence a planet’s climate. Especially when what you’re ignoring is an extra Sun in the solar system. Even if the second star only accounts for 1% of the total heat being delivered to a habitable planet, that 1% could have an important effect on the fate of the planet.
So I decided to see what would happen to planets in the habitable zone around Alpha Centauri B if I added in the starlight of Alpha Centauri A. When I added Alpha Centauri A to a simple climate model which contains a single planet (on a variety of different orbits) and Alpha Centauri B, I found that that small bit of extra sunlight can make a real difference. It can make some planets more habitable, and some less habitable. In fact, it can make the mean temperature of these planets fluctuate by up to 3 degrees C. Now these climate models I run are very simple, but remember that a few degrees C has important consequences – this is why world leaders are trying to agree to reduce C02 emissions.
Life on these planets with wobbling climates would have some unique characteristics. Circadian rhythms (such as our “body clock”) are calibrated to sunlight and the length of the day. But planets around Alpha Centauri B have two types of day – a normal day like we have on Earth, due to the planet spinning on its axis (a “B-day”), and a second type of day defined by when the planet undergoes a full spin on its axis relative to the distant ACen A (an “A-day”). Now the strength of sunlight from A will change as A orbits around B, because the orbit of A around B is elliptical, and quite elliptical at that.
So Life around Alpha Centauri B could have two separate sets of circadian rhythms – one calibrated to star B, and one to star A. Migration patterns will be affected by the fluctuations in temperature – imagine birds that only live for a few years suddenly upping sticks and heading north or south because of a deeply ingrained instinct to move every 70 years because part of the planet’s surface suddenly becomes uninhabitable!
There’s still more to do, of course – these models don’t incorporate the effects of Alpha Centauri A’s gravity, which will slightly change the planets’ orbit, and produce an extra set of periodic fluctuations known as Milankovitch cycles. In any case, we can see once again that looking for habitable planets is not as simple as it may first look.