How the orbits change when the planet is charged

Discovered 90 years ago: Initiative wants to make Pluto a planet again

Discussions in astronomy are seldom as emotional as the classification of the planet that was once furthest from the sun: Since 2006 the International Astronomical Union (IAU) introduced a new definition for planets, Pluto is "only" a dwarf planet. The decisive factor for the "degradation" was the fact that was established at the time that planets are only considered as such if they have cleared their orbits of other objects, which is not the case with Pluto in the opinion of many astronomers.

Since then, those responsible for this decision have been exposed to downright hostility, especially in the USA. For example, the California Institute of Technology scientist Mike Brown was one of the operators of the downgrade and, ironically, tweets under the name "Plutokiller". To this day he has had to put up with rude insults from Pluto fans online. To mark the 90th anniversary of the discovery of Pluto, the Hamburg planetarium has now started a counter-initiative, "Pluto for Planet".

Pluto Festival in Flagstaff

The distant ice world was tracked down by Clyde Tombaugh on February 18, 1930 at the Lowell Observatory in Flagstaff, Arizona, which is holding its own Pluto festival on the anniversary of its discovery. "The discovery of Pluto was of great importance as it was the first foray into the Kuiper Belt in the outer regions of the solar system," emphasizes IAU spokesman Lars Lindberg Christensen. The Kuiper Belt beyond the planet Neptune is a kind of icy archive. Millions of primeval fragments from the early days of the solar system cavort in it - some of them of similar size and mass as Pluto, as we know today.

"Pluto was our first indication that the solar system has an enormous pool of frozen celestial bodies that were left over from the formation of the planets," says US astronomer Mike Brown, whose discovery of the dwarf planet Eris had given the final impetus to downgrade Pluto . "Although it took more than 60 years to understand its meaning, we now know that this collection of celestial bodies contains important references to our earliest history and the events since then."

Underestimated for the size

Upon discovery, Pluto was classified as the ninth planet in our system. At that time, however, astronomers also considered it to be significantly larger, at least as big and heavy as the earth. The Mars researcher Percival Lowell (1855–1916) had already postulated in 1905 that there must be a planet beyond Neptune whose gravity should be responsible for slight irregularities in Neptune's orbit.

Lowell looked in vain for this "Transneptune" at his own observatory. His successor Vesto Slipher finally hired the young amateur astronomer Tombaugh in 1929 to continue the search. In fact, the then 24-year-old came across the previously unknown celestial body after just a few months - a coincidence, as it later turned out. Because contrary to the original assumptions, Pluto is tiny, only about a third as big and a fifth as heavy as our moon and therefore cannot influence Neptune's orbit. His position could therefore not be calculated from the observation of Neptune.

The news of the discovery, announced on March 13, 1930, on Lowell's 75th birthday, aroused great interest worldwide. After all, it was the first addition to the planetary system since Neptune had been spotted 84 years earlier. Pluto achieved a kind of pop star status: Walt Disney named the cartoon dog of his character Mickey Mouse after the new planet, the element plutonium owes its name to it and even an Antarctic glacier is named after him.

Baptized by an eleven year old girl

The name Pluto came neither from Tombaugh, who made a career as an astronomer and university professor after the discovery, nor from any other scientist, but from an eleven-year-old girl from England: Venetia Phair from Oxford had the discovery in 1930 from her grandfather at breakfast in the newspaper get read aloud. "I had read about the Greek and Roman legends in children's books and knew the solar system and the names of the other planets," the 87-year-old Phair told the US space agency Nasa in 2006. "So I thought this name didn't exist yet." Phair's grandfather shared the idea with an astronomer friend, who passed it on to the discoverers at the Lowell Observatory.

For 76 years, Pluto was considered the ninth planet in our solar system. During this time, however, astronomers came across more and more similarly sized objects in the icy climes of the Kuiper Belt. The discovery of Eris, as big as Pluto and even a little heavier, ultimately required a decision: either Eris was the tenth planet, or Pluto had to lose its planetary status. The general assembly of the IAU in Prague in 2006 decided to downgrade and introduced the new group of dwarf planets. These are celestial bodies that, in contrast to full-fledged planets, have not cleared their orbit of other large objects.

"Pluto for Planet"

To this day, parts of the public in particular, but also some researchers, are struggling with this downgrade. "The current definition of the IAU is based on a snapshot of the planetary system that is neither meaningful nor scientifically advanced," argues the Hamburg planetarium director Thomas Kraupe on the website of his initiative, plutoforplanet.de. "We are committed to preventing young people's spirit of research from being pushed in an arbitrary manner. Countless other planets beyond Pluto are waiting to be discovered and explored." Kraupe advocates creating a subclass of dwarf planets within the planets, just as gas giants and rock planets are already there.

The chief scientist of the Pluto probe New Horizons, Alan Stern from the US space agency Nasa, would like to see Pluto continue to be part of the official group of planets. And Nasa boss Jim Bridenstine recently admitted at the 70th International Astronautics Congress in Washington: "I'm here as Nasa boss to tell you that I believe that Pluto is a planet, and I will continue to tell everyone that Pluto is a planet. " The findings from New Horizons, which visited Pluto in 2015, required a reassessment of the rating, he argued.

Diverse landscapes

In fact, the NASA probe revealed an astonishingly varied landscape on Pluto, which in its diversity can certainly compete with parts of the earth: On Pluto there are lowlands, glaciers and mountainous mountains made of water ice, reddish shimmering organic compounds, probably an active geology , Nebulae in an extremely thin atmosphere and possibly even an underground ocean. The small world is therefore more diverse and more active than expected. However, whether this is a criterion for a scientific reassessment remains open. "The IAU has not received any formal requests to change Pluto's status or the definition of the planet," reports spokesman Lindberg Christensen.

"The correct category for Pluto is not simply a problem of language, but a really profound question of classification," emphasizes Eris discoverer Brown. "Classification is important. It's the first thing scientists do when trying to understand something. Good classification leads to good questions, like why the solar system has eight dominant planets and a large collection of smaller objects while poor classification obscures understanding. " (red, Till Mundzeck, APA, February 18, 2020)