# Are we alone in the Universe? Habitable planets

A revolution has occurred in the last two decades in the world of astrophysics. It all started in the mid ’90s with the first discovery of new worlds around other stars. The term “Extrasolar planet” (or Exoplanet) became widely used to identify planets orbiting a star other than the Sun. A planet is a celestial body massive enough to be bounded by its self gravity (unlike a rock or an asteroid, that are kept together by electromagnetic forces), but not massive enough to produce energy through nuclear fusion (as stars do). Planetary scientists have confirmed the existence of more than 1500 exoplanets and have identified a few thousand exoplanet candidates that require more investigation before they can join the planet club (see exoplanets.org for the most recent figures). The most remarkable discoveries came only in the last couple of years thanks to the Kepler space telescope. This amazing instrument has been patiently looking for the extremely tiny dimming induced by the passage of a planet in front of its host star. The wealth of data provided by Kepler has revealed an astonishing fact: “When you wish upon a star, you are wishing upon a star with planets” (W. Borucki). There is on average one planet orbiting every star in the Universe (Swift et al., 2013; Cassan et al., 2012). Just in our Galaxy this means we have 100 billion planets. Since we have about 100 billion galaxies in the Universe, there are about 10,000,000,000,000,000,000,000 = $$10^{22}$$ planets out there.