It was in 2006 when the International Astronomical Union’s launched new criteria for determining whether a celestial being can be considered a planet or not. This resulted in the number of planets in our solar system decreasing from nine to eight, as Pluto was stripped of its status as our ninth planet based on the new criteria. It also wasn’t clear at that time whether the planets discovered outside our solar system – all 5,000 of them as of present time – were qualified to be such. But what still seems to be clear now is that Pluto still doesn’t cut the mustard as a legitimate planet.
Regardless of the IAU’s criteria, UCLA astronomer Jean-Luc Margot launched what he calls a “simple planet test” to see whether a being like Pluto is qualified to be one. Per the IAU, planets are celestial bodies that are in orbit around the sun, have enough self-gravity to remain nearly round despite body forces, and able to clear the area around its orbit. That last criteria had nullified Pluto’s planet status, instead demoting it to a “dwarf planet” in a similar fashion to Ceres, which is found in the Main Asteroid Belt. Margot, similarly, focused on the ability of a planet to rule its region of the universe when trying to ascertain the planet-hood of beings outside of our solar system.
Margot said his test can “determine whether a body can clear a specific region around its orbit within a specific time scale, such as the lifetime of its host star.” He added that it is an easy-to-implement test that allows “99 percent of all known exoplanets” to be classified either as planets or as something different. Only star mass, planet mass, and orbital period estimates are needed, with all the data quickly available through Earth- or space-based telescopes. And as it turned out, Pluto still isn’t any more of a planet than it was back in 2006.
The test, however, was frowned upon by NASA New Horizons team leader Alan Stern, who felt it lacking in several areas. “I think that there’s pretty clear appreciation among planetary scientists that Pluto is much more similar to the planets of our solar system than anything else,” he commented. “Astronomers don’t seem to understand that attributes matter.”