The Earth is an exoplanet for the extraterrestrials. That's what they would see

The study of exoplanets has matured considerably in the last ten years. During this period, the majority of the more than 4000 exoplanets known to date have been discovered. It was also at this time that the process began to move from the discovery process to the characterization. In addition, the next-generation instruments will allow studies revealing many things about the surfaces and atmospheres of exoplanets.

This naturally raises the question: what would a sufficiently advanced species be if it studied our planet? Using Earth data of several wavelengths, a team of scientists from Caltech was able to create a map of what Earth would look like for distant alien observers. In addition to responding to the itch of curiosity, this study could also help astronomers reconstruct surface features of "Earth-like" exoplanets in the future.

The study describing the team's findings, entitled "The Earth, an exoplanet: a two-dimensional extraterrestrial map," was recently published in the journal Science Mag and is scheduled for publication in The letters of the astrophysical journal. The study was led by Siteng Fan and included many researchers from the California Institute of Technology's Division of Geological and Planetary Sciences (GPS) and NASA's Jet Propulsion Laboratory.

When they search for potentially habitable planets beyond our solar system, scientists are forced to take an indirect approach. Since most exoplanets can not be observed directly for their atmospheric composition or surface characteristics (aka Direct Imaging), scientists must be satisfied with the indications of how similar a planet is to the Earth.

As Fan told Universe Today by email, this reflects the limitations currently facing astronomers and studies of exoplanets:

"Firstly, current studies on exoplanets have failed to determine the minimum requirements for livability. Some criteria are offered, but we do not know if they are sufficient or necessary. Secondly, even with these criteria, the current observation techniques do not confirm the habitability, in particular for terrestrial exoplanets, because of the difficulty of detecting and constraining them. "

Given that Earth is the only planet we know that is capable of supporting life, the team hypothesized that remote observations of the Earth could serve as a substitute for a habitable exoplanet as observed by distant civilization. "The Earth is the only planet we know that contains life," Fan said. "Studying what Earth looks like for remote observers would tell us how to find potential living exoplanets."

One of the most important elements of Earth's climate (and which is essential to any form of life on its surface) is the water cycle, which has three distinct phases. These include the presence of water vapor in the atmosphere, clouds of condensed water and ice particles, and the presence of water masses on the surface.

Consequently, their presence could be considered as potential indications of habitability and even indications of life (or biosignatures) that can be observed from a distance. Ergo, to be able to identify surface features and clouds on exoplanets would be essential to impose constraints on their habitability.

To determine what the Earth would look like in the eyes of distant observers, the team compiled 9740 images of the Earth, which were taken by NASA's Deep Space Climate Observatory (DSCOVR) satellite. The images were taken every 68 to 110 minutes over a two-year period (2016 and 2017) and captured the light reflected from the Earth's atmosphere at different wavelengths.

Fan and his colleagues then combined the images into a 10-point time-based reflection spectrum, which was then integrated on the Earth's disk. This effectively reproduced what the Earth might look likenot observer many light years away if they observed the Earth over a period of two years.

"We found that the second major component of the Earth's light curve is strongly correlated with the Earth's illuminated hemisphere fraction (r ^ 2 = 0.91)," Fan said. "By combining with visualization geometry, map reconstruction becomes a linear regression problem."

After analyzing the curves obtained and comparing them to the original images, the research team determined which parameters of the curves corresponded to the terrestrial and cloud cover. They then selected the parameters most closely related to the terrestrial area and adjusted them to the 24-hour rotation of the Earth, which gave them a contoured map (shown above) representing the Earth's light curve. . Light years a way.

The black lines represent the surface feature parameter and roughly correspond to the coasts of the main continents. These are still colored green to give an approximate representation of Africa (center), Asia (top right), North and South America (left) ) and Antarctica (bottom). What lies between the two represents the oceans of the Earth, the shallower parts being indicated in red and the deepest in blue.

This type of representation, applied to the distant exoplanet light curves, could allow astronomers to determine if an exoplanet has oceans, clouds and ice caps – all the elements necessary for an "earthly" exoplanet. As a fan conclusionded:

"The analysis of light curves in this work has implications for the determination of geological features and climatic systems on exoplanets. We have found that the variation of the Earth's light curve is dominated by clouds and the earth / ocean, both of which are essential to life on Earth. Therefore, terrestrial-type exoplanets that harbor such features would be more likely to harbor life. "

In the future, new generation instruments such as James Webb Space Telescope (JWST) will allow the most detailed exoplanet surveys to date. In addition, ground-based instruments put online over the next decade – such as the Very large telescope (ELT), the Telescope of thirty meters (TMT), and the Giant telescope of Magellan (GMT) – should allow to directly study the imagery of smaller and more rocky planets that revolve around their stars.

With the help of studies that help to solve surface problems and atmospheric conditions, astronomers can at last be able to say with confidence which exoplanets are habitable and which are not. With a little luck, the discovery of an Earth 2.0 (or several lands elsewhere) could be imminent!

Further reading: Science, arXiv