Most of the exoplanets that we have discovered over the last decade were spotted thanks to the transiting method. They were caught passing in front of their star, briefly dimming the starlight, and thus giving away their existence. This approach can also be used to study a planet’s atmosphere, telescope-power permitting. So a question that astronomers have been pondering is this: if aliens were looking at Earth transiting across the Sun, what would they see? Now, thanks to last year's January total lunar eclipse, we have a better idea.
During a total lunar eclipse, the Earth is between the Moon and the Sun. The light from the Sun is filtered through our planet's atmosphere, giving the Moon a red coloration. Our natural satellite becomes 20,000 times dimmer during such an eclipse, so telescopes were employed to study the components usually hidden in this light, which are referred to as lines in the spectrum
“We are most interested in Earth-like planets and whether we could detect more complex molecular signatures in an exo-Earth transmission spectrum possibly even hinting for life”, explained lead author Klaus Strassmeier from the Leibniz Institute for Astrophysics in Potsdam (AIP), in a statement.
“While not yet doable for any Earth-like exoplanet transit, a total lunar eclipse, which is a total solar eclipse when seen from our own Moon, is nothing else than a transit of our own Earth, and indirectly observable.”
Reporting in the journal Astronomy & Astrophysics, the team combined observations from the Large Binocular Telescope (LBT) in Arizona with the high spectral resolution of the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) to peer at the sunlight filtered through Earth's atmosphere, and reflected by the Moon during the total lunar eclipse. This allowed them to clearly detect signatures indicating oxygen and water vapor in the Earth’s atmosphere, in the form of narrow molecular bands at optical and near-infrared wavelengths.
This is not a guaranteed “there must be life there” detection, but to a distant observer it would certainly show that our planet is capable of supporting life, at least life as we know it. We would certainly be excited to see an Earth-sized planet with an atmosphere rich in oxygen and water.
The researchers say that using Earth as a habitable prototype, this technique can verify the presence of biogenic and chemical elements, and so could be used as a test-case to detect other potential exo-Earths. Next-generation observatories could potentially have the acuity to spot these signals in the atmospheres of distant planets, and help us find other worlds where life might be possible.