Astronomers have used the Hubble Space Telescope to observe a total lunar eclipse in ultraviolet and from space for the first time. The observations took place on January 21, 2019, and the goal was a peculiar one: To study Earth’s atmosphere in an indirect way to see what kind of chemicals can be detected in this way.
When the Moon is completely in Earth's shadow, it appears red. This is because the light of the Sun is filtered through our planet’s atmosphere and reflected by the Moon. As reported in The Astronomical Journal, by taking advantage of the total lunar eclipse, Hubble was able to detect the ozone present in Earth’s atmosphere.
"Finding ozone is significant because it is a photochemical byproduct of molecular oxygen, which is itself a byproduct of life," lead researcher Allison Youngblood of the Laboratory for Atmospheric and Space Physics in Boulder, Colorado, explained in a statement.
It might not seem super exciting that Hubble discovered a byproduct of life on Earth. After all, we know our planet is teeming with life. But this approach can be used to study planets outside the Solar System. When these exoplanets pass in front of their stars, the filtered light is influenced by the chemical composition of the atmosphere. Using Earth as a proxy, we might be able to identify other worlds with similar atmospheres.
An exoplanet sporting ozone would be an interesting find, but they can form in ways that don’t require life, so more chemicals than that identified here will have to be observed before we could confirm the presence of life beyond the Solar System.
"One of NASA's major goals is to identify planets that could support life," Youngblood said. "But how would we know a habitable or an uninhabited planet if we saw one? What would they look like with the techniques that astronomers have at their disposal for characterizing the atmospheres of exoplanets? That's why it's important to develop models of Earth's spectrum as a template for categorizing atmospheres on extrasolar planets."
Studying the atmospheres of Earth-size exoplanets remains beyond our current instruments as they are very small and their atmospheres very thin. But it is important that we know what we should be looking for, so future observatories can be built with that in mind.
That said, Hubble has been able to study the atmosphere of large exoplanets, although they were not yet discovered when it was launched. It's quite remarkable and a true testament of the many people working on the telescope over the last three decades that this was achieved.