Planet Classification

With the discovery of Sedna (among others), there's a fiery debate going on in astronomical circles about what constitutes a "planet". I read an article not too long ago about space scientists trying to come to a consensus on what to call planetary bodies, and what characteristics they need to have in order to fit into a certain class.

Should Pluto be considered a planet, or a planetoid? Same thing for Sedna.

I guess I don't understand what the fuss is about. Can't we just develop a system of planet "classes" ala Star Trek? A Class M planet is like Earth, rocky, with an mild atmosphere and a temperate climate. A Class G planet is like Jupiter... gas giant, etc...

[edit on 1/5/2006 by AlphaMail]

Well, the problem is not classifying what the planetoid is like, but why we should consider something a planet and not something else. For example, what is the major difference between a large asteroid, and a moon? Both make regular orbits, both are made of rock, and both have weaker gravitational influences than their host planet. But where do we draw the line between rock and moon? Is a moon something that orbits a planet? We call those satellites. Or does it have to do with size? If so, why do we consider Charon, or Phobos for that matter, to be moons? They are tiny.

Or does it have to do with their orbits? If so, you could consider many of the asteroids in the asteroid belt to be planets or moons - which we would clearly say they are not.

Or is it distance? Perhaps you need to be close to the sun in order to qualify as a planet of that sun. Otherwise you're a ball of rock.

Myself, I don't consider Sedna or Pluto to be planets of our solar system. They're cosmic debris, and that's really it. Leftovers that formed together to make a new object roughly the size of a small planet.


It is just a matter of terms and subjects and words, but it's one that we look at so that confusion is kept low when scientists work together. We need to be on a common base in order to discuss things properly.

Personally, I figure a planet is any astronomical body roughly spherical, as big or bigger than Pluto, but that's just what how I'va always classified planets to myself.

Originally posted by iori_komei
Personally, I figure a planet is any astronomical body roughly spherical, as big or bigger than Pluto, but that's just what how I'va always classified planets to myself.

In that case the Moon is a planet. It's about 1200 km larger in diameter.

the simplest thing would be to have a defined diameter/mass density limits. and also the fact it cannot be a moon, that is a natraul body orbiting a planet.

Some moons could be planets if they didn't orbit around planets.

Pluto and charon are almost double planets, and selina might be a planet too

Originally posted by cmdrkeenkid
In that case the Moon is a planet. It's about 1200 km larger in diameter.

Ok, yeah I should have said to, "Not orbiting a larger stellar body.".

Originally posted by cmdrkeenkid

Originally posted by iori_komei
Personally, I figure a planet is any astronomical body roughly spherical, as big or bigger than Pluto, but that's just what how I'va always classified planets to myself.

In that case the Moon is a planet. It's about 1200 km larger in diameter.

So? I don't know the diameter of earth in km off the top of my head, but it is about 4k miles right, what is the diameter of Jupiter? If you compare Earth to the outer planets such as Saturn, Neptune, Uranus and Jupiter, Earth looks less and less like a planet and more like a rock inhabited by somewhat sentient beings debating whether or not Sedna is a planet because of its diameter.

Or...

Maybe we should come up with new names for the outer planets. A new classification.

Psuedoplanet? protoplanet? planetoid?

Earth is about 12,700km and Jupiter is like 140,000km in diamater. I think the naming body agreed to stop calling anything a planet out past Pluto. They even wanted to pull Pluto for a time but it's been in for so long they just leave it.

I would go with some % of placement within the orbital plane with some minimum mass number but we have enough already. We just tend to like to name things instead of giving them numbers which they do initially get. Look at Mars - whole thing is named up and so is the Moon....

Originally posted by iori_komei

Originally posted by cmdrkeenkid
In that case the Moon is a planet. It's about 1200 km larger in diameter.

Ok, yeah I should have said to, "Not orbiting a larger stellar body.".

a star is not a planet; but it still falls into these classifications hehe

i know what u mean; tho, sorry to nitpick hehe

or is a star a planet after all? just the central planet of the system; which happens to be so massive it becomes plasma?

if you want my philisophical opinion on how we Should classify planets and asteroids; heres my take on it

Planets - Major objects orbiting a star
Asteroids = minor objects orbiting a star
Star = largest object/s in system; planets and asteroids orbit around this, usually emits large amounts of energy radiation , example : Light

honestly to be precise we will have to make our definitions very in depth
even my definitions are incomplete and leave much room for error