The Universe and Galaxies: Pluto

Discovery Space: Pluto

Pluto was discovered in 1930 in a review of astrophotographic images, but since 1992 more and more celestial bodies have been detected in the region of its orbit. Consequently, it is now considered only one of the many icy objects located on the far side of Neptune’s orbit.

Pluto, whose diameter is only two-thirds that of the moon’s, was previously known as the planet furthest from the sun—the ninth planet in our solar system-until August 24, 2006. On this date the International Astronomical Union redefined the concept of “planet” to include the condition that a true planet must have cleared the path of its own orbit during its development phase. Because Pluto did not do so, it is now defined as a dwarf planet, just one of many objects orbiting the sun in the so-called Kuiper belt, a disk-shaped region beyond Neptune’s orbit.

These celestial bodies are also called trans-Neptunian objects. In comparison with the planets in the solar system, Pluto’s orbit is considerably more elongated. The largest section lies beyond the nearly circular orbit of Neptune, while another part lies within it. Pluto has not yet been visited by a space probe. What we know about Pluto comes largely from observations made from the Hubble telescope, the Infrared Astronomical Satellite (IRAS) and from Earth.

According to previous findings, it is likely that Pluto consists of a rocky core surrounded by a mantle of ice. In addition, it appears to be covered with several layers of frozen methane, nitrogen, and carbon monoxide. When Pluto passes near the sun, these icy layers evaporate into gases, building an extremely thin atmosphere. These gas layers freeze again when Pluto’s orbit takes it further away from the sun.

Three moons

Pluto has at least three moons: Charon, Hydra, and Nix. Charon was discovered in 1978, but the others were not found until 2005. Charon appears to be covered with ice and is about half the size of Pluto, which is proportionally very large for a moon. These two objects revolve around a common gravitational center in the space between them, rather than a gravitational center within Pluto.

Due to the brightness of the other two moons, their diameters have been estimated to be only around 105 miles (170 km) in length. New Horizons will be the first space probe to make the journey to Pluto. The probe, which belongs to the U.S. space agency NASA, was launched in 2006 and passed very close by Jupiter the following year, using a flyby technique that utilized the planet’s gravity to boost its speed. It will not reach Pluto and its moons until 2015.

As an extension of the mission, the probe will then proceed further into the Kuiper belt in order to investigate one or more of its small celestial bodies and shed light on the nature of the trans-Neptunian objects. These celestial bodies form their own group, in addition to the terrestrial (rocky) planets and the gas giants. It is possible that after the results of the investigation are evaluated, we will understand more about the development of the solar system.


Pluto and its moons belong to the group of celestial bodies in the Kuiper belt on the far side of Neptune’s orbit (known as Kuiper belt objects, KBOs, or trans-Neptunian objects) Numerous objects consisting of rock and ice, possibly including many comet cores as well, revolve within this ring-shaped area.

It is likely that the KBOs were formed in parallel with the planets, and the gravitational forces associated with Neptune pulled a few of them into elongated orbits. The largest KBOs known so far have dimensions and characteristics similar to Pluto. One example is Eris, which is actually some- what larger than Pluto. Eris has its own moon and is also classified as a dwarf planet.


DIAMETER .it Pluto: 1.485 miles (2.390 km)

DISTANCE from Pluto to the sun 2 757-4.583 million miles (4.437-7.376 million km)

TIME it lakes Pluto to revolve around the sun: 247 years and sight months

TIME it takes Pluto to rotate once on its axis: 153 hours and 18 minutes