Planets beyond Neptune
Pluto loses Planet X title

Year
Mass
Notes
1931
1 Earth
Nicholson & Mayall
1942
0.91 Earth
Wylie
1948
0.1 (1/10 Earth)
Kuiper
1973
0.025 (1/40 Earth)
Rawlins
1976
0.01 (1/100 Earth)
Cruikshank, Pilcher, & Morrison
1978
0.002 (1/500 Earth)
Christy & Harrington
2006
0.00218 (1/459 Earth)
Buie et al.

Question: What were the earliest measurements of the mass of Neptune?
Answer: In 1931, Nicholson and Mayall calculated Neptune's mass to be equal to 1 Earth, a value similar to the 0.91 Earth mass calculated in 1942 by Lloyd R. Wylie.