Register to reply

Why is there only 8 planets now

by poppop24
Tags: planets
Share this thread:
Aug30-06, 07:57 PM
P: 2
Please explain why there is no pluto any more
Phys.Org News Partner Astronomy news on
The Great Debate over whether the universe is small or large
Smallest known galaxy with a supermassive black hole found
Mystery of rare five-hour space explosion explained
Aug30-06, 08:09 PM
Sci Advisor
PF Gold
chroot's Avatar
P: 10,427
The International Astronomical Union recently changed the official definition of "planet." Pluto does not meet this new definition. Pluto still exists, but it is no longer considered a planet.

- Warren
Aug31-06, 02:27 AM
P: 1,157
I thought that 12 planets met the new criterion - including Ceres and the one which orbits around a mutual point with Pluto...

<read it in the Guardian the other week>

Can't find the link to my story yet. Here's an official 8 planet one:

Ahhh... the 12 planet thing seems older:

or not:

Aug31-06, 06:09 AM
Sci Advisor
P: 789
Why is there only 8 planets now

The alternatives were (basically) to include three new planets, or take away Pluto, and the later was chosen...
Aug31-06, 08:10 AM
Sci Advisor
P: 2,510
I doubt it will last for long, though. The new definition is very sketchy, and depends upon a body having "cleared out" the immediate vicinity of its orbit. Pluto was not disqualified because of its size, or the eccentricity of its ellipse, or its inclination to the orbital plane, but because it crosses Neptune's orbital radius. By this definition, Neptune (having failed to "clear out" Pluto from its orbit) should also be disqualified.

In fact, because there are still thousands of NEO's whizzing about the inner solar system, many of them crossing inside the orbit of Venus, the new definition may reduce the entire solar system to only one planet: Mercury!
Aug31-06, 08:34 AM
Sci Advisor
PF Gold
SpaceTiger's Avatar
P: 2,977
Please explain why there is no pluto any more
Extensive discussion of these issues has already occurred here:

Twelve Planets
Now Its Official: Eight Planets (Pluto got bumped)
Pluto is no longer a planet
The new definition of a planet

Please read them before asking further questions about the new planetary classification scheme.

Register to reply

Related Discussions
Outer planets always gas planets? Astronomy & Astrophysics 12
Exo Planets Astronomy & Astrophysics 3
Gas planets Astronomy & Astrophysics 9
Favoured theory for planetary formation Astronomy & Astrophysics 7
Influence of planets on planets Astronomy & Astrophysics 3