What makes Pluto the Most Fascinating Dwarf Planet in our Solar System?

Regions where water ice has been detected on Pluto (blue regions)

Pluto is an icy 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 dwarf planet.

Pluto came into science books due to the discovery by Clyde Tombaugh in the early 19th Century as the ninth planet from the Sun. After 1992, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, a dwarf planet in the scattered disc which is 27% more massive than Pluto, was discovered. This led the International Astronomical Union (IAU) to define the term “planet” formally in 2006, during their 26th General Assembly.

The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body which:

  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
  3. has “cleared the neighborhood” around its orbit.

That definition excluded Pluto and reclassified it as a dwarf planet. That definition excluded Pluto and reclassified it as a dwarf planet.

It is the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris. Like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU (4.4–7.4 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance (39.5 AU).


Pluto’s origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, Triton. This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto never approaches Neptune in its orbit.

How Long is a Day on Pluto?

Pluto is quite slower than Earth. Naturally, a day on Pluto is much longer than a day on Earth. Pluto takes 6.4 Earth days (6 days 9 hours and 36 minutes) to complete one rotation, so this is how long a day is on Pluto. After 1992, it has been proposed that Pluto may have formed as a result of the agglomeration of numerous comets and Kuiper-belt objects.

Pluto’s moons

Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. Pluto and Charon are sometimes considered a binary system because the centre of their orbits does not lie within either body.

How is Pluto Observed?

Pluto’s visual apparent magnitude averages 15.1, brightening to 13.65 at perihelion. see it, a telescope is required; around 30 cm (12 in) aperture being desirable.  It looks star-like and without a visible disk even in large telescopes, because its angular diameter is only 0.11″.

The earliest maps of Pluto, made in the late 1980s, were brightness maps created from close observation of eclipses by its largest moon, Charon. Observations were made of the change in the total average brightness of the Pluto–Charon system during the eclipses. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Computer processing of many such observations can be used to create a brightness map. This method can also track changes in brightness over time.

Atmosphere of Pluto

Pluto has a tenuous atmosphere consisting of nitrogen (N2), methane (CH4), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto’s surface. According to the measurements by New Horizons, the surface pressure is about 1 Pa (10 μbar), roughly one million to 100,000 times less than Earth’s atmospheric pressure. It was initially thought that, as Pluto moves away from the Sun, its atmosphere should gradually freeze onto the surface; studies of New Horizons data and ground-based occultations show that Pluto’s atmospheric density increases, and that it likely remains gaseous throughout Pluto’s orbit. New Horizons observations showed that atmospheric escape of nitrogen to be 10,000 times less than expected. It was also discovered that even a small increase in Pluto’s surface temperature can lead to exponential increases in Pluto’s atmospheric density; from 18 hPa to as much as 280 hPa (three times that of Mars to a quarter that of the Earth). At such densities, nitrogen could flow across the surface as liquid. Just like sweat cools the body as it evaporates from the skin, the sublimation of Pluto’s atmosphere cools its surface. The presence of atmospheric gases was traced up to 1670 kilometres high; the atmosphere does not have a sharp upper boundary.


  1. NASA
  2. New Horizons
  3. Wikipedia

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