1930: Astronomers at the Lowell Observatory in Flagstaff, Arizona, discover a ninth planet in the solar system and name it Pluto. It is the first "new" planet since Neptune was sighted in 1846. The discovery is made with mankind's most-advanced tool for space exploration: a telescope.

Today: Humans have walked on the moon, first landing m 1969. Unmanned spacecraft have explored the outer reaches of the solar system, mapping the surfaces of planets known only as points of light in 1930. The satellite-mounted Hubbell telescope was launched into space in the mid-1990s, providing astronomers with the most in-depth space pictures to date. No probes have yet made close observations of Pluto, though in 1978, the planet was discovered to have a moon, named Charon.

1930: The world's population reaches two billion, with the great majority of people living in rural areas

Today: Global population passed the five billion mark...