Sky & Telescope - March 2022 - 12

COSMIC RELIEF by David Grinspoon
Interstellar Stowaways
Could microorganisms travel between the stars?
One study involves small earthworms,
Pontodrilus litoralis, that burrow
into beaches, mudflats, and other
coastal habitats around the world.
Birds don't carry them, so how do they
travel across oceans? Keryea Soong of
Taiwan's National Sun Yat-sen University
and two colleagues found that
these little guys are adept at tunneling
into, and living off of, driftwood,
so they can float from place to place,
munching on their rafts to stay alive.
The other study is titled " Pansperp.
14), we've recently detected objects
entering our solar system from other
star systems. Those fleeting visits
naturally make me wonder about
interstellar panspermia, the phenomenon
by which - it's hypothesized - life
might spread between worlds orbiting
different stars. Now two new studies
have me thinking afresh about this.
mia in a Milky Way-like Galaxy. " A
team of Chilean, Korean, and French
scientists used sophisticated models to
simulate four steps of a hypothetical
process: ejection of spores from planets,
gravitational escape from stellar
systems, transit between stars, and
survival of spores across interstellar
distances. The team concludes that life
is more likely to originate in new stellar
systems than be shuttled between
AGE-OLD SPECULATION that life
might hitchhike between planets
revived in the 1980s with our discovery
that meteorites from Mars
occasionally pelt Earth. Add to that
the discovery of extremophile species
that can withstand huge extremes of
temperature, desiccation, and radiation.
However unlikely such an origin
might seem for planetary life, we have
to weigh it against the (still completely
unknown) probability that life can
readily spring from the native chemistry
of a young world.
As I've discussed (S&T: Mar. 2018,
the stars. However, they also admit
that both probability estimates depend
critically on unknown quantities.
Driftwood transport works
between islands here because tiny
critters can drift on timescales that
are short compared to their life
expectancy. But to take evolutionary
advantage of an interstellar dissemination
vector, natural selection would
require multiple generations using
impact explosions to loft spores from
planet to planet.
I've tried (on the back of an envelope)
to model creatures that can
evolve to do so. You'd need spores to
be spread widely enough on a planet,
and survive for long enough, to have
a decent chance of being carried to
another world after the next impact
event. I couldn't make it work, and I
doubt nature could either.
Spacefaring bugs, then, would have
to accidentally evolve in a way that
makes them fit for interstellar travel.
Some of the hardiest Earth organisms
- think tardigrades or extremophilic
bacteria - can last for a few years
desiccated and irradiated in space. But
could anything survive for millions of
years in such conditions? Our limited
experimental data suggest that, even
though the extreme cold of space
might greatly increase survival against
radiation damages, much shorter survival
times are likely.
So, could our universe be full of
Tardigrades, or water
bears, such as the one seen
here in a colored scanning
electron micrograph, are one
of our planet's most resilient
creatures. Could such microorganisms
survive a journey
among the stars?
12 MARCH 2022 * SKY & TELESCOPE
life that has spread naturally over
time from star to star, hiding like
worms in driftwood, inside ejected
rocks? Ultimately, we're still comparing
unknowns. It seems more likely
that life forms indigenously, and that's
where I'll place my bets for now. But
until we find some examples of ET life
and learn how and if we're related to
it, we have to allow for the intriguing
possibility of life drifting throughout
the galaxy on interstellar flotsam.
¢ Contributing Editor DAVID GRINSPOON
is an astrobiologist at the
Planetary Science Institute.
STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Sky & Telescope - March 2022

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