Three New Alien Planets the Size of Earth Discovered

The tally of planets beyond our solar system that are about the same size as Earth just got a little higher thanks to three new planets discovered about 160 light-years away.

The new planets aren't twins of Earth. Even based solely on the limited data scientists have so far, a year on these planets lasts between five and 10 Earth days. But they're the intriguing residents of two just-identified solar systems described in a new paper posted on the pre-print server and scheduled for publication in the journal Monthly Notices of the Royal Astronomical Society.

Scientists found the exoplanets using data gathered by NASA's Kepler Space Telescope during its second mission, called K2. K2 began after mechanical problems made Kepler's original task—staring at a fixed region of the sky—too difficult. Instead, it began flitting across the sky, looking at many regions for shorter periods of time.

Read more: Mysterious objects near supermassive black hole at galaxy's center discovered by astronomers

The telescope's basic approach remained the same: Watch individual stars carefully, looking for tiny periodic dips in their brightness caused by a planet passing between the star and the telescope. From that information, scientists can not only recognize a new planet but also estimate its size and how long it takes to orbit its sun.

In the case of the three new Earth-sized planets, their sizes are each almost exactly the same as our planet. But they take much less time to orbit their sun: One takes a little more than five days, one takes almost eight days and the other takes just over 10 days.

An artist's depiction of exoplanets orbiting their star. Instituto de Astrofísica de Canarias

The same team also discovered two new planets orbiting a second star. These planets are much larger, about twice the size of Earth, and they take six and 20 days to orbit their sun respectively.

All of these planets are much closer to their suns than Earth, or even Mercury, is to ours, hence the super-short orbits. That's possible in part because, in both of these solar systems, the star is much smaller and less powerful than our sun. Stars like these, called red dwarfs, are the best hunting grounds for rocky planets for this reason.

K2 won't spot planets forever—the telescope is expected to run out of fuel within the next few months. Until then, scientists are trying to gather as much data as possible in hopes of finding even more solar systems to study.