If the Earth were an exoplanet, would aliens be able to determine that there is life on it?

(ORDO NEWS) — What would the Earth look like to alien astronomers? What would their observations tell them about the Earth if, like us, they searched the skies for signs of habitability?

This experiment is not just fun: It is instructive from a scientific point of view. In many ways, it’s easier to study our planet and what it looks like, and then extrapolate those results as far as possible.

A new study shows that the search for evidence of life on Earth may depend on the time of year in which astronomers observe alien inhabitants.

There is almost nothing in space science that generates as much excitement as the discovery of a potentially habitable planet.

So far, we have received only a few signs and hints of exoplanets that may be habitable. We have a long way to go.

It will take a lot of scientific research and innovative reasoning before we can say, “Yes. This distant planet is habitable.”

A new study could be part of this journey if we study the appearance of the Earth at different times of the year.

The study is titled “The Earth as an Exoplanet: II. Time Variable Thermal Emission of the Earth and Its Atmospheric Seasonality of Bioindicators”.

It is available from the preprint site arXiv.org and its lead author is Jean-Noel Mettler. Mettler is a doctoral student in the Department of Physics at ETH Zurich, studying exoplanets and habitability.

The historical roots of this type of research go back to the 1970s, when spacecraft visited the planets of our solar system. Pioneer 10 and 11 (Jupiter and Saturn) and Voyager 1 and 2 (Jupiter, Saturn, Uranus and Neptune) made flybys of some of the home planets.

This was the beginning of a deeper study of other planets. By measuring ultraviolet and infrared radiation, scientists have learned a lot about the properties of planetary atmospheres, surfaces, and overall energy balance.

But today we live in the age of exoplanet science. We extend the same type of observation to planets that are light years away from us.

The amazing variety of planets we’ve discovered is interesting in itself, but if there’s a Holy Grail in exoplanet science, it’s habitability. We want to know if there is other life out there somewhere.

As technology advances, astronomers are getting more and more powerful tools to study distant planets. A technological civilization elsewhere in the Milky Way would likely do the same.

This study examines the Earth’s infrared spectrum, the effect of different observing geometries on these spectra, and what these observations would look like to a much more distant observer.

The researchers also assessed how the changing seasons affect the spectra. “We learned that there is significant seasonal variability in the Earth’s thermal spectrum, and the strength of the spectral signatures of bioindicators such as N2O, CH4, O3, and CO2 is highly dependent on both season and survey parameters.”

The study looked at four different observing geometries: one each centered on the North and South Poles, one on the African Equator, and one on the Pacific Equator.

The spectra were observed using the atmospheric infrared probe aboard NASA’s Aqua satellite.

The researchers found that there is no single, representative sample of the Earth’s thermal radiation spectrum. Seasonal changes make this impossible.

“Instead,” the article says, “there is significant seasonal variability in the Earth’s thermal spectrum, and the absorption strength of biosignatures is highly dependent on both season and observation geometry.”

The researchers also found that thermal radiation varies greatly depending on the geometry of the observation. The variability in readings was much greater over land than over oceans. The African Equatorial Survey and the North Pole Survey focused on land masses and showed great variability.

“Specifically, the Northern Hemisphere Pole View (NP) and the Equatorial View Centered on Africa (EqA) showed an annual variability of 33% and 22% at the Earth’s peak wavelength of ≈10.2 µm, respectively,” the conclusion reads.

But the thermal stability of the oceans meant less variability.” “On the other hand, survey geometries with a high sea fraction, such as the Southern Hemisphere Pole Survey (SP) and the Pacific Centered Equatorial Survey (EqP), show less annual variability due to great thermal inertia of the oceans”.

The general conclusion from this study is that a living, dynamic planet like the Earth cannot be characterized by a single spectrum of thermal radiation. There is too much going on on Earth, and this study didn’t even look at clouds and their influence.

“Future work is needed to study how cloud fraction, cloud seasonality, and their thermodynamic phase properties affect the detection and outcome of atmospheric seasonality,” the authors write.

The authors say some of the variations are subtle and would be difficult to detect when observing distant planets. Bad data can hide them.

“Even for Earth, and especially for equatorial species, the variations in flux and strength of extinction features in disk-integrated data are small, typically 10 percent. Separating these variations from noise in future exoplanet observations will be a challenge.”

The complexity of the Earth makes it a difficult target for this type of observation, and the authors acknowledge this.

“This complexity makes remote characterization of the planetary environment a very difficult task,” they explain.

“Using the Earth as a testing ground, we realized that the planet and its characteristics cannot be described by a single thermal emission spectrum, but multi-epochal measurements are required, preferably in both reflected light and thermal emission.”

Most of the exoplanets we have discovered are based on several transits of these planets in front of their stars. This has its limitations.

The James Webb Space Telescope is aiming to study the spectra of some more powerful exoplanets, so the day is approaching when we need to better understand what we see.

This study tested a new method for observing exoplanets in mid-infrared rather than reflected light. Despite seasonal fluctuations and changes in observation geometry, “we found that our result is relatively insensitive to diurnal or seasonal effects, in contrast to reflected light measurements.”

Mettler and his co-researchers believe their method could make a unique contribution to reflected-light observations of exoplanets.

“Therefore, we concluded that the observation of exoplanets using thermal radiation can provide unique and additional information needed to characterize terrestrial planets around other stars.”


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