When it all comes together in the 2020s, the GMT will be able to resolve an object the size of a dime at 60 miles away, which is an order of magnitude more resolving power than Hubble. Such viewing power opens a universe of possibilities.
"Our favorite science use for GMT is looking at planets in habitable zones," says Jared Males, assistant astronomer at the University of Arizona's Steward Observatory. "What we want to do is look for biosignatures. We want to look for life on these planets."
Sophisticated spectrometers will measure the light collected by GMT to identify the compositions of celestial objects. When an exoplanet passes in front of the star it orbits from our perspective, the giant telescope's instruments will be sensitive enough to separate the light passing through the planet's atmosphere from the rest of the starlight. The light that passes through the atmosphere will have gaps in the electromagnetic spectrum—absorption lines that reveal the presence of specific molecules in that world's skies.
"A really good biosignature is diatomic oxygen," says Patrick McCarthy. "All the oxygen in Earth's atmosphere is due to life."
Photosynthetic life is the only reason molecular oxygen exists in such abundance in Earth's atmosphere, but there are also natural geologic processes that can produce significant amounts of atmospheric oxygen (though if this were the case on another planet, other clues would tell us the oxygen wasn't from a biological source). More problematic, however, is the fact that life existed on Earth for more than a billion years before photosynthesis filled our skies with oxygen, and similar anaerobic microbes on other planets could happily thrive with no O2 to speak of.
If that life is anything like the early life on Earth, however, it will produce methane, another intriguing potential biosignature for alien worlds. In fact, a recent study in Science Advances found that a combination of methane and carbon dioxide in an exoplanet's atmosphere would be a convincing biosignature of pre-photosynthetic life.
This artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018.
NASA/JPL-CALTECH
The GMT will turn its gaze toward nearby exoplanets, such as Proxima b orbiting the closest star to us and the seven Earth-sized planets orbiting the star TRAPPIST-1 less than 40 light-years away. The scope will also look to thousands of other planets that astronomers are discovering all across the Milky Way to search for signs of life in alien atmospheres.
"You can ask whether the sky is blue on an exoplanet," says McCarthy. An atmosphere similar to Earth's would cause blue light to scatter more than other wavelengths.
The enormous telescope, perched in the high Atacama, will also be able to detect incredibly dim objects with its vast light-collecting surface. Early star formation will be studied with unprecedented precision, and galaxies will be discovered that are farther away, older, and less bright than any previously detected. In addition to searching the galaxy for life, the GMT will allow astronomers to study the most ancient eons of the universe, probing the firmament to test our fundamental laws of physics.