This is huge. We've discovered a lot of exoplanets, including rocky Earth-like ones, in far-away star systems. But the European Space Agency announced this afternoon that our solar system's nearest neighbor, a brilliant trinary star system called Alpha Centauri, has a planet that's roughly the mass of Earth. That's right — just 4.3 light years away, there's a planet that might be a lot like our own.
Well, it might be like our own in terms of its mass, which is quite similar to Earth's. But this planet has a year that's just a little over three days long, which means it's orbiting really close to its sun. The place is likely a superheated ultra-Mercury. Reports the ESA:
European astronomers have discovered a planet with about the mass of the Earth orbiting a star in the Alpha Centauri system - the nearest to Earth. It is also the lightest exoplanet ever discovered around a star like the Sun. The planet was detected using the HARPS instrument on the 3.6-metre telescope at ESO's La Silla Observatory in Chile. The results will appear online in the journal Nature on 17 October 2012.
Alpha Centauri is one of the brightest stars in the southern skies and is the nearest stellar system to our Solar System - only 4.3 light-years away. It is actually a triple star - a system consisting of two stars similar to the Sun orbiting close to each other, designated Alpha Centauri A and B, and a more distant and faint red component known as Proxima Centauri. Since the nineteenth century astronomers have speculated about planets orbiting these bodies, the closest possible abodes for life beyond the Solar System, but searches of increasing precision had revealed nothing. Until now.
Not only is this a big deal symbolically — there's a planet practically next door! — but it's also a huge breakthrough scientifically. Over at Bad Astronomy, Phil Plait explains:
The reason this is a big deal is twofold. For one, Alpha Cen is the closest star system in the sky. Because of that it's very bright, and well studied. Planets searches have looked there for decades, and in fact for a while it was thought the dinky red dwarf Proxima might have a planet. Those earlier findings have been shown to be wrong, though. If it has a planet, it's too small or too far out from the star (or both) to detect it easily.
The other reason this is important is that the signal from the planet is incredibly weak. It was found through its gravity. As it orbits Alpha Cen B, the planet tugs on the star, like two children holding hands and swinging each other around. This sets up a very small but detectable Doppler shift in the starlight. The more massive the planet is, the harder it tugs on the star, and the bigger the signal (making it easier to detect). Also, the closer in a planet is, the larger the signal is… and you get the added benefit of a short orbital period, so you don't have to observe as long to see the cycle of the Doppler shift.
In this case, the planet is low mass but very close in. The Doppler shift in the starlight amounts to a mere half meter per second – slower than walking speed! When I read that I was stunned; that low of a signal is incredibly hard to detect.