The International Astronomical Union defined in 2006 what it takes to be a planet: the object needs to be in orbit around the Sun, it needs to be a nearly round shape, and it needs to gravitationally dominate its neighborhood – or what astronomers like to call “clear its orbital neighborhood.” The catch? This definition does not apply to exoplanets, planets outside of our solar system. With the recent flood of exoplanets discovered, now numbering over 5,000, there is a pressing need to develop a new definition and classification system.
In a recent article in the Astronomical Journal, I proposed a planet test that is easy to implement and allows for the immediate classification of 99 percent of all known exoplanets. This test requires estimates of the star’s mass, the planet’s mass and orbital period – all of which can be obtained with Earth-based or space-based telescopes. Using these three estimates, it is straightforward to determine whether a celestial body can clear its orbital neighborhood within the relevant time frame, i.e., the lifetime of its host star.
In the article, I show that a celestial body that has sufficient mass to clear its orbital neighborhood – and therefore pass the planet test – also has sufficient mass to overcome material strength and pull itself into a nearly round shape. This finding is important because it is not possible to obtain images of exoplanets with sufficient resolution to evaluate roundness, the second requirement in the International Astronomical Union’s planet definition. To obtain the necessary images, we would need a teleportation device. My proposal is to drop the roundness requirement from the definition, because gravitationally dominant bodies will almost certainly be round anyway.
When applying the new approach to the Solar System, the disparity between planets and non-planets is striking – clearly placing eight planets into one distinct category and all other bodies – including Ceres, Pluto and Eris – into another.
I do not know whether the International Astronomical Union will consider my proposal for a new classification system. Its adoption would require several steps, including evaluation by at least one committee and vote during a general assembly – the next one being in 2018. I do hope that my proposal will prompt the Union to extend the planet definition to exoplanets – hopefully in a way that does not require teleportation.
Jean-Luc Margot is an Astronomer, Professor, and department Chair for the UCLA Department of Earth, Planetary, and Space Sciences. His interests include dynamics, geophysics, and geodesy of planetary bodies, the architecture of planetary systems, and the search for extraterrestrial intelligence, or SETI.