The study of exoplanets has matured significantly in the last decade. During this time, most of the more than 4,000 exoplanets we know are currently being discovered. During this time, the process began to shift from the discovery process to characterization. In addition, next-generation instruments will enable studies that tell a great deal about the surfaces and atmospheres of exoplanets.
This, of course, raises the question: what would a sufficiently advanced species see if they examined our planet? Using Earth's multi-wavelength data, a team of Caltech scientists was able to create a map of what the Earth would look like to distant extraterrestrial observers. In addition to addressing the itching of curiosity, this study could help astronomers reconstruct the surface features of "Earth-like" exoplanets in the future.
The study, which describes the results of the team's title "Earth as Exoplanet: A Two-Dimensional Alien Map", recently appeared in the journal Science Mag and is for publication in The Astrophysical Journal Journal Letters . The study was led by Siteng Fan and included several researchers from the Department of Geological and Planetary Sciences (GPS) of the California Institute of Technology and the NASA Jet Propulsion Laboratory.
In search of potentially habitable planets outside our solar system, scientists must choose the indirect approach. Since most exoplanets can not be directly observed to determine their atmospheric composition or surface features (also known as direct imaging), scientists must be content with clues that show how "earthlike" a planet is.
As Fan said to Universe Today via e-mail, this reflects the limitations that astronomers and exoplanet studies currently have to address:
"First, current exoplanet studies have failed to find out what minimum standards of habitability exist. There are some suggested criteria, but we are not sure if they are sufficient or necessary. Second, despite these criteria, current observation techniques are not good enough to confirm habitability, especially on Earth-like exoplanets, as it is difficult to detect and constrain.
Considering that Earth is the only planet we know The team is able to support life, and theorized that remote earth observations could serve as a substitute for a habitable exoplanet, like one of them distant civilization is observed. "The Earth is the only planet we know to contain life," Fan said. "If we examine what the Earth looks like to distant observers, we can locate possible habitable exoplanets."
One of the most important elements of Earth's climate (and vital to all life on its surface) is the water cycle, which has three phases. These include the presence of water vapor in the atmosphere, clouds of condensation and ice particles, and the presence of surface waters.
Therefore, the presence of these could be considered as potential signs of habitability and even signs of life (aka biosignatures) that could be observed remotely. Ergo, it would be essential to be able to identify surface features and clouds on exoplanets in order to limit their habitability.
To determine what the Earth would look like to distant observers, the Team 9740 created Earth images taken by NASA's DSCOVR (Deep Space Climate Observatory) satellite. The images were taken every 68 to 110 minutes over a period of two years (2016 and 2017) and they were able to capture light reflected from the earth's atmosphere at multiple wavelengths.
Fan and his colleagues then combined the images into a 10-point reflection spectrum that was plotted over time and then integrated across the Earth's disk. This effectively reproduced what Earth might look like for a
"We found that the second major component of the Earth's light curve correlates strongly with the land portion of the illuminated hemisphere (r ^ 2 = 0.91)," said Fan. "In combination with the Viewing geometry makes the reconstruction of the map a linear regression problem. "
After analyzing the resulting curves and comparing them to the original images, the research team determined which parameters of the curves corresponded to the land and cloud cover. They then selected the parameters most likely to be related to the land area and adapted them to the Earth's 24-hour rotation. Thus, they received a contoured map (see above) that depicted how the Earth's light curve would look
The black lines represent the surface parameters and roughly correspond to the coastlines of the main continents. These are further colored green to allow a rough representation of Africa (center), Asia (top right), North and South America (left) and the Antarctic (bottom). What lies in between represents the oceans of the earth, with the shallower sections marked red and the deeper blue ones.
Such representations could allow astronomers, when applied to the light curves of exoplanets, to assess whether an exoplanet has the oceans, clouds, and ice caps – all necessary elements of an Earth-like (also known as habitable) exoplanet. Fan
"The analysis of the light curves in this work has implications for the determination of geological features and climate systems on exoplanets. We found that the variation of the Earth's light curve is dominated by clouds and land / ocean, both of which are crucial to life on Earth. Therefore, earth-like exoplanets with such characteristics would rather harbor life. "
In the near future, next-generation instruments, such as the James Webb Space Telescope (JWST), will enable the most detailed exoplanet surveying to date. In addition, ground-based instruments such as the Extremely Large Telescope (ELT), the Thirty Meter Telescope (TMT) and the will go online over the next decade. Giant Magellan Telescope (GMT) – to facilitate direct imaging studies of smaller, rocky planets orbiting closer to their stars.
Using studies that help resolve surface features and atmospheric conditions, astronomers may finally be able to confidently say which exoplanets are habitable and which are not. With a bit of luck, the discovery of Earth 2.0 (or more Earths) could be just around the corner!
Reference: Science, arXiv