rakshathedemon shared this article with me.http://the-toast.net/2016/01/19/the-illegitimacy-of-aragorn/
I found it an amusing read though I don't agree with it.
Aragorn was also descended from Anarion through Firiel and there were no other heirs. He proved his claim by summoning the Army of the Dead, which only Isildur's heir could do and also had the heirlooms and healing powers of Elendil's line.
I see Aragorn also as a Messiah like figure whose destiny and greatness were prophesied. Gandalf's support and the finding of the White Tree suggest he is supported by the Valar.
It should be remembered too that Aragorn is only crowned after the people support his claim.