The discovery of Pluto
In early 1930, an astronomer working at the Lowell Observatory in the United States made a discovery that would ultimately initiate a dramatic change in the way we look at our Solar System. The young astronomer was Clyde Tombaugh, an observing assistant working at the observatory made famous by the great astronomer Percival Lowell. Tombaugh was continuing the search for an elusive planet — Planet X — that astronomers of the time had incorrectly believed to be responsible for perturbing the orbits of Uranus and Neptune.
After spending numerous nights at the telescope and months tediously scanning his data for the telltale signs of a planet – slight movements of the same body between two images (in those days, on photographic plates) of the sky taken at different times — Tombaugh made a discovery. While comparing two photographic plates to search for this slight movement, Tombaugh noticed a small object moving only a few millimetres near the constellation Gemini. Tombaugh had found his new planet! (Stern & Mitton, 2005)
The developing landscape of the Solar System
The object Tombaugh discovered was eventually named Pluto, a name officially adopted by the American Astronomical Society, the Royal Astronomical Society in the UK, and the International Astronomical Union. It is a frigid world, billions of kilometres from the Sun, and 30 times less massive than the then-smallest known planet, Mercury. But Pluto is not alone: its five satellites were later discovered. The largest, Charon, was discovered in 1978 (Buie et al., 2006). The smaller four were discovered using the Hubble Space Telescope between 2005 and 2012 (Stern et al. 2018) and are officially named Nix, Hydra, Kerberos and Styx by the IAU (Aksnes, 2006; Showalter et al., 2013).
In the decades following Pluto’s discovery, astronomers postulated that there might be a belt of objects beyond the orbit of Neptune. In 1992, this was finally validated by David Jewitt and Jane Luu from the University of Hawaii. They discovered (Jewitt and Luu 1993) the first in a special class of objects orbiting beyond Neptune, known as Kuiper Belt Objects, which have major implications for Solar System formation theory. Today, we know of more than 1000 objects that orbit in the so-called transneptunian region; these bodies are often referred to as Trans-Neptunian Objects (TNOs).
Due to the influx of TNO discoveries, it was inevitable that one or more might be found to rival Pluto in size. In 2003, Mike Brown (Caltech), Chad Trujillo (International Gemini Observatory) and David Rabinowitz (Yale University) searched the edge of our Solar System from the Palomar Observatory in the United States. They imaged a region of the sky that showed an object moving relative to the background stars, just as Clyde Tombaugh did decades earlier. Later analysis showed that they discovered another cold world: slightly larger than Pluto and orbiting the Sun (Brown et al., 2004). Subsequent observations (Brown, 2006) showed that the new object, initially named 2003 UB313 according to the IAU's naming protocol, was more massive than Pluto and that it, too, had a satellite. The team found several objects that did not resemble any of the 8 planets we consider today but were large enough to be compared to Pluto. These discoveries prompted astronomers to ask the question: "What constitutes a planet?"
The process of defining a planet
The IAU has been responsible for naming planetary bodies and their satellites since the early 1900s. As Professor Ron Ekers, former President of the IAU, explained in the newspaper of the 2006 IAU General Assembly:
Such decisions and recommendations are not enforceable by any national or international law; rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned. (p. 4)
The IAU created a committee to gather opinions from a broad range of scientific interests, with input from professional astronomers, planetary scientists, historians, science publishers, writers and educators. Thus the Planet Definition Committee of the IAU Executive Committee was formed to carefully gather these opinions and deliberations over the course of two years. The Committee prepared a draft resolution to present to the members of the IAU. After the final meeting in Paris, the draft resolution was completed. Professor Owen Gingerich, Chair of the IAU Planet Definition Committee, described one crucial aspect of the resolution: “we wanted to avoid arbitrary cut-offs simply based on distances, periods, magnitudes, or neighbouring objects” (Gingerich, 2006). The adopted definition is, instead, based on our understanding of the physics of planet formation.
The definition of a planet
The word planet comes from the Greek word for wanderer: planets were originally defined as objects that moved in the night sky with respect to a background of fixed stars. It wasn’t until the 2006 IAU General Assembly in Prague that astronomers attempted to agree upon a formal definition of the word. Modern science provides a wealth of information, and many scientific studies have been used to help astronomers formulate their definition. For example, observations in the outer regions of the Solar System found objects that are comparable in size to Pluto. These discoveries, and more, called into question whether these newly found objects should also be considered planets or if they should comprise their own class of objects in the Solar System.
The draft proposal for the definition of a planet was debated vigorously by astronomers at the 2006 IAU General Assembly, and a new version slowly took shape. This new version was more acceptable to the majority and was presented to the members of the IAU for a vote at the Closing Ceremony of the General Assembly. By the end of the Prague General Assembly, IAU members voted that the definition of a planet in the Solar System would be as follows:
A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (p. 1)
More generally, a planet:
- orbits its host star, just as the Earth and Jupiter orbit the Sun,
- is large enough to be mostly round, and
- must have an important influence on the orbital stability of the other objects in its neighbourhood.
Newly discovered objects will be classified by a review committee within the IAU. The review process will include an evaluation based on the best available data that demonstrate whether the physical properties of the object satisfy the IAU definitions. It is likely that for many objects, several years may be required to gather sufficient data.
Today, the resolution remains in place and is a testament to the fluid nature of science and how our view of the Universe continues to evolve with changes made by observations, measurements and theory. The continued and vibrant discussions on the topic also show how hard it is to classify objects into well-defined classes. The Universe does not like to be put in “boxes”. Everything is a continuum: comets can behave like asteroids, and brown dwarfs can be stars. So, too, can dwarf planets share the same qualities as planets.
The definition of a dwarf planet
The 2006 IAU Resolution means that the Solar System officially consists of eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. In the same resolution, a new distinct class of objects, called dwarf planets, was also defined. A dwarf planet is an object in orbit around the Sun that is large enough to pull itself into a nearly round shape but has not been able to clear its orbit of debris. Generally, dwarf planets are smaller than Mercury. A dwarf planet may also orbit in a zone that has many other objects in it. For example, an object within the asteroid belt is in a zone with many other objects that are all about the same size. Pluto now falls into the dwarf planet category because it resides within a zone of other objects that might cross its orbital path, known as the Trans-Neptunian region. Pluto is additionally recognised as an important prototype of a new class of Trans-Neptunian Objects: plutoids. Defining this class of objects helps astronomers distinguish different types of dwarf planets in our Solar System.
The term small Solar System body was introduced along with dwarf planet in the 2006 IAU General Assembly resolution. This is a term to encompass all objects orbiting the Sun that are too small (that is, not sufficiently massive) to satisfy the definition of a planet or dwarf planet.
Dwarf planets in our Solar System
Aside from Pluto, there are four currently recognised dwarf planets in our Solar System: Ceres, Haumea, Makemake and Eris.
When Ceres was first discovered orbiting within the asteroid belt between Mars and Jupiter in 1801, it was called a planet. However, due to the technology at the time, astronomers could not resolve the size and shape of Ceres, and as numerous other bodies were discovered in the same region, Ceres lost its planetary status. As technology improved, astronomers discovered that Ceres is a bit less than half the size of Pluto and is large enough to have self-gravity, pulling itself into a nearly round shape (Thomas, 2005). Because it orbits the Sun within the asteroid belt, Ceres is a good example of an object that does not orbit in a clear path: there are many asteroids that can come close to the orbital path of Ceres.
In the early 2000s, astronomers used the Hubble Space Telescope to observe a TNO originally named 2003 UB313 but later renamed Eris, after the Greek god of discord and strife. The data showed that Eris is as large as, or perhaps larger than, Pluto (Brown, 2006) and, importantly, that it has a satellite, later named Dysnomia after Eris’s daughter, the Greek demon of lawlessness. By tracking the motions of Dysnomia around Eris, astronomers were able to measure the mass of Eris very accurately. They found that Eris is almost 30% more massive than Pluto! Because Eris orbits in the Trans-Neptunian region and other TNOs might come close to its orbit, Eris is considered a dwarf planet.
Around the same time, Haumea and Makemake were also discovered. Like all TNOs, and according to IAU naming guidelines, these two objects were eventually named for mythological beings associated with creation: Makemake is the creator of humanity to the Rapa Nui, and Haumea is the Hawaiian goddess of fertility and childbirth. Haumea is thought to be oblong, covered in ice, have two moons (Hi’iaka and Namaka), and a ring. Makemake is a round, reddish world with a single moon, currently designated S/2015 (136472) 1. It is the second brightest — and second largest — Kuiper Belt object, other than Pluto.
This isn’t the end of the story for dwarf planets in our Solar System. In the coming years, some of the largest asteroids and some Trans-Neptunian Objects may be considered for dwarf planet status. The precise number is still unknown, as we are always discovering new small bodies in our Solar System. There may be dozens or perhaps even more than a hundred waiting to be discovered!
On 14 July 2015, NASA's New Horizons spacecraft flew past Pluto (Young et al., 2008), providing numerous observations that have dramatically altered our knowledge about the dwarf planet Pluto and its five moons. The images established that Pluto’s diameter is actually larger than that of Eris, despite what was previously thought. As such, Pluto regained its spot as the largest TNO. The images also revealed a remarkable landscape containing a variety of landforms, including broad plains, mountain ranges several kilometres high, and evidence of volcanoes.
Pluto is unusual for its diversity of surface compositions and colours. Some regions are as bright as snow, and others are as dark as charcoal. Colour imaging and other data revealed a highly complex distribution of surface ices, including nitrogen, carbon monoxide, water, and methane, as well as their chemical byproducts produced by prolonged exposure to solar radiation. In addition, scientists have found that some surfaces on Pluto are entirely free of visible craters, indicating that they have been created or have changed in the recent past. On the other hand, other surfaces are heavily cratered and appear to be extremely old. Pluto is shrouded by a cold, nitrogen-dominated atmosphere that contains a thin haze layer that extends about 150 km above Pluto’s surface.
New Horizons also got a good look at Pluto’s moons. It found that the largest moon, Charon, displays impressive tectonics, a puzzlingly dark polar terrain, a potentially differentiated internal structure, and no signs of an atmosphere (Moore et al., 2016). New Horizons discovered puzzling new facts (Lakdawalla, 2015) about the smaller satellites Hydra and Nix, too: their surfaces are brighter than expected, meaning their surfaces contain a high amount of water ice. No new satellites were detected, and no rings were found. The environment around Pluto is clearly a fascinating one!
These results raise fundamental questions about how a small, cold object can remain active over the age of the Solar System. They demonstrate that dwarf planets can be every bit as scientifically interesting as planets.
We would like to acknowledge the contributions of Division F President, Antonella Barucci, Division F Past President, Nader Haghighipour, Organizing Committee Members of Commission F4 Asteroids, Comets & Transneptunian Objects, Joe Masiero and Jorge Marcio Ferreira Carvano, and the IAU Director of Communications, Lars Lindberg Christensen (originator of the first version of this Theme) and OAO Deputy Director Kelly Blumenthal.
Frequently Asked Questions
Q: What new terms are used in the official IAU definition?
A: There are three new terms adopted as official definitions by the IAU. The terms are planet, dwarf planet and small Solar System body.
Q: Does a body have to be perfectly spherical to be called a planet?
A: No. For example, the rotation of a body can slightly distort the shape so that it is not perfectly spherical. Earth, for example, has a slightly greater diameter measured at the equator than measured at the poles.
Q: Is that all, only eight planets?
A: No. In addition to the eight planets, there are also five known dwarf planets: Pluto, Ceres, Eris, Makemake, and Haumea. Many more dwarf planets are likely to be discovered soon.
Q: Jupiter and Saturn, for example, have large spherical satellites in orbit around them. Are these large spherical satellites now to be called dwarf planets?
A: No. All of the large satellites of Jupiter (for example, Europa) and Saturn (for example, Titan) orbit around a common centre of gravity (called the barycentre) that is deep inside their massive planet. Regardless of the large size and shapes of these orbiting bodies, the location of the barycentre inside the massive planet is one of the key factors that defines large orbiting bodies such as Europa and Titan as satellites rather than planets.
Q: Jupiter has asteroids leading and trailing its orbit. Should Jupiter be considered a dwarf planet because it has not cleared its orbit?
A: No. One of the main theories for the formation of this class of asteroids is that they were captured by Jupiter as it migrated to its present location early in the history of the Solar System. The asteroids are clustered around two stable points in Jupiter’s orbit and have been corralled into their present orbits by Jupiter’s gravitational force. These objects clearly have very different properties than those found in the “unclear” orbits of dwarf planets.
Q: What is an object called that is too small to be either a planet or dwarf planet?
A: All objects that orbit the Sun that are too small to form a nearly spherical shape are now defined as small Solar System bodies. This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars and Jupiter Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs) and comets.
Q: Is the term minor planet still to be used?
A: The term minor planet may still be used. Generally, however, the term small Solar System body is preferred.
Q: Are there additional planet candidates currently being considered?
A: No. None appear likely in our Solar System. However, there are planet discoveries galore around other stars.
Q: What are plutoids?
A: Plutoids (Bowell et al., 2008) are celestial bodies in orbit around the Sun at a distance greater than Neptune that are sufficiently large to form a nearly spherical shape and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they are massive enough to be nearly round. The two known and named plutoids are Pluto and Eris. We expect that more plutoids will be named as science progresses and new discoveries are made.
Q: Can a satellite orbiting a plutoid be a plutoid too?
A: No, according to the IAU Resolution B5, a dwarf planet cannot be a satellite, even if it is massive enough that its shape is dictated by self-gravity.
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