The three planets were detected based on the dimming of the star they orbit, which has been named TRAPPIST-1 for the telescope it was discovered with (TRAnsiting Planets and PlanetesImals Small Telescope). As the three planets orbit, transiting in front of TRAPPIST-1 from the telescope's perspective, the star blinks and dims out of our view. Scientists use this dimming to calculate the size of the planets and their distance from their host star. Because planet composition is closely tied to these metrics, scientists can say with confidence whether a world is rocky, like Earth — as opposed to a massive planet composed of gas or ice — and whether it could host liquid water.

The three worlds are close by, located just 40 light years away in the constellation Aquarius — the water bearer. But the fact that they're Earth-like and promising doesn't mean these planets actually hold water.

The researchers, led by Michael Gillon and Emmanuel Jehin of the University of Liège in Belgium, believe the worlds are tidally locked, which means they have one hot side that faces their star and one cold side plunged in eternal darkness. The planets are incredibly close to their sun, orbiting it in just a matter of days. They likely don't receive more than a few times as much radiation as Earth does, but the first two planets are still probably a smidgen too close to have water covering their surfaces. Still, it's possible that certain regions of the planet are just right for water, which could allow some simple life to evolve. Other factors, such as cloud cover and geothermal activity, could tip the scales toward habitability as well. And the third, most-distant planet's orbit has yet to be determined, meaning it could fall into the perfect orbital range for water and life.

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But it's the star at the center of this foreign system that makes the planets so exciting.

The three worlds orbit an ultra-cool dwarf star — one just one-eighth the mass of our sun, and much cooler. It's more similar to Jupiter, a planet so massive it's almost star-like, than it is to our own sun. Generally speaking, the holy grail for an Earth-like planet is one with a star just like ours. After all, the more conditions a world has in common with our own, the more likely it is that similar life could evolve there. But when it comes to detecting the kind of atmosphere that would support life as we know it, smaller stars are better. This is the first time scientists have detected rocky planets in a system like this one, making the worlds great candidates for study.

When a planet passes in front of its star, the star's light has to pass through the planet's atmosphere in order to reach our telescopes. Scientists can use the wavelengths of the resulting light to analyze the molecular makeup of the atmosphere — revealing the presence of water and life-giving organic molecules. But big stars shine too bright for small, Earth-like planets to give off a signal that the Hubble can detect. That's why scientists have previously only used this method for studying the atmospheres of gas giants and so-called super-Earths, which are rocky but too close to their suns to host life as we know it.

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"Relatively speaking, the amplitude from these planets will be the same as that coming from giant planets and more solar-type stars," study lead Gillon told The Post.

The team has already started to collect atmospheric measurements from the three planets using the Hubble, and they hope to have an approximate assessment of their atmospheres soon. When the James Webb Space Telescope, which is much larger than the Hubble, launches in 2018, the team will collect even more precise data.

Around 15 percent of the stars near the sun are ultra-cool dwarf stars like TRAPPIST-1, so if this glut of Earth-like planets is "normal" for such a system, we could have lots of potential targets in the search for life. TRAPPIST (the telescope, not the star) focuses on these ultra-cool dwarfs, so the team hopes this is just the first system of many.

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"While such a 'cold' star might sound exotic, many, if not most, of the stars in our Milky Way Galaxy are of this cool, red, small and dim variety," study author Adam Burgasser of University of California in San Diego said in a statement. "If Earth-like planets around these stars turn out to be common, there may be many more habitable planets out there than current estimates predict."

And that brings us back to the question that all of these surveys hope to answer: How common is life in the universe?

"We know that life on Earth requires water, requires energy, requires organic compounds, and we know these three elements are very frequent in the universe," Gillon said. "But what we don’t know is whether these three elements are enough, or if life is some kind of cosmic accident."

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