NASA’s Kepler space telescope

Overview: The Kepler space telescope has run out of fuel and will be retired after a 9-1/2-year mission.

• Currently orbiting the sun 156 million km from the earth, the spacecraft will drift further from our planet when mission engineers turn off its radio transmitters.

About Kepler Mission:

• Launched in 2009, the Kepler mission is specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets.
• Since the launch of the observatory in 2009, astronomers have discovered thousands of extra-solar planets, or exoplanets, through this telescope alone. Most of them are planets that are ranging between the size of Earth and Neptune (which itself is four times the size of Earth). Most of these planets were discovered in a small region of the constellation Cygnus, at which Kepler was pointed for the first four years of its mission.
• As of March 2018, Kepler had found 2,342 confirmed planets; add potential planets, and its find of exoworlds stands at 4,587. 

What is the habitable zone?

If a planet is too close to the star it orbits, any water on the surface quickly boils off, forming a steam atmosphere. If the planet is too far from the star, any water on the surface freezes.

• The habitable zone (or “Goldilocks zone”) is the range of orbital distances from a star at which liquid water can exist on the surface of a planet.
• This range of distances changes depending on the size and temperature of the star.
• Earth is in the habitable zone of the sun – one of the reasons our planet has liquid water like oceans and lakes.

Sources: the hindu.