Size comparison of Kepler's recent planet discoveries
The techniques employed to detect planets have historically been limited to transitions in front of a parent star - effectively a dip in brightness, and small shifts in the position of the parent star due to the pull of the planet's gravity as it orbits. The gravity technique is more adept to finding heavier planets and has a limited capability to detect the likes of an Earth-like planet. Kepler refines the transition technique with a much higher resolution and the dark environment of space.
This is a very exciting find coming from the analysis of just 43 days of data. There is over 8 months of data available to date, so expect a lot more announcements from Kepler in the near future.
Happy Planet Hunting, Spacers!
Image credit: NASA