20 October 2012
Science fiction fans have been awaiting this news for decades: new results from the European Southern Observatory’s HARPS (High Accuracy Radial Velocity Planet Searcher) reveal that there is an Earth-sized planet orbiting the nearest star system to Earth.
For those of you who are not geeky enough to have the vital stats of the Alpha Centauri system memorized, allow me to introduce you the sun’s nearest neighbor. Or should I say neighbors? Alpha Centauri is actually a triple star system! The sun-like yellow star Alpha Centauri A is the largest, with a mass 110% that of the sun. Orbiting Alpha Centauri A at a distance comparable to the distance between our sun and the outer planets is the slightly orange star Alpha Centauri B, which weighs in at around 90% of the sun’s mass. The third star in the system is the tiny, far-flung Alpha Centauri C (a.k.a. Proxima Centauri), a red dwarf with only about one eighth the mass of the sun. Proxima Centauri keeps its distance from the other two, orbiting at around 0.24 light years from their center of mass. This wide orbit makes Proxima Centauri the closest star to the Earth at a distance of 4.24 light years away. But for this post, we are most interested in Alpha Centauri B, 4.37 light years from Earth and home to the newly discovered planet with the creative name Alpha Centauri Bb.
The new planet is extremely close to its host star, orbiting in just over three days. In binary systems like Alpha Centauri AB (we’ll leave Proxima out for the rest of this post because it’s so far from the other two), there are two areas where you can have a stable orbit for planets: really close, so that one star’s gravity completely swamps the other’s, or far enough away that the planet can orbit both stars without being perturbed. In between is a no-planet’s land, where any planet would get kicked around as it came close to one star, then the other until it got slignshotted out into a more stable orbit. So it’s not surprising that Alpha Cen Bb is so close to its host. The only other choice would be far enough away from its stars that we couldn’t detect the planet.
Alpha Cen Bb is a small planet: only 1.13 times the mass of Earth. But Earth-sized does not mean Earth-like. This new planet is so close to its star that its surface temperature is a toasty 1200 degrees C, so don’t pack your bags yet.
Well yes, but when a planet is this close to its star, the intense heat and radiation would easily rip away any atmosphere, which is kinda useful for habitability. Plus, this planet is so close to its sun and so hot that I would guess there is some vigorous convection inside it, transferring heat from the sunny side to the dark side. So even though the new planet is only a bit larger than the earth, it is probably a nasty lava world rather than a habitable oasis.
So why all the excitement about a nasty little ball of space lava? Well, we now know that the Alpha Centauri system had enough “metals” (annoyingly, astronomers call anything other than H or He a metal) to form a rocky planet, and in general if there’s one rocky planet in a system, it’s likely there are more of them. And one of those might be in the habitable zone. We’ll have to use larger telescopes and longer-duration studies to find planets orbiting farther out from the central stars, but now we at least know it’s worth a very close look.
In the meantime, if we want a closer look at the planet(s) of Alpha Centauri, we had better get going. Our fastest robotic probe, Voyager 1, would take about 82,000 years to reach Alpha Centauri B.