Kepler-186f
Rocky planet · First Earth-sized world in a habitable zone
Kepler-186f was the first Earth-sized planet discovered in the habitable zone of another star. It gets about a third of the energy Earth gets from the Sun, so if it has water, it may be cold – but not necessarily frozen.
- 580 light-years awayEven light – the fastest thing there is – needs 580 years to get here.
- ~1.1 × EarthAbout 1.1 times as wide as Earth.
- 130 daysYear
Take a closer look
Noon like sunset
Kepler-186f’s star is a red dwarf about half the size of the Sun. Standing on the planet at midday, its light would look about as bright as sunlight here an hour before sunset. Everything would be lit by a warm, orangey glow, all day long.
Size known, weight unknown
Kepler measured the planet’s size from the tiny dip in starlight each time it passed in front of its star. But the star is too faint to measure its wobble, so we don’t know the planet’s mass. Without that, scientists can’t be certain it is made of rock like Earth.
Old light
Kepler-186f is about 580 light-years away, so the light Kepler saw had been travelling for nearly six centuries. It set out in the 1400s, around the time the printing press was invented in Europe. We see this star system as it was back then, not as it is now.
How we found out
- 2009
NASA launches the Kepler space telescope to stare at one patch of sky in Cygnus and Lyra.
- 2014
Elisa Quintana’s team announces Kepler-186f, the outermost of five planets around its star.
- 2018
Kepler retires after finding more than 2,600 confirmed planets.
What makes a planet a good home for life?
Liquid water, a protective atmosphere, the right temperature and probably a magnetic field. Being Earth-sized helps – but it doesn’t guarantee any of these.
See it yourself
Only with large telescopes. It lies in Cygnus, the Swan.
From the encyclopedia
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