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.
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