The portrait was painted just after Newton published Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy") (1687) which presented Newtonian physics and the first physically-correct description of Solar System (in the Book III: De mundi systemate (On the System of the World):

Newton was the greatest genius that ever existed, and the most fortunate, for we CANNOT find more than once a system of the world to establish.
---Joseph-Louis Lagrange (1736--1813) quoted from Joseph-Louis Lagrange Quotes.