ESO/M. Kornmesser/N. Risinger

For a long time, scientists have been focusing on a certain type of exoplanet—or a planet that exists outside of our solar system—as possible hosts of alien life.

Researchers are interested in those exoplanets that exist in what is called a habitable zone: An orbit that is neither too close nor too far from the sun. In the habitable zone, any water on the planet would remain in its liquid form—a necessary ingredient for life.

It can be tricky to track these exoplanets down. Often, the stars they orbit are so bright our telescopes can't detect the possible exoplanets in their galactic neighborhood. But recently, a team of scientists decided to look for habitable exoplanets orbiting the supercool dwarf TRAPPIST-1; a small, dim star—the type of star that previous researchers thought couldn't pull habitable planets into its orbit.

What they found was a system of three Earth-like exoplanets: Just the type of planets that could sustain life.

This chart shows the naked eye stars visible on a clear dark night in the sprawling constellation of Aquarius (The Water Carrier). The position of the faint and very red ultracool dwarf star TRAPPIST-1 is marked.
ESO/IAU and Sky & Telescope

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A new paper published today in Nature details the discovery of the three exoplanets. In a statement on the findings, co-author Julien de Wit commented on the importance of the news. In his words: "This is a jackpot for the field."

Another co-author, Adam Burgasser, said "The kind of planets we've found are very exciting from the perspective of searching for life in the universe beyond Earth." The European Southern Observatory, or ESO, offered: "[The exoplanets] are the best targets found so far for the search for life outside the Solar System."

Scientists from The Massachusetts Institute of Technology (including de Wit), The University of California, San Diego (including Burgasser), Belgium's University of Liège, and others used the TRAPPIST (or the TRAnsiting Planets and PlanetesImals Small Telescope) to track 2MASS J23062928-0502285—the star now known as TRAPPIST-1.

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They started tracking TRAPPIST-1 in September of last year and spent months paying close attention to it, and noticed a pattern. Over specific intervals, the small star's infrared signal got weaker, suggesting that during those moments planets were passing between the star and the telescope. Ultimately, the team determined that three discrete planets were indeed the cause of the trend.

And the scientists were able to determine some encouraging details about the planets' orbits. Again, MIT explains:

The two innermost planets orbit the star in 1.5 and 2.4 days, though they receive only four and two times the amount of radiation, respectively, as the Earth receives from the sun. The third planet may orbit the star in anywhere from four to 73 days, and may receive even less radiation than Earth. Given their size and proximity to their ultracool star, all three planets may have regions with temperatures well below 400 kelvins, within a range that is suitable for sustaining liquid water and life.

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One of the key reasons to get excited about the discovery of these exoplanet is that TRAPPIST-1  exoplanets, at 40 light years away, are relatively close to Earth. "These planets are so close, and their star so small, we can study their atmosphere and composition, and further down the road, which is within our generation, assess if they are actually inhabited,” says de Wit, adding, “All of these things are achievable, and within reach now."

If scientists were to find life on one of these planets, it wouldn't look like human life. Author Michaël Gillon told The Atlantic that the microbes that might exist on one of TRAPPIST-1's exoplanets "would receive no light in the optical range… it would have to develop techniques to get energy from the infrared.” This extraterrestrial life would look much more like deep-sea extremophiles than any other form of Earthly life.

Plus, MIT explains that only one of the planets is likely to be fully be in the habitable zone:

The scientists determined that all three planets are likely tidally locked, with permanent day and night sides. The two planets closest to the star may have day sides that are too hot, and night sides too cold, to host any life forms. However, there may be a “sweet spot” on the western side of both planets—a region that still receives daylight, but with relatively cool temperatures—that may be temperate enough to sustain conditions suitable for life. The third planet, furthest from its star, may be entirely within the habitable zone.

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The question, now, is who will learn more about the three exoplanets first. The Atlantic predicts that new telescope will be key to future analysis of the planets:

The James Webb Space Telescope—the successor to the Hubble—is scheduled to launch in 2018, and it will be equipped to tease spectrographic results out of the TRAPPIST-1 syste.  The next generation of giant, ground-based telescopes will come online in the 2020s, and will be able to extend the Webb’s capacities into detailed spectroscopy.

Guess we'll have to wait patiently.

Danielle Wiener-Bronner is a news reporter.