In The Silmarillion (3, Of the Coming of the Elves and the Captivity of Melkor), we're told how Varda '...made new stars and brighter against the coming of the Firstborn...', and Nénar is listed third among those new bright stars. Taken by itself, this reference would seem to imply that Nénar was one of the brightest stars (or possibly planets) in the sky.
In his Index to volume 10 to The History of Middle-earth, however, Christopher Tolkien presents notes that seem to unambiguously associate several of Varda's new stars with planets of the Solar System. In this scheme Nénar was originally 'N' (Neptune) but this was cancelled in favour of Luinil. This left the planet Nénar unidentified, a fact which suggests a connection with Uranus (the only remaining unidentified planet in this scheme).
An obvious problem with this is that, even allowing the convention that lists Uranus as a star, it is by no means bright; indeed, under most conditions it is invisible without the aid of a telescope. To speculate, this perhaps implies that Tolkien originally envisioned Nénar as a bright star, then later co-opted the term when he wished to associate names with the planets.