Full Idea
What Aristotelian essentialism says is that you can have open sentences Fx and Gx, such that ∃x(nec Fx.Gx.¬nec Gx). For example, ∃x(nec(x>5). there are just x planets. ¬nec(there are just x planets)).
Gist of Idea
Aristotelian essentialism says a thing has some necessary and some non-necessary properties
Source
Willard Quine (Three Grades of Modal Involvement [1953], p.176)
Book Reference
Quine,Willard: 'Ways of Paradox and other essays' [Harvard 1976], p.176
A Reaction
This is a denial of 'maximal essentialism', that all of a things properties might be essential. Quine is thus denying necessity, except under a description. He may be equivocating over the reference of 'there are just 9 planets'.