海衛八是由旅行者2號在1989年飛經海王星所攝取的圖像而發現的。 它當時被暫命名為S/1989 N 1。和於1989年7月7日的IAUC 4806會議上宣布了海衛八的發現。根據公布說明的「21天中獲取的17幅圖像」來推算,海衛八的發現日期是6月16日略前。海衛八在1991年9月16日被正式命名。[9]
^ In the earlier papers slightly different dimensions were reported. Thomas and Veverka in 1991 reported 440×416×404 km.[3][4] Croft in 1992 reported 430×424×410 km.[5] The difference is caused by the use of different sets of images and by the fact that the shape of Proteus is not described well by a triaxial ellipsoid.[3]
^ The mass was calculated by multiplying the volume from Stooke (1994)[3] by the assumed density of 1,300 kg/m3. If one uses slightly larger dimensions from the earlier papers the mass will increase to 5×1019 kg.[6]
^Surface gravity derived from the mass m, the gravitational constantG and the radius r: Gm/r2.
^Escape velocity derived from the mass m, the gravitational constantG and the radius r:2Gm/r