To find an exoplanet, first focus on Earth

Astronomers used the Hubble Space Telescope, Earth and a total lunar eclipse to answer an important question: What might signs of life look like on a planet hundreds of light-years away?

One sign, hidden within the data, could be broad, wiggly lines indicative of ozone.

“When I finally was able to see the ozone feature, that was pretty exciting because it was like, ‘Hey, we detected life on Earth,’ ” said Allison Youngblood, a Houston-born research scientist who now works at the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder. “The most exciting thing was just imagining Earth as if it were an exoplanet.”

Planets outside our solar system are called exoplanets, and NASA says there are more than 4,200 of these (with another 5,481 considered exoplanet candidates). Three-quarters of these worlds were found by looking at a star and watching for dips in brightness when a planet passes in front of it.

Could these planets foster life? With current technologies, astronomers must answer that question by looking for clues in a planet’s atmosphere. They’ve done this with larger exoplanets, but studying the atmosphere of rocky worlds about the same size as Earth will require more powerful telescopes.

For now, researchers are examining Earth’s atmosphere (where they know life exists) so they can recognize these clues on other planets. Youngblood and five oth-

ers studied the Earth in January 2019 during a total lunar eclipse, where our planet moves between the sun and moon. Their findings were published this month in the Astronomical Journal.

‘Strange’ method

Youngblood wanted to see Earth as if she were on the surface of the moon with a telescope. During the eclipse, the Earth passed in front of the sun and caused a dip in brightness somewhat

similar to what astronomers see when observing exoplanets.

But since there isn’t a telescope on the moon, the team of researchers pointed the Hubble Space Telescope in Earth’s orbit at the moon itself and used its surface as a mirror reflecting light from the sun. This allowed them to observe the Earth as it passed in front of the sun.

“This is not the method of choice for Earth atmospheric scientists,” Youngblood said. “This is not what they use to study the Earth. It’s kind of strange, but it’s the tool that we have

right now that works the best for exoplanets.”

Ultraviolet reveals

Youngblood collected data from the sunlight filtering through Earth’s atmosphere at two different wavelengths (electromagnetic energy travels in waves that span a continuum from very long radio waves to very short gamma rays, and studying planets at different wavelengths can provide different clues to their atmospheres).

The first wavelength was visible light that can be seen with the naked eye. This allowed researchers to see a

sloping curve in their data created by molecular nitrogen in the Earth’s atmosphere.

The second wavelength was ultraviolet light, and this part of the electromagnetic spectrum presented the dramatic wiggly lines that indicated ozone was absorbing the light emitted from the sun.

“Hubble has ultraviolet sensitivity, and that part of the spectrum is ideal to look for the spectral fingerprints of ozone,” said Ray Villard, a spokesman with the Space Telescope Science Institute, which performs the science operations for Hubble. “It was a nice use of Hubble.”

Multiple molecules

Ozone is important for life on Earth because it provides protection from the sun’s harmful ultraviolet radiation. It also means there is molecular oxygen.

On our planet, molecular oxygen is created from photosynthesis. But that might not be the case on other planets. For instance, certain types of starlight can break down carbon dioxide, and then those free-flying oxygens could combine to create molecular oxygen without the presence of life.

“There’s not one telltale molecule that says there is life here on this planet,” Youngblood said. “You have to evaluate multiple molecules.”

And knowing how those molecules look on Earth could help researchers as technology matures and they get better glimpses of these far-away worlds.

“As astronomers are gearing up in the next decade or two to actually observer Earth-like planets, we want to know what to expect,” Youngblood said. “It’s a good idea to make sure we understand what Earth would look like with the same tools we have for observing exoplanets.”