Pluto, once considered the ninth planet in the Solar System, is now classified as a dwarf planet. This change in status occurred in 2006 when the International Astronomical Union (IAU) redefined what constitutes a planet. Despite its demotion, Pluto remains a fascinating world with a unique set of characteristics, including a complex icy surface, geology, atmosphere, and its relationship with its largest moon, Charon. Despite its reclassification, Pluto remains one of the most intriguing and well-known objects in the Solar System.

1. 1. Size and Composition

• • • Diameter: Pluto’s diameter is about 2,377 km (1,477 miles), making it roughly two-thirds the size of Earth’s Moon.
    • Mass: It is 0.2% the mass of Earth, making it relatively small even for a dwarf planet.
    • Composition: Pluto is composed mostly of rock and ice, with a surface that includes nitrogen, methane, and carbon monoxide ices. Its rocky interior is surrounded by layers of ice, and beneath the surface, there may be a subsurface ocean of liquid water.

 

1. 2. Orbit

• • • Orbit around the Sun: Pluto has a highly elliptical orbit, taking about 248 Earth years to complete one full revolution around the Sun. Its orbit is tilted compared to the orbits of the eight planets, and it crosses inside Neptune’s orbit for part of its journey.
    • Distance from the Sun: On average, Pluto is about 5.9 billion km (3.7 billion miles) from the Sun. It ranges from 30 to 49 AU (astronomical units) from the Sun, with 1 AU being the distance from the Earth to the Sun.
    • Eccentric orbit: Due to its elliptical path, Pluto’s distance from the Sun varies significantly, and for about 20 years of its orbit, Pluto is closer to the Sun than Neptune.

 

1. 3. Surface and Atmosphere

• • • Surface features: Pluto has a varied and complex surface, with regions of mountains, valleys, plains, and craters. Its most prominent feature is Sputnik Planitia, a heart-shaped ice plain made mostly of nitrogen and methane ices. This region is thought to be geologically active, reshaping itself over time.
    • Mountains: Some of Pluto’s mountains are as tall as 3-4 km (2-2.5 miles) and are composed primarily of water ice, as water behaves like rock at the extremely cold temperatures found on Pluto.
    • Atmosphere: Pluto has a thin, tenuous atmosphere composed primarily of nitrogen, with traces of methane and carbon monoxide. The atmosphere expands and contracts as Pluto moves closer to and farther from the Sun during its orbit, potentially freezing and falling to the surface as frost when it is farthest from the Sun.
    • Weather: Though extremely thin, Pluto’s atmosphere shows evidence of haze layers and complex weather patterns, including wind and possible frost cycles.

 

1. 4. Temperature

Pluto’s surface temperature varies between -375°F and -400°F (-225°C to -240°C), making it one of the coldest known objects in the Solar System. At such low temperatures, many gases like nitrogen and methane freeze solid.

 

1. 5. Moons

• • • Pluto has five known moons, the largest and most significant being Charon. The other moons are Nix, Hydra, Kerberos, and Styx.
    • Charon: Nearly half the size of Pluto, Charon is so large that Pluto and Charon are sometimes referred to as a binary (double) system because they orbit a common centre of mass located above Pluto’s surface. Charon is thought to have a relatively young surface with evidence of tectonic activity and possibly an ancient underground ocean.
    • Nix and Hydra: Two smaller moons discovered in 2005; they orbit Pluto at greater distances.
    • Kerberos and Styx: The smallest moons, discovered in 2011 and 2012, respectively. They are irregularly shaped and much less studied than Charon.

 

1. 6. Interior

• • • Pluto likely has a differentiated interior, meaning it has a rocky core surrounded by a mantle of water ice, and a thin icy crust on the surface.
    • Beneath the surface, it is thought that Pluto may harbour a subsurface ocean, possibly still in a liquid state due to heat generated from radioactive decay in its core.

 

1. 7. Discovery and Classification

• • • Discovery: Pluto was discovered in 1930 by American astronomer Clyde Tombaugh at the Lowell Observatory in Arizona. It was originally classified as the ninth planet in the Solar System.
    • Reclassification: In 2006, Pluto was reclassified as a dwarf planet by the IAU due to its size and the fact that it has not cleared its orbit of other debris, a key criterion for full planet status.
    • Dwarf Planet Definition: Pluto meets two of the three criteria for being a planet: it orbits the Sun and has enough mass to be nearly round. However, it does not dominate its orbit, as other objects in the Kuiper Belt share the same orbital region.

 

1. 8. Kuiper Belt and Beyond

• • • Pluto is part of the Kuiper Belt, a region of the Solar System beyond Neptune filled with small icy bodies and dwarf planets. It is one of the largest known objects in this distant region, but there are other similarly sized objects, such as Eris, another dwarf planet.
    • The Kuiper Belt is thought to be the source of many comets, and studying it provides insight into the early formation of the Solar System.

 

1. 9. Exploration

• • • Pluto was visited by NASA’s New Horizons spacecraft in 2015, marking the first and only close-up study of the dwarf planet. New Horizons provided a wealth of data, including detailed images of Pluto’s surface, atmosphere, and moons, and fundamentally changed our understanding of this distant world.
    • New Horizons also studied Pluto’s atmosphere and discovered a range of surface features, including glaciers, cryovolcanoes (ice volcanoes), and a surprisingly active geology.

 

1. 10. Interesting Facts

• • • Binary system with Charon: Pluto and Charon are so close in size and mass that they are often considered a double dwarf planet system. They are tidally locked, meaning the same sides of Pluto and Charon always face each other.
    • Pluto’s heart-shaped region: Pluto’s heart-shaped feature, Tombaugh Regio, named after its discoverer, became iconic after the New Horizons flyby. The left lobe of the heart is Sputnik Planitia, an ice-rich basin that may be geologically active.
    • Lonely planet: Due to its small size, distance from the Sun, and isolation in the Kuiper Belt, Pluto is sometimes described as a “lonely planet.” Its orbit keeps it far from both the Sun and the other planets for most of its journey.