The Earth-sized exoplanet just discovered so close to us in space that we could basically hit it with a tossed rock, Alpha Centauri Bb, has been the topic of discussion this week with unmanned probes coming up as a real possibility. With this planet – also known as 51 Pegasi b – we’re to understand that it’s both slightly too close to its star to be habitable by humans and nearly identical to our planet in mass. The system in which this planet exists contains two stars, Centauri A and B, and exists just a bit more than four light years away from us right this minute.