The Alpha Centauri system has always been complex, and last month, it became more so. The two main stars in the system are Alpha
Centauri A and Alpha Centauri B, a pair of stars of about solar mass, orbiting around each other at a distance of around 10 astronomical units. There is also Alpha Centauri C, a dim red dwarf star that may or may not be gravitationally bound to the larger two members, 10,000 astronmical units away.
And there is a newly discovered fourth member of this grouping: a light, small exoplanet orbiting very closely around Alpha Centauri B. In keeping with exoplanetary nomenclature, it is desginated Alpha Centauri Bb.
The technical skill needed to detect this planet is incredible. It was found by the European Southern Observatory, using ground-based telescopes in South America. Using the technique of observing doppler shift in Alpha Centauri B, they detected a slight wobble in its motion. This motion was only 1.8 kilometers an hour, which they helpfully rendered as "the speed of a baby crawling". From this, they calculated the probable size and orbit of Alpha Centauri Bb.
The good news is that the planet is around the mass of the earth. It is almost certainly a terrestrial world, with a solid surface. The bad news is that it is very close to its star, 4% of the distance from the sun to the earth. At that distance, its surface is probably hot enough to be perpetually molten, as well as being constantly bathed in ultra violet and X-Ray radiation. In other words, there is no way that life as we know it could possibly exist on this planet. For that matter, if we could figure out a way to get there, we couldn't even land a spacecraft on its surface.
But if there is one planet in the Alpha Centauri system, there may be more. Although the mechanics of keeping multiple planets orbiting in a dual (or triple) system are complicated, it could be possible that there are other earth-size planets in Alpha Centauri's habitable zone.
So while the existence of Alpha Centauri Bb might not immediately change much (since it is an uninhabitable planet orbiting a star we can't currently reach), it is still a heartening discovery due to the technical expertise used to discover it, and because there might be other, more hospitable planets, in the system.
The initial report of the ESO can be found here:
http://www.eso.org/public/news/eso1241/