Science Byte: on Pluto

Written by Marie Brown

Why is Pluto a dwarf planet?

Pluto was, for 76 years, considered the 9th planet of the solar system, however, in 2006 Pluto was stripped of its title, downgraded to a mere ‘dwarf planet’. Despite the simple, logical reasoning behind the reclassifying of this celestial body, some are still unreasonably upset about its planetary status.

Pluto is small, only 2,370 kilometres (1,473 miles) wide, rendering it smaller than Earth’s moon, and about half the width of the United States. Nonetheless, Pluto’s size is not enough to define it as less than a planet, although its mass does play an important part in its description.

The definition of a planet was set in 2006 by the International Astronomical Union (IAU). This resolution states that, in the Solar System, a planet is a celestial body which:

  • Is in orbit around the sun
  • Has sufficient mass to assume hydrostatic equilibrium (a nearly round shape)
  • Has ‘cleared the neighbourhood’ around its orbit

A non-satellite body fulfilling only the first two of these criteria is classified as a ‘dwarf planet’.

Now back to the Pluto problem: Why classify it as a dwarf planet?

Despite its great distance from the sun, Pluto is not as isolated as it may appear; many icy entities in the region of Pluto orbit the sun in an area called the Kuiper belt. As these objects orbit in close proximity to Pluto, it has ultimately failed to meet the third requirement of planetary status – to clear the neighbourhood around its orbit.

Sorry, Pluto.

About the author

Marie Brown


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