Pluto is a dwarf planet that was once considered the ninth planet of our solar system. This distant, icy world offers unique insights into the outer reaches of our solar system.
- Pluto is the ninth or eighth large body in the Solar System. Its position relative to Neptune varies.
- Pluto is smaller than Earth’s Moon.
- Because Charon is nearly half the size of Pluto, they orbit as a binary system.
- One day on Pluto is around 6.4 Earth days, while a year is about 248 Earth years.
Discovery of Pluto
Pluto was discovered on February 18, 1930, by Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona. The search for a ninth planet, known as “Planet X,” was initiated by Percival Lowell in the early 20th century. Tombaugh’s systematic photographic search led to the identification of a moving object that was later confirmed to be Pluto.
Naming Pluto
After its discovery, the newly found celestial body needed a name. The Lowell Observatory got over a thousand candidates for names. The top choices were Minerva, Cronus, and Pluto. The name “Pluto” was suggested by Venetia Burney, an 11-year-old schoolgirl from Oxford, England. She proposed the name after the Roman god of the underworld, aligning with the tradition of naming planets after mythological figures. The name was officially adopted on March 24, 1930.
Size, Distance, and Location
Pluto is much smaller than the planets in our solar system. It has a diameter of about 2,377 kilometers (1,477 miles), making it roughly 18.5% the size of Earth. It resides in the Kuiper Belt, a region of the solar system beyond Neptune filled with small icy bodies. Pluto’s average distance from the Sun is about 5.9 billion kilometers (3.7 billion miles).
Why Pluto is a Dwarf Planet
In 2006, the International Astronomical Union (IAU) reclassified Pluto from a planet to a dwarf planet. The criteria for a full-fledged planet include orbiting the Sun, being spherical due to its own gravity, and having “cleared the neighborhood” around its orbit. While Pluto meets the first two criteria, it does not clear its orbit of other debris, leading to its reclassification.
Orbit and Rotation
Pluto has an eccentric and inclined orbit, varying in distance from the Sun between 4.4 billion kilometers (2.7 billion miles) at perihelion and 7.4 billion kilometers (4.6 billion miles) at aphelion. This eccentricity means that Pluto is sometimes closer to the Sun than Neptune. Pluto’s rotation period is about 6.4 Earth days, and it rotates on its side with an axial tilt of 119.5 degrees. The axial tilt relative to the planet’s north pole and the ecliptic plane (the plane of Earth’s orbit around the Sun) is about 57.5 degrees. Like Venus and Uranus, Pluto spins from east to west with retrograde rotation.
Moons of Pluto
Pluto has five known moons: Charon, Styx, Nix, Kerberos, and Hydra. The moons take their names for mythological figures connected with the underworld. Charon is the largest, with a diameter of 1,212 kilometers (753 miles), making it nearly half the size of Pluto. The other moons are much smaller, with diameters ranging from 10 to 40 kilometers (6 to 25 miles).
Orbital Mechanics with Charon
Pluto and Charon share a unique relationship known as a binary system. They orbit a common center of mass located outside of Pluto, which causes them to show the same face to each other—a phenomenon known as mutual tidal locking.
Potential for Life on Pluto
The extreme cold and lack of liquid water on Pluto make it an unlikely candidate for life as we know it. Surface temperatures average around -229°C (-380°F). However, there may be subsurface oceans, which could potentially harbor microbial life.
Rings Around Pluto
Currently, there is no evidence that Pluto has rings. However, there is a possibility of small, transient ring systems, especially given the complex gravitational interactions with its moons.
Formation of Pluto
Pluto likely formed about 4.6 billion years ago during the early solar system’s formation. Researchers think it coalesced from the solar nebula, a disk of gas and dust left over from the Sun’s formation, in a region now known as the Kuiper Belt.
Composition and Surface Features
Pluto consists primarily of rock and ice. Its surface features include vast plains of nitrogen ice, mountain ranges made of water ice, and regions covered with methane and carbon monoxide ices. One of the most striking features is the heart-shaped Tombaugh Regio.
Atmosphere of Pluto
Pluto has a thin atmosphere consisting mainly of nitrogen, with traces of methane and carbon monoxide. This atmosphere is seasonal and collapses as Pluto moves further from the Sun, freezing onto the surface. This is similar to the behavior of comet atmospheres.
Magnetosphere
Pluto does not have a significant magnetosphere. If it has a magnetic field, it is too weak to be detected by current instruments.
Missions to Pluto
The most significant mission to Pluto was NASA’s New Horizons, which flew by Pluto on July 14, 2015. This mission provided unprecedented detailed images and data about Pluto and its moons, greatly enhancing our understanding of the dwarf planet.
Unanswered Questions
Despite the wealth of data from New Horizons, many questions about Pluto remain. Scientists are curious about the exact nature of its subsurface ocean, the detailed composition of its atmosphere, and the full extent of its geological activity.
How to See Pluto
Pluto is so small and distant that it is not visible to the naked eye. It has an apparent magnitude of around 14, while the human eye ideally sees objects of about 6 under ideal dark-sky conditions. However, Pluto is visible through a telescope with an aperture of at least 8 inches (20 cm) and a magnification of at least 150x. You still need ideal conditions, including clear skies and minimal light pollution. Use astronomy software or a star chart, since picking the dwarf planet out against faint stars poses a challenge.
Table of Key Pluto Facts
Here’s a table of key facts about Pluto:
| Feature | Details |
|---|---|
| Discovery | Clyde Tombaugh, February 18, 1930 |
| Naming | Venetia Burney, March 24, 1930 |
| Minor Planet Category | Dwarf Planet |
| Diameter | 2,377 km (1,477 miles) |
| Aphelion | 7.4 billion km (4.6 billion miles) |
| Perihelion | 4.4 billion km (2.7 billion miles) |
| Eccentricity | 0.2488 |
| Orbital Period | 248 Earth years |
| Inclination | 17.16 degrees |
| Rotation Period | 6.4 Earth days |
| Axial Tilt | 119.5 degrees |
| Surface Gravity | 0.62 m/s² |
| Average Surface Temp. | -229°C (-380°F) |
| Atmosphere Composition | Nitrogen, methane, carbon monoxide |
| Largest Moon | Charon (diameter: 1,212 km / 753 miles) |
| Number of Moons | 5 (Charon, Styx, Nix, Kerberos, Hydra) |
| Distance from the Sun | 5.9 billion km (3.7 billion miles) (average) |
| Geological Features | Nitrogen ice plains, water ice mountains |
| Missions | New Horizons (flyby: July 14, 2015) |
| Potential for Life | Unlikely, but possible subsurface ocean |
| Magnetosphere | No significant magnetosphere detected |
References
- Bierson, Carver; et al. (2020). “Evidence for a hot start and early ocean formation on Pluto”. Nature Geoscience. 769 (7): 468–472. doi:10.1038/s41561-020-0595-0
- Buie, Marc W.; Grundy, William M.; Young, Eliot F.; et al. (2006). “Orbits and photometry of Pluto’s satellites: Charon, S/2005 P1, and S/2005 P2”. Astronomical Journal. 132 (1): 290–298. doi:10.1086/504422
- Nimmo, Francis; et al. (2017). “Mean radius and shape of Pluto and Charon from New Horizons images”. Icarus. 287: 12–29. doi:10.1016/j.icarus.2016.06.027
- Stern, S.A.; Grundy, W.; McKinnon, W.B.; Weaver, H.A.; Young, L.A. (2017). “The Pluto System After New Horizons”. Annual Review of Astronomy and Astrophysics. 2018: 357–392. doi:10.1146/annurev-astro-081817-051935
- Tombaugh, Clyde W. (1946). “The Search for the Ninth Planet, Pluto“. Astronomical Society of the Pacific Leaflets. 5 (209): 73–80.