42 | NewScientist | 8 June 2013In 1993, geoscientist James Kasting of
Pennsylvania State University in State College
set out to pin down a lot more precisely where
the Goldilocks boundaries lie. He and his
colleagues examined how varying the
intensities and wavelengths of sunlight falling
on an idealised Earth affected its atmosphere
and surface temperature. Increasing the
incident sunlight by some 10 per cent –
equivalent to moving Earth inwards from
its present position of 1 astronomical unit
(AU) from the sun to 0.95 AU – produced a
temperature rise that sent water vapour
soaring high up into the atmosphere, where
it dissipated into outer space. Over tens of
millions to hundreds of millions of years,
such a “moist greenhouse” would entirely
desiccate the Earth and eradicate all surface
life (Icarus, vol 101, p 108).
When Kasting tried to place the habitable
zone’s outer limit – the point where the fall in
temperature is enough to cause irrecoverable
global glaciations – he found it to be about
1.67 AU from the sun, slightly beyond the orbit
of Mars. Already, these early calculations
began to crack Earth’s Goldilocks facade. Earth
is not slap bang in the centre of the Goldilocks
zone, but significantly towards its inner edge.Kasting’s climate model was in some
respects rather basic, simulating a single,
uniform strip of atmosphere essentially
devoid of clouds and weather systems. In
other ways it was quite elaborate, for example
incorporating the sort of positive feedback
effects by which increasing atmospheric water
vapour leads to a runaway greenhouse effect.
It could also replicate many of the climatic
quirks of other solar system planets.
Extrapolating its conclusions, Kasting was able
to chalk in the inner and outer boundaries of
the habitable zone around a wide variety of
stars of different sizes and luminosities. These
have been the gold standard for hunters of
habitable planets ever since.
Until now. At the beginning of this year,
working with Kasting and a few others, Penn
State researcher Ravi Kopparapu updated the
calculations for the first time in two decades.
A lot has changed, Kopparapu points out.
Above all, we now know that water vapour
and CO 2 are better at absorbing certain
wavelengths of infrared light than we used to
think. That affects the potency of each gas’s
greenhouse heating. Rerunning the models
produced a simple, unambiguous result: for
a planet like Earth, the habitable zone aroundall types of stars lies slightly further out than
we had assumed (arxiv.org/abs/1301.6674).
That has knock-on effects as we search for
promising habitats around other stars. A few
previously discovered extrasolar planets have
been struck from the list, whereas others are
beginning to look more promising (see
diagram, right). It also enhances the prospects
for life around the small, cool “M-dwarf” stars
that are the most prevalent in our immediate
neighbourhood, shifting their habitable zones
outwards towards the sort of distances where
most small, rocky planets have so far been
found. “It looks like nearly half of all M-dwarfs
should have an approximately Earth-sized
planet in their habitable zones,” says
Kopparapu. As new planet hunts focusing on
M-dwarfs start off over the next few years,
we should expect to find at least three or four
possible Goldilocks worlds right next door.Teetering on the brink
For the original Goldilocks planet, however,
the implications are much murkier. The inner
edge of the solar system’s habitable zone
moves outwards from 0.95 AU to 0.99 AU.
In other words, were Earth just 1 per cent closer
to the sun, its water could begin to steam off
into space as a moist-greenhouse effect kicks
in. Rather than being at a comfortable distance
from the edge of the Goldilocks zone, we are
teetering on the brink.
That portends an alarming future. As our
sun ages, it is fusing hydrogen at higher and
higher temperatures and becoming more
luminous, pushing the inner edge of the
Goldilocks zone outwards. “It suggests the end
could come sooner than we thought,” says
Kasting. Earth could technically begin to lose
water “as early as tomorrow”, he says. More
likely is that we can lop a few hundred million
years off Earth’s generally accepted remaining
habitable time of a billion years or so.
Raymond Pierrehumbert, a climate scientist
at the University of Chicago, remains
sanguine. Although he acknowledges that the
calculations are useful for establishing
reference points for habitable zones, he thinks
the prospect of imminent desiccation is a
mirage that reflects the model’s deficiencies.
“Most of what it leaves out arguably makes
a planet more habitable, not less,” he says.
Take the effects of clouds. In atmospheres
with amounts of water vapour close to the
moist or runaway greenhouse limits, clouds
are more likely to form at lower altitudes,
reflecting more sunlight back into space and
cooling the surface beneath. In fact, it is hard
to know exactly how an atmosphere that is
perhaps 50 per cent water will behave. “A big” The result was unambiguous: the habitable
zones are all further out than we assumed”
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