© NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Learn about Pluto’s Structure here.

Pluto, once considered the ninth planet in our solar system, has long captivated the human imagination with its enigmatic status and distant location in the far reaches of our celestial neighbourhood. Despite its reclassification as a dwarf planet in 2006, Pluto remains fascinating for scientists and space enthusiasts alike. has only one-sixth the mass of Earth’s Moon, and one-third its volume.

The planet is located in the Kuiper Belt, a zone beyond the orbit of Neptune brimming with hundreds of thousands of rocky, icy bodies each larger than 62 miles (100 kilometers) across as well as 1 trillion or more comets.

Pluto follows an orbit that is both moderately eccentric and inclined, spanning from 30 to 49 astronomical units (which translates to 4.5 to 7.3 billion kilometers or 2.8 to 4.6 billion miles) away from the Sun. At its farthest point, it takes 5.5 hours for sunlight to reach Pluto, which orbits at a distance of 39.5 AU (equivalent to 5.91 billion km or 3.67 billion mi). Despite its eccentric path occasionally bringing it nearer to the Sun than Neptune, a stable orbital resonance prevents any potential collisions. The dwarf planet remains beyond the reach of the unaided human eye, rendering it invisible in the night sky.

Pluto’s largest moon, Charon, is so proportionally similar in size that it is commonly referred to as a dual system along with the planet itself. Due to its remoteness and diminutive size, even the most advanced telescopes on Earth and in Earth’s orbit could only discern limited details of its surface. For decades, fundamental data such as its radius and mass proved challenging to ascertain. It wasn’t until the U.S. spacecraft New Horizons made its historic flyby of Pluto and its companion Charon in July 2015 that many critical questions about this celestial body and its surroundings were finally addressed.

PLUTO’S STATUS

Pluto’s status as a member of the solar system has been a subject of debate and controversy in the field of astronomy. For most of the 20th century, Pluto was classified as the ninth planet in our solar system. However, in 2006, the International Astronomical Union (IAU) redefined the criteria for classifying celestial objects as planets, which led to Pluto’s reclassification as a “dwarf planet.” This decision was based on several factors:

SIZE

The dwarf planet is relatively small compared to the eight recognized planets. It has a diameter of approximately 2,377 kilometers (1,474 miles), which is much smaller than the Earth’s Moon and even smaller than some other objects in the Kuiper Belt.

ORBIT

The planet follows an elliptical and inclined orbit that is significantly different from the relatively flat and circular orbits of the eight major planets. Its orbit also crosses that of Neptune.

NEIGHBOURHOOD

Pluto is located within the Kuiper Belt, a region beyond Neptune that is populated by numerous small, icy objects. This neighborhood is distinct from the inner solar system where the eight planets reside.

CLEARING THE ORBIT

One of the key criteria set by the IAU for planetary status is that a celestial object must have “cleared the neighbourhood” around its orbit, meaning it should gravitationally dominate its orbital zone. The dwarf planet shares its orbit with other objects and has not “cleared” its neighbourhood in the same way as the major planets.

While Pluto no longer holds the designation of a “planet” in the traditional sense, it is still considered a significant and fascinating member of the solar system. As a dwarf planet, it is part of a growing family of celestial objects in the Kuiper Belt, which are valuable for understanding the early solar system’s formation and evolution. The planet’s exploration by the New Horizons spacecraft in 2015 provided valuable insights into the nature of these distant icy worlds and has expanded our knowledge of the outer solar system.

CHARACTERISTICS

MASS	(1.303±0.003) ×1022 kg
VOLUME	(7.057±0.004)×109 km3
SURFACE AREA	1.774443×107 km2
MEAN RADIUS	1,188.3±0.8 km
SURFACE PRESSURE	1.0 Pa
DENSITY	1.854±0.006 g/cm3
ESCAPE VELOCITY	1.212 km/s
SURFACE GRAVITY	0.620 m/s2
ABSOLUTE MAGNITUDE	-0.44
SATELLITES	5
RINGS	NO
MEAN TEMPERATURE	-229°C
SEMI-MAJOR AXIS	39.482 AU
ORBIT PERIOD	247.94 years
PERIHELION	29.658 AU
APHELION	49.305 AU
MEAN ORBITAL VELOCITY	4.64 km/s
MAXIMUM ORBITAL VELOCITY	6.10 km/s
MINIMUM ORBITAL VELOCITY	3.71 km/s
ORBIT INCLINATION	17.16°
ORBIT ECCENTRICITY	0.2444
SIDEREAL ROTATION PERIOD	-153.2928 hours
LENGTH OF DAY	153.2820 hours
MINIMUM DISTANCE FROM EARTH	4284.7 ×106 km
MAXIMUM DISTANCE FROM EARTH	7528.0 ×106 km
MAXIMUM VISUAL MAGNITUDE	13.65

ORBIT

Pluto’s orbit is distinct and unique among the objects in our solar system.

ECCENTRICITY

The planet’s orbit is highly eccentric, meaning it is significantly elongated and deviates from a perfect circle. This eccentricity is one of the highest among all celestial bodies in the solar system. At its closest approach to the Sun (perihelion), Pluto comes about 4.43 billion kilometers (2.75 billion miles) from the Sun. At its farthest point (aphelion), it moves to approximately 7.38 billion kilometers (4.58 billion miles) away. This results in substantial variations in its distance from the Sun throughout its orbit.

INCLINATION

The planet’s orbit is also highly inclined relative to the plane of the solar system. Its orbital inclination is approximately 17.14 degrees. In contrast, the orbits of the eight major planets in our solar system lie relatively close to the solar system’s equatorial plane. The planet’s steep inclination sets it apart from the rest of the planetary system.

ORBITAL PERIOD

Pluto takes a long time to complete one orbit around the Sun. Its orbital period, or Plutonian year, is approximately 248 Earth years. This extended orbital period is a result of its vast distance from the Sun and the slow pace at which it moves along its elliptical path.

NEPTUNE CROSSINGS

Pluto’s unique orbit intersects with that of Neptune, the eighth major planet in our solar system. While Pluto is sometimes closer to the Sun than Neptune, it is currently moving farther away from the Sun and will not cross Neptune’s orbit again for several centuries.

SEASONAL VARIATIONS

The combination of Pluto’s eccentric orbit and its axial tilt leads to extreme seasonal variations on the dwarf planet’s surface. As the planet moves closer to the Sun in its orbit, it experiences short and relatively mild summers, while its long and harsh winters occur when it is farther from the Sun. These seasonal changes have a significant impact on the planet’s surface and climate.

INTERACTION WITH CHARON

Pluto’s largest moon, Charon, plays a role in its orbital dynamics. The two objects are tidally locked to each other, meaning they always show the same face to each other as they orbit. This synchronous rotation results in a shared “Plutonian day” and “Plutonian year,” with one orbit around the Sun taking the same amount of time as one rotation on their axes.

ROTATION

Pluto’s rotation is distinctive among celestial bodies in our solar system.

RETROGRADE ROTATION

The dwarf planet rotates on its axis in a retrograde motion, which means it spins from east to west. This is the opposite direction of rotation compared to most of the major planets in our solar system, including Earth. On Earth, the Sun rises in the east and sets in the west due to our prograde rotation, while on Pluto, it would appear to rise in the west and set in the east.

ROTATION PERIOD

Pluto’s rotational period, or Plutonian day, is relatively slow compared to Earth. It takes approximately 6.4 Earth days for Pluto to complete one full rotation on its axis. This extended day-night cycle contributes to extreme temperature variations on the dwarf planet’s surface.

SYNCHRONOUS ROTATION WITH CHARON

Pluto and its primary moon, Charon, are tidally synchronized with each other. This signifies that they consistently present the same side to one another while revolving around their shared center of mass. The gravitational interaction between Pluto and Charon has resulted in a synchronous rotation, where their rotational periods are equal to their orbital period around each other. As a result, one side of Pluto continuously faces Charon, and one side of Charon continuously faces Pluto.

AXIAL TILT

Pluto’s axial tilt, or the angle between its rotational axis and its orbital plane, is approximately 119.6 degrees. This axial tilt is similar to Earth’s, which is around 23.5 degrees. This tilt results in significant seasonal variations on the planet, as different parts of its surface receive varying amounts of sunlight over its long year.

EFFECTS ON CLIMATE

The planet’s retrograde rotation and axial tilt contribute to its extreme seasonal changes. As the dwarf planet orbits the Sun along its elongated path, its distance from the Sun varies, leading to significant temperature fluctuations. The combination of a slow rotation and axial tilt creates long-lasting seasons on the planet.

OBSERVATIONAL CHALLENGES

Pluto’s slow rotation and small size made it challenging to observe details of its surface from Earth before the New Horizons mission. Its surface features were difficult to discern due to the extended daylight and nighttime periods.

RELATION WITH NEPTUNE

Pluto’s relationship with Neptune is primarily defined by its orbital dynamics in the outer solar system.

ORBITAL INTERSECTION

Pluto’s orbit, while highly eccentric and inclined, crosses the orbital path of Neptune, the eighth and farthest major planet from the Sun in our solar system. This means that there are periods in Pluto’s orbit when it comes closer to the Sun than Neptune.

NEPTUNE’S GRAVITATIONAL INFLUENCE

Because Pluto’s orbit crosses Neptune’s path, Neptune’s gravitational influence has an important effect on Pluto’s orbital behaviour. The gravitational pull from Neptune affects the shape and orientation of Pluto’s orbit, causing it to be significantly perturbed.

RESONANCE WITH NEPTUNE

Pluto is in a 3:2 orbital resonance with Neptune. This means that for every three orbits Pluto completes around the Sun, Neptune completes two orbits. This resonance helps to prevent close encounters and collisions between the two bodies, as they are in stable, non-intersecting orbital configurations most of the time.

PLUTO’S ORBIT VS. NEPTUNE’S ORBIT

While Pluto crosses Neptune’s orbit, it is important to note that Pluto’s orbit is highly inclined relative to the plane of the solar system and significantly tilted concerning the ecliptic, which is the plane in which most planetary orbits lie. This orbital inclination and tilt are some of the reasons why Pluto’s path and Neptune’s path do not result in frequent close encounters.

LONG-TERM BEHAVIOUR

Over very long timescales, the gravitational interactions between Pluto and Neptune can influence Pluto’s orbital parameters, including its eccentricity and inclination. These interactions are complex and can lead to variations in Pluto’s orbital behaviour over millions of years.

CURRENT POSITION

As of the present era, the dwarf planet is moving away from the Sun and is far from its closest approach to Neptune’s orbit. It will not cross Neptune’s orbit again for several centuries.

ATMOSPHERE

The dwarf planet’s atmosphere is a thin and tenuous envelope of gases that surrounds it.

COMPOSITION

The planet’s atmosphere is primarily composed of nitrogen (N2), with smaller amounts of methane (CH4) and carbon monoxide (CO). Nitrogen makes up the bulk of the atmosphere, accounting for about 98% of its composition.

THIN AND TENUOUS

Pluto’s atmosphere is extremely thin compared to Earth‘s. At its surface, the atmospheric pressure is only about 1/100,000th that of Earth’s. This thin atmosphere makes it challenging for the planet to retain its gases, especially given its small size and low gravity.

VARIABLE THICKNESS

Pluto’s atmosphere undergoes seasonal variations in thickness due to its elliptical orbit around the Sun. When the dwarf planet is closer to the Sun in its orbit (perihelion), some of the nitrogen and other volatiles on its surface can sublimate (transition from a solid to a gas), temporarily thickening the atmosphere. As it moves farther from the Sun (aphelion), the atmosphere thins.

TEMPERATURE PROFILE

Despite the thinness of the planet’s atmosphere, it can still have a significant impact on the dwarf planet’s surface temperature. The temperature in the upper atmosphere can drop to extremely low levels, reaching as low as -230 degrees Celsius (-382 degrees Fahrenheit). The surface temperature is influenced by the sublimation and condensation of nitrogen and methane ice.

HAZE AND AEROSOLS

Pluto’s atmosphere contains complex organic molecules and small particles, which scatter sunlight and create a characteristic blue tint in images. The haze in the planet’s atmosphere is thought to be formed through photochemical processes involving ultraviolet sunlight and the breakdown of methane.

SEASONAL CHANGES

The planet’s elliptical orbit and axial tilt lead to significant seasonal changes in its atmosphere. These changes include variations in atmospheric pressure, temperature, and the distribution of gases.

ESCAPE OF GASES

Pluto’s low gravity makes it challenging for the dwarf planet to retain its atmosphere over geological timescales. Some gases, particularly methane, can escape into space over time. The interaction between the solar wind and the planet’s upper atmosphere can also lead to the loss of gases.

NEW HORIZON DISCOVERIES

The New Horizons spacecraft, which conducted a flyby of Pluto in 2015, provided valuable data on the composition and behaviour of the planet’s atmosphere. It observed the presence of numerous layers and haze in the atmosphere, helping scientists better understand its characteristics.

POTENTIAL FOR EXOTIC WEATHER

Its thin atmosphere and seasonal changes can lead to unique weather patterns. For example, the sublimation of ice can create temporary atmospheric conditions and the potential for haze or clouds.

Learn about Pluto’s Structure here.