According to NASA, planet Kepler-425b is remarkable because it combines a number of factors that make it the most likely candidate for habitability found outside the Solar System so far. It's not only roughly Earth-sized, but it also orbits in the habitable zone of a sun-like star.

To date, the Kepler mission has confirmed the existence of 1,030 exoplanets, but so far the search for a planet similar enough to Earth to support life has been an exercise in extremely rough approximations. The technique used by the unmanned Kepler space telescope (launched in 2009) to detect exoplanets relies on measuring the dip in a star's light as a planet passes in front of it. This tends to favor finding large planets orbiting very close to their stars, which means that the easiest ones to find have been very hot super-Jupiters.

Another problem is that even if a small rocky planet like Earth is found, it has to be in its star's habitable zone. That is, its distance from its star is such that the possible surface temperature could allow liquid to exist, so it can't be too close or too far away. However, one particular sticking point until today is the kind of star these planets revolve around. The most common type of stars in our galaxy are red dwarfs, which are K or M types. Since they're so numerous, it's no surprise that most exoplanets are found orbiting them, but these are small, old, and cold stars, which poses many problems for the habitability of their planets.

Earth, on the other hand, orbits a young, medium-sized G2 star, which is relatively warm and stable with a lifespan long enough for life to establish itself. Since so little is known about other star systems, and ours is the only one known to have life, scientists tend to favor sun-like stars as the best candidates for habitable planets.