From Planet X to “minor planet 134340 Pluto”

On this day 80 years ago (1 May 1930), Pluto was announced as the official name for the body identified as the the ninth planet in our solar system. The name was proposed by Venetia Burney, an 11-year-old schoolgirl from Oxford, who was rewarded with five pounds for her contribution.

Venetia’s suggestion was chosen from a shortlist of three potential names. The other two contenders were Minerva (which had already been used for an asteroid) and Cronus (which was suggested by an unpopular astronomer called Thomas Jefferson Jackson See). Pluto was chosen in part to evoke the initials of the astronomer Percival Lowell.

Pluto’s existence had only been confirmed a few weeks earlier by the Lowell Observatory in Arizona, which was searching for a Planet X. The observatory’s founder, Percival Lowell, had begun searching for Planet X in 1906 as a result of observations that the orbit of Uranus was being disturbed by another planet apart from Neptune.

Pluto was later shown not to be Lowell’s Planet X, because its mass (about 0.2 per cent of the Earth’s) is too small to account for discrepancies in Uranus’s orbit. Most scientists today agree that Planet X does not exist.

Pluto was considered the solar system’s ninth planet until 2006. Its status began to be questioned in the 1970s, following the discovery of minor planet 2060 Chiron in the outer solar system, and the recognition of its low mass.

The matter was resolved when the International Astronomical Union defined the term “planet” for the first time in 2006. Pluto fails to meet the definition because its mass is only 0.07 times that of the mass of the other objects in its orbit (Earth is 1.7 million times the remaining mass in its own orbit). The IAU simultaneously defined a dwarf planet category, into which Pluto fits, along with Eris and the Eridian moon, Dysnomia. Pluto’s minor planet catalogue designation is 134340 Pluto.

Pluto is the largest member of the Kuiper belt, a stable ring of objects between 30–50 astronomical units from the sun (an astronomical unit is the mean distance from the Earth to the sun — nearly 93 million miles).

Like other objects in the Kuiper belt, Pluto shares features with comets. For example, the solar wind is gradually blowing its surface into space, so that if it were closer to the sun it would develop a comet-like tail.