The first habitable zone planet
What happened: Kepler finds the first planet where liquid water could exist When it happened: December 2011
December 2011 was a huge moment in exoplanet science when NASA announced
Kepler found the first confirmed planet in the habitable zone of another star. This is the region in which temperatures are not too hot or cold for water to exist, meaning that liquid water could be present on the surface of a planet, and possibly life.
The planet was Kepler-22b, estimated to be about 2.4-times the size of Earth and located about 620 light years from us. It completed an orbit of its star every 289.9 days, with an average temperature expected to be similar to that of Earth.
Initially the discovery caused a huge amount of excitement, with some suggesting it could be an ocean world that contained signs of life. Today the true composition of the planet remains unclear, with some suggesting it could be a gas planet with a rocky core – although the chances of it being a terrestrial world have been ruled out.