It was 15 years ago this month that once certified member of the solar system Pluto was kicked out of the club as it was downgraded to dwarf planet status. And a lot of people are still pretty upset about it.

Even Dr Mike Brown, the man credited with it – or culpable for it – accepts the blame. Indeed, his Twitter handle is PlutoKiller. Enough said.

So why was Pluto kicked out of the solar system?

Pluto as photographed in 2015 by NASA’s New Horizons spacecraft. Picture: NASA/AFPSource:AFP

The discovery of Pluto was announced with much excitement and fanfare back on February 18, 1930. It was found by astronomer Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona, with contributions from William H. Pickering.

Meanwhile a schoolboy growing up in the US in the 1970s fell in love with “this funny little oddball at the edge of the solar system”.

“It seemed like the coolest place to think about, literally, in the solar system,” Dr Mike Brown told the BBC. Dr Brown is better known as the Pluto Killer. He even titled his memoir How I Killed Pluto And Why It Had It Coming.

But it was that cool kid in the solar system which inspired the astronomer to seek out other objects like it.

“It was very clear to I think everyone that it was likely that there were objects larger than Pluto to be found out there.

“So in the early 2000s I started searching the skies with these brand new digital cameras and we started finding bigger and bigger and bigger objects.”

Among the many objects he saw in the sky was the planet which was named Eris, discovered on January 5, 2005.

Eris was discovered to be larger than Pluto. Picture: NASA/APSource:News Limited

If it hadn’t been for Eris, Pluto could possibly have been swept under the rug forever. There was no reason to change the solar system. The problem is, after they discovered Eris and they thought Eris was more massive than Pluto even later it was proved that it wasn’t.

The International Astronomical Union (IAU) voted to downgrade Pluto to dwarf planet status in August 2006. But it was a close thing.

Many objects further out than Pluto have been found in the Kuiper Belt.Source:Supplied

Ultimately Pluto was reclassified to being a dwarf planet because it did not meet the three criteria the IAU uses to define a full-sized planet.

So, the three criteria of the IAU for a full-sized planet are:

  1. It is in orbit around the Sun.
  2. It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
  3. It has “cleared the neighbourhood” around its orbit.

Essentially Pluto meets all the criteria except one—it “has not cleared its neighbouring region of other objects.”

Pluto meets only two of these criteria, losing out on the third. In all the billions of years it has lived there, it has not managed to clear its neighborhood. You may wonder what that means, “not clearing its neighboring region of other objects?”