Humans have used radio waves to communicate across Earth for more
than 100 years. Those waves also leak out into space, a fingerprint of
our presence propagating through the cosmos.

In
more recent years, humans have also sent out a stronger signal beyond
our planet: communications with our most distant probes, like the famous
Voyager spacecraft.

According to popsci.com,
scientists recently traced the paths of these powerful radio
transmissions from Earth to multiple far-away spacecraft and determined
which stars—along with any planets with possible alien life around
them—are best positioned to intercept those messages.

The research
team created a list of stars that will encounter Earth’s signals within
the next century and found that alien civilizations (if they’re out
there) could send a return message as soon as 2029. Their results were
published on March 20 in the journal Publications of the Astronomical
Society of the Pacific.

“This
is a famous idea from Carl Sagan, who used it as a plot theme in the
movie Contact,” explains Howard Isaacson, a University of California,
Berkeley astronomer and co-author of the new work.

However, it’s
worth taking any study involving extraterrestrial life with a grain of
salt. Kaitlin Rasmussen, an astrobiologist at the University of
Washington not affiliated with the paper, calls this study “an
interesting exercise, but unlikely to yield results.” The results, in
this case, would be aliens contacting Earth within a certain timeframe.

As
radio signals travel through space, they spread out and become weaker
and harder to detect. Aliens parked around a nearby star probably won’t
notice the faint leakage from TVs and other small devices.

However,
the commands we send to trailblazing probes at the edge of the solar
system—Voyager 1, Voyager 2, Pioneer 10, Pioneer 11, and New
Horizons—require a much more focused and powerful broadcast from NASA’s
Deep Space Network (DSN), a global array of radio dishes designed for
space communications.

The
DSN signals don’t magically stop at the spacecraft they’re targeting:
They continue into interstellar space where they eventually reach other
stars.

But electromagnetic waves like radio transmissions and
light can only travel so fast—that’s why we use light-years to measure
distances across the universe. The researchers used this law of physics
to estimate how long it will take for DSN signals to reach nearby stars,
and for alien life to return the message.

The process revealed
several insights. For example, according to their calculations, a signal
sent to Pioneer 10 reached a dead star known as a white dwarf around 27
light-years away in 2002. The study team estimates a return message
from any alien life near this dead star could reach us as soon as 2029,
but no earlier.

More opportunities for return messages will pop up
in the next decade. Signals sent to Voyager 2 around 1980 and 1983
reached two stars in 2007: one that’s 26 light-years away and a brown
dwarf that’s 24 light-years away, respectively. If aliens sent a message
right back from either, it could reach Earth in the early 2030s.

This
work “gives Search for Extraterrestrial Intelligence researchers a more
narrow group of stars to focus on,” says lead author Reilly Derrick, a
University of California, Los Angeles engineering student.

Derrick
and Isaacson propose that radio astronomers could use their star lists
to listen for return messages at predetermined times. For example, in
2029 they may want to point some of Earth’s major radio telescopes
towards the white dwarf that received Pioneer 10’s message.

But
other astronomers are skeptical. “If a response were to be sent, our
ability to detect it would depend on many factors,” says Macy Huston, an
astronomer at Penn State not involved in the new study. These factors
include “how long or often we monitor the star for a response, and how
long or often the return signal is transmitted.”

Our radio transmissions have only reached one-millionth of the volume of the Milky Way.

There
are still many unknowns when considering alien life. In particular,
astronomers aren’t certain the stars in this study even have
planets—although based on other exoplanet studies, it’s likely that at
least a fraction of them do. The signals from the DSN are also still
incredibly weak at such large distances, so it’s unclear how plausible
it is for other stars to detect our transmissions.

“Our puny and
infrequent transmissions are unlikely to yield a detection of humanity
by extraterrestrials,” says Jean-Luc Margot, a University of California,
Los Angeles radio astronomer who was not involved in the recent paper.
He explains that our radio transmissions have only reached one-millionth
of the volume of the Milky Way.

“The probability that another
civilization resides in this tiny bubble is extraordinarily small unless
there are millions of civilizations in the Milky Way,” he says. But if
they’re out there, there might be a time and place to capture the
evidence.

Source: popsci.com