Why is the task of astronomy difficult to accomplish

A matter of definition: what is a planet?

What conditions does a celestial body have to meet in order to be called a planet? There has only been agreement on this since 2006

For a long time people referred to all points of light that wandered across the night sky as planets (Greek. planáomai = to wander around) - regardless of whether it was Venus, Mars, the moon or asteroids. In modern times, the title was only allowed to be used by the great celestial bodies that circled the sun but were not moons - that is, they did not orbit another planet.

When astronomers began to discover new objects on the outskirts of the solar system from 1992 onwards, some of them similar in size to Pluto (up until then the ninth planet), the International Astronomical Union felt compelled to define for the first time what exactly a planet is.

After heated discussions, the astronomers passed resolution B5 in 2006. Accordingly, a planet must meet three criteria: It must orbit the sun. It must have enough mass that it has assumed an almost round shape under its own gravity. And it must have cleared the area around its orbit.

Objects that come close to it on its orbit are "swallowed" in a collision or thrown into another orbit. Pluto, Eris and other large celestial bodies are now counted among the dwarf planets because they do not manage to clear their orbit, but instead share them with other objects. As things stand, eight planets orbit the sun.

Astronomers divide them into the four terrestrial planets Mercury, Venus, Earth, Mars (they are often called stony planets because of their solid surface) and the four Jovian - Jupiter-like - planets Jupiter, Saturn, Uranus, Neptune (due to their composition often as Called gas planets or gas giants).

Uranus and Neptune are sometimes also described as “ice giants”, as they contain less hydrogen than Jupiter and Saturn, but more frozen methane, water and ammonia.

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