A Major Discovery: A Planet in the Alpha Centauri System

A major astronomical discovery was announced in the past few days, one that was not all that widely reported (or at least I didn't see a lot of reports on it). A team of astronomers announced that they had discovered a planet in orbit around Alpha Centauri B, one of the three stars in the Alpha Centauri system. The stars of Alpha Centauri, at a distance of 4.3 light years, are the closest stars to us other than the Sun itself. What's more, the two main stars, Alpha Centauri A and Alpha Centauri B are very similar to the Sun, so a planet orbiting at the right distance from either of them might very well be habitable. The recently discovered planet is certainly not habitable by life as we know it, as it orbits far to close to its star and so is probably hot enough to melt rock. But where there's one planet, there may be more. The prospect of habitable planets around one or both of these relatively close, Sun-like stars is very exciting.

Alpha Centauri A is a yellow star of the same spectral class as the Sun, though it is very slightly hotter and more massive. Alpha Centauri B is a little cooler and less massive than the Sun, and is orange in color. These two stars orbit each other at a distance at a distance ranging from that between the Sun and Saturn to that between the Sun and Pluto. A third star, a small, cool red dwarf named Proxima Centauri, appears to orbit the other two at a great distance (it is somewhat closer to us, and so is the closest star other than the Sun). The planet orbiting Alpha Centauri B, designated Alpha Centauri Bb, is slightly larger than Earth (making it the smallest planet yet discovered orbiting a normal star other than our Sun) and orbits the star in only a little over 3 days, as compared with 88 days for Mercury in our solar system and 365+ days for Earth. Since it is so close to Alpha Centauri B, it is certainly very hot, probably several times as hot as Venus, which has surface temperatures of over 400 degrees Celsius. If there is a habitable planet orbiting Alpha Centauri B, it would have to be much further out. It is even possible that both Alpha Centauri A and Alpha Centauri B have habitable planets, which would be an exciting prospect. If such planets do exist, we may discover them within the next decade or two, or even sooner, though it is easier to detect planets that are very close to their stars with our current technology.

However, even if we do discover planets in the habitable zones of Alpha Centauri A and Alpha Centauri B, it will take further advances before we will be able to look for indications that life exists on them (such as the presence of molecules that are unlikely to produced by non-organic processes). Supposing that we do find signs of life on a planet in the Alpha Centauri system a few decades from now. What then? The obvious answer would be to go and visit them, or at least send a probe to explore them. But while Alpha Centauri is relatively close, the key word here is "relatively". Even the closest stars are still incredibly distant. It takes light 4.3 years to reach them from the Sun traveling at 300,000 km per second, while light from the Sun reaches the Earth in only 8 minutes and reaches Pluto in about 5 hours. Our current rockets can't reach even 1% of light speed, so it would take them thousands of years to get to Alpha Centauri. While propulsion systems have been proposed that might get us up to a few percentage points of light speed, it will take quite a bit of work, including further technological advances and lots of money, to actually build them. So it's not likely that we will be able to visit Alpha Centauri anytime soon. But if habitable planets are actually discovered there, especially ones with signs of life, it might provide the incentive needed for humanity to undertake the long-range project of trying to get to them. This discovery may be the first step along that road.

Here's an infographic providing more information on Alpha Centauri and the newly discovered planet.