What is the NEW definition of a planet?

Answer 1

DEFINITIONS

The International Astronomical Union held a meeting in the Czech capital, Prague on 24th August at which it passed three new definitions, of a planet, a dwarf planet and a Small Solar System Body. The IAU definitions are:

PLANET: a celestial body having all the following attributes as a "planet". It:

(1) is in orbit around a star or stellar remnants;
(2) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape;
(3) is above the minimum mass/size for a planet in our solar system*;
(4) if in our solar sytem it has cleared the neighbourhood around its orbit; and
(5) is below the limiting mass for thermonuclear fusion of deuterium.

(*What this mass/size is has not been specifically defined. However, it must be somewhere between the mass required in (2) above, and the mass of the planet Mercury.)

Our solar system is considered to have eight planets under this definition: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Three bodies in orbit around our Sun and which only fulfil conditions (1) and (2), but not (4), and are not natural satellites, are now classified as dwarf planets: Ceres, Pluto and Eris. To date, there have been more than two hundred planets discovered orbiting other stars.

The IAU further defines any celestial object having any of the following attributes as NOT being a planet:

(1) it is below the minimum mass/size for a planet in our solar system;
(2) it is above the limiting mass for thermonuclear fusion of deuterium; or
(3) it is a free-floating object in a young star cluster.

(MY INTERJECTION: the point about fusing deuterium is to distinguish planets from brown dwarf stars, which can be as small as about twelve Jupiter masses.)

DWARF PLANET: a dwarf planet as an object 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
c) Has not "cleared the neighbourhood" around its orbit
d) Is not a satellite of a planet, or other nonstellar body

SMALL SOLAR SYSTEM BODY: A small solar system body (SSSB) is a term defined in 2006 by the International Astronomical Union to describe solar system objects which are not planets or dwarf planets:

"All other objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies" ... These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies."

Therefore it refers to these objects that can be further classified based on their orbit or composition:

all known minor planets that are not dwarf planets, i.e.:
i) the classical asteroids (except the largest one, 1 Ceres);
ii) the Centaurs and Neptune Trojans;
iii) the smaller Trans-Neptunian Objects (except dwarf planets such as Pluto and Eris);
iv) all comets;

It is not yet clear whether there will be a lower bound on the group of small solar system bodies, or if it will encompass all material down to the level of meteoroids.

Some of the larger "small solar system bodies" may be reclassified in future as dwarf planets, pending further examination to determine whether or not they are in hydrostatic equilibrium.

MY COMMENT: All rather legalistically phrased and a little dry to read, but that is because these were formal motions put to a General Assembly for approval.

Well, you did ask!

Answer 2

http://www.iau.org/fileadmin/content/pdfs/Resolution_GA26-5-6.pdf

the new definition of planet is essentially a body that orbits the sun, is massive enuf to be ball-shaped, and has an isolated orbit (no other similar bodies have similar orbits). i don't know how long this will drag on tho, many planetary astronomers are not satisfied that the definition is rigorous enuf. i can accept that the definition is flawed, but i can not accept that pluto is a planet. i felt somewhat satisfied with the reclassification, but this haggling is disappointing.

i have been waiting for this since i was about ten when i learned that pluto did not fit the pattern set by the other bodies in the solar system. it is an anomaly. it is "out of place". now that astronomers have discovered hundreds of other similar bodies with similar orbits, classifying pluto as a planet is even more irrational. this does not change anything about the solar system or pluto. it just corrects the mistake of classifying pluto as a planet initially. this is the right thing to do.

Answer 3

The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as 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.

This means that the Solar System consists of eight "planets" Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. The first members of the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name). More "dwarf planets" are expected to be announced by the IAU in the coming months and years. 


Pluto fails criterion c because it has many neighbors of similar mass.

Aloha

Answer 4

1) Planet — A celestial body that orbits the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape, and has cleared the neighbourhood around its pathway.

2) Dwarf planet or planetoid — A celestial body that orbits the sun, has sufficient mass for its self-gravity to overcome rigid body forces to assume a nearly round shape but has not cleared the neighbourhood around its orbit, and is not a satellite.

3) Small solar system bodies — All other objects orbiting the sun.


I hope this answers your question.

Answer 5

Not some squitty little thing like Pluto.