Definition of planet

• 1. Introduction

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  The definition of planet has changed several times since ancient Greek astronomers first coined the term. They called wandering lights in the night sky asteres planetai, "wandering stars", distinguishing them from the fixed stars that never seemed to move relative to one another. For centuries the concept seemed perfectly adequate. Then in January 2005, astronomers discovered a body beyond Neptune more massive than Pluto, and the question that had always lurked beneath the surface could no longer be avoided: what exactly is a planet? The answer, it turned out, was far from obvious. It drew in ancient philosophers, Renaissance astronomers, rival scientific factions, children writing hate mail to a planetarium director, and a room of 424 astronomers in Prague casting a vote that still divides scientists today.

• 2. Wandering Stars

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  Plato, writing in his Timaeus in roughly 360 BCE, referred to "the Sun and Moon and five other stars, which are called the planets." His student Aristotle made the same distinction in On the Heavens, and the poet Aratus, versifying a treatise by the philosopher Eudoxus written in roughly 350 BCE, described five other orbs wheeling through the twelve figures of the Zodiac. The five bodies the Greeks had in mind were those visible to the naked eye: Venus, Jupiter, Mercury, Mars, and Saturn, listed from brightest to dimmest.

  Graeco-Roman cosmology counted seven planets in total by including the Sun and the Moon, though many ancient astronomers were ambivalent about that grouping. The 19th-century German naturalist Alexander von Humboldt captured the ancient tension in his work Cosmos, noting that only five of the seven cosmical bodies wore the appearance of stars, while the Sun and Moon were classed apart because of the size of their disks, their importance to human life, and their place in mythological systems.

  Marcus Manilius, a Latin poet who lived during the time of Caesar Augustus, referred to "the five stars called wanderers" in his poem Astronomica, one of the principal texts for modern astrology. Cicero's Dream of Scipio, written around 53 BCE, took the seven-planet view: the spirit of Scipio Africanus declares that "seven of these spheres contain the planets, one planet in each sphere." Pliny the Elder, writing in 77 CE, called them "the seven stars, which owing to their motion we call planets, though no stars wander less than they do." The phrase is a neat paradox that hints at how slippery the concept always was.

• 3. Copernicus Reshapes the Sky

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  Nicolaus Copernicus, who rejected the geocentric model, was himself ambivalent about where the Sun and Moon belonged. In De Revolutionibus he clearly separates "the sun, moon, planets and stars", yet in his dedication of the work to Pope Paul III he refers to "the motion of the sun and the moon... and of the five other planets", as if old habits of thought persisted even in the act of overturning them.

  Once the heliocentric model was accepted, Earth was placed among the planets, and the Sun and Moon were reclassified. The historian of science Thomas Kuhn, in his 1962 book The Structure of Scientific Revolutions, described the shift in terms of what a convert to Copernicanism would have to say about the Moon: "I once took the moon to be a planet, but I was mistaken." Galileo made the new position explicit in the Dialogue Concerning the Two Chief World Systems, writing that the Earth, "no less than the moon or any other planet, is to be numbered among the natural bodies that move circularly."

  With Earth now a planet, the Moon needed a new category. When Galileo discovered his four satellites of Jupiter in 1610, he called them "four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness." Christiaan Huygens, discovering Saturn's largest moon Titan in 1655, used a cluster of terms for it: planeta, stella, luna, and satellite, the last of which was a word coined by Johannes Kepler. Giovanni Cassini described his discoveries of Saturn's moons Iapetus and Rhea in 1671 and 1672 as Nouvelles Planetes autour de Saturne, "New planets around Saturn." The word satellite would eventually win out, but it took decades.

• 4. The Asteroid Problem

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  In 1781, William Herschel was searching for stellar parallaxes when he spotted what he took to be a comet in the constellation of Taurus. His colleague, the British Astronomer Royal Nevil Maskelyne, wrote to him saying, "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis." The object turned out to be the seventh planet, eventually named Uranus after the father of Saturn.

  Uranus appeared to validate Bode's law, a mathematical function predicting the sizes of planetary orbits. The law also predicted a body between Mars and Jupiter that had never been observed, and astronomers began looking. In 1801, Giuseppe Piazzi found Ceres at just the predicted location. It was hailed as a new planet. Then Heinrich Olbers discovered Pallas in 1802 at roughly the same orbital distance. Juno followed in 1804, and Olbers found Vesta in 1807.

  Herschel proposed giving these four worlds their own classification, asteroids, meaning "starlike", because they were too small for their disks to resolve. Most astronomers preferred to call them planets, and science textbooks in 1828, after Herschel's death, still listed them as such. The reclassification came not from a governing body but from a practical numbering system. When the asteroid count reached 15 by 1851, astronomers began affixing numbers before the names in order of discovery. Ceres became (1) Ceres, Pallas became (2) Pallas, and so on. The numbering inadvertently placed the asteroids in their own category. By the 1860s the known count had passed a hundred, and observatories in Europe and the United States began calling them collectively "minor planets" or "small planets."

• 5. Pluto and the Kuiper Belt

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  Clyde Tombaugh discovered Pluto in 1930, and it was named a planet shortly afterward. Uranus and Neptune had earned their planet status through circular orbits, large masses, and proximity to the ecliptic plane. Pluto had none of those qualities. It was tiny and icy, its orbit carried it high above the ecliptic and even inside Neptune's at times, and it sat in a region of gas giants where it did not belong by any obvious logic.

  In 1978, astronomers discovered Pluto's largest moon, Charon, which gave them a way to calculate Pluto's mass. The result was startling: Pluto was only one-sixth the mass of Earth's Moon. Then, beginning in 1992, astronomers began detecting large numbers of icy bodies beyond Neptune similar to Pluto in composition, size, and orbital characteristics. They had found the Kuiper belt, sometimes called the Edgeworth-Kuiper belt, a band of icy debris that serves as the source for short-period comets, those with orbital periods of up to 200 years.

  Mike Brown of the California Institute of Technology proposed that a planet should be redefined as any body more massive than the total mass of all other bodies in a similar orbit. In 1999, Brian G. Marsden of Harvard University's Minor Planet Center suggested giving Pluto the minor planet number 10000 while letting it retain its official status as a planet. The prospect of Pluto's demotion sparked a public outcry, and the International Astronomical Union clarified at the time that it was not proposing to remove Pluto from the planet list. That position would not hold.

• 6. Prague, August 2006

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  On the 29th of July 2005, Mike Brown and his team announced the discovery of Eris, a trans-Neptunian object confirmed to be more massive than Pluto. NASA briefly described it in a press release as a tenth planet. The International Astronomical Union could no longer avoid the definitional question.

  In October 2005, a group of 19 IAU members who had been working on a definition since the discovery of Sedna in 2003 narrowed their choices to three options using approval voting. The first, requiring a diameter greater than 2,000 km, received eleven votes. The second, requiring a shape stable under the object's own gravity, received eight. The third, requiring dominance of the object's immediate neighbourhood, received six. No consensus emerged, so all three went to a broader vote at the IAU General Assembly in Prague.

  On the 24th of August 2006, the IAU put a final draft to 424 astronomers. The result combined elements of two proposals and created a new middle classification, the dwarf planet, sitting between a full planet and a small Solar System body. Pluto, Ceres, and Eris all fell into it. The IAU further resolved that planets and dwarf planets are two distinct classes of objects, meaning dwarf planets would not count as planets despite the name. On the 13th of September 2006, Eris, its moon Dysnomia, and Pluto were entered into the Minor Planet Catalogue as (134340) Pluto, (136199) Eris, and (136199) Eris I Dysnomia.

  Neil deGrasse Tyson, director of the Hayden Planetarium, had already moved in this direction years earlier. When the planetarium underwent a renovation costing roughly $100 million in the early 2000s, Tyson chose to group planets by their commonalities rather than listing Pluto as the ninth planet. He received large amounts of hate mail for it, primarily from children. In 2009 he wrote a book detailing Pluto's demotion.

• 7. Clearing the Neighbourhood

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  Alan Stern, head of NASA's New Horizons mission to Pluto, circulated a petition arguing that fewer than 5 percent of astronomers voted for the IAU definition, making it unrepresentative of the full community. Stern objected that it is impossible and contrived to draw a dividing line between dwarf planets and planets, and pointed out that Earth, Mars, Jupiter, and Neptune have not entirely cleared their regions of debris either.

  Mike Brown countered that the major planets do not merely share their orbital zones passively but completely control the orbits of other bodies within them. Jupiter coexists with the Trojan asteroids, but those bodies remain in Jupiter's orbit only because they are held in place by the planet's gravity. Neptune long ago locked Pluto and the other Kuiper belt objects called plutinos into a 3:2 resonance, meaning they orbit the Sun twice for every three Neptune orbits.

  Steven Soter, professor of astrophysics at the American Museum of Natural History, made the quantitative case in the January 2007 issue of Scientific American. If each planet occupies an orbital zone, Mars, the least orbitally dominant planet, is larger than all other collected material in its orbital zone by a factor of 5,100. Ceres accounts for only about one third of the material in its orbit. Pluto's ratio is even lower, at around 7 percent. Brown said this difference left "absolutely no room for doubt about which objects do and do not belong."

  In October 2015, astronomer Jean-Luc Margot of the University of California, Los Angeles, proposed a metric for orbital clearance based on the mass of the host star, the mass of the body, and the body's orbital period. An Earth-mass body orbiting a solar-mass star can clear its orbit at distances of up to 400 astronomical units. The metric applies to extrasolar systems as well as the Solar System, and it leaves Pluto as a dwarf planet.

• 8. Brown Dwarfs and Free Floaters

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  Extrasolar planets, detected since 1992, pushed the definitional problem beyond the Solar System entirely. Many such planets approach the mass of small stars, while many newly discovered brown dwarfs are small enough to raise questions about whether they qualify as planets. The generally accepted difference between a low-mass star and a large gas giant rests on formation history: stars are said to have formed from the top down out of gases in a nebula through gravitational collapse, while planets formed from the bottom up through accretion of dust and gas around a young star.

  In 2003, an IAU working group released a position statement defining an extrasolar planet as any object with a true mass below the limiting mass for thermonuclear fusion of deuterium, currently calculated at 13 Jupiter masses, that orbits a star, brown dwarf, or stellar remnant. A free-floating object below that threshold is a sub-brown dwarf, not a planet, even if the two objects are in all other respects identical. Location, not composition or formation, becomes the determining characteristic.

  That threshold has since been questioned. A 2010 paper by Burrows, David S. Spiegel, and John A. Milsom showed that a brown dwarf of three times solar metallicity could fuse deuterium at as low as 11 Jupiter masses. As of 2016 the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, and the limit for that catalogue was later raised to 60 Jupiter masses based on mass-density relationships.

  In 2012, Philippe Delorme of the Institute of Planetology and Astrophysics of Grenoble announced the discovery of CFBDSIR 2149-0403, a free-floating object of 4-7 Jupiter masses likely part of the AB Doradus moving group, less than 100 light years from Earth. In October 2013, astronomers led by Dr. Michael Liu of the University of Hawaii discovered PSO J318.5-22, a solitary free-floating object estimated at only 6.5 times the mass of Jupiter, making it the least massive sub-brown dwarf yet found. Whether objects like these are rogue planets ejected from their star systems, or whether they formed as stars do from gravitational collapse, remains an open question.