Over the past ten years, there has been a sharp increase in interest and research in planets outside the solar system, or exoplanets.
During this time, most of the 4 thousand exoplanets known to us today were discovered. It was during this period that the process gradually began to move from the discovery stage to the learning stage. Moreover, in the coming decades, next-generation instruments will allow research that will provide accurate information about the surface structure and atmosphere of exoplanets.
Naturally, the question arises here: what would more advanced civilizations see if they studied our planet? Using Earth's multi-band radiation data, a team of scientists at the California Institute of Technology was able to create a map that gives an idea of what the Earth might look like to distant observers from outside the solar system. In addition to satisfying simple scientific curiosity, this study may also help astronomers in the future to reconstruct the surface features of "Earth-like planets" suitable for life.
The research paper, which describes the findings of the group, is published in the Astrophysical Journal Letters titled Earth as an Exoplanet: A 2D Map for Aliens. The research team, led by Siteng Fan, also included several scientists from the Department of Geological and Planetary Sciences (GPS) of the California Institute of Technology and NASA's Jet Propulsion Laboratory.
In the search for potentially habitable planets outside the solar system, scientists today are forced to use an indirect method. Considering that most exoplanets are not possible to study directly, that is, to obtain a direct image in order to find out the composition of their atmosphere or surface features, scientists have to be satisfied with indicators by which one can judge how much a planet is similar to Earth.
As Siteng Fan told Universe Today, this is due to many of the limitations that astronomers exploring exoplanets have to put up with at present.
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“Firstly, current exoplanet research does not yet provide a clear idea of the minimum requirements that a planet must meet that is suitable for human life. There are certain criteria, but we are not sure if they are sufficient or necessary. Secondly, even with these criteria in place, current observation methods cannot be called effective enough to confirm the potential suitability for life, especially when it comes to exoplanets like Earth, due to the difficulty of detecting them."
Based on the fact that Earth is the only planet capable of supporting life, a team of scientists has hypothesized that remote observations of the Earth could provide the information needed to detect habitable planets. “Earth is the only planet we know of that has life,” Fan said. “Examining what it might be seen by observers from a distant point in the universe gives us direction and guidance in our search for potentially habitable exoplanets.”
One of the most important elements of the Earth's climate, which is critical for all life on its surface, is the three-phase water cycle. We are talking about the presence of water vapor in the atmosphere, clouds, which are accumulations of condensed water and ice, as well as water bodies on the planet's surface.
Thus, they can be seen as potential signs of suitability for life, or even signs of its existence, which can be observed from a great distance. It follows from this that the ability to identify the surface structure and the presence of clouds on exoplanets is a key requirement that research must meet in order to establish their potential habitability.
To determine what the Earth would look like to distant observers, scientists collected about 10,000 images taken by NASA's Deep Space Climate Observatory (DSCOVR) satellite. Pictures were taken over a two-year period (2016-2017), on average, every 68-110 minutes. They were able to capture light reflected from the earth's atmosphere in various wavelengths.
Phan and his colleagues then combined the images to form a 10-point reflectance spectrum, which was then integrated with the earth's disk. The resulting image is consistent with what the Earth might look like to an observer many light years away if he studied the Earth over a two-year period.
After analyzing the curves obtained and comparing them with the original images, the scientists found out which parameters of these curves correspond to the earth's surface and cloud cover. They then selected those indicators that are most closely related to land and adjusted them to account for the Earth's 24-hour turnover. The result was a contour map, shown in the figure, which roughly corresponds to the view of the Earth from a distance of several light years.
The black lines represent surface features and roughly correspond to the coastline of the major continents. The green zones roughly represent the positions of Africa (center), Asia (top right), the Americas (left), and Antarctica (bottom). What lies between them represents the world's oceans, where shallower areas are marked in red and deeper areas in blue.
Such visualization, applied to the light curves of distant planets, can allow astronomers to assess whether the exoplanet has oceans, clouds and ice caps, that is, find out everything that is necessary to recognize it as potentially habitable.
Siteng Phan concluded, “The analysis of the light curves in this work is important in determining the geological features and climatic systems on the exoplanet. We found that changes in the Earth's light curve are mainly driven by clouds and the land-ocean boundary. Both of these factors are critical to the possibility of life on Earth. Thus, exoplanets like Earth, which have such features, are most likely to be habitable."
In the near future, next-generation instruments such as the James Webb Space Telescope (JWST) will enable the most detailed studies of exoplanets. In addition, ground-based instruments that will enter service in the next decade, such as the Extremely Large Telescope (ELT), the Thirty Meter Telescope (TMT) and the Giant Magellanic Telescope (GMT), are expected to enable direct exploration small rocky planets orbiting their stars.
With research that helps determine surface structure and atmospheric conditions, astronomers can finally say with confidence which exoplanets are habitable and which are not. In other words, with a bit of luck, the discovery of Earth-2, or, for that matter, even several Earths, may not be far off.
Igor Abramov
