Why was Pluto a planet for 76 years?

Chapter 12 (‘Beyond Earth’) of the Grade VI science textbook (NCERT, 2024-2025) introduces students to the concept of a planet as being: “…a large, nearly spherical object that revolves around the Sun”. In the same chapter, students learn about the eight planets in the solar system. They also learn how an object that scientists believed to be the ninth planet for nearly 76 years is now classified as a dwarf planet. According to the textbook, this change in Pluto’s status happened in 2006, when “the International Astronomical Union (IAU)… redefined the requirements of an object to be called a planet”.¹ This brief description may leave curious students and teachers with questions like: What led scientists to believe Pluto was a planet? Was this belief supported by scientific evidence? The answers are tied to the story of Pluto’s discovery (see Box 1).

The search begins

By the early 19th century, seven planets of the solar system were known.² But astronomers observed that the actual path (orbit) that Uranus took around the Sun did not match their predictions (based on Newton’s laws). What could be causing this difference? In 1841, the British astronomer John Couch Adams proposed that Uranus was being gravitationally pulled from its expected path by an undiscovered outer planet.³

In 1846, the French astronomer Urbain Le Verrier used mathematical calculations to predict the likely position of this hypothetical planet. He sent these details to the German astronomer Johann Gottfried Galle. On the night he received Le Verrier’s letter, Galle and his student Heinrich Louis d’Arrest located the planet within one degree of its predicted position. This planet was named Neptune.³

But calculations of Neptune’s mass did not seem to fully account for Uranus’ deviations from its predicted orbit. Astronomers therefore considered the possibility that another object was pulling on Uranus. In 1902, the wealthy American astronomer Percival Lowell proposed that this object was the ninth planet of the solar system. He called it ‘Planet X’ (‘X’ here is a letter of the English alphabet, not the Roman numeral).⁵

Lowell’s Planet X

Born in 1855, Lowell was the son of a wealthy businessman. In 1876, he graduated from Harvard College, with distinction in mathematics. In 1893, Lowell read a description of ‘canals’ on the surface of Mars in the book ‘La planète Mars’ written by the French astronomer Camille Flammarion. His determination to explore these canals led Lowell to a career in astronomy.^6,7 Within a year, he had used his personal wealth and influence to establish the Lowell Observatory at Flagstaff, Arizona, USA. It was here, in 1906, that he started his search for Planet X.

In the first phase of this search, Lowell, like Le Verrier, used mathematical calculations to identify the most likely position of the new planet. Astronomers at the Lowell Observatory used its 42-inch telescope to photograph these specific regions of the sky on different days. Lowell used a hand magnifier to painstakingly examine every inch of these photographic glass plates (see Box 2). By 1910, Lowell’s team had taken close to 200 photographs and recorded the positions of thousands of stars, but found no evidence for Planet X.

Lowell decided to change his approach. First, he hired a team of ‘human computers’—people, mostly women, who performed complex mathematical calculations. Led by the American astronomer Elizabeth Williams, their work allowed Lowell to more accurately predict regions of the sky where Planet X was likely to be found. Second, Lowell borrowed a 9-inch wide-field telescope from Sproul Observatory, Pennsylvania. He believed that its wider field of view would allow larger regions of the night sky to be photographed on a single plate, making the search for Planet X more efficient. Lastly, Lowell bought a specialised optical device, popularly called a ‘blink comparator’ (see Fig. 1). This instrument allowed him to rapidly alternate between two photos of the same region of the sky taken days apart. An object that had changed position between the two photographs would appear to “jump” or “blink” back and forth. This process was popularly called ‘blinking’ (see Box 3).

Early versions of this instrument could hold 6-inch x 7-inch plates. However, Lowell’s team wanted to use larger 14-inch × 17-inch plates to cover more sky. The American astronomer Carl Lampland modified the blink comparator with sliding frames to examine these larger plates quarter by quarter.⁵

Using this combined mathematical and observational approach, Lowell searched tirelessly for Planet X until his sudden death from a stroke in 1916.⁷ His wife contested his will, which left most of his million-dollar estate to the observatory. This led to a long legal battle, during which the Lowell Observatory’s search for Planet X was put on hold. In 1927, the litigation was resolved in the observatory’s favour and the search for Planet X resumed.^7,9 In 1929, the American astronomer Dr Vesto Slipher, the acting director of the observatory, assigned the task of finding Planet X to a 23-year-old astronomer called Clyde Tombaugh.

Enter Tombaugh

Born in the year Lowell started his search for Planet X, Tombaugh was the son of a farmer. He graduated from high school in 1925, but could not afford college. In 1924, Tombaugh read a magazine article titled ‘The Drift of Jupiter’s Markings’ by the amateur astronomer Latimer J Wilson. Fascinated by Wilson’s drawings of the markings on the surface of Jupiter, Tombaugh wanted to see these features himself. So, in 1926, at the age of 20, Tombaugh built his first telescope with discarded car parts and farm machinery. Not satisfied with it, he spent the next two years honing the skill of building telescopes, grinding his own mirrors for them. In 1928, Tombaugh built a telescope that allowed him to observe markings on the surface of Jupiter and Mars (see Fig. 2).

To control temperature and airflow, Tombaugh operated this telescope from a pit (24 feet long, 8 feet deep, and 7 feet wide) that he dug himself. He sent detailed drawings of his observations to the Lowell Observatory.^10,11 Impressed with his keen observation skills, Slipher offered Tombaugh a position with a three-month trial period.

At the observatory, Tombaugh began a systematic survey of the night sky. Learning from Lowell’s efforts, Tombaugh would fit a 14-inch x 17-inch photographic plate in a powerful 13-inch telescopic camera and take long exposures of the night sky (see Box 4). He focused on observing regions where Lowell had predicted Planet X’s presence. Often, he would need three hours to take one photo. He would then fix a fresh plate in the camera, focus on an adjacent region of the sky, and take another photo. Every few days, he would revisit the same regions of the sky and photograph them. During the day, he would develop the plates and use the blink comparator to compare photos of the same region taken on different days.⁵ Like Lowell, he would examine each inch of these photos for any tiny dot that had changed position. This was a tedious task and demanded extreme concentration. Every half hour, Tombaugh would take a short break by walking away from the comparator. The stakes were too high to afford any lapse in concentration.

By January 1930, Tombaugh had spent thousands of hours on the blink comparator and had scanned the positions of about 15 lakh stars. He had observed many dots that had moved, but at speeds that suggested they were too close to Earth to be Planet X. Finally, on February 18, 1930, Tombaugh noticed a tiny object that was in different positions in the plates taken on January 23 and January 29. The image was too sharp to be that of a comet and its speed was too slow to be that of an asteroid. But it appeared far enough from Earth to be beyond Neptune.^10,12 This was just as Lowell had predicted.

Announcing the discovery of Planet X

When Tombaugh reported this observation to Slipher, the entire team at the Lowell Observatory became involved in checking whether this object matched Lowell’s predictions for Planet X. Based on Lowell’s calculations, the mass of this object would be seven times that of Earth and it would be at a mean distance of 43 astronomical units (AU) from the Sun. This is how Chapter 12 of the Grade VI textbook introduces this unit of distance: “The distance of the Sun from the Earth is about 150 million km. A useful unit for expressing distances within the solar system is ‘astronomical unit’ (au) which is approximately the distance between the Sun and the Earth”.¹ Tombaugh took more images of the same region of the sky with his 13-inch telescope and the 24-inch telescope that Lowell had used to observe Mars. Based on his observations, Tombaugh estimated the mass of this object to be roughly the same as that of Earth. Lampland tried to get a clearer image of the object by photographing it with the 42-inch telescope that Lowell had used in the first phase of his search. But its images remained faint and did not have the disc-like appearance seen in images of other planets.¹³ To determine its path around the Sun, Tombaugh and Lampland recorded its position every day till May 26. Since no one at the Lowell Observatory had any experience calculating planetary orbits, Slipher asked astronomers from other observatories for help with this task.⁵

Finally, on March 12, 1930, Slipher sent a telegram with details of this discovery to the Harvard College Observatory. Based on the evidence the team at the Lowell Observatory had so carefully collected, the discovery was announced on March 13. This would have been Lowell’s 75th birthday. As news of this discovery spread, the Lowell Observatory received more than a thousand suggestions for names. The most popular choices were Minerva, Persephone, and Cronus.^9,10 The name ‘Pluto’ (the Roman god of the underworld) was suggested by 11-year-old Venetia Burney. Her grandfather, Falconer Madan, loved the name and shared it with the Lowell Observatory through an astronomer friend.¹⁴ When the three-member naming committee at the Lowell Observatory voted, they unanimously selected ‘Pluto’. The Lowell Observatory got this name approved by both the American Astronomical Society and the Royal Astronomical Society. By May 1, 1930, Planet X was officially known as Pluto.