Studies on Galileo's notebooks reveal that he may have discovered Neptune in 1612, 234 years before its official discovery by Urbain Le Verrier. According to David Jamieson, head of the Melbourne University (MU) School of Physics, evidence of his discovery is buried in notebooks that were written while observing the moons of Jupiter in the years 1612 and 1613. During such observations, he also recorded the position of a nearby star which does not appear in any modern star catalog. Jamieson says
It has been known for several decades that this unknown star was actually the planet Neptune. Computer simulations show the precision of his observations revealing that Neptune would have looked just like a faint star almost exactly where Galileo observed it.
After observing the star for several nights, Galilleo had found that the star appeared to have moved relative to an actual nearby star. A star would never change its position while planets orbit the Sun and move through the sky relative to the stars. According to Jamieson, this gives further evidence that he had actually observed Neptune.
Neptune is the farthest known planet in the solar system after Pluto lost its planet status in 2006. Neptune is never visible to the naked eye.