In the recent decades, astronomers have developed several methods and strategies that allow them to explore distant worlds in an attempt to track down alien life. One of the most popular methods involves the identification of radio signals that could be sent by other civilizations.
Other researchers scan the atmospheres of distant planets to discover bio-signatures, which could prove that life is present on them. To refine the parameters, it would be ideal for observing planets on which bio-signatures are already present. However, there is only a single known planet on which life exists: Earth. Sending a spacecraft at a distance of tens or hundreds of light-years to run scans is an impossible goal at this point due to technical limitations.
Earth can be observed as an exoplanet during lunar eclipses
There is a suitable alternative: observing Earth during a lunar eclipse. Lunar eclipses will take place when the Moon travels through the shadow produced by Earth. Our planet will block the most of light that comes from Sun, but a small part is refracted by the atmosphere of Earth and travels towards the Moon. An observer present on the lunar surface would see a ring of flames in place of Earth.
The same effect is used to observe distant exoplanets. Exoplanets that pass in front of their stars will generate the same effect as particles found in the atmosphere will reflect specific wavelengths of light. Since different amounts of chemicals will reflect different wavelengths, this phenomenon is used to trace molecules like water or carbon dioxide.
A team of scientists exploited a lunar eclipse that took place in January 2019 to observe the atmosphere of the Earth as an exoplanet by studying the spectrum of light, which was reflected by the surface of the Moon. The data collected by the team inferred the abundant presence of water and oxygen. A paper has been published in a scientific journal.