By Mike Wall
While astronomers didn't bag that elusive first "alien Earth" in 2013, they made plenty of exciting exoplanet discoveries during the past year.
Here's a list of the top exoplanet finds of 2013, from a tiny world about the size of Earth's moon to a blue gas giant on which it rains molten glass:
The smallest exoplanet
In February, astronomers announced the discovery of Kepler-37b, the smallest alien world ever found around a sun-like star. The planet is about 2,400 miles (3,900 kilometers) wide, making it just slightly larger than Earth's moon. [9 Exoplanets That Could Host Alien Life]
Kepler-37b, which was spotted by NASA's prolific Kepler space telescope, lies about 215 light-years from Earth. The exoplanet is likely far too hot to host life as we know it; it zips around its parent star once every 13 days.
The most Earthlike world yet
Also this year, researchers found the closest thing to an Earth twin in size and composition, though it's far too hot to support life as we know it.
Kepler-78b is just 20 percent wider and about 80 percent more massive than our planet, with a density nearly identical to that of Earth. The alien world, which is about 400 light-years from Earth, lies just 900,000 miles (1.5 million km) from its host star and completes one orbit every 8.5 hours. Surface temperatures on Kepler-78b likely top 3,680 degrees Fahrenheit (2,000 degrees Celsius), researchers say.
1,000 alien planets
Astronomers found the first-ever planets orbiting a star other than our sun in 1992. And in 2013, barely two decades later, they notched alien world number 1,000 — at least according to some tallies. . . .
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