Neptune
Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. Nearly 4.5 billion kilometers from the Sun, Neptune orbits the Sun once every 165 years. It is invisible to the naked eye because of its extreme distance from Earth. It is a little smaller than Uranus, so its scale size also about the size of a large grapefruit.
Info from NASA.gov
Neptune is one of the two planets that cannot be seen without a telescope. The other is Pluto. Neptune is about 30 times as far from the Sun as is Earth. Pluto is the only planet farther from the Sun than Neptune. But every 248 years Pluto moves inside Neptune's orbit for about a 20-year period, during which it is closer to the Sun than Neptune. Pluto last crossed Neptune's orbit on Jan. 23, 1979, and remained within it until Feb. 11, 1999. [+]Read more at NASA.govSun | Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune | Pluto