Seems like an easy question, right? It was, until the 1990s. Before the 1990s, a planet was defined as any large body orbiting the sun that was not an asteroid or moon. But then astronomers starting discovering objects in the Kuiper Belt – a disk of icy bodies in the furthest reaches of our solar system.
We thought Pluto was unique. An icy world beyond four rocky worlds and four massive gas giants. Then, scientists began discovering other worlds even further than Pluto. Sedna is almost as big as Pluto. Eris is. Pluto wasn’t the unique icy world we thought. It was just one icy world among dozens, perhaps even hundreds of others.
It was these discoveries that trimmed the number of planets in our solar system to 8. In 2006, the International Astronomical Union (IAU), the governing body responsible for astronomical definitions and naming objects, defined what a planet is for the first time.
(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.
For dwarf planets, the main sticking point is: has not cleared the neighborhood around its orbit. By the IAU’s definition, Pluto became a dwarf planet.
What about exoplanets?
In recent years, the number of exoplanets discovered has skyrocketed. And, there isn’t an official definition on what to call them. Remember, the IAU definition only covers planets “in orbit around the Sun.” This has created a “definitional limbo,” says UCLA professor Jean-Luc Margot.
Margot’s test needs just three measurements. Estimates of the star’s mass, the planet’s mass and the orbital period. Margot needed a simpler method so planets could be classified using measurements by Earth-based or space-based telescopes.
“One should not need a teleportation device to decide whether a newly discovered object is a planet,” says Margot.
And when you apply Margot’s method to our solar system, nothing changes. We still have 8 planets, and Pluto is on the outside looking in.
“The disparity between planets and non-planets is striking,” Margot said. “The sharp distinction suggests that there is a fundamental difference in how these bodies formed, and the mere act of classifying them reveals something profound about nature.”
What about exoplanets? Margot’s method places every every ‘confirmed’ or ‘candidate’ Kepler object in the planet category. That’s not a surprise. The ability for a planet to “clear the neighborhood around its orbit,” is directly tied to its mass. And nearly all Kepler exoplanets are much more massive than Earth.
Margot also touches on the spherical requirement. “When a body has sufficient mass to clear its orbital neighborhood, it also has sufficient mass to overcome material strength and pull itself into a nearly round shape,” he said.
Will Margot’s method be adopted by the IAU? The IAU is a governing body, so don’t expect changes anytime soon. The earliest we could see a change is at the next IAU general assembly in 2018.
And I don’t care what anyone says. Pluto is a planet to me, dammit.