Testing techniques that one day could be used to detect the presence of biosignatures in exoplanet atmospheres, the Hubble Space Telescope took advantage of a total lunar eclipse to collect ultraviolet spectra of sunlight passing through Earth’s atmosphere on its way to the moon. Hubble was able to detect the presence of ozone in sunlight reflecting off the eclipsed moon, a first for a space-based telescope.
“Finding ozone is significant because it is a photochemical byproduct of molecular oxygen, which is itself a byproduct of life,” said Allison Youngblood of the Laboratory for Atmospheric and Space Physics in Boulder, Colorado, who led the research.
The spectra of sunlight passing through the atmosphere of an exoplanet as it moves in front of its parent star is affected by interactions with the chemicals in that atmosphere. Similarly, the spectra captured by Hubble included the tell-tale effects of ozone.
Ozone forms when oxygen is exposed to strong ultraviolet radiation, which triggers the necessary chemical reactions. Photosynthesis over billions of years generated high oxygen levels and the thick ozone layer protecting the surface of the planet from dangerous solar radiation. Finding ozone or oxygen in an exoplanet’s atmosphere could be an indicator of biological activity.
Similar signatures have been seen by ground-based telescopes, but Hubble’s observations during a lunar eclipse in January 2019 were the first for a space telescope. Because the sunlight did not pass through Earth’s atmosphere twice – once on the way to the moon and then again when reflected back to ground-based instruments – the ozone signature seen by Hubble was the strongest observed to date.
“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 characterising the atmospheres of exoplanets? That’s why it’s important to develop models of Earth’s spectrum as a template for categorising atmospheres on extrasolar planets.”