The Former Ninth Planet

Pluto’s status as the ninth planet from the sun has been a topic of scientific debate for decades, captivating the imaginations of astronomers and laypeople alike. Once considered a full-fledged member of our solar system’s planetary family, Pluto’s classification took a dramatic turn in 2006 when the International Astronomical Union (IAU) reclassified it as a dwarf planet. But what led to this change, and what exactly does it mean to be a dwarf planet? Embark on a journey to understand the fascinating story behind Pluto’s new designation.

The Defining Characteristics of a Planet

Before we delve into Pluto’s reclassification, let’s establish what defines a planet. According to the IAU, a planet must meet three criteria:

  1. It orbits the Sun.
  2. It is spherical or nearly spherical in shape.
  3. It has cleared its orbit of other objects.

Meeting these criteria ensures that a celestial body is gravitationally dominant in its orbital neighborhood, a property known as "clearing the neighborhood."

Pluto’s Controversial Orbit

Pluto’s orbit stood out as an anomaly among the other planets. Its elliptical path brought it closer to the sun than Neptune at times, challenging the traditional view of planetary orbits. Additionally, Pluto’s highly inclined orbit tilted it significantly from the ecliptic, the plane in which the other planets orbit. These orbital peculiarities hinted that Pluto might not fully satisfy the criteria of «clearing the neighborhood.»

The Discovery of Pluto’s Celestial Neighbors

As astronomers continued to explore the outer reaches of our solar system, they discovered numerous icy bodies similar to Pluto. These objects, collectively known as Kuiper Belt objects, shared similar orbits with Pluto, suggesting that they might be part of a larger population. The presence of these celestial neighbors raised questions about whether Pluto was unique enough to be considered a full-fledged planet.

The IAU’s Decision: Demoting Pluto

In 2006, the IAU convened a meeting to address the growing debate surrounding Pluto’s status. After careful consideration, the organization introduced a new category called «dwarf planet,» defined as celestial bodies that meet two of the three criteria for planethood but have not cleared their neighborhood. Pluto, along with Eris, Ceres, Haumea, Makemake, and several other objects, were reclassified as dwarf planets.

Defining Dwarf Planets

Dwarf planets occupy an intermediate category between planets and small solar system bodies like asteroids and comets. They exhibit the following characteristics:

  1. Orbit the Sun.
  2. Have sufficient mass to be spherical or nearly spherical.
  3. Have not cleared their neighborhood of other objects.

Pluto fits neatly into this classification, as it meets the first two criteria but has not cleared its orbit of other Kuiper Belt objects.

Pluto’s Unique Features

Despite its reclassification, Pluto remains a fascinating and unique object in our solar system. It possesses several distinctive features that set it apart from other dwarf planets:

  • Eccentric Orbit: Pluto's elliptical orbit takes it from just inside Neptune's orbit to as far as 49 astronomical units (AU) from the sun.
  • Tilted Axis: Pluto's axis of rotation is tilted by over 120 degrees, giving it extreme seasonal variations.
  • Icy Composition: Pluto is primarily composed of rock and ice, with a thin atmosphere of nitrogen, methane, and carbon monoxide.
  • Multiple Moons: Pluto has five known moons, the largest of which is Charon, which is nearly half the size of Pluto itself.


Pluto’s journey from a full-fledged planet to a dwarf planet highlights the evolving nature of scientific understanding. As our knowledge of the solar system expands, so too does our understanding of its celestial inhabitants. Pluto, with its unique characteristics and intriguing history, serves as a compelling reminder that the universe is full of surprises.

Frequently Asked Questions

  1. Why was Pluto reclassified as a dwarf planet?
    Pluto was reclassified as a dwarf planet because it meets only two of the three criteria for planethood: it orbits the sun and is spherical. However, it has not cleared its neighborhood of other objects, as its orbit is populated by numerous Kuiper Belt objects.

  2. What other objects are classified as dwarf planets?
    Besides Pluto, Eris, Ceres, Haumea, and Makemake are also classified as dwarf planets. These objects share similar characteristics with Pluto, such as orbiting the sun, being spherical, and not clearing their neighborhood.

  3. Is Pluto still considered a planet by some astronomers?
    There is an ongoing debate among some astronomers regarding the definition of a planet and whether Pluto should be reinstated as a full-fledged planet. However, the IAU's definition of a planet remains the standard classification used by the scientific community.

  4. What are the unique features of Pluto?
    Pluto has several distinctive features, including its eccentric orbit, tilted axis, icy composition, thin atmosphere, and multiple moons. Its largest moon, Charon, is nearly half the size of Pluto itself, making the Pluto-Charon system an intriguing binary system.

  5. Can Pluto be seen with a telescope?
    With a large enough telescope, it is possible to see Pluto as a faint dot in the sky. However, due to its distance from Earth, it appears very small and requires specialized equipment to observe.



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