We now know that the universe is filled with planets. According to one estimate, there are more than 20 billion Earth-like worlds in our galaxy alone. But how many of them are likely to have lives? And how would we know if they did? If they don’t send us a very clear message directly, we will most likely discover the life of the exoplanets by looking at their atmosphere.
We have already discovered atmospheres around some large exoplanets, and when the James Webb Telescope is launched we should be able to study the atmospheres of Earth-sized exoplanets. But what would we have to see to confirm the presence of life?
Life on earth depends on a thin layer of oxygen-rich air. Photo credit: NASA
One of the strong candidates was oxygen. On earth, it is produced by photosynthesis by living organisms and makes up about 21% of our atmosphere. Oxygen is also fairly easy to spot by its spectral signature. It is also a reactive element so it needs to be replenished by living beings. So if we were to find oxygen and water in the atmosphere of an Earth-sized exoplanet in a habitable zone, it would surely be compelling evidence of extraterrestrial life. But as a recent study shows, finding oxygen and water is not enough.
There are several ways for a planet to achieve an oxygen-rich atmosphere devoid of life. Photo credit: J. Krissansen-Totton
The study shows that we cannot rely on Earth as the standard model for a planet’s atmospheric evolution. The early Earth had an oxygen depleted atmosphere. Early life forms produced oxygen as a waste product. Only after the development of photosynthesis did atmospheric oxygen become abundant. For the earth, the increase in oxygen is a direct result of earthly life. However, our path to oxygen is not the only possible path, especially for planets orbiting a red dwarf star.
While hydrogen is by far the most abundant element in the universe, oxygen is expected to be abundant on rocky planets alongside carbon and nitrogen. Hence, we will likely find compounds such as water (H2O), carbon dioxide (CO2), and nitrogen (N2) in the atmosphere of almost every potentially habitable world. Free oxygen could be biologically released from water and carbon dioxide, just like on Earth, but the researchers found three scenarios in which free oxygen appears geologically.
Artist’s view of the TRAPPIST-1 system. Photo credit: NASA / JPL-Caltech
The team focused on the planets of the red dwarf stars. Red dwarfs make up about 75% of the stars in our galaxy, so most potentially habitable worlds will likely orbit a red dwarf. But red dwarfs are very different from Earth’s sun. They are smaller and therefore take longer to go from a protostar to a main sequence star. They can send out large solar flares that could destroy a planet’s atmosphere in close orbit. They also emit much less ultraviolet light, which can ionize atoms and break molecules apart.
It turns out that this changes the evolution of a planet’s atmosphere dramatically. If a red dwarf planet has a high carbon to nitrogen ratio, it is likely going into an out of control greenhouse state with a thick atmosphere of carbon dioxide and free oxygen. If it is a water world, oxygen is released from water vapor in the upper atmosphere. If the planet is dry, there will be nitrogen and CO2 as well as oxygen in the atmosphere. All three cases would have an oxygen signature if observed by our telescopes, even though they do not necessarily have life. For red dwarf planets, the presence of oxygen could be a false positive.
Interestingly, the study also showed that these scenarios are much less likely for larger, more Sun-like stars. If we can find free oxygen around a planet orbiting a sun-like star, it could be strong evidence of life. The result of this research is that finding life on other planets is complicated. As we gather tantalizing evidence over the next few years, we must remember to be careful if we claim too quickly that we have found signs of life.
Reference: Krissansen-Totton, Joshua et al. “Novel false positive oxygen levels on habitable zoned planets.” AGU Advances 2.2 (2021): 2576.
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