Yes, Pluto is a planet, says a NASA scientist at the site of its discovery 91 years ago this week

Is Pluto a planet? This is one of the most controversial astronomical issues since the degradation of the then ninth planet to the status of a “dwarf planet” was voted at the meeting of the International Astronomical Union (IAU) in August 2006.

Scientific planetary scientists not only completely ignored the IAU’s definition of the planet 15 years ago, but the use of voices made science act arbitrarily and politically, undermining confidence in science itself.

So says Dr. Alan Stern, a planetary scientist leading NASA’s New Horizons mission that explored Pluto’s 2015 system. He spoke at the I Heart Pluto Festival 2021, a virtual program of lectures and events organized by the Lowell Observatory in Flagstaff, Arizona – a look at Pluto’s discovery on February 18, 1930 by astronomer Clyde Tombaugh.

Stern’s argument against the decision to link Pluto to the status of a “dwarf planet” boils down to this: it makes no scientific sense.

The IAU dropped Pluto after a flurry of new discoveries of small planets in the outer solar system – especially Eris 2005 – so the IAU felt it had to create a firmer, more exclusive definition of the planet:

  1. It orbits the Sun.
  2. It has enough mass (and therefore gravity) to be round.
  3. He “cleaned up the neighborhood” around his orbit.

Pluto does not knock that last frame because it was influenced by Neptune’s gravity. It also shares its orbit with other objects in the Kuiper Belt.

Thus Pluto was reclassified as a “dwarf planet” to sit next to Eris, Ceres, Haume, and Makemake.

Stern thinks the definition of a planet is poorly worded. “The definition of the IAU was created by non-experts – astronomers – who study stars, galaxies and black holes,” he said. “They forgot.”

He thinks the IAU exaggerated because they were appalled by the idea that there could be hundreds of small planets outside the orbit of Neptune in the Kuiper belt. “The purpose of the IAU in creating this definition was to limit the number of planets in our solar systems, so that school children would not have to remember long lists of planets,” he said. “Astronomers are frightened of an astronomically large number.”

This is a shame because one of the revolutions in planetary science in the last 40 years is the discovery that the Kuiper belt – that “third zone” in the solar system outside the orbit of Neptune – is occupied by comets, planetesimals and small planets like Pluto. “They are now more numerous than the terrestrial and gaseous planets, and it is expected that there will be hundreds of them when the research is completed,” Stern said.

He also points out that the solar system is full of asteroids to the extent that no celestial body has “cleansed the neighborhood” around its orbit.

Therefore, planetary researchers ignore the IAU definition of planets in favor of a geophysical definition that is completely agnostic for the total number of planets in the solar system:

  1. It has enough mass (and therefore gravity) to be round.
  2. There is not enough mass in its interior for nuclear fusion.

In this way, says Stern, Pluto easily qualifies as a planet – like all “dwarf planets”.

However, Stern also accused the IAU of harming science itself when it publicly voted to eject Pluto in 2006. “Voting is a terrible mechanism for dealing with science,” he said. “We are not voting on the theory of relativity. We are not voting on quantum mechanics. The image of the IAU that voted was the only most damaging pedagogical event in science in probably a century, as it was easy for many people to come to the conclusion that science is arbitrary or political, and it is not. ”

The argument about Pluto relates to semantics. After all, where astronomers draw the line between planets, dwarf planets, planetoids, and moons is basically arbitrary and means nothing in reality. Even Stern’s main speech – provocatively titled “Why Pluto is a planet, a disgrace to the IAU and why it came to them” is an invitation to the book “How I Killed Pluto and Why It Came”, by Mike Brown, one of Eris’s discoverers.

Stern’s argument is only that science should be agnostic. There are countless stars and countless planets – and who cares? It’s just data, “he said. “We have to give up the old notion of the 20th century that we have to remember the names of all of them – this is a great science and there is a lot of data. As scientists, we are reductionists and we want to have classification systems, but we don’t need memory – it’s collecting stamps. ”

I wish you clear skies and wide eyes.