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When you hear the term
“habitable zone” you might outrageously think
that this is a zone around a
star in which planets are in fact,
habitable. But this turns out to be a
whole lot of “nope”. Nope, nope, nope, nope. And
nope to you too. Rather, the habitable zone is
the worst name ever in the
history of naming anything. The problem is that the perfect
amount of starlight needed to
keep a planet’s surface temperate and comfortable
depends on the planet itself. The Earth’s atmosphere
contains greenhouse gases such
as carbon dioxide, which trap
heat. This is like taking a planet
and giving it a coat. If our planet was stripped of
its atmospheric coat, our surface temperature would
plummet reaching an average surface
temperature of -18°C or
-0.4°F. Alternatively, if our planet
was a bit bigger, then its
stronger gravity would attract
a thicker atmosphere. This would be like wearing a
much thicker coat and
conditions on our surface
would then become too hot for
water. The water would evaporate,
taking with it the best
solvent we know for building
biological molecules. This situation is like growing
up in Hawai’i, with a wardrobe
full of light summer shirts and then deciding to visit
Canada in January whereupon you instantly
declare the location far too
cold for life. Meanwhile, Canadians are
sitting outside in their
thick, winter-appropriate
jackets munching on timbits and
considering whether an ice
cream might just hit the spot. and your British pals have
declared both Hawai’i and
Canada completely
uninhabitable as neither location is
suitable for the raincoat that
is glued to their body. What this all means is that
the ideal orbit for temperate
conditions depends strongly on
the atmospheric coat the
planet is wearing. Planets with thin atmospheres
will need to orbit closer to
the star to have similar
surface temperatures as
planets with thick winter
coat-grade atmospheres. But when we observe an
exoplanet, we normally only
measure the planet’s total
size and we are not able to tell
what the atmosphere is like. This leaves us with no way of
knowing if the starlight a
planet is receiving is the
right amount for a temperate
surface. This lack of knowledge about
individual planets means that
we define the habitable zone
for the Earth. That is, the location around a
star where the Earth’s
atmosphere is the right type
of coat to keep our seas
liquid on the planet surface and therefore, fulfil at
least one of the conditions
needed for life. If the planet orbiting in
the habitable zone has a
thinner atmosphere than the
Earth, then it will be too
cold for surface water to be
liquid. And if the planet has a
thicker atmosphere than the
Earth, then the higher surface
temperatures may cause the
oceans to evaporate entirely. Now if most planets in the
habitable zone will be kitted
out in inappropriate
atmospheric fashion wear for
the conditions, why do we still get excited
by planets found orbiting in
this region? This is because if another
planet like our own is out
there, it will be orbiting in
the habitable zone. It is true that life may
exist in many different forms but our best chance of
recognising its signature will
be if that life is similar to
our own. Identifying a region where
Earth-like planets will hang
out is therefore important for
prioritising future
observations aimed at finding
life. But this doesn’t mean that
all or even most of the
planets in this region will be
anything like our own. Different names have been
suggested for the habitable
zone to try and express its
meaning more accurately, but none have really hit the
spot. The “temperate zone” has
been suggested to move away
from promises of
habitability. But a planet is still only
temperate in the temperate
zone if it’s wearing the
Earth’
s atmospheric coat. Calling the habitable zone
the “hunting zone” emphasises
that this is a region to hunt
for another Earth. Buuuut the image that
conjures up… isn’t quite the
one we want either… So how about the “ECHaLWOTS
zone”? Earth Could Have Liquid
Water On the Surface. It’s much more accurate, if
not very pithy. But maybe we could claim it
was the name of an obscure
river god that no one has
heard about and that might
catch on. Or we could keep the name
habitable zone but remember
it’s the region where only the Earth’s
atmospheric coat is the right
choice in outfit.

James Carver

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