By Dame Jocelyn Bell Burnell, discoverer of Pulsars, Oxford Unversity, UK

Pluto is the second-most-massive known dwarf planet in the Solar System (after Eris) and the tenth-most-massive body observed directly orbiting the Sun. Originally classified as the ninth planet from the Sun, Pluto was recategorized as a dwarf planet and plutoid due to the discovery that it is one of several large bodies within the newly charted Kuiper belt.

Artist's impression of Pluto with the Sun and Charon, one of its moons. Click to enlarge.

From its discovery in 1930 until 2006, Pluto was classified as a planet. In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's relatively low mass, its status as a major planet began to be questioned. In the late 20th and early 21st century, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object Eris in 2005, which is 27% more massive than Pluto.[14] On August 24, 2006, the International Astronomical Union (IAU) defined what it means to be a "planet" within the Solar System. This definition excluded Pluto as a planet and added it as a member of the new category "dwarf planet" along with Eris and Ceres. After the reclassification, Pluto was added to the list of minor planets and given the number 134340. A number of scientists continue to hold that Pluto should be classified as a planet.

[From Wikipedia]

Images of Pluto's surface. Click to enlarge.

ABOUT THE LECTURER

The radio astronomer Dame Susan Jocelyn Bell Burnell discovered the first pulsar (stars that release regular bursts of radio waves) in 1967. Susan Jocelyn Bell (Burnell) was born in Belfast, Northern Ireland, on July 15, 1943. Her father was the architect for the Armagh Observatory, which was close to their home. Her early interest in astronomy was encouraged by the observatory staff. She studied at the Mount School in York, England, from 1956 to 1961. She earned a B.S. in physics at the University of Glasgow in 1965. That same year, she began work on her Ph.D. at Cambridge University. There, under the supervision of Antony Hewish, she constructed and operated a 81.5 megaherz radio telescope. 

She studied interplanetary scintillation of compact radio sources. Bell Burnell detected the first four pulsars. The term "pulsar" is an abbreviation of pulsating radio star or of rapidly pulsating radio sources. Pulsars represent rotating neutron stars that emit brilliant flashes of electromagnetic radiation at each revolution, like beacons from a lighthouse. 

She has since played very active role in developing the science of radio-astronomy and in the world of Astronomy and Astrophysics and has championed the status of women in science.  In 2006 she chaired the meeting of the IAU in Prague where Pluto got demoted to the status of Minor Planet. In 2007 she was honoured with a DBE - Dame Commander of the order of the British Empire. She has just finished her tenure as president of the Institute of Physics.

She was recently featured on BBC tv in the fascinating "Beautiful Minds" series.