There were once nine planets. Everyone learned them, sometimes aided by a mnemonic: “My Very Excellent Mother Just Sent Us Nine Pizzas.”

But back in 2006, the International Astronomical Union (IAU), arbiter of what is and what isn’t a planet, stripped Pluto of its status, saying it was too small to pack sufficient gravitational punch. It was downgraded to a new, second-class status: “dwarf planet.”

So then there were eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, or “My Very Excellent Mother Just Served Us Nachos.”

The decision did not sit well with the public. Some amateur stargazers and some astronomers thought it rather arbitrary. As the Harvard-Smithsonian Center for Astrophysics put it in a press release, “a dwarf fruit tree is still a small fruit tree, and a dwarf hamster is still a small hamster.”

But recently the Harvard-Smithsonian Center did something about it: It held a debate — pro and con — and let the audience vote. The result: “Pluto IS a planet.”

The debate centered around the IAU’s demands of a planet — that it must:

be in orbit around the Sun,

be round or nearly round, and

be shown to have “cleared the neighborhood” around its orbit, be gravitationally dominant in its area — the big kid on the block.

Pluto was originally kicked out because it did not “clear the neighborhood.” It is indeed small. It has a radius of about 750 miles — less than 20 per cent of the Earth’s radius. Its circumference is about 4,500 miles, which makes it smaller than the moon. You could fly around its equator faster than flying from Washington, DC, to Hawaii.

NASA: Space in pictures Show all 12 1 /12 NASA: Space in pictures NASA: Space in pictures NASA A false colour image of Cassiopeia A comprised with data from the Spitzer and Hubble Space Telescopes and the Chandra X-Ray observatory Nasa NASA: Space in pictures NASA The Barred Spiral Galaxy (NGC 6217) in the Ursa Minor constellation is pictured in Space NASA: Space in pictures NASA A team of astrophysicists has detected so-called gravitational waves – predicted by Albert Einstein a century ago – which are the first tremors of the Big Bang when time and space began about 13.7 billion years ago Rex NASA: Space in pictures NASA The barred spiral galaxy M83, also known as the Southern Pinwheel. The Hubble photograph captures thousands of star clusters, hundreds of thousands of individual stars, and 'ghosts' of dead stars called supernova remnants NASA: Space in pictures NASA Acosmic creepy-crawly known as the Tarantula Nebula in infrared light NASA: Space in pictures NASA A spiral galaxy ESO 373-8 - together with at least seven of its galactic neighbours, this galaxy is a member of the NGC 2997 group NASA: Space in pictures NASA A massive galaxy cluster Abell 2744, according to NASA these are some of the faintest and youngest galaxies ever detected in space NASA: Space in pictures NASA A giant cloud of solar particles, a coronal mass ejection, explodes off the sun, lower right, captured by the European Space Agency and NASA's Solar and Heliospheric Observatory NASA: Space in pictures NASA Current conditions of the quiet corona and upper transition region of the Sun NASA: Space in pictures NASA First color image of the Earth taken by the Apollo 8 astronauts in 1968 NASA: Space in pictures NASA Fog forming over the the US Great Lakes area and streaming southeast with the wind. A swirling mass of Arctic air moved south into the continental United States NASA: Space in pictures NASA Astronaut Mike Hopkins, Expedition 38 Flight Engineer, is shown in the second of two spacewalks designed to allow the crew to change out a faulty water pump on the exterior of the Earth-orbiting International Space Station

According to a release from the centre, Owen Gingerich, who chaired the IAU planet definition committee, presented the historical viewpoint. He said Pluto is a planet, and “a planet is a culturally defined word that changes over time.” Gareth Williams, associate director of the Minor Planet Center, presented the IAU’s viewpoint — that Pluto is not a planet. And Dimitar Sasselov, director of the Harvard Origins of Life Initiative, “presented the exoplanet scientist’s viewpoint.”

Sasselov argued, among other things, that the criteria for planethood was sun-centric, excluding planets beyond our solar system, or so-called “exoplanets.” He offered an alternative definition: A planet, he argued, is “the smallest spherical lump of matter that formed around stars or stellar remnants.” That opened up a lot of possibilities, one of them clearly being the reinstatement of Pluto.

When the unrecorded voice vote was taken, Sasselov’s new definition prevailed.

Of course, the vote doesn’t bind anyone. But, for Pluto enthusiasts, it’s a start.