Unmasking 'Trickster' Exoplanets with Bogus Signatures of Alien Life

Methane—chemical formula CH₄—is a hugely significant molecule for us Earthlings. On the one hand, methane helps to serve our energy needs as natural gas; on the other, it is altering our climate as a potent greenhouse gas. The colorless, odorless substance's importance doesn't end there: methane might just prove the key for discovering alien life on a distant world.

Because methane breaks down in Earth-like atmospheres, the gas must be replenished somehow. And here on Earth at least, almost all methane traces back to life. The gas is released in bulk from living organisms—for instance, by bacteria in wetlands and by ruminants such as cows—as well as from dead organisms when their hydrocarbon remains are burnt as fossil fuel.

Given this special origin, astrobiologists have accordingly zeroed in on atmospheric methane as a potential sign of extraterrestrial life. Part of this work has necessarily involved understanding and accounting for abiotic processes—meaning not linked to life—that can also pump out notable quantities of methane. On Earth, there are not many examples (though some chemical reactions involving magma and where crustal plates meet have been studied). For scientists to truly suspect they have the real deal when a telescope detects methane, "false positive" abiotic sources will have to be rigorously ruled out.

A major knowledge gap in this arena? The potential sources of abiotic methane on potentially habitable worlds that are not perfectly Earth-like. The rich range of planets and moons just in our solar system attest to the wild variation exoplanets are expected to possess. Coupled with the fact that the nearest analog known to Earth in terms of size and mass is Venus, a hellish world utterly unlike ours climatologically, it stands to reason that Earth-sized exoplanets—once we know more about them—will likewise display a vast spectrum of properties.

Edwin Kite, the recipient of funds from The Kavli Foundation from the 2021 Scialog meeting, co-leads a project to filling this knowledge gap. Kite is an Associate Professor in the Department of the Geophysical Sciences at the University of Chicago (which is also the host university of the Kavli Institute for Cosmological Physics). With co-leads Jennifer Glass at the Georgia Institute of Technology and Smadar Naoz at UCLA, the team is striving to identify nontraditional abiotic methane sources that could potentially be in play on exoplanets of interest. Those worlds, like Earth, are both rocky and found in their stars' habitable zones—the temperate bands where life as we know it could plausibly evolve.

"As the research community has come up with models for abiotic origins of gases that could mimic biosignatures, we've really focused on source processes that could operate on Earth-like worlds," says Kite. "However, habitable-zone rocky exoplanets could have compositions and processes dramatically different from Earth. We want to consider a wider range of possible abiotic methane sources to make sure we don't get fooled by 'trickster' planets that emit false positive biosignatures."

Kite and colleagues are initially exploring three different scenarios that could generate copious methane without life's animating spirit. One such scenario: exoplanets that experience far greater bombardment by space rocks than ours does. These meteorites could deliver significant quantities of kerogen, a kind of organic matter that reacts to form methane when heated. The inferno of initial meteorite impact would spew out some methane, and then more methane would seep out over time as buried kerogen compresses and heats up.

A second scenario is exoplanets that end up with graphite-rich crusts, unlike Earth. If the initial abundances of elements available for planet formation are a bit different than in our solar system, such a planet could plausibly develop. When rocky planets form, they are initially molten, and during this time, the fledgling worlds undergo a process called differentiation. During this process, heavier materials tend to sink toward the forming planet's core while lighter materials rise and eventually harden into planetary crust. If a more graphitic crust were to come about, such a surface layer could then react with the available atmosphere to readily produce biosignature-confounding methane.

The third scenario Kite and colleagues are examining is exoplanets that possess large stores of frozen methane-rich clathrate hydrates. These methane-rich materials are common in our solar system, appearing underground right here on Earth at continental edges, and likely in bulk in the crusts of the mega-moon Titan and the dwarf planet Ceres. Over time, those frozen methane-rich clathrate hydrates could decompose and release substantial methane into the atmosphere. One mechanism would be the natural brightening and warming of the planet's host star—a slow, eons-long evolution that our own sun is likewise undergoing. A second mechanism is planetary migration, where worlds that initially form farther out from their stars move inward as a result of accumulating gravitational interactions with other planets in their solar systems. An apparently Earth-like planet in a habitable zone could in fact be a migrated Titan-like world, belching methane as it thaws.

"There are a number of ways that trickster planets can make false positive biosignatures," says Kite. "We have to keep open minds about what we could find out there."

Kite and colleagues are using software programs to run models of these scenarios. The researchers' goal is to equip astrobiologists to sort the wheat from the chaff as observations of habitable zone rocky worlds wonderfully accumulate. The next-generation James Webb Space Telescope, which launched in December 2021, is powerful enough to gather the first of these highly anticipated observations and should do so in the coming months and years.

Kite's research takes on additional importance in light of the next-next generation of telescopes. The most recent Decadal Survey—a priority list compiled by a committee of experts facilitated by the National Academy of Sciences on behalf of NASA, the National Science Foundation, and the Department of Energy that reflects the consensus goals of the astronomical community—has called for a new 6-meter infrared/optical/ultraviolet space telescope. This observatory would scan for biosignatures from at least a couple dozen habitable-zone exoplanets. Future ground-based telescopes will also contribute to chemically characterizing exoplanet atmospheres, indicating the need for thoughtful assessment of what the data haul might contain.

"We don’t know much about the origin of life nor about how life is distributed in the universe," says Kite. "If the scientific community is to make a claim that alien life has been discovered through biosignature detection, it is our responsibility to ensure that we've thought long and hard about all the ways biosignatures could in fact be abiotic geosignatures."

"With this Scialog research that The Kavli Foundation and the Research Corporation for Science Advancement is funding," Kite continues, "we're making headway in this pursuit."