If you go far enough into the past you will find that people believed Earth was the only planet capable of supporting life. Now that assumption is in doubt as we know there are likely billions of planets orbiting millions of stars, and there is a chance some of them orbit in the habitable zone and look like Earth. Now researchers at the Harvard-Smithsonian Center for Astrophysics have determined that such a planet may be as near as 13 light years away.
The researchers arrived at this figure after reexamining data from the Kepler Space Telescope, which hunts for planets. Their analysis determined that approximately 60% of red dwarf stars should have a planet smaller than Neptune orbiting them. Most of these planets are likely too large and too far from the star to support life, but some 6% should be approximately the size of Earth and in the star's habitable zone. Red dwarfs are the most common kind of star and roughly 75% of the stars near us are red dwarfs, so even at just 6%, there is a good chance the nearest Earth-like planet is just 13 light years away.
Of course this planet would be very different from our own as it will be orbiting the smaller and cooler star much more closely, which means they have a very short year. The researchers did identify three red-dwarf systems that are likely to have a planet in the habitable zone, and the longest year amongst them was just 56 days.