Would Planet Nine pass the planet test?

Although Planet Nine has not been detected, many people have asked whether it would qualify as a planet. Using the theoretical estimates of the orbit and mass of the putative planet (Batygin and Brown 2016), it is possible to show that it satisfies a quantitative criterion for orbit-clearing (Margot 2015). According to this criterion, Planet Nine will qualify as a planet, if and when it is detected.

Batygin et al. 2019 revised their estimates for the range of masses and locations of Planet Nine. They suggest that the putative planet is 5-10 Earth masses and resides 400-800 astronomical units from the Sun. All of these possibilities still pass the planet test.

Planet Nine dynamically dominates its orbital zone

Objects above the solid line are planets: they have sufficient mass to dynamically dominate their orbital zone. The ability of a planetary body to clear its orbital zone can be quantified with the criterion Π. The solid line represents Π = 1. Objects above the line satisfy Π > 1 (planets) and objects below the line have Π < 1.

For further reading

  • J. L. Margot. A Quantitative Criterion for Defining Planets. Astronomical Journal 150, 185, 2015.