I was either born at one of the best possible times for an
exoplanet enthusiast or much too early. The first conclusive proof of a planet
outside our home solar system was found in 1992, around the time my brain was
figuring out how to make its first memories that could last a lifetime. The
pace has accelerated, and instead of a universe with eight or nine known
planets we now live in a universe with thousands of confirmed planets and a
reasonable expectation that the number of planets in the Milky Way is of the
same order of magnitude as the number of stars. That wasn’t always something we
could take for granted.
That’s the case for enthusiasm. The case for frustration is
that virtually nothing is known of most of these planets but their mass and
cross-sectional areas. We “know” these planets in the sense you know someone
whose name and number you looked up in a phone book. So long as human
civilization doesn’t regress in the next century or two we’ll certainly know
more about many of these worlds in that timeframe, but during my lifetime they
may never be anything more than the faintest points of light shining on a
telescope’s imaging chip. Even that won’t be for a decade or more to come.
In the meantime, the theorists had time to figure out how to
make do with a world’s signature compressed into a single pixel of data.
Supposing James Webb or a later telescope can directly image a planet like
Proxima b or one of the companions of TRAPPIST-1, what could we learn from that
faint light? It’s not much, but it would allow for spectroscopy of the
atmosphere, and perhaps a detailed understanding of what gases exist around
that distant world.
An analysis of Earth’s atmosphere would reveal that it’s a
very odd world indeed. Nitrogen, argon, and water vapor are ordinary enough,
but the enormous concentration of oxygen and significant trace of methane in
our atmosphere are like a rotating beacon announcing the presence of life to the
rest of the universe. If a similar mix is found blanketing another planet or
moon it would be difficult to find any plausible explanation besides life much
like that on Earth. But we know so little of the diversity of life in the
universe that there may be many other avenues it takes than the dominance of
the photosynthesizers on Earth. It won’t do to miss out on life due to oxygen
chauvinism. How else might the signature of life be written in another sky?
A variety of other compounds, the astrobiologists speculate,
could serve as plausible or likely biosignatures. These are chemicals that have
no known or theoretical processes that can make them other than the movement of
life and that break down readily if not constantly replenished. The list
includes the molecule phosphine. Phosphorous, hydrogen, hydrogen, hydrogen. It
can be made deep in the superfluid bowels of the atmospheres of gas giants, in
the laboratory, or by microorganisms living freely and in the guts of some
animals. Supposing we found a clear signature of phosphine in the atmosphere of
Gliese 581 g, might that be enough to say with confidence that life on Earth is
not alone in the universe?
It’s a useful hypothetical to consider in the context of
where phosphine actually was discovered earlier this year. The team at ALMA and
the James Clerk Maxwell Telescope didn’t discover it in the constellation
Centauri or in the Kepler zone of study. They found it at Earth’s nearest
neighbor in space, Venus.
One of the two ancient Latin names of Venus was Lucifer,
which has been co-opted in the English language as a shorthand for the devil.
It’s a cliché to call Venus hellish, but maybe a brief review is helpful. The
surface of Venus is the hottest solid surface in the solar system, averaging
around 450 degrees Celsius. The pressure at an average point on the surface is
roughly what you’ll find a kilometer beneath the surface of Earth’s oceans. A
few Soviet spacecraft made it to the surface during the Cold War, and all
ceased functioning within hours from the extreme conditions. The entire globe
is surrounded by a layer of concentrated sulfuric acid haze many kilometers up.
A continuous rain of acid falls from this haze that boils before it can reach
the rock below. Above this there are overcast and scattered layers of clouds
that are 75-95% sulfuric acid.
Since Venus’s atmosphere was first rigorously studied with
Earth-based radio telescopes at the beginning of the space age it seemed like a
safe bet to assume that life was impossible on a world like this. While Mars
and the icy moons of Jupiter and Saturn have been painstakingly investigated
for clues leading toward water and life, Venus has been largely ignored from
the standpoint of astrobiology. That seemed fair, given that a layer of highly
acidic clouds was as hospitable as the planet gets. Then again, that phosphine
is real and it has to come from somewhere.
The suggestion that there is life on Venus constantly replenishing
a faint trace of phosphine in the atmosphere is scandalous. It’s crazy talk, if
your mind is used to thinking about planetary science as we understood it
before a couple of months ago. You might just as well launch an expedition to
the north pole for elves as you would search the clouds of Venus for bacteria. So
the natural inclination is to look for another explanation, any halfway
plausible explanation, for where the phosphine came from that doesn’t involve
life.
The idea that an unknown abiotic process is generating the
phosphine observed on Venus is not without merit. The history of science is
humbling to anyone who claims to know all the possible ways that something can
be done. The planets are literally other worlds, with otherworldly chemistry.
Maybe Venus thought of something that Earth and Mars haven’t imagined, and that
acid virga or the plate tectonics-free volcanoes can put three hydrogen atoms
on a phosphorous without the magic spell of biology. It wouldn’t be the first
time we’ve all been surprised by the creativity of geology and meteorology.
And yet, how would the announcements read if a direct image
of Proxima b revealed a clear signature of phosphine in the atmosphere? I doubt
it would be treated with such suspicion. We’d likely be popping champagne and
marveling at a truth revealed that was long suspected but never known for sure,
that Earth is not a stranger in a completely strange land, but one of many
companions in cradling life. But Venus? No. We’re not reading it right.
Which way ought the arrow of logic point? Does finding a
biosignature on Venus reshape our view of that world’s habitability, or does it
mean we need to go back and get smart about what a biosignature really is? These
are concrete questions with definite answers that will be found in time. In the
meantime, though, it’s impossible not to speculate.
It may well be that we’ve been too obsessed with the surface
of Venus, which really is a bit of Gehenna. Go up 50 kilometers and there are
no oceans to speak of, but the temperatures and pressures are nearly identical
to Earth’s surface. We’ve found bacteria and achea in the mot outrageous of places,
boiling hot springs and hydrothermal vents, irradiated by nuclear reactors and
compressed under kilometers of rock. Some extremophiles are known to thrive in water
so acidic it would desiccate most living things to charcoal. Those cool clouds
of Venus are more acidic still, but it could be that life on Earth simply hasn’t
had the right evolutionary pressure to find a way to make a living there. Could
be.
There’s a scene in The Lion, the Witch, and the Wardrobe
where Peter and Susan have to grapple with Lucy’s claim to have been whisked
away to another realm by a piece of furniture in the spare room. C.S. Lewis
wasn’t known for subtlety, and it’s obvious that this is a stand-in for his
famous trilemma, that Jesus must either have been insane, a demon, or really
the son of God, as Pilate asked if he was. Lucy wasn’t crazy or a rogue from
Hades, but that she had an overactive imagination seemed the better fit to Occam’s
razor than a magic wardrobe. I think the dilemma of the phosphine bears more
than a passing resemblance to this scene.
Science works well when the theorists muse clearly on how
the universe might be and the experimentalists faithfully and patiently test
these ideas. The voice of some witnesses seem trustworthy immediately. The
eclipse photography of Eddington, the neutrons found by Chadwick, and the X-ray
crystallography of Franklin come to mind. Then again, there are the many
detailed observations of planet Vulcan in the 19th Century to
consider. Is the voice of Venus a reliable witness or not? I think either
interpretation is valid, but if you assume that any new discovery from Venus
cannot be a trace of life by virtue of the planet being Venus I’d suggest you
reconsider your priors before being so sure.
Two comments regarding the ancient names of Venus are worth
noting, one flippant and one possibly profound. Lucifer, the Latin name of the
morning star, was a literal translation of the Greek for light-bringer,
Phosphorous. How apposite, then, that the biosignature discovered was such.
By the time of the Romans it was understood that the morning star and the evening star were the same object in space, but to the Greeks of antiquity they each seemed unique and received a unique name. The Romans maintained this convention and called Venus in the evening sky Vesper. This name, too, has survived to modern English, but with a very different connotation than the morning star Lucifer. Each evening the cathedrals, monasteries, and convents of Earth ring their bells, calling the brothers and sisters to pray vespers, the evening prayer of the liturgy of the hours. What is life but a prayer?
