Although Planet Nine has not been detected, many people have asked whether it would qualify as a planet. The theoretical estimates of the orbit and mass of the putative planet (Batygin and Brown 2016, Batygin et al. 2019, Brown and Batygin 2021) indicate that Planet Nine 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 suggested that the putative planet is 5-10 Earth masses and resides 400-800 astronomical units from the Sun. The lowest Pi value for this range is 2.2, almost 100 times larger than Pluto's Pi value.

Brown et al. 2021 suggested that the putative planet is 6.2+2.2−1.3 Earth masses and resides 380+140−80 astronomical units from the Sun. The lowest Pi value for this range is 3.5, more than 100 times larger than Pluto's Pi value.

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.

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