The exoplanet, dubbed GJ 1132b, is about the size of Earth and lives in a solar system roughly 39 light-years from Earth, a team of scientists reported in the November 12, 2015, issue of the journal Nature.
Furthermore, the exoplanet is the closest rocky Earth-sized exoplanet ever discovered, by far.
The next closest is about three times farther away.
GJ 1132b's size and distance are what have astronomers like Drake Deming at the University of Maryland - who was not part of the study - saying that this planet is "arguably the most important planet ever found outside the solar system," he told The Guardian.
The reason is because GJ 1132b is close enough for astronomers to point their telescopes at it and sniff out any traces of an atmosphere.
Atmospheric signatures of life
Studying the atmospheres of exoplanets for signs of life is the next big step in the search for extraterrestrial life beyond our solar system.
But it's an extremely difficult project because most Earth-sized planets - the exoplanets that astrobiologists think are most likely to harbor life - are too distant to study in any detail.
One of the ways astronomers determine the chemical composition of gases on planets in our own solar system is by studying the light passing through their atmospheric layers, like Earth's shown in the photo below with our crescent moon in the foreground:
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But when an object is hundreds of light-years away, it's nearly impossible to differentiate the light that's passing through an exoplanet's atmosphere from the light that's emanating from the larger, brighter host star.
A significant stepping stone
Now, GJ 1132b has changed that, offering astronomers a perfect specimen to test their instruments and methods for detecting life.
"It's nearby, it's Earth-like, and its star won't interfere," Deming told The Guardian.
This newly discovered exoplanet is, however, just a stepping stone.
The possibility of finding life on it is practically zero. GJ 1132b might be only 16% larger than Earth, but it's surface temperature is a balmy 450 degrees Fahrenheit.
"Our ultimate goal is to find a twin Earth, but along the way we've found a twin Venus," David Charbonneau, a co-author on the new paper and an astronomer at the Harvard-Smithsonian Center for Astrophysics, said in a press release. "We suspect it will have a Venus-like atmosphere, too, and if it does we can't wait to get a whiff."
The reason for these toasty temperatures is because GJ 1132b sits just 1.4 million miles from its host star - 26 times closer than Mercury is to our sun.
It takes just 1.6 days for GJ 1132b to complete a single orbit around its star, which is very different from our sun.
The most common star in the galaxy
The star is what is called a red dwarf, which astronomers suspect is the most common star throughout our home galaxy, the Milky Way.
Red-dwarf stars are cooler, smaller, and dimmer than our sun. The one that GJ 1132b is orbiting is about one-fifth the size of our sun and only emits about 1/200th the amount of light.
Here's a diagram comparing, to scale, the size of our sun with a red dwarf (second from the left) and Jupiter (far right):
Just because they're different from our sun, however, doesn't mean life couldn't spawn around them. It just means that the habitable zone - the region in space where liquid water could exist on a planet's surface - is closer to the star than it is for our sun.
Some astronomers estimate that red-dwarf stars comprise 75% of all stars in the galaxy. This means that most of the exoplanets in our galaxy are likely orbiting red-dwarfs.
Whether any of those planets are habitable, however, is a subject of intense debate, which is why the discovery of GJ 1132b is so exciting. And while GJ 1132b is not likely habitable, it's possible that it has some neighbors that are.
To first spot GJ 1132b as it passed in front of the star, the researchers used the MEarth-South array, a group of telescopes at the Cerro-Tololo Inter-American Observatory in Chile.
Now that they know it's there, the team has requested time on the Hubble Space Telescope (which observes the same type of light we see with our eyes) and the Spitzer Space Telescope (which observes longer wavelengths than Hubble, in the infrared regime) to study GJ 1132b more fully.
With the viewing powers of Hubble and Spitzer combined, the scientists could get a much broader spectrum of light to study even more chemical compounds than they otherwise would with just one of the telescopes.