Q&A: The IAU's Proposed Planet Definition

These questions were answered by the International Astronomical Union (IAU). They pertain to a draft resolution (which has not yet been voted on) for a new planet definition.

See Also:

• Full Story
• Gallery: The 12 "Planets"
• Read the Draft Resolution

Q: What is the origin of the word "planet"?
A: The word "planet" comes from the Greek word for "wanderer", meaning that planets were originally defined as objects that moved in the sky with respect to the background of fixed stars.
 
Q: Why is there a need for a new definition for the word "planet"?
A: Modern science provides much more knowledge than the simple fact that objects orbiting the Sun appear to move with respect to the background of fixed stars. For example, recent new discoveries have been made of objects in the outer regions of our Solar System that have sizes comparable to and larger than Pluto. (Noting that historically Pluto has been recognized as "the ninth planet.") Thus these discoveries have rightfully called into question whether or not they should be considered as new "planets."
 
Q: How did astronomers reach a consensus for a new definition of "planet"?
A: The world's astronomers, under the auspices of the International Astronomical Union, have had official deliberations on a new definition for the word "planet" for nearly two years. The results of these deliberations were channelled to a Planet Definition Committee comprising seven persons who were astronomers, writers, and historians with broad international representation. This group of seven convened in Paris in late June and early July 2006. They culminated the two year process by reaching a unanimous consensus for a proposed new definition of the word "planet."
 
Q: What new terms are proposed as official IAU definitions?
A: There are two new terms being proposed for use as official definitions of the IAU. The terms are: "planet" and "pluton".
 
Q: What is the proposed new definition of "planet"?
A: The new definition of "planet" recognizes and utilizes the capabilities of modern science. The new definition is based on the principle that we no longer need to rely on the simple definition from past millennia that an object is a "planet" if it moves against the background of fixed stars. Instead we can utilize our modern ability to measure the physical properties of an object to determine its true nature. An object is thus defined as a planet based on its intrinsic physical nature. Two conditions must be satisfied for an object to be called a "planet." First, the object must be in orbit around a star, while not being itself a star. Second, the object must be large enough (or more technically correct, massive enough) for its own gravity to pull it into a nearly spherical shape. The shape of objects with mass above 5 x 10 20 kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation.
 
Q: What is the exact wording of the official IAU proposed definition of "planet" in "Resolution 5 for GA-XXVI"?
A: "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."
 
Q: Does an object have to be in orbit around a star in order to be called a "planet"?
A: Yes.
 
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 from pole to pole.
 
Q: Based on this new definition, how many planets are there in our Solar System?
A: There are currently 12. Eight are the classical planets Mercury through Neptune. Three are in a newly defined (and growing in number) category called "plutons", for which Pluto is the prototype. One is Ceres, which may be described as a dwarf planet.
 
Q: What are the 12 planets?
A: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313 (provisional name).
 
Q: How are these 12 planets categorized?
A: There are eight "classical planets": Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Ceres is a planet, but because it is smaller than Mercury, one may describe it as a "dwarf planet". A new category of planet is now defined: "plutons". Pluto, Charon, and 2003 UB313 fall into the growing category of planets called "plutons".
 
Q: What are the classical planets?
A: The classical planets are those recognized by sky watchers and astronomers starting from the beginning of human history until the year 1900 A.D. The classical planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Note that the term "classical planet" is only a historical reference and not an IAU definition.
 
Q: What is a dwarf planet?
A: A dwarf planet is a term generally used to describe any planet that is smaller than Mercury. Note that the term "dwarf planet" is simply a descriptive category and not an IAU definition. Terms such as "terrestrial planets" and "giant planets" are additional examples of descriptive categories that are not IAU definitions.
 
Q: What is a "pluton"?
A: A pluton is a new category of planet now being defined by the IAU. A "pluton" is an object satisfying the technical (hydrostatic equilibrium shape in the presence of self-gravity) definition of "planet." Plutons are distinguished from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete (i.e. they orbit beyond Neptune). Plutons typically have orbits that are highly tilted with respect to the classical planets (technically referred to as a large orbital inclination). Plutons also typically have orbits that are far from being perfectly circular (technically referred to as having a large orbital eccentricity). All of these distinguishing characteristics for plutons are scientifically interesting in that they suggest a different origin from the classical planets.
 
Q: Where does the name "pluton" come from?
A: The name "pluton" comes from Pluto itself. Pluto is the first object discovered that is a large spherical body, and therefore a planet, orbiting beyond Neptune.
 
Q: Is a "pluton" a planet?
A: Yes.
 
Q: Is Pluto a "pluton"?
A: Yes.
 
Q: Is Pluto a planet?
A: Yes. In fact, Pluto's large companion named Charon is also large enough and massive enough to satisfy the definition of "planet". Because Pluto and Charon are gravitationally bound together, they are actually now considered to be a "double planet."
 
Q: Is Pluto a dwarf planet?
A: If one describes any planet smaller than Mercury as a "dwarf planet", then Pluto could be called a dwarf planet. Note that in terms of an IAU definition, Pluto is a "pluton." "Dwarf planet" is simply a descriptive category like "terrestrial planet" and "giant planet." None of these descriptive categories are formally defined by the IAU.
 
Q: What is a "double planet"?
A: A pair of objects, which each independently satisfy the definition of "planet" are considered a "double planet" if they orbit each other around a common point in space that is technically known as the "barycentre". In addition, the definition of "double planet" requires that this "barycentre" point must not be located within the interior of either body.
 
Q: What is a "satellite" of a planet?
A: For a body that is large enough (massive enough) to satisfy the definition of "planet", an object in orbit around the planet is called a "satellite" of the planet if the point that represents their common centre of gravity (called the "barycentre") is located inside the surface of the planet.
 
Q: The Earth's moon is spherical. Is the Moon now eligible to be called a "planet"?
A: No. The Moon is a satellite of the Earth. The reason the Moon is called a "satellite" instead of a "planet" is because the common centre of gravity between the Earth and Moon (called the "barycentre") resides below the surface of the Earth.
 
Q: Jupiter and Saturn, for example, have large spherical satellites in orbit around them. Are these large spherical satellites now to be called 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 of their massive planet. Regardless of the large size and shapes of these orbiting bodies, the location of the barycentre inside the massive planet is what defines large orbiting bodies such as Europa, Titan, etc. to be "satellites" rather than planets.
 
Q: Why is Pluto-Charon a "double planet" and not a "planet with a satellite"?
A: Both Pluto and Charon each are large enough (massive enough) to be spherical. Both bodies independently satisfy the definition of "planet". The reason they are called a "double planet" is that their common centre of gravity is a point that is located in free space outside the surface of Pluto. Because both conditions are met: each body is "planet-like" and each body orbits around a point in free space that is not inside one of them, the system qualifies to be called a "double planet."
 
Q: Pluto has at least two recently discovered additional satellites that are smaller than Charon. If these smaller satellites also orbit the "barycentre", does this make Pluto a "quadruple planet"?
A: No. The two newly discovered smaller bodies in orbit around Pluto are too small and not massive enough for their self-gravity to force them in to a spherical shape. Therefore neither of these bodies independently satisfies the definition of "planet." The fact that their size (mass) and shapes does not qualify them as planets implies that they must be called satellites, even though the center of gravity (called the "barycentre") about which they orbit is located outside the surface of Pluto.
 
Q: Can there be triple planets or quadruple planets?
A: Yes, but none are currently known.
 
Q: Is Ceres a planet?
A: Yes.
 
Q: Didn't Ceres used to be called an asteroid or minor planet?
A: Historically, Ceres was called a "planet" when it was first discovered (in 1801) orbiting in what is known as the asteroid belt between Mars and Jupiter. Because 19th century astronomers could not resolve the size and shape of Ceres, and because numerous other bodies were discovered in the same region, Ceres lost its planetary status. For more than a century, Ceres has been referred to as an asteroid or minor planet.
 
Q: Why is Ceres now being called a "planet"?
A: An object in orbit around a star is now being called a "planet" if it is large enough (or more technically, massive enough) for its own gravity to pull it into a nearly spherical shape. Recent Hubble Space Telescope images that resolve the size and shape of Ceres show it to be nearly spherical. More technically, Ceres is found to have a shape that is in a state of hydrostatic equilibrium under self-gravity. Therefore Ceres is a planet because it satisfies the IAU definition of "planet." [Published reference for shape of Ceres: P. Thomas et al. (2005), Nature 437, 224-227. Dr. Peter Thomas is at Cornell University.]
 
Q: Is Ceres a dwarf planet?
A: If one describes any planet smaller than Mercury as a "dwarf planet", then Ceres could be called a dwarf planet. Note that "dwarf planet" is simply a descriptive category like "terrestrial planet" and "giant planet." None of these descriptive categories are formally defined by the IAU.
 
Q: Is Ceres a "pluton"?
A: No.
 
Q: What is 2003 UB313?
A: "2003 UB313" is a provisional name given to a large object discovered in 2003 that resides in an orbit around the Sun beyond Neptune.
 
Q: Is 2003 UB313 a planet?
A: Yes.
 
Q: Why is 2003 UB313 a planet?
A: Recent Hubble Space Telescope images have resolved the size of 2003 UB313 showing it to be as large as, or larger than Pluto. Any object having this size, and any reasonable estimate of density, is understood to have sufficient mass that its own gravity will pull it into a nearly spherical shape determined by hydrostatic equilibrium. Therefore, 2003 UB313 is a planet because it satisfies the IAU definition of "planet."
 
Q: Will the new planet 2003 UB313 receive a name? When?
A: Yes. The International Astronomical Union has the official authority to assign names to objects in space. This object has been popularly called "Xena", but this is not an official IAU name. A decision and announcement of the new name are likely not to be made during the IAU General Assembly in Prague, but at a later time.
 
Q: Has the IAU ever named a planet?
A: No, so far not.
 
Q: Is 2003 UB313 a "pluton"?
A: Yes.
 
Q: Is 2003 UB313 a dwarf planet?
A: If one describes any planet smaller than Mercury as a "dwarf planet", then 2003 UB313 could be called a dwarf planet. Note that "dwarf planet" is simply a descriptive category like "terrestrial planet" and "giant planet." None of these descriptive categories are formally defined by the IAU.
 
Q: What is an object called that is too small to be a "planet"?
A: All objects that orbit the Sun, which are too small (not massive enough) for their own gravity to pull them into a nearly spherical shape are now collectively referred to as "small Solar System bodies." This collection includes the category of objects we continue to call asteroids and comets. This collection also currently includes, near-Earth objects (NEOs), Mars- and Jupiter-Trojan asteroids, most Centaurs and most Trans-Neptunian Objects (TNOs). In the new system of IAU definitions, the term "minor planet" is no longer used.
 
Q: Is the term "minor planet" still to be used?
A: No. The term "minor planet" is no longer to be used for official IAU purposes. The term will be replaced by "small Solar System bodies."
 
Q: Why is the term "minor planet" being replaced by "small Solar System bodies"?
A: Under the new definition of "planet", nearly all objects currently called "minor planets" are not planets. For IAU purposes, a definition and name is needed that clearly distinguishes between objects that are officially recognized as planets and those that are not.
 
Q: For any newly discovered object, how will a decision be reached on whether or not to officially call it a "planet."
A: The decision on whether or not an object is officially a "planet" will be made by a review committee within the IAU. The review process will be an evaluation, based on the best available data, of whether or not the physical properties of the object satisfy the definition of "planet." It is likely that for many objects, a period of time of several years may be required in order for sufficient data to be gathered.
 
Q: Are there additional "planet" candidates currently being considered?
A: Yes.
 
Q: Does this mean there will be more than 12 planets in our Solar System?
A: Almost certainly yes.
 
Q: When will additional new planets likely be announced?
A: When the responsible committee has had time to work on the issues after the resolution has been voted upon. Most likely any further new planet announcements will not be made until after the General Assembly in Prague.
 
Q: How many more new planets are there likely to be?
A: Perhaps as many as a dozen or two new planets in the IAU category called "plutons" remain to be discovered. We estimate this number based on our understanding of the current discovery statistics. This understanding is subject to change as new data continue to be gathered. [Some astronomers say there are hundreds of round objects beyond Neptune awaiting discovery.]
 
Q: When is an object too large to be called a "planet"?
A: The new definitions proposed by the IAU seek only to define the lower boundary between an object that is a "planet" or a "small Solar System body." At this time there is no official IAU definition in place or proposed that defines the upper limit for when an object is, for example a "planet" or a "brown dwarf." This limit is generally thought to be about 13 times more massive than Jupiter, but is subject to discussion.
 
Q: Is the new definition for "planet" intended to apply also to objects discovered in orbit around other stars?
A: Yes.
 
Q: Are objects that have planetary sizes and masses, but which are free floating in space (and not orbit a star) officially "planets" by the IAU definition?
A: No. At this time there is no official IAU definition in place that addresses this class of objects.
 
Q: Is a "pluton" a dwarf planet?
A: If one describes any planet smaller than Mercury as a "dwarf planet", then any pluton smaller than Mercury could be called a dwarf planet. Note that "dwarf planet" is simply a descriptive category like "terrestrial planet" and "giant planet." None of these descriptive categories are formally defined by the IAU.
 
Q: What is the difference between a "pluton" and a "plutino"?
A: A "plutino" is a small body that has a similar orbit to Pluto, that is, it has the same 248 year orbital period as Pluto. Plutino is not an official IAU description. All currently known plutinos are too small to be planets. Plutinos are not plutons.
 
Q: Is Pluto the ninth planet?
A: Historically it was indeed the ninth planet to be discovered, but now Pluto is also known as the first pluton, with its moon Charon being the second pluton. The classical planets can be numbered by their distance from the Sun, and there is no change in their order. Plutons, on the other hand, may due to their high eccentricity change their relative distances from the Sun with time (and hence their order).