Planets then and now
Planets, the "wandering stars"
The original term "planet" comes from the Greek word "lunatic star" - describing a „wandering star" in the apparently unchangeable pattern of the constellations. According to this definition, one also counted sun and moon at that time. These "Mighty 7" were considered mighty gods, to each of whom was assigned one of the seven days of the week. Only later it was noticed that these wandering stars/ planets do not orbit around the earth as originally assumed, but in elliptical orbits around the sun and that the earth itself is a planet, whereas the sun and moon itself are separate categories of objects.
Until the invention of the telescope, the number of planets remained small: Mercury, Venus, Earth, Mars, Jupiter and Saturn. The telescope was used to discover Uranus in 1781 and Neptune in 1846 - both are large, icy gas planets. In between was the discovery of Ceres (1801), a rock between Mars and Jupiter that seemed to fill the gap between these two planets. Ceres was first classified as a planet. Only when many more rocks were found in this area (which is now known as the asteroid belt) was this assignment removed.
Due to irregularities in the orbits of Uranus and Neptune, the search for a responsible celestial body began on the edge of the then known solar system. The calculations by the American astronomer Percival Lowell were particularly pioneering. When he died in 1916, they formed the basis for further Planet hunting endeavors . Until the young Clyde Tombaugh finally found what he was looking for on February 18, 1930. In search of another planet that was supposed to move against the fixed stars, he compared hundreds of photo plates and came across the new "wandering star" in the constellation Gemini the twins.
On March 13, 1930, on the 75th birthday of Percival Lowell’s, who had died in 1916, the discovery of the 9th planet and its name “Pluto” were announced to the public. The name follows the mythological system of naming for planets: Pluto was named after the god of the underworld. It is the Roman equivalent of the Greek god Hades. With a diameter of around 2,300 kilometers, Pluto proved to be the smallest planet - but was still much larger than Ceres (952 kilometers).
NASA New Horizons Mission
On January 16, 2006, NASA's "New Horizons" spacecraft was launched. A bold project, propelled and managed by Dr. Alan Stern to explore Pluto and other transneptunian objects. After nearly ten years of flight, the probe reached the point of closest approach to Pluto on July 14, 2015. At a distance of approximately 12,500 kilometers, spectacular high-resolution images could be made for the first time in the history of astronomy. The mission then continued to explore other Kuiper belt objects. On January 1, 2019, the probe flew past Arrokoth (originally known as "Ultima Thule") at a distance of only 3,500 kilometers. The probe is currently almost seven billion kilometers from the sun.
How many planets?
"Mein Vater erklärt mir jeden Sonntag unsere neun Planeten ." With this beautiful German sentence, since the discovery of Pluto (1930), children and parents have been able to memorize the sequence of the nine planets of our solar system in the correct order, starting from the central sun (the initial letters stand for the initial letters of the respective planet). But it didn't stop there. Other celestial bodies, similar to Pluto, were discovered - including "Eris", almost as large as Pluto. Now good advice was needed: either you had to accept objects like Eris as a planet or Pluto had to contest the status of the planet. But with what reason? Should an arbitrary size limit be introduced?