Pluto and beyond: Widening views of what makes a planet

Since being found in 1930 Pluto has been a headache for astronauts trying to define what a planet is. Now with a flood of discovery of earth-like planets outside of our solar system the definition of a planet is once again up for debate.

Planets outside our solar system will require new ways to define them

Pluto has been a headache for astronomers trying to define what a planet is, ever since it was found in 1930. Now with a flood of discovery of Earth-like planets outside our solar system, debate has intensified over the definition of a planet.

“Pluto was mishandled even before it was discovered,” says Gareth Williams, associate director of the International Astronomical Union (IAU) Minor Planet Centre, the organization tasked with defining what makes a planet.

The organization defines a planet as a celestial body that is spherical or nearly spherical, orbits the sun and has “cleared the neighbourhood” around its orbit, meaning that it is not surrounded by any similar objects.  

“Pluto has not cleared its orbit,” says Williams.

In 2006 the astronomical union demoted Pluto from a planet to the second-class status of dwarf planet.

Pluto's demotion came after the discovery of a number of other big rocks just like it out beyond the eighth planet, Neptune, all considered too small to be identified as a planet.

“If we treated Pluto as a planet we would have to treat all those other Plutinos, or Pluto-like objects and trans-Neptunian objects beyond Neptune as planets as well,” says Williams.

He says the Lowell Observatory in Arizona credited with Pluto’s discovery simply made an error in the telegraph that it sent to the Harvard College Observatory in March 1930 calling Pluto a planet.  

According to Williams, even with the primitive technology available at the time, scientists realized that the new so-called planet was much smaller than anticipated and was perhaps not in the same league as the other planets in our solar system.  

New definitions for an expanding galaxy

Now with a flurry of new discoveries of other possible planets outside of our solar system, scientists are re-examining Pluto’s characteristics to shed light on how to define new discoveries.  

As a flurry of discoveries of other possible planets outside our solar system are made, scientists like Dimitar Sasselov, director of Harvard’s origins of life initiative, are re-examining Pluto’s status as a planet to shed new light on how to define new discoveries. ( Jon Chase/Harvard University News Office)
“We know that our solar system is not representative of the planets that are out there. Science has discovered a large diversity of planets that we never even imagined before,” says Dimitar Sasselov, director of Harvard’s origins of life initiative and author of the book, The Life of Super-Earths: How the Hunt for Alien Worlds and Artificial Cells Will Revolutionize Life on our Planet.

“If you looked at Pluto as a member of our solar system, it is not like other planets,” says Sasselov.

“All the other planets are in an orbit that was established when they were formed, they never cross each other's orbits, but Pluto is different because it does cross the orbit of other planets,” he adds.

Sasselov says this characteristic of Pluto becomes noteworthy when looking outside our solar system. New discoveries outside of our solar system show planets that don’t orbit the stars or planets that orbit more than one star simultaneously.

“I argue that this definition doesn’t work in context of all the new planets that we are discovering now. These definitions are premature,” says Sasselov.

Sasselov says that we should abandon our “solar-system-centric perspective.”  

Williams says that new, more nuanced definitions of planets will likely emerge to address the findings beyond our solar system.

“What I suspect will eventually occur is an overarching planets category with three sub classes: classical or major planets like Mercury and Neptune, dwarf planets like Pluto and Eris and minor planets that could include asteroids and comets,” says Williams.


In the past decade, astronomers have made tremendous leaps in charting our universe. We have jumped from a list of nine known planets any school child could rhyme off to a flurry of new planets with obscure designations like 51 peg b.

Sasselov says that with this pace of discovery we are on the brink of another possible Copernican moment, such as when Copernicus and his followers realized that a better view of the solar system was to see the sun as the centre rather than the Earth.

“There are types of planets out there called Super-Earths, meaning that they are Earth-like but bigger than our Earth,” says Sasselov.

“We are now technically capable to find out if there’s life on them. And if we find life on them, that will be a shift in our very Earth-centric frame of reference," adds Sasselov.