The Discovery of Pluto 90 Years Later

2020 marks the 90 year anniversary of the discovery of Pluto. Pluto was officially discovered on February 18th, 1930, by Clyde W. Tombaugh of the Lowell Observatory in Flagstaff Arizona.

Founded by Percival Lowell in 1894, the Lowell Observatory started looking for what was called at the time Planet X in 1906. The reason for Lowell to look for Planet X was the result of Lowell theorizing that wobbles in the orbits of Uranus and Neptune were caused by the gravitational pull of an unknown planetary body. Considering that the discovery of Neptune was done using mathematics and confirmed using a telescope and several prominent astronomers had surmised there was something affecting Neptune’s orbit, this idea wasn’t totally bizarre.

By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. At his new observatory, Lowell conducted his search until his death in 1916, but to no results. Unknown to Lowell at the time, his surveys had captured two faint images of Pluto on March 19 and April 7, 1915, but they were not recognized for what they were.

However, in 1929, using the calculations of Lowell and W.H. Pickering as a guide, the search for Pluto was resumed at the Lowell Observatory in Arizona. On February 18, 1930, Tombaugh discovered the tiny, distant planet by use of a new astronomic technique of photographic plates combined with a blink microscope. His finding was confirmed by several other astronomers, and on March 13, 1930–the anniversary of Lowell’s birth and of William Hershel’s discovery of Uranus–the discovery of Pluto was publicly announced.

For his part in the discovery of Pluto, some of Clyde Tombaugh’s ashes were placed inside the New Horizons probe that was sent to study the dwarf-planet.

Is Pluto A Planet?

In 1992, the status of Pluto as a planet was starting to be scientifically questioned as a growing number of objects in the Kuiper Belt were discovered. A notable Kuiper belt object was discovered in 2005, named Eris, was a dwarf planet that scientists discovered to be 27% more massive than Pluto. The discovery of Eris directly led the International Astronomical Union, IAU, to define the term “planet” formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet.

This led to a public debate as to what was a planet and there were some passionate arguments on both sides of the isle. However, the decision to demote Pluto to a dwarf-planet has remained, much to the chagrin of some.

Imaging And Visiting Pluto

Before space based telescopes were available to provide detailed images of Pluto, the best available viewing option was to utilize a 30 cm, 12 inch aperture telescope. With a telescope of this size, Pluto looks star-like and without a visible disk even in large telescopes, which is a result of its angular diameter being only 0.11″.

The earliest maps of Pluto, made in the late 1980s, were brightness maps created from observations of eclipses by its largest moon, Charon. Better maps were produced from images taken by the Hubble Space Telescope, which offered higher resolution, and showed considerably more detail. The context of the increased detail means resolving variations several hundred kilometers across, including polar regions and large bright spots.

The best images of Pluto come from the New Horizons Probe, which was launched in 2006. Flying by July 2015, is the first and so far only attempt to explore Pluto directly by humanity. Observations were conducted using a remote sensing package that included imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments. The scientific goals of New Horizons were to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition, and analyze Pluto’s neutral atmosphere and its escape rate. 

Due to the length of time to get to objects in the outer solar system, these objects are rarely studied by satellites; however, there are advocates for an orbiter mission that would return to Pluto to fulfill new science objectives. These new objectives include mapping the surface at 30 feet per pixel, observations of Pluto’s smaller satellites, observations of how Pluto changes as it rotates on its axis, and topographic mapping of Pluto’s regions that are covered in long-term darkness due to its axial tilt. The last objective could be accomplished using laser pulses to generate a complete topographic map of Pluto. 



New Horizons Probe

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