YSM Issue 94.3

COUNTERPOINT
WHAT MAKES A HABITABLE PLANET?
BY NATHAN WU
IMAGE COURTESY OF PIXABAY
We all know how the story goes. A mysterious
spaceship is detected in the atmosphere. Humans
try to communicate with the aliens on it. Aliens are
hostile and attempt to conquer Earth. Pandemonium ensues.
The “alien invasion” trope and extraterrestrial beings in
general have been parts of movies, books, and other media for
decades, from H. G. Wells’s The War of the Worlds to the cult
classic film Independence Day to everyone’s favorite quarantine
video game, Among Us. The idea of encountering aliens has
captured our imaginations. However, in scientific communities,
the search for extraterrestrial life has yet to find success.
Traditionally, scientists have looked towards planets with
conditions like ours in their search for life. Whether a planet
has appropriate conditions for liquid water has been a primary
concern. These planets can neither be too close nor too far
from the star they orbit: this famed “Goldilocks” region is
usually considered to be the habitable zone for a star. An
additional constraint is that the models used to predict the
bounds of this region assume a small, rocky planet with an
Earth-like atmosphere filled with nitrogen gas, oxygen gas,
and carbon dioxide. However, two recent studies tell us that
we may not be looking in the right places.
Nikku Madhusudhan and his team at the University of
Cambridge proposed a new type of potentially habitable planet.
These planets, known as “Hycean worlds,” are composed of massive
oceans with surrounding atmospheres made mostly of hydrogen
gas. Madhusudhan’s team first explored the range of masses and
radii that Hycean worlds can take on and then determined the
range of temperatures (and, by extension, distances from various
stars) that allow for habitable Hycean surfaces.
Madhusudhan’s team found that Hycean planets offer several
advantages over Earth-like ones when it comes to the search for
life. Hycean worlds can be much larger than rocky, terrestrial
ones, and their thick atmospheres provide insulation that allows
for liquid water far away from a star: some “Cold Hycean”
planets may not need any stellar irradiation at all, with their
only heat source being internal. The increased range of sizes
and distances from a star that Hycean planets have could mean
that scientists can broaden their search for extraterrestrial life.
Meanwhile, Noah Tuchow and Jason Wright of Penn State
questioned the habitability of planets in the traditionally
defined habitable zone. They noted that, while the traditional
definition considers whether liquid water could exist under
current conditions, a planet’s habitability is dependent on
whether it has existed in the habitable zone ever since life there
began. Planets currently observed in a star’s habitable zone may
have entered the zone relatively recently, either due to changes
in a star’s luminosity or planetary migration. These “belatedly
habitable” planets are unlikely to gain the ability to host life: if
Venus somehow took Earth’s spot in our solar system, entering
the “habitable zone,” it would never regain liquid water.
Identifying the “belatedness” of a planet’s habitability is a
difficult task. It requires knowledge of both a star’s life history
as well as when and how planetary formation occurs. However,
while no simple model can tell us which planets we can ignore,
Tuchow and Wright’s research will guide future extraterrestrial
exploration. Considering belated habitability for planets may
change how we approach future mission design, as many planets
found in habitable zones will merely be belatedly habitable.
These two studies are challenging our traditional ideas of
what makes a planet habitable. Our current definition of the
habitable zone, centered around the possibility of finding
liquid water on Earth-like planets, ignores other types of
potentially habitable planets and fails to consider the impact
of stellar history on habitability. These studies teach us that
our initial conceptions about science are often false: life in
the universe need not look like life on Earth. Our current
definition for “habitable zone” may be less useful than we once
thought, and it may be time to reconsider it. Perhaps applying
a new definition will help us find those aliens we’ve fantasized
about for so long—let’s just hope they aren’t as hostile as those
in all the movies. ■
38 Yale Scientific Magazine October 2021 www.yalescientific.org