Neptune

Neptune Eighth planet from the Sun. The mass, orbit, and position of an unseen planet had been calculated by Leverrier and, independently, by British astronomer John Couch Adams (1819–92). Neptune was first observed in 1846, and is invisible to the naked eye. Through a telescope it appears as a small, greenish-blue disc with very few details. The upper atmosphere is c.85% molecular hydrogen and 15% helium. Its predominant blue colour is due to a trace of methane, which strongly absorbs red light. Several different atmospheric features were visible at the time of the fly-by of the Voyager 2 probe in 1989. There were faint bands parallel to the equator, and spots, the most prominent of which was the oval Great Dark Spot (GDS), c.12,500km (8000mi) long and 7500km (4500mi) wide, which is a giant anticyclone. White, cirrus-type clouds of methane crystals cast shadows on the main cloud deck c.50km (30mi) below. Neptune has the highest wind speeds recorded on any planet – more than 2000km/h (1250mph) in some places.

http://lpl.arizona.edu/nineplanets/nineplanets/neptune.html; http://wr.usgs.gov