Are you familiar with the names Giuseppe Piazzi, Heinrich Olbers* and Karl Harding? These eminent astronomers discovered the four largest celestial bodies immediately beyond Mars between 1801 and 1807. They were searching for the ‘missing planet’ thought to orbit between Mars and Jupiter, but instead found these small objects they termed ‘minor planets’.

How about Clyde Tombaugh? You may know about him, but probably more because the planet he discovered in 1930, Pluto, was ‘demoted’ by the IAU (International Astronomical Union) to dwarf planet status in 2006. However, ‘demoted’ is an unjustifiable emotional term.

The discovery of three dwarf planets beyond Pluto and confirmation of the Kuiper Belt puts Pluto centre stage as the first and nearest of the Kuiper Belt objects, and the only one to be visited by a spacecraft – until New Horizons passes by 2014 MU69 (‘Ultima Thule’) on 1st January 2019.

Anomalies in the orbits of distant objects beyond the Kuiper Belt led in 2016 to astronomers to begin searching for a postulated ‘mini-Neptune’, ten times more massive than Earth in a highly elliptical orbit lasting around 15,000 years – dubbed ‘Planet Nine’.

Then, twelve years after the IAU decision, in August this year a group of 35 (mostly US) astronomers published a message criticising the use of the term ‘Planet Nine’ in the search for a ‘true planet’ beyond Neptune. They effused about the “incredible accomplishment of the discovery of Pluto” by Tombaugh, and went further claiming that using this term is “insensitive to Professor Tombaugh’s legacy”, and should be replaced with “culturally and taxonomically neutral terms” such as “Planet X, Planet Next or Giant Planet Five”.

The minor planets between Mars and Jupiter were reclassified as ‘asteroids’. This lazy term, meaning ‘star-like’, persists today, even though we know them to be rocky debris from the primordial solar nebula.

When the largest asteroid, Ceres, was reclassified as a dwarf planet along with the change to Pluto’s status there was no great clamouring to elevate Piazzi’s reputation because his asteroid had been ‘promoted’ to dwarf planet status.

To my mind, yes, Clyde Tombaugh found Pluto. But he didn’t actually ‘discover’ it.

Neptune had been found in 1846 following perturbations discovered in the orbit of Uranus. Various astronomers in the late nineteenth century made predictions of a ninth planet that was apparently contributing to these disturbances. Percival Lowell, of Mars canals infamy, and his observatory, in association with William Pickering, searched for ‘Planet X’ up until Lowell’s death in 1916 without success. Two images of Pluto had in fact been acquired in 1915, but were missed by the observatory. Legal disputes by Lowell’s widow halted the search until 1929.

The promising young 23-year-old Clyde Tombaugh at his first position as an astronomer was given the task at Lowell Observatory of systematically photographing the night sky and analysing pairs of images in the search for a ninth planet moving against the background stars. The object was spotted after a year of diligent work.

But does he deserve all the credit for Pluto’s discovery?

Rivalry between French and British astronomers over who deserved credit for the discovery of Neptune led firstly to consensus of joint equal credit, then to questioning in the 1960s and finally to a full review by the Royal Observatory at Greenwich. They decided that “credit belongs only to the person who succeeded both in predicting the planet’s place and in convincing astronomers to search for it.”

Using the same criteria, Tombaugh should at least share credit with the astronomers who predicted its existence, and with Percival Lowell, William Pickering and the Lowell Observatory, who began the search that Tombaugh completed but had missed Pluto in two photographs taken in 1915. There are 13 other ‘precovery’ images of Pluto, the earliest being found on photographic glass plates at Yerkes Observatory taken in 1909. 

I grew up with a 9-planet Solar System. However, astronomers continued to search beyond Pluto for ‘Planet X’, the tenth, unknown, planet. For example, in the 1980s the controversial US observatory at Black Birch in Marlborough allotted 10% of its observing time to searching for Planet X.

Following the IAU decision on dwarf planets, Clyde Tombaugh’s widow said about his legacy that “… he was a scientist. He would understand [astronomers] had a real problem when they start finding several of these things flying around the place.”

It’s not about Pluto, and it’s not about Tombaugh. The debate should be about the best way to classify the myriad objects that inhabit our Solar System, i.e. cubewanos, plutinos, twotinos, scattered disc objects, detached objects, trans-Neptunian objects, dwarf planets, Kuiper Belt objects, planetesimals, Oort Cloud objects, planetoids, centaurs, sednoids, ice giants…  And that’s without mentioning the >3,700 planets confirmed orbiting other stars, which fall outside the current definition of a planet.

One group has put forward a proposal that would classify 110 objects in our Solar System as “fully fledged” planets, including the largest moons. Another proposal would see “all round objects smaller than stars” classified as planets, including moons!

Ultimately, the members of the IAU will reach a consensus and let us know what we should be teaching everyone as part of education’s ‘life-long learner’ drive.

Let’s hope they stop the bickering sooner rather than later.

David Britten, Astronomy Educator

* Known for Olbers’ Paradox, which asks why the night sky is dark if there are an infinite number of stars across the sky seen to an infinite distance.

Further reading?

Neptune discovery claim was a British ‘crime’

Patricia Tombaugh about Clyde’s attitude:

USNO Black Birch Astrometric Observatory (1984-96) in Marlborough contributed to the International Reference Stars Catalog; now superseded by the Hipparcos Catalog: here and here