Why is Pluto not a planet

(ORDO NEWS) — Pluto was discovered on February 18, 1930 by the American astronomer Clyde Tombaugh at the Lowell Observatory (Flagstaff, Arizona, USA). For a long time, Pluto was considered a full-fledged planet, like the other eight planets that we knew existed.

But as telescopes became more powerful, astronomers began to find many other large objects – in the asteroid belt between the orbits of Mars and Jupiter and in the Kuiper belt beyond the orbit of Neptune – similar in size to Pluto.

As long as these objects were smaller than Pluto, nothing threatened its status.

But when cosmic bodies were discovered that seemed larger than Pluto, it became obvious that it was necessary to clarify which objects should be classified as planets and which should not. Surprisingly, up to this point, we did not have an accurate definition of the planet.

The word “planet” comes from the Greek “wandering star” and originally referred to bright objects in the sky that behaved differently from other stars. People of the past knew only about Mercury , Venus , Mars , Jupiter and Saturn , planets that could be seen with the naked eye (Uranus was discovered in 1781, and Neptune in 1846).

Scientists did not need to clarify the term “Planet” until large and powerful telescopes were at their disposal, which made it possible to detect many pluton-like objects moving around the Sun. On August 24, 2006, after much thought and discussion, the International Astronomical Union decided to define the planet.

To be a full-fledged planet, any object must meet three criteria:

  • The object must revolve around the Sun (if it revolves around the planet, then we have a satellite);
  • The object must have sufficient mass to be in a state close to hydrostatic equilibrium (have a spherical shape);
  • The object must clear its orbit of other objects (either by crashing into them and absorbing them, or by throwing them aside with its gravity).
Why is Pluto not a planet 2
The Sputnik Planitia is an ice plain on the surface of Pluto, with a diameter of 1492 kilometers

Dwarf planets meet two of the three criteria – they orbit the Sun and have a spherical (or almost spherical) shape. But what they failed to do was clear their orbit of other objects.

So, objects that are large enough to become spherical under the influence of gravity, but that share their orbit with other bodies, are called dwarf planets. At the time of writing, five dwarf planets are officially recognized by the International Astronomical Union: Ceres , Pluto, Haumea, Makemake, and Eris.


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