When NASA's New Horizons Pluto probe launched on January 19, 2006, Pluto was still officially considered a planet.
By the end of 2006, it was no longer ranked alongside Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. It had been downgraded to the status of a dwarf planet. Now, as the world celebrates the, some are asking: Is it time to bring Pluto back to the planet fold?
Pluto was officially discovered in 1930 by Clyde Tombaugh, and was initially thought to have a mass equivalent to that of Earth. It was, therefore, reasonable to declare Pluto the solar system's ninth planet.
Over the years, though, Pluto's mass was found to be smaller and smaller. In 1948, astronomer Gerard Kuiper revised its mass. It was, he said, just one tenth the mass of Earth. In 1978, this was revised to 1/500 of Earth's mass. In 2006, the final number was 1/459.
But the final nail in the coffin was the discovery of Eris, a trans-Neptunian object -- that is, a minor planet that orbits the sun at a greater average distance than Neptune -- in 2005, by astronomer Mike Brown. Eris's mass was 27 percent greater than Pluto's, and it was thought to be larger than Pluto.
This meant one of two things: Eris could also be declared a planet or Pluto could be stripped of its planet status. In the 2006 General Assembly of the International Astronomical Society, a consortium of International Astronomical Union members laid out a new definition for planets:
A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
Pluto, it was declared, did not meet the third criterion. It shares its orbital neighbourhood with other objects in the area of the Solar System beyond Neptune called the Kuiper belt.
A vote was counted. 237 members voted in favour of downgrading Pluto to the definition of dwarf planet, 157 against, 17 abstaining.
"Pluto is dead," Brown said, announcing the verdict that would earn him the nickname Pluto killer. "Pluto is not a planet. There are finally, officially, eight planets in the solar system."
When compared with other objects in the solar system, the verdict seems to make sense.at 1,473 miles (2.370 kilometres), making it bigger than Eris, which clocks in at 1,445 miles (2,326 kilometres). But even with its revised size it is still smaller than Earth's moon, which comes in at 2,159 miles (3,475 kilometres).
However, Alan Stern, leader of the New Horizons mission, disagreed with the verdict.
"This definition stinks, for technical reasons," he told Space.com in 2006. "It's patently clear that Earth's zone is not cleared. Jupiter has 50,000 trojan asteroids."
A new petition on Change.org hopes to restore Pluto's planetary status. The petition claims that the IAU meeting in August would be a good time to revisit Pluto's planetary classification.
"This is about much more than planetary definitions. When this new definition of a planet was instituted, future generations lost millions of dollars in funding for space exploration," the petition reads.
"If thousands of people demand that Pluto be reclassified as a planet at the same time as New Horizons sends down the data to prove it, we can reignite a generation of future planetary exploration."
Emily Lakdawalla of the Planetary Society, however, has compiled a handy infographic of the non-planet objects in the solar system that have been visited by spacecraft, such as the moon and dwarf planet . Some are even the focus of upcoming missions, such as and . Many are larger than Pluto.
It may seem counter-intuitive classifying something smaller than a moon as a planet, but Stern has an answer to that: reclassifying large moons as "satellite planets." This would put them in the planet category -- which, in turn, would make it more reasonable for the solar system's dwarf planets, which are smaller but not satellites, to be considered planets. This could mean Ceres, Eris, Haumea and Makemake -- the solar system's four other known dwarf planets -- would also be eligible for planet status.
But Brown, who in 2006 said the decision was "the right thing to do," remains firm. He estimates that there are 376 objects in the outer solar system that could be dwarf planets. If the definition of what constitutes a planet changes to incorporate dwarf planets, that could mean hundreds of new planets in the solar system alone.
And he maintains that Pluto doesn't need to be classified as a planet to be worth our time and attention.
"Every time someone says Pluto looks really interesting thus should be a planet, [Saturn's moon] Titan sheds an alkane tear," he said on Twitter.