How pollution can help find extraterrestrial life – 01/26/2024 – Science

One day, about ten years ago, a colleague knocked on Gonzalo González Abad’s door with an unexpected question.

“If you were looking for signs of a technologically advanced extraterrestrial civilization light years away from us, how would you try to find them?”

González Abad is an atmospheric scientist at the Harvard-Smithsonian Center for Astrophysics, in the United States. He thought for a moment and replied: “CFCs”, chlorofluorocarbons.

On Earth, various objects, such as aerosol cans and refrigerators, spent years releasing these gases in immense volumes — until we discovered that CFCs were destroying the ozone layer.

“They last for a long time — and they are certainly not produced by nature,” says González Abad.

Therefore, if some alien population polluted their world as we did ours in the 20th century, our telescopes could simply detect the presence of CFCs in the atmospheres of other planets.

This would potentially be an indication of a technology-rich culture elsewhere in the universe — what scientists call a “technosignature.”

In other words, maybe we’re not the only ones… to mess up a planet. In the same way that people’s trash can reveal their secrets, extraterrestrial civilizations can let themselves be revealed through sheer negligence.

Over the years, researchers have focused a lot on this subject. They found several possibilities for technosignatures — from excess light to space fragments or harmful gases in the atmosphere of an extraterrestrial planet.

Telescopes powerful enough to detect technosignatures are emerging. And many scientists hope they can be used in the coming decades to try to find life on other planets.

Trash is out there, waiting to be found. Or not.

In search of pollutants

In 2014, González Abad published a study with other authors that discussed the possibility of finding extraterrestrial beings through CFC emissions.

The researchers calculated that if the concentration of these gases in the atmosphere of a distant planet reaches about 10 times their concentration on Earth, it may be possible to detect their presence with the James Webb Space Telescope, which begins operations in 2022.

Basically, CFCs can remain in a planet’s atmosphere for tens of thousands of years. This means that an extraterrestrial civilization would not necessarily need to produce them for a long time to leave traces of activity.

The chlorine in Earth’s atmosphere is currently there due to the emission of CFCs from decades past. Today, these gases are banned worldwide, but there is still a lot of work to be done to completely eliminate them.

It may be possible to detect CFCs with the James Webb Space Telescope if the polluted planet orbits a small white dwarf star, González Abad and his colleagues indicate. This would increase the chances of sufficient light reaching Earth.

Scientists can look for CFCs and several other substances in the atmospheres of distant planets by studying the spectra — specific wavelengths — of light reflected from alien worlds.

Because some light is absorbed by certain substances as it passes through the atmosphere, the exact characteristics of the emitted light can reveal which substances are present on a distant body.

Many of the categories of extraterrestrial technosignatures already discussed are inspired by pollutants created by humans here on Earth.

Scientists have already considered, for example, the possibility of finding extraterrestrial civilizations by detecting immense amounts of residual heat emitted by industrial sources.

Others have suggested that, in the case of aliens who may have succumbed to a large-scale nuclear war on their planet, we could observe the bright flashes of their warhead explosions from Earth.

And there is space debris. Could we detect masses of fragments orbiting another planet? After all, some of them may reach Earth one day.

Science fiction is full of stories of humans exploring space and stumbling upon extraterrestrial debris.

In the film “Alien, the Eighth Passenger” (1979), the crew of the spaceship Nostromo encountered a crashed alien megaship. But the less said about what happens next, the better.

‘Expect the unexpected’

Let’s go back to gases, which can be detected at much greater distances. Scientists are also considering the search for other gaseous pollutants, in addition to CFCs. Nitrogen dioxide (NO2) is an example.

“Most of our planet’s NO2 comes from industrial activity,” says Giada Arney of NASA’s Goddard Space Flight Center.

Arney is one of the authors of a 2021 study on the potential for discovering extraterrestrial civilizations by searching for NO2 pollution across the galaxy.

On Earth, around 65% of NO2 comes from emissions generated by cars, ships, planes and power plants, among other anthropogenic sources.

Unlike CFCs, NO2 does not remain in the atmosphere for thousands of years, which could make it difficult to discover on other planets. But on the other hand, the ups and downs of NO2 concentrations could offer an indication of levels of industrial activity on specific planets.

“It is also possible to think about temporary technosignatures, which could provide additional information about what is happening in the planet’s atmosphere,” explains Arney.

The scientist says that she and her colleagues came up with the idea for their study when NO2 levels in the Earth’s atmosphere dropped abruptly during the Covid-19 pandemic isolation measures.

Field measurements revealed that NO2 levels dropped by around 30% in some countries with strict isolation policies. And reductions in NO2 emissions have also been observed by satellites orbiting the Earth.

Therefore, it is not out of place to suggest that an alien civilization that is observing us could notice this change.

In addition to NO2’s short lifespan in the atmosphere, there is another important issue: many natural sources produce the gas, from lightning to forest fires.

Therefore, finding NO2 in a planet’s atmosphere may not be definitive proof of the development of internal combustion engines by an alien civilization, for example, nor of any other source emitting this gas.

Arney and his colleagues argue that, on Earth, natural sources alone would not produce enough NO2 for the gas to be detectable from a distance. In other words, observing nitrogen dioxide in the atmosphere of another planet could actually indicate the existence of some type of industrial activity.

Scientists are already studying the light reflected by distant planets, to try to determine what substances may be present.

In September 2023, researchers reported having detected methane and carbon dioxide, using the James Webb Space Telescope, in the atmosphere of a planet called K2-18b.

The planet orbits a dwarf star about 120 light-years from Earth.

This was an important discovery. She suggests that the planet could be bathed by an ocean of water.

“This is the first time we have come close to saying that there is an ocean on an exoplanet,” says Nikku Madhusudhan, from the University of Cambridge, in the United Kingdom.

But he and his colleagues also found evidence of an even more interesting compound in K2-18b’s atmosphere: dimethyl sulfoxide (DMS).

On Earth, only living organisms produce this compound, but it is also emitted by certain industrial sources, such as wood processing mills, for example.

Madhusudhan highlights that this is a preliminary result and it is necessary to wait for further confirmations to be sure of the existence of DMS. Even so, in principle, it could be a biosignature — a sign of life — and, who knows, even a technosignature, if extraterrestrial beings are processing materials in a similar way to what we do on Earth.

But the scientist finds it difficult to imagine an industrialized alien civilization on a world that could be completely covered by water.

Still, Madhusudhan believes that the search for life in other parts of the Universe must consider possibilities that surprise us.

“We need to expect the unexpected,” he says.

New telescopes

The great expectation of projects searching for technologically advanced civilizations using polluting gases is that, in the coming decades, our observation capabilities will be drastically increased, according to Jane Greaves, from Cardiff University, in the United Kingdom.

“It’s impressive that these trace gases are potentially detectable,” she explains.

Greaves highlights that the next telescopes to be opened will be suitable for this type of analysis.

In addition to the James Webb space telescope, which is in orbit around the Sun, there is the Extremely Large Telescope of the European Southern Observatory. It is a land-based construction in Chile that should begin operations in 2028.

NASA currently plans to develop a space telescope called the Large Infrared, Optical and Ultraviolet Surveyor —Luvoir— for the 2030s.

Arney and his colleagues considered the capabilities of this equipment when calculating how they could detect NO2 on other planets.

And there is the Habitable Worlds Observatory, a telescope specifically designed to look for biosignatures — and, by extension, perhaps technosignatures — in the late 2030s or 2040s.

For Greaves, “if we can persist for a monotonous period of a few years, it will be fascinating.”

But optical telescopes have their limitations.

Andrew Siemion, from the Seti Institute in the United States, says that most of his organization’s work focuses on detecting alien radio signals in our galaxy.

The Seti Institute was created to search for life in other parts of the Universe.

“We dedicate maybe 5% of our time and analysis to some of these other ideas,” he explains.

Siemion highlights that radio emissions can be detected “across the entire galaxy.” This means that our possibility of observing them may be greater.

He also points out that, as all possible technosignatures indicated so far are essentially based on pollutants produced by human beings, there is a risk of considering that extraterrestrial civilizations would be very similar to ours.

And there is no guarantee that this is true.

As Madhusudhan says, “expect the unexpected”, without taking an anthropocentric view of life throughout the Universe.

With this in mind, the Seti Institute has increasingly focused on searching for anomalies in the data rather than specific traits that we might consider coming from an alien population, according to Siemion.

Any odd spot in a dataset of cosmic observations could be the indication we’re looking for.

“We don’t need to get into the minds of ETs, so to speak, and imagine what they could do, nor demand that they have the same type of technological evolution as humans,” explains Siemion.

In other words, you don’t even need to know exactly what to look for. We just need to find something unexpected that deserves further research.

Perhaps all of these proposed technosignatures, while potentially on the right track, are yet another reflection of our growing awareness that humans are a messy, polluting species.

Perhaps there is even a degree of guilt built into this assumption. We console ourselves by imagining that aliens would make the same mistakes we did.

As Arney says, truly intelligent life forms may not produce pollutant-based technosignatures like CFCs for long periods of their history. Maybe they only do it for brief moments and clean up afterward.

In this case, we would have to be lucky to look for these signatures at the right time.

“I wonder how long the pollution will express itself,” says Arney. “I like to wait for a civilization to act efficiently.”

_{Read the original version of this report (in English) on the BBC Future website.}