In two weeks, NASA's New Horizons mission will fly by Pluto, after spending nearly a decade in space. Pluto, as you may recall, was stripped of its planetary title back in 2005, for reasons that some scientists think are bogus. Now, they're hoping to welcome Pluto back to the planetary club.

In 2005, scientists discovered a large, rocky object close to Pluto. The discovery of Eris put scientists in an intergalactic pickle—should they make Eris the tenth planet in our solar system, or sacrifice Pluto and make them both dwarf planets? Because Eris is larger than Pluto, it would be inconsistent for scientists to leave well enough alone.


In 2006, the International Astronomical Union (IAU) finally reached a decision. They issued an official definition of a planet:

"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."

… and an official definition of a dwarf planet:

"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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite."

Plut is not a satellite, meaning it doesn't orbit another planet. It orbits around the sun, like other planets, and it is round.


Part (c) is where Pluto fails to make the cut. IAU's definition put the issue to rest: Eris is in the "neighbourhood around Pluto's orbit," and vice versa.

Pluto is a dwarf planet, Eris is a dwarf planet, we have eight planets in our solar system, case closed.

Except, the decision turned out to be pretty emotional, and not all that scientific. For some, the decision was a triumph. "Pluto is dead," Astronomer Mike Brown—one of the scientists responsible for ousting Pluto from our solar system—said upon hearing the IAU's decision. Brown eventually wrote a book called How I Killed Pluto: And Why It Had It Coming. His Twitter handle is @plutokiller. He's really into having helped put the kibosh on Pluto as a planet.

On the other side of the spectrum we find Alan Stern, New Horizon's principal investigator. According to Stern, Pluto's always been a planet to him. He told NewsWorks: "I don't know a single expert, a single planetary scientist, that thinks that the astronomers made a good definition…So we pretty much ignore it."

Basically, Stern adds, a planet is a large, round object in space: "We recognize big, round objects, rounded by gravity, as planets. It's about that simple." Plus, he says, once we see close-up images of Pluto it will be really hard for us not to think of it as a planet.

Former NASA scientist Phil Metzger laid the situation out rather bluntly in an interview with Deutsche Welle. When asked, "Do you think we will call Pluto a "planet" again in the near future?" he answered:

"We are free to call it a planet right now. The planetary science community has never stopped calling bodies like Pluto "planets."

Metzger said that Pluto planet advocates should start "calling Pluto a planet right now," adding: "Add to the consensus, because that's how science makes progress, by one person at a time being convinced of the truth and adopting it."

You heard him, Pluto Truthers. Make your move.

Danielle Wiener-Bronner is a news reporter.