1930

Clyde Tombaugh discovers Pluto while searching for 'planet X', predicted by Percival Lowell in the late nineteenth century to explain an apparent anomaly in the orbit of Uranus.

1976

Methane ice discovered on Pluto. Nitrogen and carbon monoxide ices are also identified 16 years later.

1978

Discovering Pluto's moon Charon allows astronomers to determine their masses. They find that Pluto is much smaller than they thought, and conclude that it is made of ice and rock.

1985-88

Pluto found to have a tenuous atmosphere, probably of nitrogen gas. Observations in 2003 show that the atmospheric pressure has since doubled, presumably caused by seasonal warming.

1989

Pluto makes its closest approach to the Sun, bringing it 29.7 astronomical units away (1 AU = the Earth-Sun separation). Its highly eccentric, 248-year orbit means that during 1979-99 it was closer to the Sun than Neptune. It will swing back out to its most distant point of 49.5 AU in about 2113.

1992

The first evidence that Pluto is just one of a band of numerous Kuiper-bell objects (KBOs) arrives with the discovery of (15760) 1992 QB. The belt had been hypothesized for decades, suggested as a repository of debris from the Solar System's formation.

2001-04

The first binary system, other than Pluto-Charon, is found in the Kuiper belt. When the relatively large KBOs Quaoar and Sedna follow in 2002 and 2004, some astronomers argue that Pluto is just one of the crowd, and not deserving of the title 'planet'.

July 2005

2003 UB313 is spied by ace planet hunter Mike Brown's team and dubbed 'the tenth planet', as it seems to be even larger than Pluto. Its small moon is unveiled to the world two months later.

October 2005

Two tiny moons are found around Pluto, each between 50 and 160 kilometres across. The find supports the theory that Pluto and its satellites formed in a massive collision, rather than a capture event.

January 2006

Astronomers in Hawaii find that Pluto's surface temperature is -230°C, ten degrees cooler than Charon. The difference is blamed on the evaporation of nitrogen ice from Pluto's surface, keeping the planet cool.

19 January 2006

The New Horizons craft launches, off to probe the Kuiper belt. It will try to discover if Pluto has any geological activity, or even an internal liquid ocean. Sampling the atmosphere should help explain why it rapidly leaks into space. With the craft goes the man who started it all: a small vial contains Clyde Tombaugh's ashes.

July 2015

After a gravitational boost from Jupiter in 2007, New Horizons will have just six months where its views of Pluto and co. are better than those of the Hubble Space Telescope, and most observations will occur during flybys over a 24-hour period. A KBO encounter is planned for two or three years later.