The Solar System currently has eight recognized planets but before 2006 when Pluto was demoted to a dwarf planet, school children were taught that there were nine planets in our star system.
Now, as a group of scientists proposes a new way to classify planets, our celestial neighborhood may possibly have more than 100 objects that could be called planets.
Demoting Pluto To Kuiper Belt Object Status
The International Astronomical Union (IAU) changed the definition of a planet in 2006. Pluto's demotion is attributed to discoveries that show it is actually a Kuiper Belt Object or KBO.
Much of the Kuiper Belt was yet unknown when Pluto was discovered in 1930 and included as the ninth planet of the Solar System. Skeptics, however, eventually emerged after the discovery of several objects in the Kuiper Belt whose sizes are comparable to that of Pluto's. Skeptics pointed out the existence of bigger objects in Pluto's surrounding.
The discovery of Eris, a dwarf planet 27 percent larger than Pluto led IAU to come up with a formal definition of a planet in 2006 during its 26th General Assembly.
What Is A Planet?
One of IAU's new criteria for defining a full planet requires having a clear neighborhood around the body's orbit.
"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," reads the IAU criteria.
Because part of this formula requires that a planet and its natural satellite move alone through their orbit, Pluto is not classified as a planet and is now considered as a mere dwarf planet.
Scientists Propose New Criteria For A Planet
Kirby Runyon of Johns Hopkins University and colleagues proposed that the factors that need to define whether or not a celestial object is a planet should rely on the body itself and not just other things such as location. Runyon said that no planet has actually totally cleared its orbit. Jupiter, the largest planet in the Solar system, for instance, even has clouds of asteroid.
Runyon and colleagues defined a planet as "a sub-stellar mass body that has never undergone nuclear fusion." The object also needs to have enough gravitational heft to retain a roughly round shape. The group's definition is different from that of the IAU definition in that it did not make reference to the surroundings of the celestial body.
More Than 100 Planets In The Solar System
Based on the team's definition, Jupiter's moon Europa and even the Earth's moon would be classified as a planet. Both moons are larger than Pluto, which Runyon and colleagues claim to be no less of a planet than Mars, Jupiter, Neptune, and Earth.
Although it would likely take a long shot to have the team's version of planetary definition to be adopted, its official acceptance could significantly increase the number of recognized planets in the solar system from eight to about 110.