The largest , and the only one big enough to have the shape of a sphere is Triton , ( pronounced : Ë traÉ tÉ n ) discovered by William Lassell just 17 days after the discovery of Neptune itself . The largest by far , comprising more than 99.5 percent of the mass in orbit around Neptune and the only one massive enough to be spheroidal , is Triton , discovered by William Lassell just 17 days after the discovery of Neptune itself .