This Day in History (18-Feb-1930) – Clyde Tombaugh Discovers Pluto

Looking over some photographic plates from the month before as part of his research into asteroids, astronomer Clyde Tombaugh noticed a peculiar object on February 18, 1930: the ‘Planet X’. Part of a larger cloud of debris called the Kuiper belt, the small orb 4.6 billion miles from the sun has become a controversial figure in the scientific community. Lurking among the distant rocks sprayed across space beyond the orbits of Uranus and Neptune, Percival Lowell — the founder of the observatory Tombaugh worked at — first theorized the presence of a “Planet X” affecting the two outermost planets and predicted its location in 1909. Tombaugh carried on the research after Lowell’s death and diligently spent each day taking photographs of the night sky and looking them over with a blink comparator, an optical device that allows the researcher to quickly flip back and forth between two images.

The news shocked the public, resulting in a flood of more than a thousand letters arriving at the Lowell Observatory with ideas for naming the planet. Venetia Burnley, an 11-year-old girl from the English city of Oxford, suggested Pluto, Roman name for the Greek god of the underworld, Hades (world of deads). Taking 248 Earth years to make one pass around the sun, Pluto’s orbit does not fit the regular pattern exhibited by the other eight planets. At times, it is within the path trotted by Neptune, a factor in debates about the true status of the object.

Following more than 75 years of being regarded as a planet, the International Astronomical Union announced Pluto no longer fit the bill in August 2006. Citing the shape of its orbit and the various asteroids it shares the Kuiper belt with, experts decided to downgrade it to a dwarf planet. (True planets have to “own” their path around the sun.)

There are now officially only eight planets in our solar system. Pluto  is much smaller than any of the official planets. It is smaller than seven of the solar system’s moons including moon of the earth. Pluto has not yet been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface (left and above). A spacecraft called New Horizons was launched in January 2006. If all goes well it should reach Pluto in 2015.