What is the difference between a dwarf planet and a regular planet?

Answer 1

The other answers up to this point are wrong. The International Astronomical Union (IAU) defines a dwarf planet as one that is
* is in orbit around the Sun
* has sufficient mass for its self-gravity to allow it to have a near-spherical shape
* has not cleared the neighborhood around its orbit
* is not a satellite

#3 is an important one and is the main difference between a dwarf and a regular planet. If there is a lot of asteroidal or cometary debris that shares a body's orbit -- i.e., if the object didn't have a enough influence to sweep things out -- then it is a dwarf planet, not a regular one. That is the test that causes Pluto to fail the "regular planet" test.

Answer 2

A regular planet is an object that is round, has lots of mass, and orbits around the Sun (or a star) in a regular orbit. One example is our home planet Earth. A dwarf planet, however, is an object that is round (or potato-shaped), has little mass, and travels in an irregular orbit. One example is Pluto, which is very small (even smaller than Earth's Moon) and orbits irregularly beyond Neptune's orbit.

Answer 3

So far, all known dwarf planets are beyond the orbit of Neptune, in the Kuiper belt. I believe there is a size issue, however, I don't know what the limit is. The diameter of Pluto, Eris (formerly called Xena), Sedna and Quaoar are below 3000 km. By comparison, Mercury has a diameter of about 5000 km.

Answer 4

Size. A dwarf planet is smaller than a planet but bigger than an asteroid.

Answer 5

A dwarf planet it too small to be a planet but either too large to be a moon or isn't orbiting another planet.

Answer 6

It follows the general orbital rotation patterns of a planet, yet is too small to be considered a planet by definition.

Answer 7

They don't have their own orbit unlike a regular planet.

Answer 8

size

Answer 9

At the turn of the 21st century, the definition of "planet" became the subject of intense debate. Although the word itself dates back millennia, it had never had a formal definition until, beginning in 2005, a confluence of circumstances forced the astronomical community to take action.

From the end of the 19th century, "planet" had, without being defined, settled into a comfortable working term. It only applied to objects in the Solar System; a number small enough that any differences could be dealt with on an individual basis. After 1992 however, astronomers began to discover many additional objects beyond the orbit of Neptune, as well as hundreds of objects orbiting other stars. These discoveries not only increased the number of potential planets, but also expanded their variety and peculiarity. Some were nearly large enough to be stars, while others were smaller than Earth's moon. These discoveries challenged long perceived notions of what a planet could be.

The issue of a clear definition for "planet" came to a head in 2005 with the discovery of the trans-Neptunian object Eris, a body larger than the smallest then-accepted planet, Pluto. In its 2006 response, the International Astronomical Union (IAU), recognised by astronomers as the world body responsible for resolving issues of nomenclature, released its decision on the matter. This definition, which applies only to the Solar System, states that a planet is a body that orbits the Sun, is large enough for its own gravity to make it round, and has "cleared its neighbourhood" of smaller objects.

Pluto does not qualify as a planet under this definition, and the Solar System is thus considered to have eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The new category of dwarf planet was created, currently including Pluto, Eris, and Ceres. The IAU's decision has not resolved all controversies, however, and some in the astronomical community have rejected it outright. The issue of what constitutes a planet will likely remain contentious at least until 2009, when the IAU holds its next Congress in Rio de Janeiro.

The word "planet" has meant many different things in its long life, often simultaneously. Throughout its history, use of the term was never strict and its meaning has twisted and blurred to include or exclude a variety of different objects.

While knowledge of the planets likely predates history, the word "planet" itself dates to ancient Greece. Greek astronomers employed the term asteres planetai, "wandering stars", to describe those starlike lights in the heavens that moved over the course of the year, in contrast to the "fixed stars" which stayed motionless relative to one another. The five bodies known to the ancient world that are currently called "planets" are Mercury, Venus, Mars, Jupiter and Saturn. Greek cosmology is commonly thought to have consisted of seven planets, with the Sun and the Moon counted among them (as is the case in modern astrology); however, this does not appear to be the case. Although the Greeks, like all ancient civilisations, believed the Sun and the Moon to be in orbit around a stationary Earth along with the planets in accordance with the geocentric model, the main astronomical and astrological sources for the time appear to agree that their conception of the universe contained five planets, with the Sun and Moon considered separate. In his Almagest, Ptolemy refers to "the Sun, Moon and five planets," as does Plato in his Timaeus ("the Sun and Moon and five other stars, which are called the planets"). In his On the Heavens, Aristotle too makes a similar distinction; "The movements of the sun and moon are fewer than those of some of the planets". Hyginus explicitly mentions "the five stars which many have called wandering, and which the Greeks call Planeta." Marcus Manilius, a Greek philosopher who lived during the time of Caesar Augustus and whose poem Astronomica is considered one of the principal texts for modern astrology, says, "Now the dodecatemory is divided into five parts, for so many are the stars called wanderers which with passing brightness shine in heaven."

The seven-planet conception was not unheard of, even in classical times. Nonnus, the 5th century Greek poet, says in his Dionysiaca, "I have oracles of history on seven tablets, and the tablets bear the names of the seven planets."

Later medieval and Renaissance writers appear to have accepted the idea of seven planets. In his Confessio Amantis, 14th century poet John Gower, referring to the planets' connection with the craft of alchemy, writes, "Of the planetes ben begonne/The gold is tilted to the Sonne/The Mone of Selver hath his part...", indicating that the Sun and the Moon were planets. Even Nicholas Copernicus, who rejected the geocentric model, was ambivalent concerning whether the Sun and Moon were planets. In his De Revolutionibus, Copernicus clearly separates "the sun, moon, planets and stars"; however, in his Dedication of the work to Pope Paul III, Copernicus refers to, "the motion of the sun and the moon... and of the five other planets."

Eventually, when Copernicus's heliocentric model was accepted over the geocentric, Earth was placed among the planets and the Sun was dropped. It could be said, therefore, that Earth was the first planet of the modern era.

In 1781, the astronomer William Herschel was searching the sky for elusive stellar parallaxes, when he observed what he termed a comet in the constellation of Gemini. Unlike stars, which remained mere points of light even under high magnification, this object's size increased in proportion to the power used. That this strange object might have been a planet simply did not occur to Herschel; the five planets beyond Earth had been part of humanity's conception of the universe since antiquity. However, unlike a comet, this object's orbit was nearly circular and within the ecliptic plane. Before Herschel announced his discovery of his "comet", his colleague, 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. I have not yet seen any coma or tail to it." The "comet" was also very far away, too far away for a mere comet to resolve itself. Eventually it was recognised as the seventh planet and named Uranus after the father of Saturn.

Gravitationally induced irregularities in Uranus's observed orbit led eventually to the discovery of Neptune in 1846, and presumed irregularities in Neptune's orbit subsequently led to the search which ultimately located Pluto in 1930. Initially believed to be roughly the mass of the Earth, observation gradually shrank Pluto's estimated mass until it was revealed to be a mere five hundredth as large; far too small to have influenced Neptune's orbit at all. in 1989, Voyager 2 determined the irregularities to be due to an overestimation of Neptune's mass.

When Copernicus placed the Earth among the planets, he also placed the Moon in orbit around the Earth, making the Moon the first natural satellite to be discovered. When Galileo discovered his four satellites of Jupiter in 1610, they lent weight to Copernicus's argument, since if other planets could have satellites, then the Earth could too. However, there remained some confusion as to whether these objects were "planets"; Galileo initially intended to name them the "Medicean planets", in honour of his patrons, the Medicis. Similarly, Christiaan Huygens, upon discovering Saturn's largest moon Titan in 1655, employed many terms to describe it, including "planeta", (planet) "stella" (star) "Luna" (moon), and the more modern "satellite". Giovanni Cassini, in announcing his discovery of Saturn's moons Iapetus and Rhea in 1671 and 1672, described them as Nouvelles Planetes autour de Saturne ("New planets around Saturn"). However, when the "Journal de Scavans" reported Cassini's discovery of two new Saturnian moons in 1686, it referred to them strictly as "satellites". When William Herschel announced his discovery of two objects in orbit around Uranus, he referred to them as "satellites" and "secondary planets."

One of the unexpected results of William Herschel's discovery of Uranus was that it appeared to validate Bode's law, a mathematical function which generates the size of the semimajor axis of planetary orbits. Astronomers had considered the "law" a meaningless coincidence, but Uranus fell at very nearly the exact distance it predicted. Since Bode's law also predicted a body between Mars and Jupiter that at that point had not been observed, astronomers turned their attention to that region in the hope that it might be vindicated again. Finally, in 1801, astronomer Giuseppe Piazzi found a miniature new world, Ceres, lying at just the correct point in space. The object was hailed as a new planet.

Then in 1802, Heinrich Olbers discovered Pallas, a second "planet" at roughly the same distance from the Sun as Ceres. That two planets could occupy the same orbit was an affront to centuries of thinking; even Shakespeare had ridiculed the idea ("Two stars keep not their motion in one sphere"). Some years later, another world, Juno, was discovered in a similar orbit. Over the following decades, several more were discovered, all within relatively the same orbital distance.

Herschel suggested that these worlds be given their own separate classification, asteroids (meaning "starlike" since they were too small for their disks to resolve and thus resembled stars), though most astronomers preferred to refer to them as planets. Science textbooks in 1828, after Herschel's death, still numbered the asteroids among the planets. By 1851, the number of asteroids had increased to 15, and a new method of classifying them, by affixing a number before their names in order of discovery, was adopted, inadvertently placing them in their own distinct category. Ceres became " Ceres", Pallas became " Pallas", and so on. By the 1860s, observatories in Europe and the United States began referring to them as "minor planets", or "small planets", though it took the first four asteroids longer to be grouped as such. To this day, "minor planet" remains the official designation for all small bodies in orbit around the Sun (whether asteroid or not), and each new discovery is numbered accordingly in the IAU's Minor Planet Catalogue.

The long road from planethood to reconsideration undergone by Ceres is mirrored in the story of Pluto, which was named a planet soon after its discovery by Clyde Tombaugh in 1930. Uranus and Neptune had been declared planets based on their circular orbits, large masses and proximity to the ecliptic plane. None of these applied to Pluto; a tiny, icy world in a region of gas giants with an orbit that carried it high above the ecliptic and even inside that of Neptune. However, it was, as far as anyone could tell, unique. Then, beginning in 1992, astronomers began to detect large numbers of icy bodies beyond the orbit of Neptune that were similar in composition and size to Pluto. They concluded that they had discovered the long-hypothesised Kuiper belt (sometimes called the Edgeworth-Kuiper belt), a band of icy debris that is the source for "short-period" comets—those, like Halley, with orbital periods of up to 200 years.

Pluto's orbit lay right in the middle of this band and thus its planetary status was thrown into question; the precedent set by Ceres in downgrading an object from planet status because of a shared orbit led many to conclude that Pluto must be reclassified as a minor planet as well. Mike Brown of the California Institute of Technology suggested that a "planet" should be redefined as "any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit." The eight planets over that mass limit would be referred to as "major planets". There was outcry at the prospect of Pluto's "demotion", and in 1999 the International Astronomical Union clarified that it was not at that time proposing to change Pluto's status as a planet.



The relative sizes of the largest trans-Neptunian objects as compared to Earth.The discovery of several other trans-Neptunian objects approaching the size of Pluto, such as Quaoar and Sedna, continued to erode arguments that Pluto was exceptional from the rest of the trans-Neptunian population. On July 29, 2005, Mike Brown and his team announced the discovery of a trans-Neptunian object confirmed to be larger than Pluto, named Eris.

In the immediate aftermath of the object's discovery, there was much discussion as to whether it could be termed a "tenth planet". NASA even put out a press release describing it as such. However, acceptance of Eris as the tenth planet implicitly demanded a definition of planet that set Pluto as an arbitrary minimum size. Many astronomers, claiming that the definition of planet was of little scientific importance, preferred to recognise Pluto's historical identity as a planet by "grandfathering" it into the planet list.

The discovery of Eris forced the IAU to act on a definition. In October 2005, a group of 19 IAU members, which had already been working on a definition since the discovery of Sedna in 2003, narrowed their choices to a shortlist of three, using approval voting. The definitions were:

A planet is any object in orbit around the Sun with a diameter greater than 2000 km. (eleven votes in favour)
A planet is any object in orbit around the Sun whose shape is stable due to its own gravity. (eight votes in favour)
A planet is any object in orbit around the Sun that is dominant in its immediate neighborhood. (six votes in favour)
Since no overall consensus could be reached, the committee decided to put these three definitions to a wider vote at the IAU General Assembly meeting in Prague in August 2006, and on August 24, the IAU put a final draft to a vote, which combined elements from two of the three proposals. It essentially created a medial classification between "planet" and "rock" (or, in the new parlance, "small solar system body"), called "dwarf planet" and placed Pluto in it, along with Ceres and Eris. The vote was passed, though only 424 astronomers took part in the ballot.

“ The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:
(1) A "planet"1 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.

(2) A "dwarf 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) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects3 except satellites orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

Footnotes:

1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2 An IAU process will be established to assign borderline objects into either "dwarf planet" and other categories.
3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

Answer 10

One only has short people on it - duh!