In December, NASA will launch the most powerful telescope ever put into space. The James Webb Space Telescope will be able to study planets outside our solar system with unparalleled detail — including checking to see if their atmospheres give any indication that a planet is home to life as we know it.

The search for life beyond Earth isn't easy, of course, and this telescope won't be able to offer up rock-solid evidence that aliens are out there. But some researchers say it's possible that this telescope could at least find hints of life on Earth-sized planets that have so far evaded close scrutiny.

Cosmic ambitions for new telescope

Searching for signs of life wasn't part of the original job description over three decades ago, when the James Webb Space Telescope, named after a former NASA administrator, was first conceived.

Way back then, no one had discovered any of the planets circling distant stars, and what scientists mainly wanted was a telescope that could capture light from the first galaxies in the universe.

Building this $10 billion instrument proved so complex and time-consuming, however, that in the meantime, a whole new scientific field has sprung up. That's the study of so-called exoplanets — planets beyond our solar system. The new generation of astronomers who work in this field are eager to take advantage of this telescope's power.

"I think the earliest discussions about James Webb were happening in the 1990s, when I was in elementary school," says Laura Kreidberg, an astronomer at the Max Planck Institute for Astronomy in Germany who studies planets beyond our solar system. She notes that the first such planet around a Sun-like star was found in 1995.

Since then, scientists have detected thousands of planets. "Now, 25 years after the first planet was discovered around another star, we know that pretty much every star, on average, has at least one planet," says Kreidberg.

The launch of James Webb will transform the ability to learn about these far-flung worlds. So far, it's been difficult to know what distant planets are really like, beyond some basic information like how massive they are and how far away they are from the star they orbit.

That's because scientists usually don't see the planets themselves. Instead, researchers detect planets indirectly. For example, they can measure how a planet's gravity makes a star wobble, or watch as a star dims because a planet has passed in front of it and blocked some of the star's light.

Already, it is sometimes possible to learn a bit about a planet's atmosphere by using a telescope like Hubble to analyze the starlight that filters through that atmosphere.

"We can do this analysis right now for the big, hot planets, with lots of gas that the light shines through," says Lisa Kaltenegger, an astronomer at Cornell University whose research focuses on new planets around other stars.

"But for the small planets like the Earth's with a little bit of an atmosphere, we need to catch more light to do the same thing," she says.

That's where James Webb comes in.

Searching for 'biosignatures' in far-off worlds

The telescope's enormous light-collecting mirror, which is 21 feet across, will catch enough light to let scientists analyze the chemical make-up of small rocky planets' atmospheres like they never have before.

That's important because if any of those planets has life as we know it, scientists would expect to see certain tell-tale combinations of different gases that they call 'biosignatures,' such as oxygen plus methane.

"The James Webb Space Telescope does have the capability to measure those key biosignatures," says Nikole Lewis, another astronomer at Cornell University who focuses on planets beyond our solar system. "It's within scope for the James Webb Space Telescope to find hints of life on rocky planets."

One intriguing planetary system that James Webb will study is about 40 light years away. A small, cool star, called TRAPPIST-1, is orbited by seven Earth-sized planets, and three of them orbit in the zone where temperatures should be mild enough to have water in liquid form.

"It's the perfect target for the James Webb Space Telescope," says Lewis.

James Webb should be able to reveal whether or not any of these planets are actually surrounded by air, says Lewis. "And then we're going to go from there--okay, what is that air made of? And is the air the same for planets that are close to the star or planets that are far from the star?"

Alien life? Not so fast

Lewis expects that if any possible atmospheric signs of life are found by this telescope, scientists will argue fiercely about what the findings really mean, and the public might end up getting the wrong idea.

"As careful as we are as scientists in trying to present our findings, when people hear things like 'habitable zone plus water,' they immediately assume that we've found aliens," says Lewis. "We're going to have to be, sort of, very careful with that."

Trying to tease out subtle combinations of gases that might mean life on other planets is an extremely difficult thing to do—especially for a telescope that was not conceived with this task in mind.

That's why some astronomers think it's a stretch to believe that James Webb will be able to find hints of alien life, if they're out there.

The telescope should definitely be able to see what gas dominates an atmosphere, says Kreidberg, "but when you start talking about combinations of different gases, particularly those that have very low abundance or not very strong spectral features, it's a really big challenge."

She's still super excited for James Webb's launch later this year, because of what this telescope should reveal about planets outside our solar system, both large and small. At the same time, though, Kreidberg is hopeful that soon other space telescopes will be built that are designed from the start to search Earth-like planets for signs of life.