How Would Aliens Detect Life on Earth?

As the universe’s only known harbor for life, Earth is arguably
one strange rock^[1]. But light-years from
our solar system, other intelligent beings on a similar planetary
oasis might be gazing in our direction and seeing us as a sign that
they’re not alone in the universe.

To date, astronomers have confirmed the existence of nearly 4,000 planets beyond
our solar system^[2], including some that
just might have the conditions necessary to support life as we know
it. As our technology improves, we should be able to learn more
about these worlds and their chances of hosting plants, animals,
and maybe even civilizations.

One Strange Rock
Hosted by Will Smith, ONE STRANGE ROCK is a mind-bending,
thrilling journey exploring the fragility and wonder of our
planet.

That means if aliens are out there, they could just as easily
discover us.

First, they’d need to find Earth from afar, either by watching
our sun wobble as the planet’s gravity tugs against it, or by
seeing the sun dim as Earth blocks a tiny fraction of sunlight
during its orbit. Nine known alien
worlds^[3] can see Earth transit
across our sun, just as we’ve seen thousands of alien planets dim
their host stars.

Once spotted, our planet would likely intrigue E.T. Our sun is
relatively stable, not prone to disastrous flares that’d rip our
atmosphere to shreds. What’s more, we fall squarely within our
sun’s habitable zone, the area around a star where liquid water can
persist on a planet’s surface. (These are just some of the things
that make life as we know it possible on Earth^[4].)

Faraway scientists might then attempt to spot our atmosphere, to
see whether life’s thumb is on the chemical scales. But what would
they be looking for? And could they really infer life’s presence
across trillions of miles?

Oxygen is a highly reactive element, glomming on to other atoms
and molecules with such alacrity that it’s tough to find it in
abundance by itself—unless something is breaking down oxygen-rich
compounds and pumping out loads of O2. On Earth, you can thank
photosynthetic plants for being generous oxygen factories.

But oxygen alone wouldn’t be enough for E.T. to prove our
existence. “We have discovered several ways in which O2 can
accumulate in the absence of life,” says Stephanie Olson^[5], an astrobiologist at
the University of California, Riverside. “High levels of O2, or the
processes culminating in high levels of O2, may actually preclude
the emergence of life on some planets.”

In addition to oxygen molecules, alien astronomers would look at
our atmosphere’s levels of nitrogen, carbon dioxide, nitrous oxide,
and methane. Only life could keep Earth chemically off-kilter
enough for all these gases to persist at once.

Beyond those chemical clues, alien astronomers with truly
massive telescopes might even be able to map Earth’s surface from
afar, down to major urban areas.

As a proof-of-concept for this telescope, called the ExoLife Finder,
or ELF^[6], Berdyugina and Kuhn
simulated how nearby aliens using the telescope would see Earth.
From 25 trillion miles away, E.T. could not only map Earth’s
continents, but they’d also be able to see signs of intelligent
life.

“The ELF telescope has the sensitivity to see a Los Angeles
basin,” says Kuhn. “We don’t see the lights, but we see the heat
signature.” Their group, the Planets
Foundation^[7], is now building a
single-mirror telescope in Hawaii^[8]
to test the underlying tech. If all goes to plan, they say it’s
possible to build ELF within a decade.

“It would be like Star Trek, the reality show,” says
Berdyugina. “We could virtually visit these planets.”

If intelligent life is nearby, the most straightforward way to
find earthlings would be to listen for us. For the last century,
human civilization has been broadcasting its existence to the
cosmos via our leaky radio transmissions. Occasionally, we’ve
broadcast messages to E.T. intentionally, and sent
golden records into the void^[9]
on the off-chance that aliens stumble across our interplanetary
spacecraft.

Aliens wouldn’t be able to detect our radio presence, much less
our golden records, unless they’re within 590 trillion miles of
Earth. If intelligent life is any farther away than that, our
earliest radio signals simply haven’t reached them yet.

But in several centuries’ time, intelligent aliens may see other
signs of our tech-savviness. In a study recently accepted
to The Astrophysical Journal^[10], for example,
astrophysicist Hector Socas-Navarro says we could find intelligent
life by looking for artificial satellites orbiting alien worlds.
And that means they could find us using similar methods.

As a satellite-fringed world drifts in front of its home star,
its satellites would block some starlight in front of and behind
the transiting planet. This metallic belt would look unnatural when
compared to known planetary rings.

Currently, Earth’s satellite network isn’t anywhere near dense
enough to be detected, nor will it be anytime soon. But our
footprint in space is growing exponentially: If we keep launching
satellites at our present pace, Socas-Navarro says in his study,
nearby aliens armed with telescopes as powerful as the ones we have
today could spot our satellites by 2200.

Of course, Earth is more than 4.5 billion years old, and life
has changed a lot over the eons. What if alien astronomers had
looked our way even a billion years ago?

In a 2018 paper in Science
Advances^[11], Olson and her
colleagues simulated how Earth’s atmosphere has changed over time.
Even three billion years ago, aliens may have been able to infer
life by sniffing out methane and carbon dioxide in the early
atmosphere. But our modern atmosphere—a literal beacon for
life—arrived only about 500 million years ago.

“For more than a billion years of Earth history, an alien
astronomer may have even been sufficiently misled to conclude that
Earth was sterile—despite the fact that life was flourishing in our
ocean at the time,” says Olson.

Still, if the aliens were advanced and committed enough, even an
early Earth would yield compelling clues for life, says study
coauthor Joshua
Krissansen-Totton^[12] of the University of
Washington.

“The presence of life on Earth has been fairly obvious for the
last 4 billion years to anyone who could build a big telescope,” he
says in an email. “If there was anything nasty out there, then they
would have extinguished life on Earth long ago. I think we are safe
inviting them over to visit and exchange notes on the cosmos.”

If aliens are anything like us, perhaps the news that they
aren’t alone in the cosmos wouldn’t be their equivalent of
earth-shattering. In a study published in Frontiers in
Psychology in February, researchers found that people, at
least, would
take the discovery of alien life in stride^[13].

“People will be able to accommodate even high-impact scientific
discoveries without their worldviews collapsing,” theologian
Ted Peters^[14] said at the time.

But like us, aliens may fear the prospect of hostile,
intelligent extraterrestrials—in this case, humans—arriving
unannounced on their cosmic doorstep.