Astronomers don’t typically observe exoplanets directly. Instead, they look for transits, or telltale blips when a planet crosses in front of its parent star. During such a time, starlight is filtered through the exoplanet’s atmosphere in the same way that, during a lunar eclipse, sunlight passes through Earth’s atmosphere before it hits the moon.
That means astronomers can treat a lunar eclipse as a proxy for an exoplanet transit. “It’s basically using the moon as a mirror to observe the Earth transiting the sun,” said Allison Youngblood, an astronomer at the NASA Goddard Space Flight Center in Greenbelt, Md.
In January 2019, Dr. Youngblood and her colleagues trained the Hubble Space Telescope on the moon during a total lunar eclipse. Because chemicals in Earth’s atmosphere should block certain wavelengths of sunlight from reaching the moon — thus leaving dips in the observed spectrum — Dr. Youngblood’s team was able to detect ozone.
“It’s kind of like a practice round,” Dr. Youngblood said. By treating Earth as an exoplanet, astronomers can double-check that they correctly detect atmospheric details when observing other stars.
But Manisha Shrestha, an astronomer at the University of Arizona, has another idea in mind. She plans to observe the lunar eclipse on Tuesday from the Bok Telescope at Kitt Peak National Observatory in Arizona with the hope of spotting not only certain chemicals within our atmosphere, but also their distribution.
This technique has never been performed on exoplanets before and could mean that future detections won’t simply reveal whether an exoplanet has clouds, but whether those clouds smother the world in a thick layer or whether they are slightly uneven, as clouds on Earth are. If those clouds were both uneven and composed of water vapor, that exoplanet just might be Earth 2.0.
But you don’t need a scientific reason to enjoy the eclipse. Astronomers agree that it’s the perfect opportunity to take a break from the politics of election season and simply ponder the cosmos.
“From the cosmic perspective, our problems are temporary things — things that are passing fancies of the human species,” Dr. Fraknoi said. “The eclipse connects you to cycles and rhythms that are much older.”
