A ticker on the official James Webb Space Telescope (JWST) website teases readers about what NASA’s next-generation, space-based telescope will be able to do: “see the first light of the universe, watch galaxies collide, see stars and planets being born, find and study exoplanets.” Indeed, just by being able to glimpse the earliest light from the dawn of the universe, JWST dwarfs the capabilities of its predecessor, the Hubble Space Telescope, which itself captured images of some of the farthest reaches of the cosmos.
But JWST is not without limits. Although JWST is a technological marvel, when it comes to studying exoplanets—especially if the aim is to uncover any signs of extraterrestrial life—recent research suggests that the telescope might have a tricky time detecting at least one telltale sign of life: oxygen in an exoplanet’s atmosphere.
Oxygen is an ingredient that much of life on Earth today needs to survive, so its presence in an exoplanet’s atmosphere could be a big clue that life is lurking there. Noah Planavsky, a biogeochemist at Yale University, and a team of researchers recently found that a planet’s atmosphere with an extremely small amount of oxygen can still support life. This finding means there could be planets that have only minute oxygen levels—but that nonetheless harbor life—that would appear to be dead to JWST.
In short, using JWST, “we have the potential to miss life,” said Planavsky.
JWST was not originally designed to scan distant planets for their oxygen concentrations. And yet, even though JWST may miss oxygen, there are other signs of life, like methane and carbon dioxide, that other new research suggests JWST will be better able to detect.
Earth Points the Way
Earth is our best and only example of a planet that harbors life, so scientists use Earth, particularly its deep geological record, to build a search image of what they should look for when they hunt for habitable exoplanets. Planavsky and his team, who published their results last month in the Proceedings of the National Academy of Sciences of the Unites States of America, discovered that about 1.87 billion years ago, Earth’s atmosphere had only about 0.1% of the oxygen that it has today (modern levels hover at about 21%).
According to Edward Schwieterman, an astrobiologist at the University of California, Riverside, who was not involved in the research, “there’s no way that JWST will be able to pick up on 0.1% [of today’s] oxygen levels.”
The problem is that the instrumentation aboard JWST, such as the Mid-infrared Instrument, can detect only light emitted in the infrared part of the electromagnetic spectrum, and oxygen is most detectable from afar in emissions of visible light.
Schwieterman explained that for 40% of Earth’s 4.54-billion-year history, oxygen levels hovered around the low value reported by Planavsky and his team. “For about 10 percent of Earth history, oxygen was roughly as high as it is today,” he said. “So a really rough guess is that, for every planet like Earth is now, there are several more with low or no oxygen in their atmospheres” but that still may have life.
JWST, he noted, will likely gloss over the latter.
Other Molecules to the Rescue
JWST’s oxygen-spotting prospects may be dim, but Joshua Krissansen-Totton, an astrobiologist at the University of Washington, thinks there is a good chance that the telescope will be able to detect two other molecules that would suggest the presence of life: carbon dioxide and methane. It will be possible to spot those molecules in a planet’s atmosphere, Krissansen-Totton explained, because they emit light in the infrared portion of the spectrum.
Carbon dioxide and methane were both prevalent for much of Earth’s—and also life’s—early history, from about 4.0 to 2.5 billion years ago. So the key, Krissansen-Totton said, is to find an exoplanet with an atmosphere that has both molecules occurring side by side: “I would argue that, if there’s enough methane in combination with CO2, that would be very difficult to explain without life.”
In research published last month in the Astronomical Journal, Krissansen-Totton and a team of researchers modeled just how successful JWST would be at detecting these two gases on a real exoplanet called TRAPPIST-1e, the most Earth-like planet of the seven planets that make up the famous TRAPPIST-1 planetary system, discovered in 2016. By measuring the wavelengths of the light that passes through a hypothetical TRAPPIST-1e atmosphere simulated to have methane and carbon dioxide at 0.5% and 5%, respectively—concentrations thought to mimic those seen early on in Earth’s history—the team determined with “90 percent confidence” that JWST would be able to detect the two gases.
But there’s another ingredient thought to be essential for life to exist: water. “Webb will definitely have the capacity to detect water on a rocky planet,” said René Doyon, an astrophysicist at the Université de Montréal who is one of the scientists in charge of building JWST. “That’s a major step forward,” he said. And a planet like TRAPPIST-1e, which JWST will likely observe, may be a prime target for Webb to scan for water because the planet is in the zone around its star where liquid water can exist.
Webb of Life
The idea for JWST was born in 1996, and the original aim for the telescope, Doyon explained, was to see farther into the cosmos than Hubble ever has. It was to be “the machine to find the very first stars, the very first galaxies that lighted up at the beginning of the big bang,” he said. That was more than 20 years ago, and back then NASA set the launch date for the telescope to be between 2007 and 2011.
“James Webb was not optimized for exoplanets, because it was designed so long ago,” said Schwieterman. The space agency has since delayed the JWST launch date to 2018 and then to 2020 and, most recently, to 2021. When it finally launches, JWST will undoubtedly help make strides toward characterizing exoplanets and determining which may be likely candidates for harboring life, explained Doyon.
But when it comes to definitively identifying life on other planets, Shawn Domagal-Goldman, an astrobiologist at NASA’s Goddard Space Flight Center, said, “I think the answer is ‘we shouldn’t be counting on it.’”
—Lucas Joel (email: [email protected]), Freelance Journalist