The hunt for extraterrestrial life on distant planets may be operating with a big blind spot.
That's because we aren't accounting for the nature of the light coming from the host star itself when we interpret the chemical signatures of life, or biosignatures, on some of the most promising candidate planets beyond the solar system. If we don't consider that crucial factor, we could be led on a wild-goose chase across the cosmos.
Despite having discovered thousands of exoplanets, we have not detected convincing evidence of alien life on any of them. But astronomy is reaching a point where powerful telescopes can zoom in on many more potentially habitable worlds. Planets that orbit M dwarfs, which are small stars cooler than the sun, are particularly promising targets because they have a unique observation window. When a planet passes in front of one of these stars, it blocks a relatively large fraction of the star's light, making the planet and its atmosphere easier to detect.
There are also a lot of them. M dwarfs are the most common type of star in the Milky Way. However, if we misunderstand how these stars interact with planetary atmospheres, we may misinterpret the significance of observations from some of the most promising planets.
The most-studied biosignatures are methane, oxygen and ozone. On Earth, these are often produced by biological processes, with ozone acting as an indirect indicator of oxygen. But even on our home planet, molecules typically associated with life are not unambiguous biological fingerprints. These molecules can be produced by geological and chemical processes, as well as via atmospheric reactions with light from the sun.
A potential biosignature detection found elsewhere in the cosmos is therefore a clue, not proof of life. We can't interpret the chemical signals coming from a potentially habitable planet without understanding the star that illuminates it. M dwarfs produce ultraviolet radiation that can break apart molecules and trigger chemical reactions in the atmosphere of an orbiting planet. The intensity and wavelength of the radiation influence which molecules form and survive, as well as how abundant they become. Two planets with otherwise identical properties could develop very different atmospheres simply because they orbit stars with different ultraviolet emissions. Radiation from a star could therefore make the same level of biological activity appear stronger on one planet than on another, or make nonbiological chemistry look like life. A recent study submitted to the preprint server arXiv Aug. 19 demonstrated this using entirely simulated planets. The researchers, led by University of California, Santa Cruz astronomy graduate student C. Evan Davis, simulated Earth-like planets orbiting two different types of M dwarf with ages ranging from 650 million to 5 billion years.
The team considered atmospheres resembling that of Earth during the oxygen-rich preindustrial era and the Archean eon (about 4 billion to 2.5 billion years ago), during which the first life-forms emerged but atmospheric oxygen was scarce. The models considered the stars' usual, or "quiescent," ultraviolet emission rather than short-lived flares. By changing the modeled stars' ages and ultraviolet radiation while keeping the planets comparable, they investigated how a star's evolution changed an atmosphere and the signals astronomers might observe.
The first convincing discovery of life on another planet will depend on an understanding of not only that planet but also the star that shaped the atmosphere we observe.
Alix Freckelton, astrophysicist
One of the clearest differences the team found appeared in methane. Simulated planets with preindustrial atmospheres orbiting 5 billion-year-old M dwarfs accumulated up to 10 times more methane than equivalent planets simulated to orbit the younger 650 million-year-old stars. The methane signals produced in the simulated data were up to 68% stronger for the older systems. The weaker UV emission from the older M dwarf allowed methane to survive longer and accumulate in the simulated planetary atmospheres. The stronger methane signal could make a planet orbiting an older star appear to support more biological activity, when this difference was actually caused by the UV emission of the host star.
An even more striking result came from ozone. For the simulated Archean Earth-like planets, which were low in oxygen and rich in carbon dioxide (CO2), the stronger UV radiation from younger M dwarfs broke apart more CO2. This kick-started reactions that created oxygen and ozone without any life involved. Some model planetary atmospheres contained more than 100,000 times as much ozone as equivalent models around older stars. A hypothetical observer could misinterpret the resulting ozone signal as indirect evidence of biologically produced oxygen. Ozone is not useless as a biosignature, but these simulations show that it cannot be interpreted confidently without knowing the UV environment that shaped the atmosphere.
Of course, astronomers are already aware that a single molecule would not prove the existence of life on an exoplanet. Researchers use atmospheric models, look for combinations of gases, and consider nonbiological explanations before describing a signal as a potential biosignature. These methods remain valuable for identifying the most promising planets for further investigation, even when our knowledge of their stars is incomplete. Some might argue that these methods provide a sufficiently reliable first assessment and we should reserve more detailed stellar observations for the strongest candidates.
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But even the best atmospheric model can mislead us if the stellar radiation isn't accounted for correctly. Ultraviolet observations of M dwarfs remain limited, so researchers often rely on estimates from similar stars. However, two M dwarfs that might appear similar can produce very different levels of UV radiation. Atmospheric models based on currently available stellar measurements may be sufficient for selecting promising targets but not for deciding whether the origin of a signal is biological. Simply acknowledging that the star matters isn't enough; we need accurate information about the specific star hosting the planet.
So before we train our telescopes on promising exoplanets, we need to study their host stars. This requires repeated UV observations, typically with different telescopes or instruments or at different time periods than you'd use to study the exoplanet. These observations can characterize how the host star's emission varies over time, while better stellar-age estimates — which can be calculated from properties such as rotation and magnetic activity— will reveal how that radiation has evolved.
Then, once we do start studying an exoplanet, potential biosignatures must be analyzed and interpreted using models informed by accurate measurements of the host star. As recommended in the preprint study, astronomers should also search for accompanying molecules, such as carbon monoxide, that could reveal whether ozone arose through reactions between light and carbon dioxide rather than from biology. The first convincing discovery of life on another planet will depend on an understanding of not only that planet but also the star that shaped the atmosphere we observe.
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