If you watched our videos about exoplanets, you must already know that scientists are
looking for oceans in the Goldilocks or habitable zone, tens to hundreds of light-years away
from Earth. But it’s difficult to imagine that a planet more than 5 billion kilometers
away from its star can have an ocean of liquid water on it. And it's even harder to believe
that there could be life on such a planet!
However, recent findings prove it’s possible. And there’s just such a world in our solar
system - Pluto.
[INTRO]
According to astronomers’ estimates, Pluto is just about 2,370 kilometers [1,470 mi]
in diameter, which makes it smaller than a number of solar system moons such as Ganymede,
Titan, Callisto, Io, the Moon, Europa, and Triton. And this made scientists wonder if
Pluto could still be considered a planet. To compare, it’s almost 60 times smaller
than Jupiter [Diameter: 143,000 km / 88,850 mi]!
So scientists were skeptical about Pluto’s status as a planet. The small world was rather
just a part of the population of small icy bodies beyond the orbit of Neptune or the
region known as the Kuiper Belt. But it wasn't until 1992 that astronomers discovered its
first 'resident'.
The space object dubbed 1992 QB1 was found using a 2.24-meter [7.34-ft] telescope located
on Mauna Kea, the island of Hawaii. The discovery of this celestial body further intensified
discussions on the status of Pluto. 8 years later, the New York Hayden Planetarium presented
a scandalous exhibition that featured just eight planets.
Then, scientists discovered more Kuiper belt objects with a mass roughly that of Pluto,
such as Quaoar, Sedna, and Eris. And the newly discovered Eris turned out to be even larger
than Pluto. First, this Kuiper belt object was even called the "tenth planet" of the
solar system. Eventually, the International Astronomical Union [IAU] created a committee
to establish a general definition of a planet. One idea was to extend the list of solar system
planets from 9 to 12. It was proposed that Pluto along with its satellite should be considered
twin planets, and Ceres and Eris had to join a planetary list as well. But the idea was
criticized and rejected.
Eventually, a new definition of a planet was created. On the last day of the meeting, August
24, members of the IAU defined criteria for a celestial body to be called a planet.
So what are these criteria?
To be a planet, a space object must be in orbit around the Sun; it must have sufficient
mass and gravity to force it into a spherical shape, and it must also be able to clear away
any other objects of similar size near its orbit. And there are just eight such planets
in our solar system, starting with Mercury and ending with Neptune.
So because Pluto only meets the first two criteria, it was reclassified as a dwarf planet
or planetoid.
It all started when the New Horizons approached the dwarf planet located almost at the edge
of the solar system. Scientists have long been interested in Pluto. Even such an advanced
device as the Hubble Space Telescope could only see a few bright spots on this planet.
And the most thrilling part about Pluto was its location, the Kuiper belt. This region
of space consists of a bunch of icy bodies of various sizes. Scientists think these are
the remnants of the solar system building blocks.
So, studying Pluto could help us better understand how our solar system and our home planet were
formed. And in 2003, scientists were able to convince NASA to organize an expedition
to Pluto. 3 years later, the New Horizons probe atop the Atlas-5 carrier rocket was
launched from the US Space Force station at Cape Canaveral. But just days before approaching
Pluto, the spacecraft stopped communicating with Earth and scientists had to urgently
reboot its software.
Then the probe faced even more issues when it approached Pluto. Despite its small size,
the dwarf planet has five satellites, the largest of which is roughly half the planet’s
size [Diameter: 1212 km / 753 mi]. But astronomers were worried that the planetoid could be also
surrounded by swarms of tiny satellites that could form rings, like those around Saturn
or Jupiter but much smaller. A collision with such a satellite or even a dust particle the
size of a grain, at a speed of almost 16 kilometers per second [9.9 m/s] would mean a disaster
for the mission. It would cause the energy of a collision equal to a large caliber gun
projectile and blow the probe to pieces. But everything worked out well, and mankind was
finally able to see the first photos of the dwarf planet.
As Project Manager Professor Alan Stern stated: “It exceeded my wildest expectations in
every way. I am delighted. The Pluto system is simply stunning.” Another participant
of the project, Professor Hal Weaver, said: “Our jaws just dropped. That was incredible".
Who would’ve thought that a cosmic body that receives just 1/1600 of sunlight compared
to Earth and is 3 billion miles away from the Sun can be so beautiful. Interestingly,
Pluto, among all other cosmic bodies in our solar system, is most similar to Earth!
Scientists were shocked to see frozen icy lakes, icy mountains the size of the Rocky
Mountains, and chasms larger than the Grand Canyon. And all that is on a planet smaller
than the moon! What’s more, like Earth, Pluto also has a blue atmosphere. But, unfortunately,
there’s no oxygen in the atmosphere of the dwarf planet, just nitrogen and numerous deposits
of methane and carbon monoxide. The pressure there is about 1 pascal [1 Pa], which is about
100,000 times less than that on our planet. The average surface temperature is minus 226
to minus 240 degrees Celsius [-375/-400°F].
Scientists were also intrigued by Pluto’s smooth surface, almost round in shape, occupying
about a third of the entire area of the planet. Its diameter is 1492 kilometers [927 mi].
The area named Sputnik Planitia has practically no craters on it, meaning there are no traces
of meteorite impact. Scientists believe it was formed relatively recently, during the
last 100 million years or so. Further analysis showed that this is a large basin filled with
frozen nitrogen.
But for now, let’s take a look at Pluto's main satellite. Charon is located at a distance
of about 19,600 kilometers [12,180 mi] from the surface of Pluto, while the distance between
the Earth and the Moon is roughly 385,000 kilometers [240,000 mi]. This means that the
tidal forces between the dwarf planet and its satellite are very powerful. And according
to the laws of celestial mechanics, under such conditions, the rotation of a satellite
becomes synchronized in time, meaning the period of its revolution around its axis and
a planet becomes the same. That is, Charon is always facing Pluto on one side.
Now, let's get back to Sputnik Planitia. This area is located on the axis connecting the
centers of Pluto and Charon, and the probability that such a large geological formation lies
on this axis by chance is about 5 percent. Most likely, sometime in the past, a pole
shift occurred on Pluto and Sputnik Planitia took the place of one of the poles. But for
this to be possible, this region should’ve had a positive gravitational anomaly. In other
words, it must be heavier than the average volume of Pluto. But Sputnik Planitia is a
hollow basin filled with light ice - frozen nitrogen. And this made scientists think that
there may be a denser substance under its surface that “adds” mass to the area.
So what is this substance?
Scientists got interested in the deep cracks that are clearly visible on the surface of
Sputnik Planitia. These cracks usually form as the planet’s surface expands. And as
the temperatures on the surface of Pluto are freezing, there must be a substance beneath
the frozen nitrogen crust that expands when it cools down. So what kind of substance can
expand rather than contract when it gets cooled? Water.
So, does this mean that there’s an ocean of liquid water hiding underneath a layer
of frozen nitrogen in this giant basin? That’s what scientists thought and later confirmed
by another observation. Geologists studied three-dimensional maps of the surface of Pluto
and identified two quite unusual objects - mountains up to 5 or 6 kilometers [3.1/3.7 mi] high.
But these aren’t just mountains. Scientists have also found a billion-year-old volcanic
craters and textures resembling traces of ancient eruptions on their surface. The two
gigantic volcanoes were named Wright Mons and Piccard Mons.
If scientists are right that these geological formations are volcanoes, Wright Mons and
Piccard Mons are the first large cryovolcanoes discovered in the solar system. But, so far,
scientists aren’t 100% certain about the nature of these objects. "They definitely
look suspicious, so we're looking at them closely," says Geoff Moore, a member of the
mission's science team. But these volcanoes are on the outskirts of Sputnik Planitia.
So it’s very likely that this is the case. Perhaps in the past, Pluto had two cryovolcanoes
that erupted with icy water instead of lava.
As for Sputnik Planitia, scientists think that it was formed as a result of a cosmic
catastrophe - a large celestial body once fell on Pluto, resulting in a crater with
an initial depth of about seven kilometers [4.3 mi]. The collision caused a significant
decrease in the thickness of the dwarf planet’s icy surface and also made water from the subsurface
ocean rise.
Geologists have also considered yet another idea. Pluto could have no subsurface ocean
at all. In this case, for a positive gravitational anomaly to occur, huge amounts of nitrogen
ice must accumulate in the impact crater. And amounts that big wouldn’t simply fit
into the crater. Scientists say that the depth of the ocean on Pluto should reach a hundred
kilometers and that it has existed for billions of years. Large quantities of ammonia and
methanol could help maintain the subsurface ocean in a "semi-liquid" state.
Scientists are almost certain that the subsurface ocean isn’t completely frozen. Because if
it were, Pluto's low temperatures and high pressure inside the planetoid would make water
ice turn into denser cubic or Ice II. This would’ve been accompanied by a decrease
in the volume of ice and compression of Pluto's icy crust, but the images taken by the New
Horizons show a different picture. This means that the ocean may still be partly liquid.
Something similar even happens here on Earth. Our planet does also have a region with very
low temperatures and there’s a reservoir with liquid water under its surface. This
is the famous Lake Vostok with a maximum depth of 1200 meters [3900 ft] and an area of 15,790
km² [6096 mi²].
But how did Pluto, despite its great distance from the Sun, get enough heat to create a
liquid ocean that survived under the icy crust up to this day?
Scientists believe two potential sources made it possible. The first one is the heat released
during the decay of radioactive elements inside the dwarf planet. And the second one is the
gravitational energy release resulting from the bombardment of Pluto’s surface. Calculations
show that if all gravitational energy was stored as heat, this source alone would be
enough to create a liquid ocean. However, for this to happen, the accretion process
must be very fast. Otherwise, a planetary body’s surface would give off a significant
part of the energy back into space. Researchers calculated that to heat up so quickly, Pluto
must have formed in about thirty thousand years.
Scientists suggest that some other large Kuiper belt objects, such as the dwarf planets Eris
and Makemake, also had a "hot start" and had liquid oceans at the very beginning of their
existence. So far, it seems humanity only looks for water millions of light-years away
while it has always been much closer, in our solar system. And water often means life.
So there’s a high probability that life could one day be found in the depths of Pluto.
Even on our planet, there are life forms that can survive without heat from the Sun. Some
bacteria live in thermal springs and survive because of the heat incoming from hot water.
In science, there’s even the concept of extremophiles or organisms capable of living
and reproducing in extreme environmental conditions, such as very high or low temperature, pressure,
acidity, oxygen, and so on. So why wouldn't there be life in Pluto's underground ocean?
The great English science fiction writer Arthur C. Clarke predicted life in the subsurface
ocean on Jupiter’s satellite Europa in his famous novel “2001: A Space Odyssey”.
This celestial body is larger than Pluto, and it receives more heat from the Sun. So
even though this is just a prediction, the same author was already right with yet another
forecast. During the New Horizons’ mission, the probe’s flight path was calculated according
to the method described by the great science fiction writer in his novel. The idea is to
use Jupiter's gravitational field as an additional source of acceleration to save fuel. According
to the creators of the New Horizons project, such a maneuver allowed the device to arrive
at Pluto three years earlier.
To scientists’ surprise, the dwarf planet turned out to be similar to our home. Like
Earth, Pluto has a blue halo of atmosphere around it; although a subsurface one, it also
has an ocean, and the existence of certain life forms there is very likely. So the debate
about whether Pluto should have a status of a planet is back on the table. Because it
seems like the planet deserves it. But it also deserves another mission. We have to
land on Sputnik Planitia, get to liquid water and finally put an end to the question of
whether life is present there or not.
This could be the greatest discovery since the beginning of the third millennium.
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