40 | NewScientist | 00 Month 2013>
From life’s point of view, Earth sits in the solar
system’s sweet spot. But will things be “just right”
for much longer, asks Lee Billings
W
E LIVE in the best of all possible
worlds. This much-lampooned idea
originated in the early 18th century,
when the German polymath Gottfried Leibniz
was pondering what sort of world a benevolent
God would make. In the past couple of decades,
however, it has acquired a new resonance as
we have spied out worlds orbiting other stars.
The question is whether any of these, too,
might support life. Underpinning each
assessment is a surprisingly simple process
that measures each of our potential planetary
twins – or, more likely, near cousins – against
a presumed earthly idyll. To host life, a world
must, like Earth, be rocky, watery and orbiting
in a star’s “Goldilocks zone”, where things are
not too hot, not too cold, but just right.
But do we actually inhabit the best of all
possible worlds? As we come to understand
better how the properties of stars, planets and
their atmospheres combine to produce life-
friendly worlds, it seems Earth’s own
habitability is more precarious than we had
assumed. That has far-reaching consequences
for the likelihood of life on other planets –
and for the fate of life on Earth.
Our assumption of Earth’s perfection has
largely rested on one fact: our planet is full
of life. Life here is invariably constructed
from carbon and reliant on liquid water, and
there are good reasons beyond egotism to
believe that, as it is on Earth, so it is in the
heavens. Carbon and water are two of the
most common substances in the universe.
In tandem, they provide an extravagance of
durable chemical products unmatched by
any other obvious combination of elements.
The requirement for liquid water means any
life-bearing planet must occupy a slim sliver
of the space surrounding a star. Too close to
the thermonuclear furnace, and water will boil
off as steam. Too far away, and it will freeze toice, consigning life to a frigid fate. Where
exactly these boundaries are in a given
planetary system depends on a star’s
mass and age, which in turn determine
how much heat and light it radiates.
Earth seems to be snugly sandwiched in
the sun’s sweet zone: the best of all
possible worlds, at least in our solar
system. The laws of physics as we
understand them are the same throughout
the universe, so presumably any other
small, rocky planet in a similarly temperate
orbit could also be a Goldilocks world.
If only it were that simple. Estimating
where the Goldilocks zone lies depends
on other assumptions about a potentially
habitable planet’s nature besides the
presence of liquid water. Based on its position
in the solar system alone, Earth’s average
surface temperature should be well below
freezing. Its saviour is a heat-trapping
atmosphere laced with the greenhouse gases
carbon dioxide and water vapour. Such an
atmosphere is thought to be a typical result of
the way rocky planets form. If Earth’s comfort
blanket were much thicker or thinner, however,
or had a different chemical make-up, the planet
could rapidly cease to be so amenable to life.
Our neighbour Venus illustrates the point.
Venus seems to have started out habitable,
with a relatively Earth-like ocean and
atmosphere. Its proximity to the sun rapidly
turned those blessings into a curse. Water began
to boil off from the oceans into the atmosphere,
where its heat-retaining qualities caused
temperatures to rise still further. The result was
a runaway greenhouse effect that sterilised the
planet as all the CO 2 was baked out of its crust
and into its atmosphere. Under its stifling sky of
almost pure CO 2 today, Venus’s surface temperature
is some 460 °C – above the melting points of tin,
lead and zinc.Goodbye
Goldilocks?
sam chivErs130608_F_Goldilocks.indd 40 31/5/13 10:40:39