Well, it's not exactly new, because it's been around for billions of years, but it's new to us.
Astronomers at the La Silla Observatory in Chile have found an Earth-sized planet orbiting one of the stars in the Alpha Centauri system, which is basically our stellar next-door neighbour!
The newly discovered planet, Alpha Centauri Bb, is just slightly larger than Earth, and orbits Alpha Centauri B every 3.24 days. That puts it very close in to the star, so it will be extremely hot. However, there is some speculation that it may not be the only planet orbiting the star, so there is still hope for planets further out.
Alpha Centauri is a triple-star system that is only 4.3 light years away (about 40 trillion kilometres). Alpha Centauri A is slightly larger than our sun, and like our Sun is a yellow (G-type) star. Alpha Centauri B is slightly smaller than our sun, and is an orange (K-type) star. These two form a binary system, orbiting each other at roughly the same distance that Saturn orbits our Sun. The third star in the system is Proxima Centauri, which is a red dwarf star that orbits far out from the other two.
Alpha Centauri Bb was discovered using the High Accuracy Radial velocity Planet Searcher (HARPS), which detects the minuscule shifts in the light from stars caused by changes in how fast they are moving away from us. In a star system that has a planet or planets, the star is not completely motionless. The force of gravity exerted by each planet orbiting the star will have an effect, however small, on the motion of the star. Basically, the star and planet(s) both orbit a point that is just slightly off from the centre of the star. The motion of the star likely won't be visible if you look at it, because the movement is so small, but the motion of the star causes tiny shifts in the light we receive from it. HARPS is able to detect these very small 'doppler shifts' in the light.
The discovery of this new planet is significant in two ways:
First, it's one of the smallest exoplanets ever found, and apparently the smallest exoplanet found using this radial velocity method (the smallest on the list is 0.008 times the mass of Jupiter, or over 2.5 times the mass of Earth), so astronomers are getting very good at sifting through the 'noise' to find planets.
Secondly, it's the closest exoplanet ever found, in a star system that would be the easiest for us to visit. Unfortunately, we're nowhere near being able to build a ship that would get us there within anyone's lifetime. The Voyager 1 probe currently holds the record for the fastest space vehicle, at over 61,000 km/h, but it would take the probe over 75,000 years to reach Alpha Centauri.
The 100 Year Starship project started this year, with the goal of building an interstellar starship within the next 100 years, but hopefully advances in technology will decrease that timeline so we can check out this planet (and any others that may be in the system) sooner!