What Defines a Planet?

  • Thread starter scott1
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In summary: It turns out that the IAU's final definition was not their original proposal, but a compromise between the eight-planet-only group and the nine-planet group. Their original proposal was to have eleven planets, and to keep the name "pluton" for the new class of objects. This compromise was reached after four days of heated debate and multiple votes.So now, in summary, the IAU has officially defined a planet as a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet. This definition includes the eight classical planets

What is a planet

  • pluto & charon is a double planet

    Votes: 4 22.2%

  • ceres is a planet

    Votes: 2 11.1%

  • 2003 UB<sub>313 </sub> is a planet

    Votes: 3 16.7%

  • Pluto & charon is not a double planet

    Votes: 4 22.2%

  • Pluto is not a planet

    Votes: 7 38.9%

  • ceres is not a planet

    Votes: 7 38.9%

  • 2003 UB<sub>313</sub> is not a planet

    Votes: 2 11.1%

  • It should be kept the same

    Votes: 3 16.7%

  • other(please explain)

    Votes: 2 11.1%
  • Total voters
    18
  • #1
scott1
350
1
The IAU therefore resolves that planets and other Solar System bodies be defined in the following way:



(1) 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) shape1, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.2



(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a "dwarf planet."3



(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call "plutons".



(4) All non-planet objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".4
http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_resolution.html"

I'm ok with Pluto and Charon being a planet and 2003 UB313but crers? what were they thinking!
 
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  • #2
What a strange poll. Your options #3 and #7 appear to be identical.
 
  • #3
OK... so how many planets are there now? The definition still seems like quite a mess >_<
 
  • #4
SpaceTiger said:
What a strange poll. Your options #3 and #7 appear to be identical.
woops it should say it's not a planet
 
  • #6
Criterion 3 says "as are one or more recently discovered large trans-neptunian objects"... so how many of those are we talking about? And how large is large? Are Sedna and Quaoar counted as planets?
 
  • #7
yenchin said:
Criterion 3 says "as are one or more recently discovered large trans-neptunian objects"... so how many of those are we talking about? And how large is large? Are Sedna and Quaoar counted as planets?
Maybe 12 are being conisder here's a picture of them:
http://www.iau2006.org/mirror/www.iau.org/iau0601/large/iau0601c.jpg
 
  • #8
yenchin said:
OK... so how many planets are there now? The definition still seems like quite a mess >_<

Depends on whom you ask. According to this article;

http://space.com/scienceastronomy/060816_planet_definition.html"


Mike Brown (discoverer of the 12th planet) puts the current count at 53 objects which would satisfy the new definition, "with countless more to be discovered".
 
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  • #9
A planet is a body that is big enough to have cleared all significantly sized objects in similar orbits out of its orbit around a star. Mercury to neptune have managed to do this. pluto is one of many smaller objects moving in their own orbits like the minor planets and is such a minor planet or asteroid. Pluto/charon is also I believe in an orbit with a gravitational resonance with Neptune so it is in some way a very loosely coupled satellite of Neptune.

Most astronomers had considered this to be likely since it was discovered but had not put the effort into looking for other similar objects. Recent interest in near Earth orbit objects and telescope technology and automation has made the search for small faint slow moving objects much easier, so expect lots more to be found in the next few years.
 
  • #12
The category pluton is in, but the name pluton is out.

Also, the "small solar system bodies" name (even SSSBs for short) reeks of the suckitude that comes from having astronomers with no sense of creativity, joy or liberal arts educations.

So, what would be better names:

For Plutons

Demoids? (from Demon on the Neptune, Pluto, Charon theme).
Plutoroids? (from Pluto and Asteroid)
Pegasi? (Pegasus, the flying horse, was a child of Neptune).
Chtons? (For the Chthonic dieties who inhabit the underworld).

For Minor Solar System Bodies

Apollites? (from Apollo god of the Sun)
 
  • #13
the BBC article
http://news.bbc.co.uk/1/hi/world/5282440.stm
said that things like Pluto will not be called "plutons" (that's a geology term already) but instead they
will be officially known as DWARF PLANETS
I don't think that is especially elegant or imaginative nomenclature and it looks like ohwilleke isn't happy with it either.

ohwilleke said:
So, what would be better names:

For Plutons

Demoids? (from Demon on the Neptune, Pluto, Charon theme).
Plutoroids? (from Pluto and Asteroid)
Pegasi? (Pegasus, the flying horse, was a child of Neptune).
Chthons? (For the Chthonic dieties who inhabit the underworld).
So I propose those smaller bodies out around and beyond Neptune should be DOLPHINS
 
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  • #14
Resolution 6 of the IAU, in addition the the main resolution 5 (about planet and dwarf planet status) says that in addition to the dwarf planet designation that dwarf planets like Pluto. Thus, the pluton thing will stay.
 
  • #15
Here's an interesting story on the vote.

http://news.bbc.co.uk/2/hi/science/nature/5283956.stm"
 
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FAQ: What Defines a Planet?

What is the new definition of a planet?

The new definition of a planet, as set by the International Astronomical Union in 2006, states that a planet is a celestial body that orbits the sun, has enough mass to be nearly round, and has cleared the neighborhood around its orbit.

Why was the definition of a planet changed?

The definition of a planet was changed due to the discovery of many objects in our solar system that were previously classified as planets, such as Pluto, that did not meet the new criteria. This new definition aims to more accurately classify objects in our solar system and beyond.

How many planets are there under the new definition?

Under the new definition, there are currently 8 planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

What are the requirements for a celestial body to be considered a planet?

To be considered a planet, a celestial body must meet three criteria: it must orbit the sun, it must have enough mass to be nearly round, and it must have cleared the neighborhood around its orbit of any other objects.

Are there any exceptions to the new definition of a planet?

Yes, there is one exception to the new definition of a planet. Dwarf planets, such as Pluto, meet the first two criteria but have not cleared their orbit. They are classified separately from planets but are still considered important objects in our solar system.

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