When the “New Horizons” NASA space probe left earth on January 19th 2006, Pluto was the 9th and final planet from the sun. When it finally arrived 9 1/2 years later in July 2015 it wasn’t! By then Pluto had been down graded to a dwarf planet. So what happened to poor old Pluto to diminish it so?

the definition of a planet was rather shockingly only formally agreed, for the first time in August 2006!

The immediate reason was that Pluto did not meet the definition of a planet that was rather shockingly only formally agreed, for the first time in August 2006! Up until then a distinctly unscientific fudge existed where no precise definition of what a plant actually was existed.

 So, What is a Planet Anyway ?

In August 2006 the International Astronomical Union (IAU) produced the following 3 conditions for an object in the Solar System to be considered a planet.

A planet is a celestial body that :
  1. Is in orbit around the Sun
  2. It must be massive enough to have become spherical under the force of its own gravity (which rules out things like asteroids and comets) d
  3. It must have “cleared its orbit”, either by absorbing other nearby objects into itself, or by kicking them out of the way with its gravity.

So at a stroke the IAU voted to kick Pluto out of the planetary club. Pluto passes the first two tests, but fails the third.

These days, it is clear that Pluto is merely one among thousands of “trans-Neptunian objects” (TNOs), itinerant hunks of rock and ice that drift around in the distant reaches of the solar system. (see our article What is a Planet? ; How should a Planet be Defined?)

 Slow Decline

To be fair the redefinition of Pluto was the result of a series of revisions of the mass of Pluto over a long time.

When Pluto was first discovered, in 1930 , its claim to ‘planethood ‘ seemed much stronger. At first, astronomers reckoned it was roughly as massive as Earth, based on calculations about how they presumed it was affecting the orbits of Uranus and Neptune. The estimate of Pluto’s mass was then repeatedly revised downwards; first to around the mass of Mars, then (after measurements of Pluto’s reflectivity) to more like 1% of the mass of Earth. The discovery of Pluto’s moon, Charon, allowed the estimate of its mass to be refined further. Today, the accepted value is about 0.2% of the mass of Earth.

As Pluto’s estimated mass fell, doubts grew about whether it should be counted as a full-blown planet. They became impossible to ignore in the mid-2000s, with the discovery of other, similarly sized objects beyond the orbit of Neptune. The final nail in “Pluto’s Planet Coffin” came in 2005 when a team at the California Institute of Technology, announced the discovery of Eris, another big TNO. Eris is almost as large as Pluto (with a diameter of 1,163 km, compared with 1,184 km for Pluto) and is about 25% more massive. If Pluto counted as a planet, there seemed no reason why Eris should not as well. And who knew how many more Eris-sized objects might lurk out in the darkness beyond Neptune? So the logical thing to do was to demote Pluto.

 Not the 1st Time This Has Happened

This isn’t the first time in history that something like this has happened. When Ceres, the most massive of the asteroids (and now another member of the club of dwarf planets), was discovered, in 1801, it too was designated a full-blown planet. Only later, as it became clear that it was merely the largest constituent of a vast asteroid belt was its planetary status rescinded.

So you could say that Pluto was kicked out of one very select club (the planets), only to join an even more select group: the club of astronomical bodies formerly known as planets.