Scientists change their minds: Pluto might be a planet

After years in ‘exile’, Pluto may be reclassified as a planet again, as the International Astronomical Union may reconsider the definition of ‘planet’.

Pluto was discovered in 1930 and ever since it has been mentioned in every science book as the ninth planet in our Solar System. Until 2006. That was the year things were about to change for the furthest discovered planet in our System, beyond Neptune.

Pluto is a planet

76 years after the discovery of Pluto was made and after years of fierce debates, scientists reached the conclusion that Pluto is much more similar to large asteroids, such as Ceres, rather than to planets. This meant that the nine-planet Solar System every generation of schoolchildren learned about had, in fact, eight planets only.

Last year, however, scientists reconsidered the definition of ‘planet’ and the new one may bring our lonely Pluto back into the fold. David Grinspoon and Alan Stern, the authors of a book entitled “Chasing New Horizons: Inside the Epic First Mission to Pluto”, were the ones that endorsed this definition.

Pluto is not a planet anymore

In 2006, the International Astronomical Union’s definition of a ‘planet’ required to meet three main characteristics in order for an astronomical object such as Pluto to be called a ‘planet’:

  1. It must be in orbit around the Sunday
  2. It must come with enough self-gravity to make a round (or at least roundish) shape
  3. The neighborhood around its orbit must be clear

Considering these characteristics and due to the fact that Pluto is surrounded with various icy Kuiper Belt objects that are similar in shape and composition to itself, the IAU reached the conclusion that Pluto simply doesn’t fit with the definition. As a result, it cannot be considered a true planet.

Pluto, a planet again

Grinspoon and Sterm agree that this definition of a ‘planet’ contained “obvious flaws” and was “hastily drawn”. According to it, some may say that Earth is not a planet due to the numerous asteroids present in its neighborhood. Additionally, this definition doesn’t consider exoplanets either and the thing is they are incredibly many and most of them are located far beyond this Solar System.

“We use ‘planet’ to describe worlds with certain qualities,” explain Stern and Grinspoon. “When we see one like Pluto, with its many familiar features – mountains of ice, glaciers of nitrogen, a blue sky with layers of smog – we and our colleagues quite naturally find ourselves using the word ‘planet’ to describe it and compare it to other planets that we know and love.”

Last March, numerous planetary scientists gathered in Houston at the annual Lunar and Planetary Science Conference. One of the presentations caught people’s attention – “A Geophysical Planet Definition”. It mentioned the following:

“In keeping with both sound scientific classification and peoples’ intuition, we propose a geophysically-based definition of ‘planet’ that importantly emphasizes a body’s intrinsic physical properties over its extrinsic orbital properties. A simple paraphrase of our planet definition – especially suitable for elementary school students – could be, ’round objects in space that are smaller than stars’.”

Obviously, not all astronomers share the same opinion, as this would mean that they should include most Moons found in our Solar System in the ‘planet’ category as well.

All in all, one thing’s for sure – the debate around whether or not to consider Pluto a planet is far from dead.