This Is How We'll Detect Life on Distant Exoplanets

By: Ian O'Neill, Ph.D.

A rendering of the Thirty Meter Telescope that will be used to seek out biosignatures on exoplanets. It could be up and running by the late 2020s. Caltech/IPAC-TMT
A rendering of the Thirty Meter Telescope that will be used to seek out biosignatures on exoplanets. It could be up and running by the late 2020s. Caltech/IPAC-TMT

The search for extraterrestrial life is arguably the most profound scientific undertaking of our time. If alien biology is found on another world orbiting another star, we'll finally know that life is possible beyond our solar system.

Searching for hints of alien biology on far-away worlds isn't easy though. But a team of astronomers is developing a new technique for use by the next generation of powerful telescopes, allowing them to precisely measure the chemicals in exoplanet atmospheres. The hope, of course, is to find evidence of extraterrestrial life.

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This profound search was thrown into the limelight recently by the discovery of seven small alien worlds orbiting the tiny, red dwarf star TRAPPIST-1. Three of these exoplanets orbit within the star's so-called "habitable zone." That's the region surrounding any star where it's not too hot and not too cold for liquid water to exist on a planetary body.

On Earth, where there's liquid water there's life, so if any of TRAPPIST-1's habitable worlds possess water, they might have life, too.

TRAPPIST-1's life-giving potential remains pure speculation, however. Even though this fascinating star system is in our galactic backyard, we have no idea if water exists in any of those worlds' atmospheres. Actually, we don't even know if they have atmospheres! All we know is how long the exoplanets take to orbit the star and their physical size. 

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Artist's impression of the view from one of the exoplanets in TRAPPIST-1.
M. Kornmesser/ESO

"The first detection of biosignatures on other worlds may be one of the most significant scientific discoveries of our lifetime," says Garreth Ruane, an astronomer with the California Institute of Technology (Caltech). "It will be a significant step towards answering one of human-kind's biggest questions: 'Are we alone?'"

Ruane works at Caltech's Exoplanet Technology Laboratory, or ET Lab, which is developing new strategies to scan for exoplanetary biosignatures, such as oxygen molecules and methane. Typically, molecules like these are highly reactive with other chemicals, meaning they quickly break down in planetary atmospheres. So, if astronomers detect the spectroscopic "fingerprint" of methane in an exoplanet's atmosphere, it could mean that alien biological processes are producing the stuff.

Unfortunately, we can't just grab the world's most powerful telescope and point it at TRAPPIST-1 to see if those planets' atmospheres contain methane.

"In order to detect molecules in the atmospheres of exoplanets, astronomers need to be able to analyze light from the planet without being completely overwhelmed by light from the nearby star," Ruane says.

Fortunately, red dwarf (or M-dwarf) stars like Trappist-1 are cool and dim, so the glare problem is less acute. And as these stars are the most common type of star in our galaxy, red dwarfs are where astronomers are looking first to make that historic discovery.

Astronomers use an instrument known as an "coronagraph" to isolate the reflected starlight bouncing off a nearby exoplanet. Once the coronagraph zeros in on the faint light of an exoplanet, a low-resolution spectrometer then analyzes the chemical "fingerprints" of that world. Unfortunately, this technology is limited to only studying the largest exoplanets orbiting far from their stars.

The ET Lab's new technique uses a coronagraph, optical fibers and a high-resolution spectrometer, all working together to strip out a star's glare while capturing an extremely detailed chemical fingerprint of any worlds in orbit. This technique is known as "high-dispersion coronagraphy" (HDC), and it could revolutionize our understanding of the diversity of exoplanetary atmospheres. Papers detailing the method are soon to be published in The Astrophysical Journal and The Astronomical Journal.

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The HDC setup in the lab the equipment is about the same size as what would be installed in a telescope, but would be arranged differently.
Caltech/IPAC-TMT

"What makes the HDC method so powerful is that the spectral signature of the planet can be picked out, even when it's still buried in the glare of the star after the coronagraph," Ruane tells HowStuffWorks. "This allows for detection of molecules in the atmosphere of planets that are extremely difficult to image.

"The trick is to split the light up into many colors and create what astronomers call a high-resolution spectrum, which helps distinguish the signature of the planet from that of residual starlight."

All that's needed now is a powerful telescope to attach the system to.

In the late 2020s, the Thirty Meter Telescope will become the world's biggest ground-based optical telescope and, when used in conjunction with HDC, astronomers will soon be able to study the atmospheres of potentially habitable worlds orbiting red dwarfs.

"The detection of oxygen and methane in the atmospheres of Earth sized planets orbiting M dwarfs similar to Proxima Centauri b with TMT will be extremely exciting," says Ruane. "We still have a lot to learn about the potential habitability of these planets, but it would perhaps indicate that there may be planets similar to Earth orbiting our nearest stellar neighbors."

An estimated 58 billion red dwarf stars live in our galaxy, and it is known that most will play host to planets, so when the Thirty Meter Telescope goes online, astronomers may be on the verge of finding that highly sought after biosignature fingerprint.

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