Washington, August 13 : A scientist has come out with a new definition for a planet, according to which, our solar system would have 13 planets in total.
According to Mark Sykes, director of the Planetary Science Institute in the US, if a non-stellar object is massive enough to be round and orbits a star, it ought to be a planet.
The key here is that once an object gets that big, important geophysical processes begin.
Under this scenario, the smallest known planet in the solar system would be Ceres, the largest and most massive object in the asteroid belt between Mars and Jupiter.
By round, Sykes doesn't mean perfectly spherical.
In scientific terms, he's talking about the shape of bodies that are in "hydrostatic equilibrium," where the pressure from an essentially fluid interior is balanced against gravity and centrifugal force at the body's surface to round the object.
This makes planetary objects fundamentally different from small, irregularly shaped asteroids and comets.
Sykes takes into account planets across the universe in general, about Pluto and other small planets in particular, and about planets in the distant reaches of our solar system yet to be discovered.
In 2006, the International Astronomical Union (IAU) devised new classifications for solar system objects and proclaimed there are only eight planets in our solar system - excluding several, including Pluto, and effectively slamming the door on future discoveries of planets beyond Neptune.
But, shortly after the IAU issued its rules for defining planets, Sykes, along with another planetary scientist, Alan Stern, former associate administrator for NASA's Science Mission Directorate, wrote a protest petition that was subsequently signed by hundreds of scientists who disagreed with the IAU action.
"The IAU damaged the public perception of science by the high-profile spectacle of imposing, by vote, a controversial definition of a commonly used term," Sykes said.
Under the planet definition supported by Sykes, our solar system would have 13 planets, although more might be found in the future beyond the orbit of Pluto.
They are: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, Eris, and recently discovered Makemake.
Pluto and Charon would be considered a "double planet" because they both satisfy the criterion for being massive enough to be round, and orbit about a point in space between them as they orbit the Sun.