Whether a planet is habitable may depend on geology as well
as distance (Credit: Michael S. Helfenbein / Yale University)
If Yale researcher Jun Korenaga is correct, then
habitable planets may be rarer than previously thought.
According to a recently published paper, the ability of
Earthlike planets to self-regulate their temperatures may
not be as simple as once believed and that a new
"Goldilocks zone" may need to be considered when looking
for worlds beyond the Solar System where life could exist.
Of all the factors that go into determining if an
exoplanet might habitable, the Goldilocks or habitable zone
ranks at the top of the list. Like the little girl who
preferred her porridge temperature to be "just right," the
Goldilocks zone is a band around a star where a planet will
receive the right amount of solar energy for liquid water to
exist. Since water is absolutely necessary for life as we
know it to exist, whether a planet sits in the zone where it
is neither too hot or too cold is paramount in determining
if it is a candidate for habitability.
However, there's more to determining the temperature of a
planet than just its distance from its sun. Venus and Mars
both sit in the Solar System's Goldilocks zone, yet both are
dead worlds. For over sixty years, geologists and planetary
scientists one key factor has been plate tectonics –
specifically, plate convection.
The idea is that as the plates of the Earth's crust float
about on the hot, plastic magma of the mantle, they act as a
kind of thermostat that automatically adjusts the planet's
temperature. This is important because between the initial
heat of the Earth radiating constantly into space and the
reheating or the Earth's core by decaying radioactive
isotopes, the interior temperature of the Earth changes over
the eons.
The self regulation works because the rate of convection
of the magma under the Earth's tectonic plates is determined
by temperature. If the Earth radiates too much heat, this
cools the magma under the plates, which slows down the
convection, and the surface warms. If Earth radiates too
little heat, the convection speeds up, and the surface
cools.
In this way, the Earth can keep itself roughly in the
temperature range of liquid water. In addition, scientists
believe that the same would hold true for most tectonically
active Earthlike planets and those that were at first too
hot or too cold would eventually regulate themselves into
habitability if they were in their system's Goldilocks zone.
But Korenaga has other ideas. According to his research,
viscosity and the early history of a planet are major
factors that make the regulating mechanism much less
certain.
"If you assemble all kinds of scientific data on how
Earth has evolved in the past few billion years and try to
make sense out of them, you eventually realize that mantle
convection is rather indifferent to the internal
temperature," says Korenaga.
According to his research, self-regulating planets are
much rarer than thought because the interior structure and
temperature of the mantle must also be "just right" or the
viscosity of the magma would be too much or too little, so
the convection would be too slow or too fast for effective
regulation. The result would be a planet too hot or too cold
for life to exist. Given the way planets form by a series of
catastrophic collisions, this is highly unlikely.
"The lack of the self-regulating mechanism has enormous
implications for planetary habitability," says Korenaga.
"Studies on planetary formation suggest that planets like
Earth form by multiple giant impacts, and the outcome of
this highly random process is known to be very diverse."
The results were published in
Science Advances.
Source:
Yale University