NASA/JPL-Caltech
An illustration of what it might look like on the surface of TRAPPIST-1f, a rocky planet 39 light-years from Earth.
- NASA launched the Kepler space telescope into orbit in 2009, which helped astronomers identify over 4,000 exoplanets - worlds outside our solar system.
- Based on that data, scientists estimate there could be up to 10 billion Earth-like planets in the Milky Way galaxy. Some are in our galactic backyard.
- In September, scientists discovered water vapor on a potentially habitable exoplanet for the first time ever.
- Here are 16 exoplanets that offer our best chance of finding alien life outside our solar system.
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Scientists detected water vapor on a potentially habitable planet last month - a major step in the hunt for alien life.
The discovery was the culmination of a decade-long search that began when NASA shot the Kepler space telescope into orbit in 2009.
The telescope used what's known as the transit method to look for exoplanets (worlds outside our solar system), launching a new era of planetary discovery. Kepler watched over 530,000 stars for tiny dips in brightness that could be caused by a planet passing in front of the star - transits, in other words. Since then, astronomers have discovered over 4,000 exoplanets, and another 4,000-plus candidates are waiting to be confirmed with further observation.
This data has taught us that Earth-like worlds may not be so rare. In fact, planets between the sizes of Earth and Neptune seem to be the most common type in the galaxy.
One recent analysis of Kepler data estimated that 20% to 50% of stars have small, possibly rocky planets orbiting in their habitable zones - the range of distances from a star in which a planet's surface temperature could allow for liquid water. That means there could be up to 10 billion Earth-like planets in our galaxy.
Kepler retired last year after the telescope ran out of fuel, but it has passed the planet-hunting torch to the Transiting Exoplanet Survey Satellite (TESS).
Here are 16 of the exoplanets we know about so far that have the greatest potential for alien life.