Today, most astronomers consider atmospheric water vapor and oxygen to be the two most important properties of a planet to host life. (Methane and small amounts of carbon dioxide are usually next on the list, with sulfur and nitrous oxide next.) If extraterrestrial astronomers were to study our planet’s atmosphere during the Archaean period, they would probably see signs of steam and methane, but no oxygen. . Therefore, life on Earth and its potential for a life-hosting planet were ignored. That’s why Young created his flowchart with this premise: to look for signals from exoplanets that might be in one of the stages of their multi-billion-year evolution, even if their atmospheres don’t resemble Earth’s today.
According to Tim Lyons, an astrobiologist at the University of California Riverside and a fan of the flowchart concept, based on Earth’s past, it is possible that oxygen will not be exposed for more than 2 billion years. It is even possible that the abundance of methane and oxygen was not enough to reveal them from long distances during the Proterozoic era.
Lyon’s research has a similar view to Yang’s. He examines this issue from a different perspective: Can an extraterrestrial astronomer, when observing the Earth, correctly determine that the Earth hosts life? This question means examining the contents of the atmosphere of the planet earth in the last four billion years and hosting it for life, as well as examining the levels of biological gases that can be detected from space.
Another group tried to figure out whether aliens could observe Earth using the same method we use to detect rocky exoplanets. Researchers believe that extraterrestrials can observe the earth while it passes in front of the sun, or in simpler terms, by blocking the starlight, and get a clue of our existence.
Currently, when evaluating an exoplanet, scientists first check its host star to make sure it isn’t emitting too many flares. Then they check the planet’s orbit to assess its stability and position in the habitable zone. In this region, the planet is neither too cold nor too hot, so that it is possible for liquid water to flow on its surface.
Then the hard part begins. With Young’s decision tree, astronomers can estimate the amount of water vapor in the atmosphere, a sign of whether water is present on a planet’s surface. In fact, they scan the planet’s atmosphere in infrared wavelengths using spectrometers like the one used in the James Webb Space Telescope.
In the next step, researchers use the spectrometer to find key molecules such as oxygen or methane. Whatever amount they find of each element determines their next target, such as carbon dioxide or ozone. For example, if the process of photosynthesis exists on other worlds, living organisms that use oxygen usually produce water and carbon dioxide, while some types of microbes, such as bacteria, produce methane.
Therefore, it is better to estimate all possible biological effects if possible; But based on the wavelength spectrum and the sensitivity of a telescope to it, the abundance of some elements can be measured better than others. Plotting all of these paths on Young’s decision tree tells astronomers whether they are looking at a world like Earth today, or whether they are observing an earlier version of our universe or a different world.