Today
marks the 86th anniversary of the discovery of planet Pluto in 1930
by 24-year-old Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona.
Sometime
around 4 PM on that day, while using a blink comparator to move between two photographic
plates depicting the same part of the sky taken several days apart a month
earlier, Tombaugh detected a tiny dot that moved against the background stars.
That
dot was Pluto. The fascinating story of its discovery is described here: http://pluto.jhuapl.edu/Participate/learn/What-We-Know.php?link=Discovery-of-Pluto
.
“I
have found your Planet X,” the young astronomer told Lowell Observatory director
Vesto Slipher, ending the decades-long search for a trans-Neptunian planet
initiated by observatory founder Percival Lowell.
Over
the past year, we have had the opportunity to do what Clyde Tombaugh could only
dream of—transform that tiny dot he found, not even large enough to be resolved
into a disk with the most powerful telescopes of the day—into a living, breathing
planet.
We
are now into the “Year of Pluto 2,” and the amazing images and information keep
coming and will continue to do so through most of this year.
This
anniversary is an appropriate occasion to re-examine, in light of all these new
findings, the claim that Pluto is somehow “different” from the solar system’s
eight larger planets.
Let’s
start with the often repeated, “Pluto is very different from the ‘big eight.’”
First,
there are no “big eight,” unless one lumps together two very different types of
worlds—the rocky, terrestrial planets on one hand, and the gas giants and ice
giants on the other.
Any
classification system that puts Earth and Jupiter in the same category but
excludes Pluto overlooks an important fact, specifically, that Earth has far
more in common with Pluto than it does with Jupiter.
Gas
giants Jupiter and Saturn and ice giants Uranus and Neptune have no known solid
surfaces. Both have extensive systems of rings and moons that are almost their
own “mini-solar systems.” Jupiter and Saturn are composed primarily of hydrogen
and helium, much like the Sun.
Like
Earth, Pluto is rocky and geologically differentiated into core, mantle, and
crust. Like Earth, it is geologically active. It has far more water ice than
previously thought, and its geological processes suggest an internal heat source
that could possibly support a subsurface ocean.
Some
scientists see evidence for such an ocean in the faults and fissures on Pluto’s
surface and in the planet’s lack of an equatorial bulge.
Equatorial
bulges are created by the spin of rotating spherical objects. Because water
moves more easily than ice, an underground ocean would reduce any bulge by acting
against rotational forces.
Both
Earth and Pluto have nitrogen in their atmospheres. The only other solar system
world with a nitrogen atmosphere is Saturn’s moon Titan.
Both
Earth and Pluto have large moons formed via a giant impact very early in the
solar system’s history.
Pluto
was initially thought to be larger than it is because until 1978, scientists
did not realize they were looking at two objects rather than one when they
observed Pluto through a telescope. The planet and its largest moon Charon,
which is half its size, are separated by just 12,196 miles (19,640 km), the
smallest separation between any planet and moon in the solar system.
And
because Pluto and Charon orbit a center of gravity, known as a barycenter,
outside of Pluto, between the two objects, they can genuinely be considered a
double or binary planet system.
New
Horizons’ findings indicate Pluto actually has more in common with Earth than
anyone imagined.
Instead
of the dead world many expected, Pluto has revealed itself to be “a world of
unexpected complexity and riches,” mission principal investigator Alan Stern
commented.
Among
the small planet’s stunning array of terrains are wind-blown dunes similar to
those on Earth and on Jupiter’s moon Europa. Worlds with atmospheres as thin as
Pluto’s do not usually have dunes, suggesting Pluto’s atmosphere may once have
been a lot thicker.
Pluto’s
famous “heart,” named Tombaugh Regio for discoverer Tombaugh, has a young
surface with no craters.
Ice
floating on the left side of side of Tombaugh Regio, known as Sputnik Planum,
flows in a manner similar to the movement of glaciers on Earth. Only two other
worlds in the solar system, Earth and Mars, experience this type of activity.
The
fact that Sputnik Planum’s terrain is constantly being reshaped suggests
tectonic forces (geological forces that cause movements of a planet’s crust)
are at work, possibly caused by internal heating produced through radioactive
decay of rocky material in Pluto’s core.
“The
Pluto system surprised us in many ways, most notably teaching us that small
planets can remain active billions of years after their formation,” Stern said.
Pluto’s
layered, atmospheric haze is similar to that seen on Titan. It is also far more
complex than scientists anticipated. Pluto’s sky appears blue at sunrise and sunset
because its haze particles scatter blue light. Which other planet has a blue sky?
Two
mountains on the Pluto’s encounter side (that observed in most detail by New
Horizons), Wright Mons and Picard Mons, appear to be ice volcanoes, also known
as cryovolcanoes. These mountains have broad, gentle slopes, characteristic of
what are known as shield volcanoes.
The
only other shield volcanoes in the solar system are on Earth and Mars.
Pluto’s
active geology and possibly cryovolcanism could be driven by a mix of ammonia
and water ice in its mantle.
Located
between the inner crust and outer core, that mantle may be experiencing
convection, a process through which hot material rises up while cooler material
sinks down.
On
Earth, convection drives the movement of tectonic plates.
Networks
of eroded valleys on Pluto’s surface, described by some scientists as “hanging
valleys,” resemble similar features seen on Earth in Yellowstone National Park.
The
point in emphasizing these detailed features is that Pluto may have more in
common with Earth than with any other solar system planet.
The
abundance of water ice on its surface and the possibility of a subsurface ocean
add Pluto to the solar system’s leading contenders for microbial life, a list
that includes Europa and Saturn’s moon Enceladus, both of which are believed to
have subsurface oceans.
Data
sent back by New Horizons just this week indicates Charon, which is also
geologically active, once had a subsurface ocean too.
Nine
and a half years ago, four percent of the IAU decided they knew how to best
classify Pluto in spite of the fact that they had never seen it up close and
knew nothing of its features. Even today, apologists for the IAU claim that
their decision stands, that Pluto is not a planet because astronomy’s “ruling
authority” said so.
Yet
Pluto’s surface and atmosphere tell a very different story.
"I
naturally refer to Pluto as a planet because that seems like the right moniker,”
New Horizons project scientist Cathy Olkin states. “It has an atmosphere; it
has interesting geology; it orbits the sun; it has moons. ‘Planet’ just seems
right to me."
