© americaherald.com
Artist's representation of an ideal exoplanet.
Scientists have found thousands of Earth-like alien worlds, called exoplanets, revolving around remote stars, since the first one was discovered in 1988.
Planetary Habitability Laboratory (PHL)
The Earth Similarity Index (ESI) is a measure of Earth-likeness for exoplanets: 1.0 means identical to Earth. Zero means no similarity.

Any exoplanet with an ESI value above 0.8 can be considered Earth-like, which means that it has a similar size and composition to Earth with a temperate atmosphere that might potentially support terrestrial life forms.
Potentially habitable exoplanets
© Planetary Habitability Laboratory @ UPR Arecibo, 2015
Potentially habitable exoplanets ranked by the Earth Similarity Index (ESI)
If confirmed, Gliese 581g has the highest ESI number so far, estimated as 0.92. The star Gliese is located 20 light years away from Earth, in the constellation Libra. The existence of its planets 581g - 0.92 (ESI) - and 581d - 0.72 (ESI) - are subject of constant debate.
Distance of exoplanets from Earth
© Sott.net
The circle's radius shows the maximum range, 2541 light years for Kepler 443-b, at which potentially habitable exoplanets were detected so far. Earth is at the center of the circle.
How do astronomers find exoplanets?: learn more about the methods used here.

Exoplanet discoveries by year
© Wikimedia Commons/Aldaron
A graph of exoplanet discoveries by year, with detection method represented by color. Green = transit, blue = radial velocity, red = direct imaging, orange = gravitational lensing.