Luinil was one of the bright stars placed in the sky by Varda at the time of the awakening of the Elves in Middle-earth. Its identity is uncertain, though the fact that its name contains the Elvish word luin, meaning 'blue', does offer a clue.
The only remote explanation offered by Tolkien was a rough and uncertain note that seems to link Luinil to the planet we call [en.wikipedia.org/wiki/Neptune Neptune]. As Christopher Tolkien notes, though, this dark and distant object, too faint to be seen by the naked eye, would hardly qualify as a 'bright star'. Another alternative would be blue [en.wikipedia.org/wiki/Rigel Rigel] in [en.wikipedia.org/wiki/Orion Orion], making Luinil the mate of red Borgil. Two other possibilities - based purely on the fact that they are both bright blue stars - would be [en.wikipedia.org/wiki/Spica Spica] in [en.wikipedia.org/wiki/Virgo Virgo] or [en.wikipedia.org/wiki/Regulus Regulus] in [en.wikipedia.org/wiki/Leo Leo].