If aliens wanted to study Earth’s atmosphere, they’d have to wait for a lunar eclipse.
This happens when the Earth moves between the Sun and the Moon perfectly enough to cast a shadow on the Moon.
If you were standing on the Moon (or were an alien in a spaceship), this would look like a kind of solar eclipse — the Earth would be passing in front of the Sun.
In this position, the Sun’s rays shine through Earth’s atmosphere, and it’s this interaction between atmosphere and sunlight that would allow alien scientists to figure out what our atmosphere is made of.
This is also how humans study alien worlds — we study how a star’s light passes through an exoplanet’s atmosphere.
When scientists are looking for potentially habitable worlds, they look for the kinds of gasses that are in our atmosphere, like oxygen, nitrogen, and water vapor.
To test this experiment, a group of scientists came up with a clever plan.
They realized they can look at the Sun’s light that the Moon reflects in order to measure the gasses in our atmosphere.
Roberto Machado Noa/Moment/Getty Images
The researchers measured the Sun’s reflected light and found traces of expected gasses: oxygen, water vapor, some sodium, potassium, and calcium.
Being able to look at our Earth like an alien planet helps scientists figure out what to look for on other planets orbiting other stars.