Why wasn't Pluto considered a planet

Aha! : Why is Pluto no longer a planet?

“Planet X” - the astronomer was after him. Percival Lowell said there had to be such a planet. Otherwise the disturbances in the orbital movements of the planets Uranus and Neptune at the edge of our solar system could not be explained. The astronomer himself did not find a suitable candidate. But soon after Lowell's death, in February 1930, just 24-year-old Clyde Tombaugh observed a wandering patch of light that matched the predicted dates: Pluto.

The new one was hailed as the ninth planet. But was the celestial body, named after the Roman god of the underworld, really big enough to distract the two giants Uranus and Neptune? Then its mass should have been significantly greater than that of the earth.

As soon as it was discovered, Pluto began to shrink. For a while it was thought to be as big as the earth, but the 12,000 kilometers in diameter were far too large. In 1950 it was only half the size of the globe, today its diameter is estimated to be around 2,300 kilometers. That would make Pluto smaller than the Earth's moon. Billions of kilometers from Earth, Pluto orbits the sun once within 250 years.

In the meantime, astronomers have discovered many similar celestial bodies out there: Quaoar or Orcus, Sedna and Makemake. They are all pitch black and hardly reflect the light of the sun. Beyond Neptune lies a whole ring of dark, icy little bodies: the Kuiper belt.

"Pluto is a victim of inflation," says Jürgen Kerp from the Argelander Institute for Astronomy in Bonn. “Only its history of discovery distinguishes it from other objects in the Kuiper Belt.” However, Pluto lost its historically determined special position after researchers discovered a celestial body in the summer of 2005 that is slightly larger than it: Eris. The following year, Pluto was stripped of its planetary status. Otherwise Eris & Co. would also have had to be called a planet.

With only eight planets left, the solar system remains clear. But what is the difference between Pluto and similar celestial bodies and real planets? They must have enough gravity to dominate their surroundings and to be able to clear them of other objects, it is now said. A vague phrase. After all, it does not rule out that the number of planets will one day rise again from eight to nine, says the sky researcher Kerp: "It is assumed that there is a still undiscovered planet in the Kuiper Belt that is as big as Earth." Thomas de Padova

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