Questions and Answers on Planets

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 night 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 and ultimately proposed to the IAU General Assembly. Continued evolution of the definition allowed a final consensus and vote.
 
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: In plain language, what is the new definition of “planet”?
A: A “planet” is an object in orbit around the Sun that is large enough (massive enough) to have its self-gravity pull itself into a round (or nearly round) shape. In addition a “planet” orbits in a clear path around the Sun – there are no other bodies in its path that it must sweep up as it goes around the Sun.
 
Q: What is the exact wording of the official IAU proposed definition of “planet”?
A: A “planet” is 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.
 
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: Based on this new definition, how many planets are there in our solar system?
A: There are eight planets in our Solar System; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. “My very educated mother just served us nachos.”
 
Q: Is that all, only eight planets?
A: No. In addition to the eight planets, there are also three known “dwarf planets”. Many more “dwarf planets” are likely to be discovered soon.

Q: What is a dwarf planet?
A: A dwarf planet is an object in orbit around the Sun that is large enough (massive enough) to have its own gravity pull itself into a round (or nearly round) shape. Generally, a dwarf planet is smaller than Mercury. A dwarf planet may also orbit in a zone that has many other objects in it. For example, an orbit within the asteroid belt is in a zone with lots of other objects.
 
Q: How many dwarf planets are there?
A: Currently there are three known dwarf planets. Ceres, Pluto, and 2003 UB 313.
 
Q: What is Ceres?
A: Ceres is (or now we can say it was) the largest asteroid, about 1000 km across, orbiting in the asteroid belt between Mars and Jupiter. Ceres now qualifies as a dwarf planet because it is now known to be large enough (massive enough) to have self-gravity pulling itself into a nearly round shape. [Published reference for shape of Ceres: P. Thomas et al. (2005), Nature 437, 224-227. Dr. Peter Thomas is at Cornell University.] Ceres orbits within the asteroid belt and is an example of a case of an object that does not orbit in a clear path. There are many other asteroids that can cross the orbital path of Ceres.
 
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 19 th 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 Pluto now called a dwarf planet?
A: Pluto now falls into the dwarf planet category on account of its size and the fact that it resides within a zone of other objects, known as the Kuiper Belt.
 
Q: Is Pluto’s moon Charon a dwarf planet?
A: For now, Charon is considered just to be Pluto’s moon. The idea that Charon might qualify to be called a dwarf planet on its own, may be considered later. (Charon may receive consideration because Pluto and Charon are comparable in size and orbit each other, rather than just being a moon orbiting a planet.)
 
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 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: What is 2003 UB 313?
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 UB 313 a planet?
A: No. It is a dwarf planet.
 
Q: Why is 2003 UB 313 a dwarf planet?
A: Recent Hubble Space Telescope images have resolved the size of 2003 UB 313 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. 2003 UB 313 also orbits within the Kuiper Belt – a region that has not been cleared out. Therefore, 2003 UB 313 is a dwarf planet. [Published reference: M. Brown et al. (2006). Astrophysical Journal643, L61-L63. Dr. Brown Michael Brown is at the California Institute of Technology.]
 
Q: Will the new dwarf 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 name. A decision and announcement of the new name are likely to be made within a few months.
 
Q: What is an object called that is too small to be a planet or dwarf 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 defined as being “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: What is a “small solar system body”?
A: The term “small solar system body” is a new IAU definition to encompass all objects orbiting the Sun that are too small (not sufficiently massive) to satisfy the definition of planet or dwarf planet.
 
Q: Is the term “minor planet” still to be used?
A: The term “minor planet” may still be used. But generally the term “small solar system body” will be preferred.
 
Q: For any newly discovered object, how will a decision be reached on whether or not to officially call it a planet, dwarf planet, or other?
A: The decision on how to classify newly discovered objects 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 definitions. 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: No. None appear likely in our Solar System. But there are planet discoveries galore around other stars.
 
Q: Are there additional “dwarf planet” candidates currently being considered?
A: Yes. Some of the largest asteroids may be candidates for “dwarf planet” status and some additional “dwarf planet” candidates beyond Neptune will soon be considered. The total number of dwarf planets to be found in the coming months and years could reach to over 100.
 
Q: When will additional new dwarf planets likely be announced?
A: Probably within a few months.
 
Q: How many more new dwarf planets are there likely to be?
A: There may be dozens or perhaps more than 100 waiting to be discovered.

Additional information
Press release (iau0603)