Why scientists want to revise the definition of 'planet'

Pluto-lovers everywhere may soon have cause to rejoice, as scientists consider reinstating the distant orb's planetary status.

Once known as the solar system's ninth planet, Pluto was demoted to dwarf-planet status in 2006, when the International Astronomical Union revised its definition of a planet. The news, which prompted thousands of angry letters from elementary school students, came just six months after NASA’s New Horizons space probe began a nine-year journey toward Pluto.

That decision ushered in a decade of questioning for the mission’s scientists. “Many members of the public, in our experience, assume that alleged ‘non-planets’ cease to be interesting enough to warrant scientific exploration,” they wrote. “A common question we receive is, ‘Why did you send New Horizons to Pluto if it’s not a planet anymore?’”

In its 2015 flyby, New Horizons found plenty to interest scientists, including snowcapped peaks and “ice blades.” Now, some of those scientists want to revise the definition of “planet” so as to re-admit Pluto – along with at least 110 other objects.

Their proposal, which they plan to present at the Lunar and Planetary Science Conference in March, isn’t just an effort to repair bruised egos. Our understanding of the solar system has been in flux for a quarter-century.

In 1992, astronomers at the University of Hawaii discovered the first of more than 1,000 known “Trans-Neptunian Objects” orbiting the sun beyond Neptune, in a region known as the Kuiper Belt. When one of these objects, later named Eris, was discovered in 2003, it was found to be more massive than Pluto and suggested as a possible 10th planet for the solar system.  

But these new worlds differed from the rest of the solar system in an important way. As part of the formation process, planets are generally understood to “clear their orbit:” either merge with or eject other bodies in their orbital vicinity.

The Kuiper Belt satellites – including Pluto – haven’t pulled this off as well as the Mercury-to-Neptune group has. Instead, their orbits overlap considerably. The IAU decided that this didn’t make the cut. At its 2006 General Assembly in Prague, it redefined a planet as:

“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.”

But over the next decade, this definition has weakened. New instruments like Hawaii’s Pan-STARRS telescope have found that Earth’s orbit is more crowded than we thought. By 2011, sky surveys had logged more than 1,200 “potentially hazardous” satellites within 4 million miles of Earth, prompting Discovery News to argue that Earth itself should lose its “planet” status.

The proposal’s authors also note that the requirement that a planet be “in orbit around the Sun” rules out the many worlds that have been found orbiting other stars or appear to be free-flying in the galaxy in recent years.

To clear things up, New Horizons principal investigator Alan Stern and his colleagues want “a geophysically-based definition of 'planet' that importantly emphasizes a body’s intrinsic physical properties over its extrinsic orbital properties.” As Dr. Stern puts it in layman’s terms, they would define planets simply as “round objects in space that are smaller than stars.”

This would bring in Pluto and its Kuiper Belt neighbors – along with Earth’s moon, other planetary moons like Jupiter and Europa, and, by the authors’ estimate, over 100 other bodies in our solar system alone.

For the moment, this new, expansive definition remains an idea. But it indicates that several decades of explorations and observations have shown the solar system to be a more complex place than previously thought – and left scientists eager to learn more.

Their new definition, they conclude, “highlights to the general public and policymakers the many fascinating worlds in our Solar System that remain unexplored and are worthy of our exploration, along with the necessary budgets.”