Rarely do astronomers get to play action heroes in real life since most of what we do is far more mundane. 

For instance, astronomers wanted to figure out how to see our Earth’s atmosphere the same way we see exoplanet atmospheres – by studying sunlight that has passed through the atmosphere and been altered in its passage. While it would have been super cool to fly a spacecraft with fancy instruments to a point in our solar system where it could watch the Earth transit the Sun, astronomers came up with a much more boring technique. 

During the January 2019 lunar eclipse, astronomers pointed the Hubble Space Telescope at the Moon. The Moon passed into Earth’s shadow, and the only light bouncing off its surface was the sunlight that had passed through Earth’s atmosphere and light pollution from Earth that had scattered into space. This is consistent with the light we would see from an alien civilization during the transit of a planet across a star. 

In studying the light reflected from the Moon, astronomers using Hubble were able to find signs of life on Earth; specifically, they were able to identify ozone. This three-oxygen molecule is most common where life of some form is producing oxygen. Left alone, oxygen will bond to other atoms, but ozone can be produced when nitrogen and oxygen are exposed to sunlight. This kind of detection quite accurately says that Earth has something oxygen-generating that could be life, but to definitively say there is life will require the detection of additional chemicals in Earth’s atmosphere. For now, the best Hubble can do is say Earth might contain life.

More Information

Hubblesite press release 

Space Telescope Science Institute press release 

“The Hubble Space Telescope’s Near-UV and Optical Transmission Spectrum of Earth as an Exoplanet,” Allison Youngblood et al., 2020, Astronomical Journal