The title of "planet" seems to be as rock solid as the surface of Neptune these days.

Pluto? Leave your membership card at the front desk. Ceres? Kicked out of the club over 150 years ago.

As science progresses and evidence becomes more concrete, the planetary model of the solar system has shrunk since 1800. Ceres was reclassified as an asteroid in the 1850s and Pluto as a dwarf planet in 2005.

But with the recent discovery of what people are calling Planet X or Planet 9, is the planetary model about to add a new member or is it even appropriate to call this new discovery a planet?

Not quite yet, says Robert Cockcroft, an astronomer and McMaster University lecturer.

"I do roll my eyes a bit (about the discovery of a new "planet") because there have been suggestions before, and each one comes to nothing. I'm approaching it with a heavy dose of skepticism," said Cockcroft.

"I'm totally excited if it turns out to be true though."

And to help quell any future overreactions to new discoveries, Cockcroft has developed a visual flowchart to help define a planet.

planet definition chart

(McMaster University)

The chart goes through a number of questions and yes/no responses that either eliminate the possibility or keep it alive. Answering "no" to the question of "Are You Round" results in the answer of "Sorry not a planet," while answering "sometimes" to the question results in the answer of "You're an asteroid. Get away from us!"

As the chart illustrates, there are three qualifiers that an object must meet in order to satisfy the International Astronomical Union's (AIU) definition of planet:

  • It's big enough to pull itself into a spherical shape
  • It orbits the sun
  • Is it big enough to have cleared celestial debris from its orbit

Planet X satisfies the first two of the three qualifiers. It is roughly ten times the size of the earth, and at that size, "it would have to be round," said Cockcroft. Science also suggests that it is orbiting the sun.

What scientists have not yet concluded is whether Planet X has cleared any celestial debris from its orbit or whether there are other objects in its orbit as well, he said.

Too specific of a definition?

The problem, Cockcroft continued, is that the IAU's definition of a planet is too specific. As more technology has improved, scientists have found that the definition does not accurately reflect all of the bodies currently defined as planets in our solar system

"As with any scientific process, the more data you get, the more you have to question whether we're doing this properly," he said.

In the case of Jupiter, it is large enough that it would have cleared everything in its orbit, but it has collected asteroids – known as Trojan asteroids – in its orbit, he said. Even the Earth has objects cross into its orbit every so often.

The definition also doesn't account for the type. The inner half of planets — Mercury, Venus, Earth and Mars — are rocky, while the outer half — Jupiter, Saturn, Uranus and Neptune — has gaseous surfaces. But Planet X likely falls in between those types, said Cockcroft.

The Unknown

Cockcroft put emphasis on the word likely, mostly because Planet X is 25 times farther away from Earth than Pluto. It's still never been seen, and its size and orbit are based on hypothesis.

Still, people gravitated towards the news, and were quick to proclaim a new entry into our planetary solar system. Cockcroft believes that it's people's sense of wonder and natural excitement to discovery that led to the proclamations of a new planet.

"It's because it's within our backyard," he said. "It captures our own interest, and if we find something else in our solar system, there's a chance we could go explore it."

Cockcroft, too, admits he still has a lot of wonder of what's out there in the far reaches of the galaxy.

"My gut instinct: it's very likely that there's lots of material we haven't discovered.

Whether that material is a planet or not is still to be determined – perhaps with a flowchart.