In the quest to discover life beyond Earth, scientists are harnessing a very large and proximate tool—the moon.
During a total lunar eclipse in January 2019, the moon acted like a giant mirror, reflecting sunlight that had passed through our atmosphere back toward Earth, reports Chelsea Gohd for Space.com. The Hubble Space Telescope, which was positioned between the Earth and moon, intercepted the reflected ultraviolet light for scientists to analyze.
Scientists from NASA and the European Space Agency studied the reflected light from a lunar eclipse during a two-day window. They reported their findings in an article published August 6 in The Astronomical Journal.
For the first time, scientists used a space telescope to capture ultraviolet wavelengths. Though similar ground-based studies have been done before, using a space telescope for this observation allows scientists to simulate future observations of exoplanets, Space.com reports.
The goal was for the telescope to detect the Earth’s ozone layer. The ozone molecule that makes up the Earth’s protective layer absorbs ultraviolet radiation. During the eclipse, Hubble detected lower amounts of UV radiation from the light reflected off the moon than is present from unfiltered sunlight, meaning the Earth’s atmosphere must have absorbed some of it, according to a NASA press release.
If scientists are able to detect an ozone layer or oxygen on a neighboring exoplanet, there’s a possibility that the planet may harbor life. On Earth, oxygen is often produced by life forms, especially those that photosynthesize. If scientists detect an oxygen-rich atmosphere on an exoplanet, especially if the amount of oxygen varies seasonally, there is a chance that it also hosts life. But scientists would need to further analyze the atmosphere using other tools before determining if it’s life-hosting, Allison Youngblood of the Laboratory for Atmospheric and Space Physics, and lead researcher of Hubble's observations, says in the press release.
"One of NASA's major goals is to identify planets that could support life," Youngblood says. "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."
To study other, Earth-sized, exoplanets, scientists plan to use a similar method as was tested in this study. When the exoplanet crosses between its parent star and the telescope, the star light is filtered through the planet’s atmosphere, creating a “halo”-like effect, NASA says. Chemicals in the atmosphere filter out certain colors of starlight, so scientists can understand the atmospheric composition based on the quality of the light that reaches the telescope.
The age of the planet should also be taken into account when determining its ability to host life. Earth had low concentrations of oxygen for more than a billion years, while organisms used photosynthesis to build the ozone layer. If other planets are in the early developmental stage, their ozone might be challenging to detect.
Still, ultraviolet may be “the best wavelength to detect photosynthetic life on low-oxygen exoplanets,” says Giada Arney of NASA's Goddard Space Flight Center and a co-author of the study, in the press release.
The Hubble telescope was launched in 1990, before astronomers discovered the first exoplanets. While its ability to observe extraterrestrial atmospheres is “remarkable,” NASA says future observations of Earth-sized planets will require much larger telescopes and longer observational periods. The James Webb Space Telescope, which is scheduled to launch in 2021, will have increased ability to detect oxygen and methane in atmospheres.