Rebel astronomers want to make Pluto a planet again

A group of NASA scientists have proposed a new definition of a planet to bring far-flung Pluto back into the heavenly canon.

But there's perhaps a Jupiter-sized problem with their proposal - it would make the moon an official planet too.

Discovered in 1930, Pluto was considered the ninth planet of the solar system until 2006, when it was reclassified as a dwarf planet. Too many other objects just as big had been found.

In that year, the International Astronomical Union (IAU) said planets had to have "cleared the neighbourhood" in their orbit, and demoted Pluto, which shares its region of space with millions of other rocks.

But as the rebel astronomers point out, if Pluto was the size of Earth it still wouldn't be able to clear its neighbourhood. Would it still then be a planet?

Led by New Horizons mission chief Alan Stern, the group wants the definition changed to: "a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters".

In other words, something in space that's round, but isn't a star. The flaw with this definition, however, is perhaps obvious.

"With the above definition of a planet, we count at least 110 known planets in our solar system," the group says - a massive number they admit will only grow as more are discovered beyond Pluto's orbit.

"Certainly 110 planets is more than students should be expected to memorise, and indeed they ought not. Instead, students should learn only a few planets of interest.

"For an analogy, there are 88 official constellations and 94 naturally occurring elements, yet most people are content to learn only a few. So it should be with planets."

All the "planets" under the proposed new definition under 100km in diameter (Planetary Society)
All the "planets" under the proposed new definition under 100km in diameter (Planetary Society)

While the nine-planet solar system might appeal to those who grew up with it, they don't think kids will have much trouble adjusting to 110.

"Teachers may introduce new moon planets to their students with phrases such as, 'In the 2020s, NASA will send a spacecraft to study the planet Europa, which orbits around Jupiter as one of its many moons.'"

The call for Pluto's promotion comes the same day NASA announced the discovery of seven new exoplanets which could harbour life.