Discovering Pluto

Young Astronomers Blog, Volume 28, Number 4.

Remember Pluto? Yes, Pluto. It was a planet for a long time, then it wasn’t. Pluto was discovered ninety years ago on February 18, 1930. This is the story.

Shortly before the turn of the century, that is the turn of the 19^th century, there were five planets, other than the Earth. These were the planets known by the ancients. They “wandered” across the night sky resulting in the name planets after the Greek word for wandering stars. In 1791 things started to change. William Herschel discovered a new planet, which eventually became known as Uranus.

Ten years later, additional objects were found; Ceres in 1801, Pallas in 1802, Juno in 1804 and Vesta in 1807. They were small, but still they wandered across the night sky, so without much thought they were considered planets. If you pick up an astronomy textbook from the early 1800s, you will find the list of the eleven planets.

Beginning in 1845 more objects were discovered and the list of “planets” became larger and larger. And, of course, Neptune was discovered in 1846. Eventually, astronomers decided enough was enough. First, the term “minor planet” was used for these new objects and by 1860 they were reclassified as asteroids or minor planets. We had survived the first planetary controversy.

Neptune was the first planet to be discovered based on mathematical analysis. The orbit of Uranus didn’t quite fit. British astronomer John Couch Adams and French astronomer Urbain Le Verrier began to calculate the possible position of another planet that would explain Uranus’ orbit. In 1846, Le Verrier sent his calculations to Johann Galle at the Berlin observatory. Galle almost immediately found Neptune exactly where it was predicted to be.

For several years, based on the orbits of Uranus and Neptune, astronomers thought there was another, possibly large, planet out beyond the orbit of Neptune. In 1905, Percival Lowell began a search for this mysterious planet X. A few years earlier Lowell had established an observatory in Flagstaff, Az. Up until his death in 1916, Lowell used the resources at his observatory to search for planet X, but to no avail.

In January 1929, Clyde W. Tombaugh joined the staff of the Lowell Observatory. Tombaugh used a device called the Blink Comparator to view two sets of photos and look for objects moving against the background of stars. On February 18, 1930, he was comparing two images taken on January 23 and 29. He noticed an object moving slowly across the sky. After some additional observations, the Lowell observatory announced the discovery of a new planet on March 13, 1930, which would have been Percival Lowell’s 75^th birthday.

The new planet needed a name. Several such as Minerva and Cronus, the other finalists, were suggested. In England, a young girl, Venetia Burney, was stilling in her kitchen with her grandfather, Falconer Madan. She had been studying Roman mythology and after learning about the discovery of the new planet, thought Pluto, the Roman god of the underworld would be a good name. Her grandfather passed on her suggestion to British astronomer Herbert Hall Turner, who sent a cable to the folks at the Lowell Observatory. They liked the name, and, on May 1, 1930, the new planet officially became Pluto. By the way, the initials for Pluto, PL, were also the initials of Percival Lowell.

At first, everyone thought Pluto was the size of the Earth, but over time, it was determined that it was small, not another Neptune sized planet X astronomers were searching for at the time. Pluto also had an unusual orbit, very elliptical and at an angle to the plane of the solar system where we find the other planets. But it was there. It orbited the Sun and it was round. So, it must be a planet.

This is the way the Solar System looked from 1930 until the turn of the century. There were four rocky inner planets (Mercury, Venus, Earth, Mars), four outer gas and ice giants (Jupiter, Saturn, Uranus, Neptune) and little Pluto all by itself out in the far reaches of the Solar System.

Around the turn of the 21^st century things started to change. Eventually, Pluto ceased to be a planet and is now classified as a dwarf planet. However, that is another story.