Is Pluto a planet or not?

NASA's New Horizons spacecraft is the first probe sent to Pluto, and it's scheduled to arrive in July 2015. This is an artist's concept of the spacecraft flying past Pluto.

New Horizons was launched from Kennedy Space Center in Florida on January 19, 2006, on a 10-year, 3-billion-mile journey to Pluto.

This was the best view we had of Pluto and its moon, Charon, before the New Horizons mission. The image was taken by the European Space Agency's Faint Object Camera on the Hubble Space Telescope on February 21, 1994.

NASA's Hubble Space Telescope discovered two more moons orbiting Pluto in 2005. The moons, called Nix and Hydra, are about two to three times farther from Pluto than Charon. Charon was discovered in 1978.

A white arrow points to Pluto in this photo taken by New Horizons Long Range Reconnaissance Imager on September 21, 2006. Pluto was about 2.6 billion miles (4.2 billion kilometers) from the spacecraft.

On its way to Pluto, New Horizons snapped these photos of Jupiter's four large "Galilean" moons (from left): Io, Europa, Ganymede and Callisto.

New Horizons snapped this photo of Jupiter and its volcanic moon Io in early 2007.

New Horizons crossed the orbit of Neptune on August 25, 2014, the last planet it will pass on its journey to fly by Pluto on July 14, 2015. New Horizons took this photo of Neptune and its large moon Triton when it was about 2.45 billion miles from the planet -- more than 26 times the distance between the Earth and our sun.

• Pluto was demoted to dwarf planet in 2006


• Harvard held a forum to debate Pluto's status


• The public says Pluto should be reinstated

(CNN) -- Poor Pluto. Is it or isn't it a planet?

Ever since astronomer Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona, discovered it on February 18, 1930, we've believed that we live in a solar system with nine planets.

Then, along came the International Astronomical Union, the group that gets to name planetary bodies. In 2006, it came up with some rules for what is and is not a planet. The group decided Pluto didn't make the grade.

It was demoted to a dwarf planet, leaving our solar system with just eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

The union says a planet is a celestial body (PDF) that:

1. Orbits the sun 2. Is round or nearly round3. Has cleared the neighborhood around its orbit

Pluto orbits the sun, and it's round. It got kicked out as a planet because of rule No. 3: The astronomical union said Pluto was too small to knock other space rocks out of its path as it orbits the sun.

But the group's definitions -- and the public's attachment to tiny Pluto -- sparked lots of debate. On September 18, the Harvard-Smithsonian Center for Astrophysics jumped into the debate: What is a planet? It had some experts discuss the definition of a planet and then let the audience vote. Guess what? They voted that Pluto is a planet.

One person who never accepted Pluto's demotion is Alan Stern, principal investigator for the New Horizons spacecraft, the first spacecraft sent to Pluto. He said Pluto was given the boot because of its distance from the sun.

"In fact, if you put Earth where Pluto is, it would be excluded!" Stern said. "Any definition of planethood that excludes Earth, in any circumstance, is deeply flawed. After all, if there is any object everyone agrees is a planet, it's Earth."

And he agrees with those in the audience at that Harvard panel discussion.

"I think the public is better suited to this than astronomers, at least," Stern said. "The IAU should never have pretended to have the expertise to enter into this debate. It's a matter for planetary scientists, not astronomers."

Don't forget that the other eight planets also have their differences: Mercury, Venus, Earth and Mars are rocky; Jupiter and Saturn are gas giants; and Uranus and Neptune are ice giants.

"This is really about a revolution in planetary science," Stern said. "We're seeing that what we used to know about the number and variety of planets was very data limited until the mid- to late 1990s. Now we know that there are lots of types of planets."

NASA's decades-old Voyager 2 mission is a preview of new mission to Pluto

When New Horizons arrives at Pluto in July, it might not end the debate over Pluto's status as a planet, but Stern says the mission is "going to be mind-blowing."

The spacecraft has to survive what Stern calls the "seven weeks of suspense," dodging asteroids as zips toward Pluto. Then, we'll finally get a detailed, amazing photos of the tiny world. Planet or not, Pluto is finally coming into focus.