Sep 14, 2023 - Science

Alien atmospheres are helping scientists search for life

Illustration of planets creating a hydrogen molecule

Illustration: Natalie Peeples/Axios

The atmospheres of planets beyond this solar system are coming into focus and helping scientists decide what constitutes evidence of life and where to look for it.

Why it matters: As increasingly sensitive telescopes and other tools study more exoplanets in search of signs of life, scientists are trying to hammer out a framework for determining — and communicating — if and when such extraordinary evidence is found.

  • "When there is a robust detection of life on another planet it will be insane," says Jessie Christiansen, an astrophysicist at the NASA Exoplanet Science Institute at the California Institute of Technology.

Driving the news: NASA scientists this week reported detecting carbon dioxide and methane and a lack of ammonia in the atmosphere of an exoplanet about 124 light years away from Earth, suggesting there could be an ocean of water on its surface.

How it works: The James Webb Space Telescope studied light from K2-18 b's star as it passed through the exoplanet's atmosphere.

  • Different molecules in the atmosphere dim the starlight at different wavelengths that can be detected by JWST.
  • The planet orbits its star in the theoretical habitable zone, a distance at which liquid water could exist on a planet's surface. But K2-18 b is not an Earth-like planet.
  • It's more than eight times the size of Earth and a lot less dense. That and the presence of carbon dioxide and methane suggest it could be a Hycean exoplanet, a hypothetical world covered in oceans and shrouded in a thick atmosphere of hydrogen.

The big picture: JWST isn't sensitive enough to study Earth-like planets around Sun-like stars. But the planets it can study may help to narrow the search for habitable worlds.

  • The telescope is best suited for looking at the atmospheres of planets around red dwarf stars, which are smaller and cooler than our Sun. They are also the most plentiful type of stars in the galaxy.
  • Scientists worry, however, that planets around these types of stars, even in their habitable zones, could be bombarded with solar flares that would make it difficult for the worlds to hold on to an atmosphere.
  • If planets around red dwarf stars can't retain an atmosphere, it will "really change the equation of how much habitable real estate there is in the galaxy," Christiansen says.

Between the lines: There is still a lot of space for people to speculate about the chemical signatures of life because there aren't ways to confirm them, Christiansen adds.

  • "One thing that feels consistent is the idea that atmospheres on planets that have life will probably be more complex than those that don't have life."
  • That means life's fingerprint very likely won't be left with one molecule. And the planet's environment and evolution matter.
  • "The context is essential because we don't know all of the configurations a purely lifeless planet can take on. We don't know the nature of the most common planet in the universe," says University of California, Riverside astrobiologist Eddie Schwieterman.
  • "[W]e're only just starting to understand the composition of those atmospheres and there is probably a suite of combinations of gases" that are signatures of life.

The intrigue: A promising biosignature on one planet may not be a sign of life on another whose physical and chemical features might offer different, non-life-related ways to create that molecule.

  • "Compelling signs of life on our planet today around another planet may have nothing to do with life," says University of California, Riverside biogeochemistry professor Timothy Lyons.
  • The chapter of a planet's history could also be a factor: methane, which is produced by several processes, may have been a potential biosignature for life on Earth during a 1-billion-year window but not in the rest of Earth's history when it was overshadowed by oxygen in the atmosphere or made from a reaction of sunlight and water, not microbes.

What's happening: Scientists are now trying to come up with a framework — basically a flow chart for confirming a discovery — that lays out what needs to be tested when one signature or another is spotted.

  • It's "likely that multiple measurements and approaches, spanning disciplines and missions, will be needed to make a convincing claim" about finding life, a group of researchers working on the standards wrote last year. "Life detection will therefore not be an instantaneous process, and it is unlikely to be unambiguous."
  • The framework is a systematic way of reporting a detection of life, Schwieterman says. It "acknowledges the various complexities and allows us to be honest with each other."
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