If Earth Were an Exoplanet, it Would Still be Tricky to Figure Out if There’s Life Here

How would Earth appear to alien astronomers? What would their observations tell them about Earth if they were searching the heavens for signs of habitability like we are? It’s a fun thought experiment.

But the experiment is more than just fun: it’s scientifically instructive. In many ways, it’s easier to study our planet and how it appears and then extrapolate those results as far as they go.

A new study shows that finding evidence of life on Earth may depend on the season alien astronomers are observing.

Almost nothing in space science generates as much widespread excitement as finding a potentially habitable planet. The headlines spread like a virus through the internet with only minor mutations from site to site. So far, we’ve only got glimpses and hints of exoplanets that might be able to support life. We’ve got a long way to go.

It’ll take a lot of science and innovative reasoning before we ever get to a point where we can say “Yes. This distant planet is habitable.” A new study might be part of getting to that point by examining Earth’s outward appearance through different seasons.

The study is “Earth as an Exoplanet: II. Earth’s Time-Variable Thermal Emission and its Atmospheric Seasonality of Bio-Indicators.” It’s available at the pre-press site arxiv.org, and the lead author is Jean-Noel Mettler. Mettler is a doctoral student at the ETH Zurich Department of Physics, studying Exoplanets and Habitability.

The historical roots of this type of research go back to the 70s when spacecraft were visiting the planets in our Solar System. Pioneer 10 and 11 (Jupiter and Saturn) and Voyager 1 and 2 (Jupiter, Saturn, Uranus, and Neptune) performed flybys of some of Earth’s siblings. It was the beginning of more in-depth characterization of other planets. By measuring UV and infrared, scientists learned a lot about the properties of planetary atmospheres, surfaces, and overall energy balance.

But today, we live in the time of exoplanet science. We’re extending the same type of observations to planets light years away. The bewildering variety of planets we’ve discovered are interesting in their own right, but if there’s a Holy Grail in exoplanet science, it’s got to be habitability. We want to know if anything else lives somewhere out there.

As our technology advances, astronomers are getting more powerful instruments to study distant planets with. A technological civilization elsewhere in the Milky Way would likely do the same thing. This study examines Earth’s infrared emission spectrum, the effect of different observation geometries on those spectra, and how the observations would appear to a much more distant observer. The researchers also assessed how the changing seasons impact the spectra. “We learned that there is significant seasonal variability in Earth’s thermal emission spectrum, and the strength of spectral features of bio-indicators, such as N2O, CH4, O3 and CO2, depends strongly on both season and viewing geometry.”

The study looked at four different observing geometries: one each centred on the north and south poles, one on the African equatorial and one on the Pacific equatorial. The spectra were observed with the Atmospheric Infrared Sounder aboard NASA’s Aqua satellite.

This figure from the study shows the four observing geometries used: North Pole, South Pole, Equatorial Africa, and Equatorial Pacific. The study measured infrared emissions rather than reflected light. Image Credit: Mettler et. al. 2022.
This figure from the study shows the four observing geometries used: North Pole, South Pole, Equatorial Africa, and Equatorial Pacific. The study measured infrared emissions rather than reflected light. Image Credit: Mettler et al. 2022.

The researchers found that there’s no single, representative sample of Earth’s thermal emissions spectrum. The seasonal changes make it impossible. “Instead,” the paper states, “there is significant seasonal variability in Earth’s thermal emission spectrum, and the strength of biosignature absorption features depends strongly on both season and viewing geometry.”

This figure from the study shows seasonal variations in Earth's thermal emission spectrum. The 365 days in an Earth year run along the bottom of the graphs. Each panel is a different potential biosignature. The different colours in each panel represent the four viewing geometries. Image Credit: Mettler et. al. 2022.
This figure from the study shows seasonal variations in Earth’s thermal emission spectrum. The 365 days in an Earth year run along the bottom of the graphs. Each panel is a different potential biosignature. The different colours in each panel represent the four viewing geometries. Image Credit: Mettler et al. 2022.

The researchers also found thermal emissions varied greatly by observing geometry. The variability in readings was much greater over time above land masses than above oceans. The African Equatorial View and the North Pole view were centred on land masses and showed greater variability. “Specifically, the northern hemisphere pole-on view (NP) and the Africa-centered equatorial view (EqA) showed annual variabilities of 33% and 22% at Earth’s peak wavelength at ? 10.2 µm, respectively,” the paper concluded.

But the thermal stability of oceans meant less variability. “On the other hand, viewing geometries with a high sea fraction, such as the southern hemisphere pole-on (SP) and the Pacific-centered, equatorial view (EqP), show smaller annual variabilities due to the large thermal inertia of oceans.”

The overall takeaway from this research is that a living, dynamic planet like Earth can’t be characterized by a single thermal emissions spectrum. There’s too much going on here on Earth, and this study didn’t even delve into clouds and their effect. “Future work is required to investigate how cloud fraction, cloud seasonality and their thermodynamical phase properties affect the detection and result of
atmospheric seasonality,” the authors write. “

The authors say that some variations are slight and will be difficult to untangle when observing distant planets. Dirty data could obscure them. “Even for Earth and especially for equatorial views, the variations in flux and strength of absorption features in the disk-integrated data are small and typically ? 10%. Disentangling these variations from the noise in future exoplanet observations will be a challenge.”

Earth’s complexity makes it a difficult target for this type of observation, and the authors acknowledge it. “This complexity makes remote characterization of planetary environments very challenging,” they explain. “Using Earth as our test bed, we learned that a planet and its characteristics cannot be described by a single thermal emission spectrum, but multi-epoch measurements, preferably in both reflected light and thermal emission, are required.”

Most of our exoplanet detections are based on a few transits of those planets in front of their stars. That has its limitations. The James Webb Space Telescope aims to study the spectra of some exoplanets with more power, so we’re approaching the day when we’ll need to understand better what we’re seeing.

This study tested a new method of observing exoplanets in mid-Infrared rather than in reflective light. Even though there’s seasonal variation and observing geometry variation, “… we find that our result is relatively insensitive to diurnal or seasonal effects, unlike in the case for reflected light measurements.”

Mettler and his co-researchers think their method can contribute unique data to exoplanet observations in reflected light. “We, therefore, conclude that observing exoplanets with thermal emission could provide unique and complementary information that is necessary for the characterization of terrestrial planets around other stars.”

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