For the first time in human history, we have the means to
answer the question.
Chris McKay's quest for extraterrestrial life started in
1976, when Viking 1 and 2 landed on Mars. Touching down on Mars for the first
time was a big deal, sure, but the then-first-year graduate student was
especially excited because the landers found what appeared to be signs of
Martian life.
The spacecraft found that something in the dirt - possibly
microbes - was taking in nutrients and producing gases like carbon dioxide. But
when instruments failed to find any organic molecules, which are the building
blocks of any organism, scientists concluded that no, aliens weren't living in
the dirt.
To this day, however, scientists like McKay are still baffled
by the Viking data, which never conclusively supported the existence of life,
but were tantalizing nevertheless. For McKay, now a planetary scientist at NASA
Ames Research Center, those results launched a career in astrobiology, despite
the warnings of other scientists at the time. "Not only did they tell me
not to," he says, "they made fun of me for being interested in
it."
Four decades later, he's enjoyed some vindication. As
robotic space probes continue to explore the solar system, visiting planets,
moons, and asteroids, they're finding watery environments where microbial life
could grab hold. Science can actually apply itself to the question instead of
it being purely philosophical.
Outside the solar system, astronomers have discovered
thousands of worlds - and they estimate our galaxy alone could be filled with
hundreds of billions of planets. Many could be similar to Earth, with oceans,
an atmosphere, and, yes, life.
In the coming decades, new space probes and telescopes will
search for signs of life in the solar system and beyond. "We have a decent
chance for finding Earth-like planets and evidence for life by sometime in the
early 2030s," says Jim Kasting, a planetary scientist at Penn State
University in the US.
Telescopes are used to eavesdrop on distant civilisations
And for the first time in human history, scientists have a
plan and the means to answer the question of whether we're alone. "The
fact that science can actually apply itself to the question instead of it being
purely philosophical is very exciting," says Jason Wright, an astronomer
also at Penn State. "It might be a long shot that we can do this, but the
question is so compelling."
Undoubtedly, the most exciting kind of alien life would be
the intelligent kind: the ETs or the ones depicted in Carl Sagan's novel
Contact. Despite Roswell and Area 51, such close encounters have yet to happen.
But scientists have been searching for decades, trying to eavesdrop on radio
signals from a distant civilisation. Today, for example, the SETI Institute
listens with the Allen Telescope Array in California.
Most recently, Wright led a hunt for super-advanced
civilisations that have colonised an entire galaxy. In the 1960s, the physicist
Freeman Dyson suggested that aliens could power their civilisation using energy
from their planet's star. Consuming that energy - to run computers, spaceships,
or whatever aliens might need - will radiate heat, like how your laptop gets
warm. If such a civilisation took over a galaxy, then you could recognise it by
searching for galaxies that radiate more heat than expected.
After scouring through images of 100,000 galaxies taken with
the WISE satellite, Wright's team came up with nothing. But that's just for the
extreme case of a super-advanced, galaxy-conquering alien. Maybe aliens stayed
local. To find out, he says, the next step would be to study the galaxies in
more detail, to see if certain regions within each galaxy are producing extra
heat. "That would be very unusual," he says. "I don't know how
we would go about getting a natural explanation for that."
Still, the search for intelligent life remains a reach.
After all, life has flourished on Earth for about 3.5 billion years, and
intelligent life (if we consider humans to be intelligent) has been around only
for the last 200,000. For most of Earth's history, life consisted of primitive
microbes. If we're ever to find life elsewhere, it will probably be microbial.
Some could even be in our own cosmic backyard.
One intriguing place to look for life is on Titan, Saturn's
largest moon. It's got a thick atmosphere and is the only other place in the
solar system covered in seas and lakes - only they're filled with liquid
methane, not water. Scientists think liquid is important for life, but the fact
that it's methane means any Titanian critters would be fundamentally different
from any Earthling.
That doesn't make life impossible, just maybe less probable.
Life on Titan would also have to survive frigid temperatures of about -180
degrees C.
For life as we know it, the most important ingredient is
still liquid water. And spacecraft are discovering the solar system to be quite
wet. In March, observations with the Hubble Space Telescope suggested that an
ocean lurks beneath the surface of Jupiter's largest moon, Ganymede. Right now,
the Dawn spacecraft is orbiting Ceres, a dwarf planet in the asteroid belt
that's 40 percent water by volume, including a possible subsurface ocean.
Among the most promising abodes for life are Mars, Saturn's
moon Enceladus, and Jupiter's moon Europa. On Mars, the best chance for life
might have been in the past, when the planet was warm and filled with rivers
and lakes. Today, Mars is barren and likely inhospitable.
Microbes might, however, be able to eke out an existence
below the surface. "I'd say it's 50/50 as to whether there's life on Mars
right now," Kasting says. If there is, though, he says it's probably
buried as deep as a kilometre underground, where temperatures are warm enough
for water to be liquid. Getting there and finding proof, however, might require
astronauts drilling on Mars.
Detecting life on Europa might also require drilling. A
thick layer of ice maybe several kilometres deep encloses a potentially
habitable ocean. Scientists have wanted to go to Europa for years, and they may
soon get their chance. The White House's requested budget for 2016 includes $30
million for such a mission. But landing and drilling is difficult and
expensive, so if the mission comes to fruition, it will probably study the
world from space.
Which is why McKay thinks Enceladus - which also might have
a subsurface ocean - is a better bet. "As people realise how difficult
Europa is and how inaccessible its ocean is, they're going to be naturally
attracted to Enceladus," says McKay, who was part of a team that recently
proposed a NASA mission to Enceladus.
The icy moon became a top destination in 2009 when the
Cassini spacecraft discovered plumes of water shooting hundreds of kilometres
into space. Those plumes, spraying straight from the ocean below, could contain
telltale signs of life. "You fly through the plumes from Enceladus,"
McKay says. "That gives you the best chance of detecting life." No
drilling required.
Such an alien-hunting spacecraft would look for two types of
molecules: lipids and amino acids. Lipids include fats and oils, and are
important for the structure and function of cells. Amino acids are the building
blocks of proteins.
The thing about an amino acid is that it can come in two
versions that are mirror opposites of each other, like a left and right hand.
Of the 20 amino acids that make up life on Earth, 19 are left-handed. Maybe,
the thinking goes, amino acids that are biological in origin must generally
have the same handedness. Discovering such molecules would certainly suggest
life. "That's a grand slam," McKay says.
Still, he admits, that's a fantasy scenario. Microbes might
not reveal themselves so easily, or they might not be there at all. Space
missions take time and money, so if one spacecraft doesn't find anything, you'd
have to wait years for another shot.
Chances might be better outside our solar system, among the
billions of other planets in the galaxy. While a mission within the solar
system can visit only one place at a time, a space telescope can easily go
through dozens or even hundreds of potentially habitable worlds. Instead of
lipids and amino acids, such telescopes will look for other molecules: oxygen
and other gases that reveal living, breathing aliens.
Building off the resounding success of the Kepler space
telescope, which has found thousands of planets, NASA will launch its
Transiting Exoplanet Survey Satellite, or TESS, in 2017. Like Kepler, TESS will
search for planets that pass in front of their stars, causing a slight dip in
starlight. But unlike Kepler, TESS will target planets closer to Earth, and
therefore easier to study and detect life.
What's got alien hunters excited is that TESS will find
targets for the James Webb Telescope, which, after launching in 2018, will
search those planets for atmospheric gases indicative of life.
The idea is this: As a planet passes in front of its star,
some of the starlight will penetrate the planet's atmosphere, which appears as
a thin outline surrounding the disk of the planet. Depending on its chemical
composition, the atmosphere will absorb certain wavelengths of light. By
measuring which wavelengths of light get through, astronomers can identify the
gases in the atmosphere.
Astronomers have already studied planetary atmospheres with
Hubble, showing their methods are sound. With the more powerful JWST, however,
they can analyse atmospheres in greater detail.
One of the gases they hope to find is oxygen, which doesn't
sit around very long before reacting with other compounds. So to maintain a lot
of oxygen in its atmosphere, a planet would need something to replenish it -
something living. On Earth, plants and bacteria do the job.
Compared to Mars or even Enceladus, this could be the most
likely way scientists find life. "If I was betting today, I would bet on
oxygen on an exoplanet," McKay says.
But oxygen is just one gas. Earthlings, for example, produce
thousands (just think of all the smells that people, animals, and plants make).
Only a handful of them are abundant enough to be detectable from space,
however, so astronomers are figuring out which ones could be realistic
indicators of life. Some proposed so far include methane and dimethyl sulfide,
which phytoplankton produce on Earth.
Of course, finding life won't simply be a matter of
detecting gases. Non-living things - such as thermal vents and volcanoes - can
spew out many of the same compounds. To determine whether a particular gas is
biological in origin, astronomers will have to study the chemistry and the
specific properties of the planet.
Even then, short of a message from ET, astronomers may only
be able to give the odds for extraterrestrial life. "We won't be sure
there's life there, but we may be able to work through all the scenarios and
assign a probability," says Sara Seager, an astronomer at MIT.
Another issue is that no one knows what alien life really
looks like, so the proposed biosignatures so far are based on Earth's life.
"You don't want to be too targeted and only look for stuff like
Earth," Wright says. "But you also can't be so general that you have
no idea what you're looking for."
To go beyond Earth-based life, Seager wants to identify any
and all gases that could be stable and abundant in an atmosphere, regardless of
whether anything on Earth makes them. To see if they're viable biosignatures,
she will work backwards, reverse engineering biological processes that could
produce those gases.
If JWST is to detect life, it will have to get lucky. The
telescope was proposed years before astronomers knew the galaxy had billions of
planets, so it wasn't designed for planet or alien hunting.
TESS will find thousands of plants, but only some will be good
targets for JWST. A suitable planet can't be too small compared to its star.
Otherwise, the glare of such a bright star swamps the image, and you can't see
the subtle signal from the atmosphere. According to Seager, observing a planet
next to its star is like picking out a firefly next to a searchlight from 1,500
kilometres away.
"It's not going to be easy," she says. "We're
only going to have a handful of planets to search for signs of life on."
TESS and JWST will also be limited because they can only
study planets that pass in front of their stars, which requires a perfect
alignment. If JWST fails to find anything, astronomers will have to wait for a
specially designed telescope that doesn't rely on transits.
Such a telescope will observe a planet directly, but for
that to work, something will have to block the light from the planet's star.
One idea called Starshade, which Seager has worked on, is a spacecraft that
unfolds like a parasol to block starlight, allowing a separate space telescope
to peer into the planet.
The telescope will be able to observe an Earth-sized planet
orbiting a sun-like star, something TESS can't do because the brightness of a
sun-like star will overwhelm the planet. With more potentially habitable
planets - and including truly Earth-like planets - the chances for detecting
life improve. "For a direct-imaging telescope, I'd say the odds are pretty
good," Kasting says.
If anything, the sheer number and diversity of planets is
reason for optimism in the quest for extraterrestrial life. "We know that
atmospheres are out there, we've studied many of them, so the possibility is
out there for the first time ever," Seager says. "It would be foolish
not to take this opportunity."
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