In August 2006 some incredible news broke: the Solar System has lost one of its planets! One would indeed be alarmed: one planet was gone today, another tomorrow, and then the Earth would follow!
But there was no cause for panic then or now. The question was only about the decision of the International Astronomical Union which after long disputes deprived Pluto of the status of a full-fledged planet. And, contrary to misconceptions, the solar system did not shrink that day, but rather expanded unimaginably.
In a nutshell:
Pluto is too small for a planet. Some celestial bodies used to be thought of as asteroids, even though they are the same size, if not bigger, than Pluto. Both they and Pluto are now called dwarf planets.
The search for wanderers
The discovery of Pluto, long thought to be the ninth planet in the solar system, has a prehistory.
Before the advent of telescopes, mankind was aware of five celestial bodies called planets (translated from the Greek as “wanderers”): Mercury, Venus, Mars, Jupiter and Saturn. Within four centuries, two more major planets were discovered: Uranus and Neptune.
The discovery of Uranus is notable because it was made by an amateur music teacher, William Herschel. On March 13, 1781, he was surveying the sky and suddenly noticed a small yellow-green disk in Gemini’s constellation. At first Herschel thought he had discovered a comet, but observations by other astronomers confirmed that a real planet with a stable elliptical orbit had been discovered.
Herschel wanted to name the planet George after King George III. But the astronomical community decided that the name of any new planet should be consistent with others, that is derived from classical mythology. As a result, the planet was named Uranus after the ancient Greek god of heaven.
Observations of Uranus revealed an anomaly: the planet stubbornly refused to follow the laws of celestial mechanics, deviating from its calculated orbit. Twice astronomers calculated models of Uranus’ motion, correcting for the gravity of other planets, and twice it “cheated” them. It was then suggested that another planet beyond its orbit influenced Uranus.
On June 1, 1846, an article by the mathematician Urbain Leverrier appeared in the journal of the French Academy of Sciences, describing the expected position of the hypothetical celestial body. On the night of September 24, 1846, at his prompting, the German astronomers Johannes Halle and Heinrich d’Array, without much time to search, discovered the unknown object, which turned out to be a large planet and was named Neptune.
Planet X
The discovery of the seventh and eighth planets in just half a century has tripled the solar system’s boundaries. Uranus and Neptune had discovered satellites, which made it possible to accurately calculate the planets’ masses and their mutual gravitational influence. From this data, Urben Leverrier built the most accurate orbital model at the time. Once again, reality differed from the calculations! The new mystery inspired astronomers to search for a transneptunian object, which became known as Planet X.
The fame of the discoverer went to the young astronomer Clyde Tombaugh, who abandoned mathematical models and set about the hard study of the sky with a photographic refractor. On February 18, 1930, while comparing January plates, Tombaugh discovered the displacement of a faint star-shaped object – it turned out to be Pluto.
Astronomers soon discovered that Pluto is a very small planet, smaller than the Moon. And it clearly didn’t have enough mass to affect the movement of the massive Neptune. Clyde Tombaugh then embarked on a massive programme to find another ‘planet X’, but despite his best efforts, they could not find it.
We know far more about Pluto today than we did in the 1930s. Thanks to years of observations and orbiting telescopes, Pluto has an elongated orbit that tilts to the ecliptic plane (Earth’s orbit) at a significant angle of 17.1°. This unusual property has led to speculation as to whether Pluto is a native planet of the solar system or whether it is accidentally attracted by the Sun’s gravity (for example, this hypothesis is considered by Ivan Efremov in his novel The Andromeda Nebula).
Pluto has small satellites, many of which have been discovered recently. There are five in all: Charon (discovered in 1978), Hydra (2005), Nicta (2005), P4 (2011) and P5 (2012). The presence of such a complex system of satellites has suggested that Pluto has sparse debris rings – the kind that always occur when small bodies collide in orbits around planets.
Maps drawn from data from the Hubble Orbiting Telescope have shown that Pluto’s surface is not homogeneous. The part facing Charon contains mostly methane ice, while the opposite side contains more nitrogen and carbon monoxide ice. In late 2011, complex hydrocarbons were discovered on Pluto, leading scientists to speculate that simple life forms exist there. In addition, Pluto’s rarefied atmosphere of methane and nitrogen has markedly ‘swollen’ in recent years, meaning there is climate change on the planet.
What has Pluto been called
Pluto received its name on 24 March 1930. Astronomers voted on a shortlist containing the final three choices: Minerva, Cronus and Pluto.
The third option – the name of the ancient god of the realm of the dead, also known as Hades and Hades – proved the most suitable. Venetia Burney, an eleven-year-old schoolgirl from Oxford, suggested it. She was interested not only in astronomy, but also in classical mythology, and decided that the name Pluto suited the dark and cold world best. The name came up in conversation with her grandfather Falconer Maydan, who had read about the planet’s discovery in a magazine. He forwarded Venetia’s proposal to Professor Herbert Turner, who in turn telegraphed it to colleagues in the US. Venetia Byrne received a prize of five pounds sterling for her contribution to the history of astronomy.
Interestingly, Venetia lived to see Pluto lose its status as a planet. When asked about her attitude to this ‘demotion’ she replied: ‘At my age I don’t care about this kind of debate any more, but I would like Pluto to remain a planet’.
Edgeworth-Coiper Belt
By all accounts, Pluto is a normal planet, albeit small. So why have astronomers been so unfavourable to it?
The search for the hypothetical ‘planet X’ has been going on for decades and has led to many interesting discoveries. In 1992, a large cluster of small bodies resembling asteroids and comet nuclei was discovered beyond Neptune’s orbit. The existence of a belt of debris, left over from the formation of the solar system, had been predicted long before by Irish engineer Kenneth Edgeworth (in 1943) and US astronomer Gerard Kuiper (in 1951).
The first trans-Neptunian object belonging to the Kuiper Belt was discovered by astronomers David Jewett and Jane Lu, observing the sky with the latest technology. On August 30, 1992, they announced the discovery of QB1, named Smiley, after the character from the popular detective John Le Carré. However, the name was not used officially as there was already an asteroid called Smiley.
By 1995, seventeen more bodies had been discovered beyond Neptune’s orbit, eight of which were in Pluto’s orbit. By 1999, the total number of recorded objects in the Edgeworth-Coiper belt had passed the one hundred mark, and by now, over a thousand. Scientists believe that in the foreseeable future, will be able to identify more than seventy thousand (!) Objects larger than 100 km. It is known that all these bodies move along elliptical orbits, like the real planets, and a third of them have the same orbital period as that of Pluto (they are called “plutino” – “plutonchiki”). Objects of the belt are still very difficult to classify – we only know that they have a size of 100 to 1000 km and their surface is dark with a reddish tint, indicating the ancient composition and presence of organic compounds.
The confirmation of the Edgeworth-Coiper hypothesis alone could not have revolutionized astronomy. Yes, we now know that Pluto is not a lonely wanderer, but neighbouring bodies cannot compete with it in size, and besides, they have no atmosphere or satellites. The scientific world could go on sleeping in peace. And then the worst happened!
Goddess of discord
Mike Brown’s finest hour was on January 5, 2005 when his team discovered a transneptunian object estimated to be 3000 km across (later measurements gave a diameter of 2326 km). Thus, a celestial body was found in the Edgeworth-Coiper Belt, the size of which was definitely larger than Pluto. Scientists were buzzing: A tenth planet had finally been discovered!
Astronomers have given the new planet the unofficial name Xena, in honour of the heroine of the fantasy television series “Xena the Warrior Princess”. And when Xena’s satellite was discovered, it was immediately named Gabrielle, the name of Xena’s companion. The International Astronomical Union could not accept such “frivolous” names, so Xena was renamed Erida (Greek goddess of discord) and Gabrielle was renamed Dysnomia (Greek goddess of lawlessness).
Erida did indeed cause discord among astronomers. Logically, Xena-Erida should have been immediately recognized as the tenth planet, and the group of Michael Brown should be entered in the annals of history as its discoverers. But no such luck! Previous discoveries had suggested that dozens more objects comparable in size to Pluto might be lurking in the Edgeworth-Coiper Belt. Is it easier to multiply the number of planets by rewriting the astronomy textbooks every couple of years, or to scrap Pluto and with it all the newly discovered celestial bodies?
The verdict was handed down by Mike Brown himself, on March 31, 2005, when he discovered the object FY9, named Makemake (the creator-god of mankind in the mythology of the Rapa Nui, inhabitants of Easter Island), measuring 1500 km in diameter. When their colleagues ran out of patience, they convened at a conference of the International Astronomical Union in Prague to decide once and for all what a planet is.
Previously, a planet could be defined as a celestial body that orbits the Sun, is not a satellite of another planet and has enough mass to take on a spherical shape. Following the debate, astronomers added another requirement: that the body “clear” the vicinity of its orbit of bodies of comparable size. Pluto did not meet the latter requirement and was stripped of its planetary status.
It was moved to the list of “dwarf planets” (from the English “dwarf planet”, literally – “gnomic planet”) under the number 134340.
Such a decision has drawn criticism and ridicule. Scientist Alan Stern, who deals with Pluto, said that if this definition is applied to Earth, Mars, Jupiter and Neptune, which are found to have asteroids in their orbits, they too should be stripped of the title of planets. Besides, he said, less than 5% of astronomers voted for the ruling, so their opinion cannot be considered universal.
However, Mike Brown acknowledged the determination of the International Astronomical Union, satisfied that the debate was finally over to everyone’s satisfaction. And indeed – the storm has subsided, the astronomers have dispersed to their observatories.
There have been differing public reactions to the International Astronomical Union’s decision: some dismissed it as irrelevant, some convinced the scientists were fooling around. The verb “to pluto”, recognized as Word of the Year 2006 by the American Society of Dialectology, appeared in English. The word means “to downgrade in meaning or value”.
The state authorities in New Mexico and Illinois, where Clyde Tombaugh lived and worked, legislated to keep Pluto as a planet and declared March 13th as the annual Planet Pluto Day. Ordinary citizens responded with both online petitions and street protests. People who had thought of Pluto as a planet all their lives found it difficult to accept the astronomers’ decision. Besides, Pluto was the only planet discovered by an American.
Who benefits?
Pluto is the only one that has lost in status. The other dwarf planets, on the other hand, were previously classified as asteroids. Among them is Ceres (named after the Roman goddess of fertility), discovered back in 1801 by Italian astronomer Giuseppe Piazzi. For a time, Ceres was considered the missing planet between Mars and Jupiter. Still, it was later classified as an asteroid (by the way, the term was specifically coined after the discovery of Ceres and nearby large objects). The Astronomical Union decided in 2006 that Ceres was to be considered a dwarf planet.
Ceres, which has a diameter of about 950 km, is located in an asteroid belt, making it very difficult to observe. It is thought to have an icy mantle or even oceans of liquid water beneath its surface. A qualitative step in the study of Ceres was the Dawn mission, which reached the dwarf planet in the autumn of 2015.