A snapshot of a distant exoplanet gives astronomers an idea of what it looks like. A snapshot of a distant exoplanet as it passes in front of its sun and light shines around it gives astronomers an idea of what the planet and its atmosphere are made of. If we assume there’s intelligent life on some of these planets, we can also assume that they’re looking back at us and trying to figure out the same thing. If we could only look back from deep space at Earth as it passes in front of the Sun, we could see the signatures of life that would tell us if an exoplanet is our cosmic twin – identical or fraternal. Last year, astronomers figured out a way to do without the need for a spaceship looking back. Instead, they looked into a giant mirror during a lunar eclipse. Wait … what?
“Observations of the Earthshine off the Moon allow for the unique opportunity to measure the large-scale Earth atmosphere. Another opportunity is realized during a total lunar eclipse which, if seen from the Moon, is like a transit of the Earth in front of the Sun. We thus aim at transmission spectroscopy of an Earth transit by tracing the solar spectrum during the total lunar eclipse of January 21, 2019.”
As described in a paper published this week in the journal Astronomy & Astrophysics and summarized in a press release, lead author Klaus Strassmeier from the Leibniz Institute for Astrophysics in Potsdam firgured out that a total lunar eclipse on January 21, 2019 – when the Earth passed directly between the Moon and the Sun — was a total ‘solar’ eclipse to anyone looking at the Sun (with protective glasses, of course) from the lunar surface. Since we know (or at least can confidently assume) that no one did that, Strassmeier and astronomers using the Large Binocular Telescope in Arizona took advantage of the dimmed yet reflective surface of the Moon and looked into a mirror image that showed Earth’s atmosphere as sunlight passed through it around the edges – a filtered light with the appropriate name “Earthshine.”
“The Earth’s atmosphere contains many by-products of biological activity, such as oxygen and ozone in association with water vapor, methane and carbon dioxide. These biogenic molecules present attractive narrow molecular bands at optical and near infrared wavelengths for detection in atmospheres of other planets.”
Our Earthshine also showed traces of sodium, calcium and potassium — all elements generated by or needed by human life on the planet. A similar planetshine would thus indicate that an exoplanet has or is capable of hosting similar life forms or even humans, once we figure out how to travel that far.
Besides the Large Binocular Telescope, the astronomers needed the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) to separate tiny spectral-line absorptions of the Earth’s atmosphere from the normal solar spectrum. That combination allowed them to find two elements that don’t belong in Earth’s atmosphere — manganese and barium. Instead of from Earth, the astronomers believe these two chemicals came from the Sun.
“To understand this phenomenon further, the researchers plan to study the Sun’s atmosphere more closely to trace those two conspicuous chemical elements. Doing so will reveal how they traveled from the center of the Sun to its limb, leaving an imprint on Earth’s upper atmosphere, the researchers say.”
In an interview with Inverse, the astronomers admitted they have much more to learn before they’ll feel confident identifying an exoplanet as a potential Earth clone. That should not take anything away from the fantastic accomplishment of using the Moon as a mirror to check ourselves out.