If Earth was an extrasolar planet, could extraterrestrials find life on it?

Have you ever wondered what the Earth looks like from the perspective of extraterrestrial astronomers? If they, like us, are looking for signs of habitability, what conclusions do they reach from the observations? This thought experiment can entertain us; But it is more than just a fun issue and can be very informative from a scientific point of view. Examining the earth from the perspective of extraterrestrials and then comparing the results seems to be an easier task.

Finding evidence of life on Earth depends on the season extraterrestrial astronomers choose to observe, according to new research. Almost nothing in astronomy is more exciting than finding life on a potentially habitable planet. If this discovery is realized, the news headlines will spread like a virus from one site to another.

So far, we have only been able to briefly investigate exoplanets with the potential for life, and we still have a long way to go. Finding the point where we say “yes. This distant planet is habitable” requires a lot of innovative and scientific reasoning.

Now, a new research has investigated the earth and the possibility of life on it from a different angle and from the point of view of possible extraterrestrials. The title of this research “Earth as an extrasolar planet: variable thermal radiation, seasonal and atmospheric changes of biological indicators” Is. You can see the pre-published version of this research in the arXiv database. Jean-Noel Mettler, a PhD student at ETH Zurich’s physics department who studies exoplanets and their habitability, is the senior author of the study.

Earth from the perspective of the International Space Station

Earth as seen from the International Space Station, June 2016

The historical roots of this type of research go back to the 1970s. At that time, a number of spacecraft were sent to investigate the planets of the solar system. Pioneer 10 and 11 (Jupiter and Saturn) and Voyager (Jupiter, Saturn, Uranus and Neptune) performed low-altitude flybys of Earth’s neighbors.

The 1970s marked the beginning of a more detailed investigation of other planets. By examining ultraviolet and infrared rays, scientists learned many things about the characteristics of the atmosphere, its surface and its overall energy balance; But today we are living in the era of exoplanet science and we are looking for the observation of planets that are several light years away from us.

The bewildering variety of discovered planets is fascinating in its own right, but it’s the habitability of planets that makes them valuable. In fact, we want to know if the creature lives outside. As technology advances, astronomers have access to more powerful tools for studying distant planets. An advanced civilization elsewhere in the Milky Way would probably do the same.

The mentioned research is devoted to the investigation of the spectrum of infrared terrestrial rays, the effect of different geometries of observation on these spectra and how the observations appear to an observer in a distant world. The researchers also evaluated the effect of seasonal changes on the spectra. According to them:

We learned that there are significant seasonal changes in the spectrum of the Earth’s thermal radiation, and the strength of the spectral features of biological indicators such as nitrous oxide, methane, ozone, and carbon dioxide strongly depend on the season and viewing geometry.

The mentioned research examined four different observation geometries: two geometries for the south and north poles, one for the African equator and one for the Pacific equator. The spectra were analyzed by the Atmospheric Infrared Depth Instrument from NASA’s Aqua satellite. The researchers concluded that no single measurement can represent the spectrum of Earth’s thermal radiation because seasonal variations make this impossible. They wrote in their article:

Seasonal changes affect the spectrum of the earth’s thermal radiation, and the strength of the absorption indicators of biological effects is strongly dependent on the two factors of the season as well as the view geometry.

The researchers also found that thermal radiation varies based on the observation geometry. For example, changes in readings of land masses over time were much greater than those of oceans. The African equatorial view and the Arctic view focused on the landmasses and showed many changes. The article concludes as follows:

The Northern Hemisphere Polar View (NP) and Equatorial African View (EqA) showed annual variations of 33% and 22%, respectively, in the peak terrestrial wavelength of approximately 10.2 μm; But thermal stability of the oceans means less variability. On the other hand, view geometries such as the Southern Hemisphere Pole (SP) and the Equatorial Pacific (EqP) view, which are largely oceanic, showed less variation due to the high thermal inertia of the oceans.

The general impression of this research was that a variable and living planet like Earth cannot be recognized by just one spectrum of thermal radiation. Many things happen on earth, and on the other hand, this research has not mentioned the role of clouds and their effect. “Future research is necessary to investigate the influence of clouds and their seasonal changes, as well as the influence of the thermodynamic phase properties of clouds on seasonal atmospheric changes,” the authors write. According to the authors, some changes are small and difficult to detect while observing distant planets. Dirty data can also hide them. They explain:

Even for land, especially for equatorial views, the flux and strength variations of integrated data assimilation combinations are small, approximately ten percent. Separating these changes from noise in future observations of exoplanets will be a big challenge. The complexity of the Earth makes it a difficult target for this type of observation. This complexity makes it very difficult to understand distant planetary environments. Earth is our test bed, and based on it, we learn that a planet and its characteristics cannot be identified solely based on a thermal radiation spectrum, but we need multi-period measurements of thermal radiation as well as reflected light.

Related articles:

Most of the discoveries of exoplanets are based on the passage of these planets in front of their star. This method has limitations. The goal of the James Webb Space Telescope is to investigate the light spectrum of some exoplanets with greater power, and as a result, one day we will reach a better understanding of what we see. The above research tested the new method of observing exoplanets in the infrared spectrum instead of reflected light. Although there are seasonal variations and changes in observation geometry, the test result is not dependent on seasonal or diurnal variations, unlike reflected light measurements. Mettler and his co-researchers believe that their method can use unique data to observe exoplanets. they say:

Therefore, we conclude that observing exoplanets using thermal radiation provides complementary and unique information that is necessary to identify rocky planets around other stars.

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