Study reveals what aliens might see if they spotted Earth from space

For more than 20 years, astronomers have observed alien planets of myriad different compositions and masses, all spread out across the cosmos. But what if someone (or something) on those exoplanets was looking back at us, what would they see?

Thanks to an intriguing new study, scientists have an answer.

A study published Monday in the journal Astronomy and Astrophysics is the first to look at the Earth as though it were a foreign exoplanet, observing it from the outside as it transits in front of the Sun.

The study’s findings not only provide a unique, new perspective on the planet we have inhabited for millions of years, but they also offer a way for scientists to verify their findings as they search for habitable worlds outside of our Solar System.

Earth as an exoplanet

To study exoplanets, scientists use what is known as the transmission spectroscopy method. As the planet transits in front of its host star during its orbit, scientists can observe the spectra of starlight as it shines through a planet’s atmosphere. Signatures in the spectra can reveal the chemistry of the planet and its atmosphere.

“If you want to observe the Earth and you’re sitting on it, it’s a funny thing,” Klaus Strassmeier, a researcher at the Leibniz Institute for Astrophysics in Potsdam, and lead author of the study, tells Inverse.

In order to solve this dilemma, the team of researchers got creative.

On January 21, 2019, a total lunar eclipse occurred. This is when the Moon hides behind the shadow of our planet, as Earth stands in between the Sun and the Moon. From Earth, the Moon appears dimmer.

But if you were to stand on the Moon, then you would get a view of Earth transiting in front of the Sun.

“If we were to stand on the Moon as the observer during a lunar eclipse, then we would see a solar eclipse,” Strassmeier says. “Not [the solar eclipse] for us earthlings with the Moon blocking the Sun, but rather the Earth blocking the Sun.”

Instead of booking a quick trip to the Moon to catch that view, the team of researchers used the image of the Earth as reflected on the Moon.

Using the Large Binocular Telescope Observatory located in Arizona, they pointed it at the part of the Moon that reflects the light of the Sun, just at the right angle to be able to view the parts of that light that has already passed through the upper layer of Earth’s atmosphere.

“We wanted to trace the sunlight as it shines through the upper layers of Earth’s atmosphere, [an area] that we otherwise cannot easily observe,” Strassmeier says.

Earth as a prototype for habitability

The researchers were able to trace chemical elements such as sodium, potassium and calcium in Earth’s upper atmosphere, in addition to oxygen, and water vapor.

But they also found traces of manganese and barium, two chemical elements that do not belong in Earth’s upper atmosphere in their free atomic form.

Instead, these two chemical elements likely came from the Sun’s atmosphere, rather than the Earth's, the researchers say.

“It modulates the light in a way we have never seen before,” Strassmeier says.

Turning our view back on Earth in this way is crucial to the search for exoplanets that may be habitable. The composition of an exoplanet’s atmosphere is one of the main ingredients for life on other worlds, therefore scientists need to ensure that their results reflect what is coming from the planet's atmosphere, versus what may be coming from its star.

“We have to verify our findings, and we have many findings on other exoplanets but we have to make sure we’re not observing radiating transfer issues or photons interacting with interplanetary material,” Strassmeier says.

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.

Abstract: 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. Time series spectra of the Tycho crater were taken with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT) in its polarimetric mode in Stokes IQUV at a spectral resolution of 130000 (0.06 Å). In particular, the spectra cover the red parts of the optical spectrum between 7419-9067 Å. The spectrograph's exposure meter was used to obtain a light curve of the lunar eclipse. The brightness of the Moon dimmed by 10.75 mag during umbral eclipse. We found both branches of the O2 A-band almost completely saturated as well as a strong increase of H2O absorption during totality. The deep penumbral spectra show significant excess absorption from the NaI 5890 Ådoublet, the CaII infrared triplet around 8600 Å, and the KI line at 7699 Åin addition to several hyper-fine-structure lines of MnI and even from BaII. The detections of the latter two elements are likely due to an untypical solar center-to-limb effect rather than Earth's atmosphere. The absorption in CaII and KI remained visible throughout umbral eclipse. A small continuum polarization of the O2 A-band of 0.12\% during umbral eclipse was detected at 6.3σ. No line polarization of the O2 A-band, or any other spectral-line feature, is detected outside nor inside eclipse. It places an upper limit of ≈0.2\% on the degree of line polarization during transmission through Earth's atmosphere and magnetosphere.