PLUTO

PLUTO
Name	Pluto
Caption	Pluto as imaged by the New Horizons spacecraft in 2015.
Discoverer	Clyde Tombaugh
Discovery date	February 18, 1930
Designations	134340 Pluto
Mp category	Dwarf planet, Trans-Neptunian object, Kuiper belt object
Named after	Pluto, Roman god of the underworld
Epoch	J2000
Semimajor km	5.90638 billion
Orbital period	247.94 years
Mean radius km	1188.3 ± 1.6
Mass kg	1.303 × 1022
Density gcm3	1.854 ± 0.006
Surface grav ms2	0.620
Escape velocity kms	1.212
Rotation period	6.387 days
Axial tilt	122.53°
Albedo	0.49–0.66 (geometric)
Magnitude	15.1
Surface pressure pa	1.0 Pa
Surface temp	33–55 K

PLUTO is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It was the first Kuiper belt object to be discovered and is the largest known trans-Neptunian object by volume. For much of the 20th century, it was considered the ninth planet from the Sun, but its status was redefined following the discovery of similar objects in its orbital neighborhood, leading to a formal reclassification by the International Astronomical Union in 2006.

Discovery and classification

The search for a ninth planet, initially prompted by perceived irregularities in the orbit of Uranus, was led by astronomer Percival Lowell at his Lowell Observatory in Flagstaff, Arizona. After Lowell's death, the search was continued, and on February 18, 1930, Clyde Tombaugh discovered Pluto by comparing photographic plates. The name was suggested by an 11-year-old schoolgirl from Oxford, Venetia Burney, and was formally adopted. For decades, Pluto was accepted as the solar system's ninth planet, but beginning in 1992 with the discovery of Albion, many similar icy bodies were found in the Kuiper belt. The pivotal discovery of the massive Eris in 2005 forced a reevaluation, culminating in the International Astronomical Union creating the new "dwarf planet" category, into which Pluto was placed.

Physical characteristics

Pluto has a complex and varied surface composed of more than 98% nitrogen ice, with traces of methane and carbon monoxide. Data from the New Horizons mission revealed vast, heart-shaped nitrogen glacier plains named Tombaugh Regio, alongside mountain ranges made of water ice, such as the Tenzing Montes and Hillary Montes. Its surface shows a wide range of colors, from charcoal black to white and reddish regions, the latter likely caused by tholins formed by solar radiation. Pluto has a tenuous but structured atmosphere primarily of nitrogen, with layers of haze extending high above the surface, which undergoes dramatic pressure changes as its orbit takes it farther from the Sun.

Orbit and rotation

Pluto's orbit is markedly different from those of the eight classical planets; it is highly eccentric and inclined relative to the ecliptic. Its average distance from the Sun is about 39.5 astronomical units, but it can come as close as 30 AU and recede to nearly 49 AU. This 248-year orbit brings it inside the orbit of Neptune for 20 years of its cycle, though a stable orbital resonance prevents a collision. Pluto rotates on its side with an axial tilt of about 120 degrees, similar to Uranus, resulting in extreme seasonal variations. A day on Pluto lasts about 6.4 Earth days.

Moons

Pluto possesses five known natural satellites. The largest, Charon, was discovered in 1978 by James Christy; it is so massive relative to Pluto that the system's barycenter lies between them, making it a binary system. The four smaller moons—Styx, Nix, Kerberos, and Hydra—were discovered between 2005 and 2012 by teams using the Hubble Space Telescope. These moons have chaotic rotations and are thought to be remnants from a giant impact that also formed Charon. Their surfaces are covered in water ice.

Exploration

For decades, Pluto was observed only from Earth, most notably by the Hubble Space Telescope. The first and only spacecraft to visit Pluto is NASA's New Horizons probe, which was launched in 2006 under the direction of the Southwest Research Institute and principal investigator Alan Stern. After a nine-year journey, it conducted a historic flyby on July 14, 2015, returning a wealth of high-resolution images and scientific data that revolutionized our understanding of its geology, atmosphere, and moons. The mission then continued into the Kuiper belt, performing a flyby of the object 486958 Arrokoth in 2019.

Formation and evolution

Pluto is believed to have formed in the early Solar System from the accretion of planetesimals in the primordial Kuiper belt. Its composition suggests it accreted from material that was abundant in the colder outer regions of the protoplanetary disk. The Pluto-Charon system is thought to be the product of a giant impact between two bodies, with the smaller moons coalescing from the resulting debris disk. Its internal structure is differentiated, likely with a rocky core surrounded by a mantle of water ice and a crust of more volatile ices. Geological activity, possibly driven by residual internal heat from radioactive decay or the freezing of a subsurface ocean, appears to have reshaped its surface much more recently than previously theorized.

Category:Dwarf planets Category:Kuiper belt objects Category:Astronomical objects discovered in 1930