The King of the Kuiper Belt
Alan Stern frequently highlights this core physical trait—having enough mass for its gravity to crush it into a spherical shape—as the primary geological definition of a planet.
What is hydrostatic equilibrium?
Hydrostatic equilibrium is the state of balance where the inward pull of gravity on a fluid or gas is equal to the outward push of internal pressure.
When this balance is reached, an object does not collapse inward or expand outward.
Under the geophysical planet definition championed by Alan Stern, Pluto is classified as a sub-category of planet, recognizing its smaller size but complex structure.
What is a dwarf planet (or geophysical planet)?
Alan Stern served as the Principal Investigator(often referred to as the principal scientist or lead) for this NASA mission, which executed a historic flyby of Pluto in July 2015.
What is New Horizons?
NASA's New Horizons is a historic robotic space mission exploring Pluto, its moons, and the distant Kuiper Belt.
In 2006, this international scientific body voted (237 in favor to 157 against) to strip Pluto of its major planet status, a decision Stern and many planetary scientists still openly reject.
What is the International Astronomical Union (IAU)?
Pluto exhibits a differentiated internal structure, meaning it has a rocky core separated from this outermost layer.
What is an icy mantle (or crust)?
Unlike simple space rocks, Pluto possesses a complex, multi-layered atmosphere made mostly of this gas, which expands when it is closer to the Sun.
What is nitrogen?
Pluto boasts a system of this specific number of moons, a count higher than that of Mercury, Venus, and Earth combined, pointing to its complexity as a planetary system.
What is five?
Images from New Horizons revealed that Pluto’s atmosphere features dozens of layers of this blue visual phenomenon, scattering sunlight just like Earth's sky.
What is haze?
Stern argues that if you look at a photo of Pluto without a caption, any person or child would instantly identify it as one of these astronomical bodies.
What is a planet?
New Horizons detected vast fields of these wind-blown structures made of solid methane ice grains, a geological feature requiring a dynamic atmosphere.
What are sand dunes?
Alan Stern argues that Pluto’s active surface geology, including flowing ice glaciers and moving dunes, makes it more dynamically planetary than this dead, cratered Earth satellite.
What is the Moon?
Pluto’s largest moon, Charon, is so massive that the two orbit a point in empty space, making Pluto part of the solar system's only known binary planet system.
What is a double planet?
Pluto features complex organic molecules called tholins on its surface, which are formed when solar ultraviolet light interacts with methane and another gas.
What is nitrogen?
Pluto changes throughout its 248-year orbit (Pluto's orbit is stretched out, so for 20 years out of its 248-year trip around the Sun, it shares an orbit with/and comes closer to the Sun than Neptune), experiencing extreme seasonal variations because its axis is tilted at a steep 120 degrees.
What is climate change (or seasonal cycles)?
Pluto’s surface features massive blocks of water-ice mountains that float like icebergs on top of denser pools of this frozen element.
What is solid nitrogen?
This massive, heart-shaped nitrogen-ice glacier discovered by New Horizons proves Pluto has a churning, geologically active interior driven by internal heat.
What is Sputnik Planitia?
is a vast, 1,000-kilometer-wide basin of frozen nitrogen ice on Pluto that forms the western lobe of the dwarf planet's famous heart-shaped feature, Tombaugh Regio.
New Horizons captured evidence of these towering features, suggesting Pluto may experience cryovolcanism, or the eruption of icy slush instead of molten lava.
What are ice volcanoes?
The chaotic orbits of Pluto’s small outer moons (Nix, Hydra, Kerberos, and Styx) indicate they were formed by a giant impact, a birth story identical to that of this planetary neighbor of ours.
What is Earth’s Moon?
Planetary scientists prefer the geophysical planet definition, which focuses entirely on an object's intrinsic physical properties rather than this external path.
What is its orbit?
The surface of Pluto shows highly varied albedo regions, meaning it has stark contrasts between incredibly bright icy plains and pitch-black equatorial terrain.
What is reflectivity?
Alan Stern objects to the IAU’s definition of a planet because it requires a world to clear its orbit, a standard that would disqualify Earth if it were placed in Pluto's distant region.
What is the neighborhood?
Pluto's elongated orbit crosses inside the path of Neptune for about 20 years every 248 years.
Data from Pluto suggests it may hide a liquid, subsurface ocean beneath its icy crust, a trait shared with major planetary moons like Europa.
What is a water ocean?
Stern points out that Pluto features a bounded magnetosphere or plasma interaction zone, interacting with the solar wind just like major planets do.
What is a bow shock (or magnetopause)?
Stern frequently notes that the solar system is populated by a third class of planets found in the Kuiper Belt, outnumbering the rocky inner planets and the gas giants combined.
What are ice dwarfs (or dwarf planets)?
Because Pluto’s atmosphere expands and leaks into space, it shares gas with its moon Charon in a planetary phenomenon known as interplanetary mass transfer.
What is atmospheric sharing (or a shared atmosphere)?