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 the world is 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.But why was Pluto kicked out of the solar system anyway? Were nine planets just one planet too many? Or was there a little more to it?Darling of the solar systemThe 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.It was a time of intense planet hunting; with explorers having mapped the continents on Earth, they turned instead to mapping the stars.And the possibility of a ninth planet in our own solar system – known as Planet X – was the most sought-after prize.The Lowell Observatory had been hunting for the illusive planet since at least 1909, with founder Percival Lowell dying empty-handed in 1916.But Lowell had actually seen Pluto, although he didn’t realise it. Unknown to him, on March 19, 1915, his observatory had captured two faint images of Pluto.They weren’t the first to photograph the planet. At lease 16 photos of Pluto have been identified that predate its official discovery. Pluto was there all along. They just didn’t realise the gravity of what they were looking at.Then in 1930, 23-year-old astronomer Clyde Tombaugh was tasked with taking photos of the night sky a few days apart so that a comparison could see which images were moving.“I looked in there [the telescope] and I spied it almost immediately, and a tremendous thrill came over me and I almost shook,” he later said.If he was excited about the find, well the world was over the moon.God of the underworld is prize of the skiesAs discoverers of the new planet, it was the Lowell Observatory’s privilege to give it a name.It opened it up to suggestions. More than 1000 names flooded in from across the globe.But one, from 11-year-old British girl Venetia Burney, was deemed the best.Venetia, from Oxford in England, had a keen interest in mythology. She suggested therefore Pluto, Roman god of the underworld. She thought it suited a dark and cold world on the edge of the solar system.She received £5 as a reward (given by her grandfather), as well as, presumably, the immense satisfaction of having named the ninth planet in our little corner of the universe.The name had one other advantage to it. The first two letters were the initials of Percival Lowell, who had spent so much of his life devoted to finding the planet.Venetia was still alive in 2006 when Pluto was told it was off the solar system team.Then Venetia Phair, a retired schoolteacher, aged 87, wasn’t too upset at the planet’s demotion.“It’s interesting isn’t it, that as they come to demote Pluto, so the interest in it seems to have grown,” she said in 2006.“At my age, I’ve been largely indifferent to [the debate] though I suppose I would prefer it to remain a planet.”The little planet that couldn’tMeanwhile 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.“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. It was fine, even if it didn’t make any scientific sense. The problem is, after we discovered Eris and realised Eris is more massive than Pluto, you gotta do something.”The International Astronomical Union (IAU) voted to downgrade Pluto to dwarf planet status in August 2005. But it was a close thing.“At one point, it seemed pretty likely the answer was going to be Pluto stayed a planet, Eris was a planet because Eris is more massive than Pluto, and many of the other things that I had also discovered beside Eris would eventually become planets too.“I would then be the only living planetary discoverer. What I felt about that was fraudulent.The correct solution is to draw the line that should have been drawn to begin with and say there are these eight planets and all these small icy things out beyond Neptune are in a different class of objects; they were all part of the Kuiper Belt,” Dr Brown said.Hit the galactic highway, PlutoPluto was a member of the planet club for just 76 years. It takes 248 Earth years for the planet to orbit the sun – for one Pluto year.In its life, it is a mere blip. It played with the cool kids for a second but it will one day be a brief memory. Indeed, does Pluto really care at all?Kids are now taught about the “eight planets” in the solar system. There are catchy songs teaching the names. But they end with Neptune.But that isn’t something kids care about, according to Dr Brown.“It’s not the schoolkids who were mad,” he said. “It’s the adults who remember themselves as schoolkids being mad. The schoolkids were delighted.“I mean, what’s more fun than coming home from school and telling your parents what they learnt in school was wrong? Kids love that,” he told the BBC.So how does he feel about “killing” the much-loved member of the solar system?“It’s not even that Pluto was demoted. It’s that Pluto was finally classified correctly.”