Aug
16
2006

From 9 planets to 12...to 53?

The 12 planets: In this artist's impression the planets are drawn to scale, but without correct relative distances.  Image courtesy the International Astronomical Union/Martin Kornmesser.
The 12 planets: In this artist's impression the planets are drawn to scale, but without correct relative distances. Image courtesy the International Astronomical Union/Martin Kornmesser.

The international panel that was formed to establish a scientific definition of a planet will make a recommendation to the International Astronomical Union (IAU) General Assembly that will increase the number of objects in out solar system that are defined as planets from nine to twelve.

The panel was originally formed to discuss the issue of Pluto’s status as a planet. Not only does the panel recommend retaining Pluto’s planetary status, but also promoting Ceres, Charon and "Xena" to planets. The new definition of a planet is that it has to orbit a star, not be a star itself, not a satellite of another planet, and massive enough that its gravitational forces compress it into a roughly round shape. This opens the door for many more objects in our solar system to be called planets, notably Vesta, Pallas and Hygiea. In fact, there may be more than 53 objects that meet the new criteria to be called a planet, and probably many more yet to be discovered.

The proposal is not final, and will be discussed on August 23 at the International Astronomical Union General Assembly meeting in Prague.

Watch a fly by of the new solar system here.

UPDATE: Turns out they changed the rules, and we now have only eight planets. Pluto doesn't qualify under the new definitions a planet. To be a “planet” it must:

1. Orbit a sun.
2. Have sufficient mass so that it assumes a nearly round shape.
3. And have sufficient mass to have "cleared the neighborhood around its orbit".

Pluto has not cleared the neighborhood of its orbit of Kuiper Belt Objects, so it is no longer considered a planet.

Your Comments, Thoughts, Questions, Ideas

James Satter's picture

One of the proposal's strengths comes from avoiding the pitfalls of some earlier criteria, which might define a planet (somewhat arbitrarily) on size, composition, distance from the Sun, relation to other objects, or even tradition.

Classifying planets became problematic at other points in time, as astronomers continued to find new planets on the outskirts of our solar system, reexamine the status of large asteroids, and just since 1995, discover more than 100 planets orbiting other stars.

posted on Fri, 08/18/2006 - 11:57am
James Satter's picture

If the top three contenders receive planetary status, the 12 planets in our solar system will follow this lineup, based on average distance from the Sun:

TERRESTRIAL PLANETS, the rocky, Earth-like planets
1. Mercury
2. Venus
3. Earth
4. Mars
5. Ceres, regaining its original status as a planet, instead of an asteroid

JOVIAN PLANETS, the Jupiter-like "gas giants"
6. Jupiter
7. Saturn
8. Uranus
9. Neptune

PLUTONS, the proposed subclassification for small, icy, Pluto-like planets
10.-11. Pluto and Charon, promoted from its present status as Pluto's moon
12. 2003 UB313

Though nicknamed Xena by its discoverer, the proposed 12th planet still awaits an official astronomical name.

posted on Sat, 08/19/2006 - 1:51am
Anonymous's picture
Anonymous says:

Honestly, don't we have enough planets? 'Oh yay, more inhabitable pieces of mass that we are forced to memorize though they do nothing for us.'

posted on Sat, 08/19/2006 - 2:42pm
Joe's picture
Joe says:

Turns out they changed the rules, but we now have only eight planets. Pluto doesn't qualify under the new rules for a planet: "a celestial body that is in orbit around 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 neighborhood around its orbit."

Pluto is disqualified because its orbit overlaps with Neptune's.

posted on Thu, 08/24/2006 - 5:31pm
Liza's picture
Liza says:

If Pluto is disqualified because its orbit overlaps Neptune's, shouldn't Neptune be disqualified, too?

(I heard there's a footnote to the definition that lists the eight classical planets, including Neptune, and that's how they get around the weirdness of the definition, but I'd love to know more...)

posted on Thu, 08/24/2006 - 8:48pm
bryan kennedy's picture

What I had heard is that Pluto is disqualified because it had a non-near-circular orbit. It overlaps with Neptune but Neptune's orbit is still near circular.

Neptune is also so much bigger. About 60 earths could fit inside Neptune, which qualifies it with the size as well.

I am fine with the new rules as long as they stay rules. We can't have anymore celestial bodies sneakin' into our cannon of planets. I mean their going to have to re-write that Animaniacs song. We can't have something tragic like that happen again.

posted on Fri, 08/25/2006 - 8:39am
James Satter's picture

The song Interplanet Janet, one of my favorites from Schoolhouse Rock, needs rewriting now too.

posted on Sat, 08/26/2006 - 1:46am
Joe's picture
Joe says:

Well, I was wrong about the overlapping of Neptune's orbit thing.

Under the new definitions a planet must:

1. Orbit a sun.
2. Has sufficient mass so that it assumes a nearly round shape.
3. And have sufficient mass to have "cleared the neighborhood around its orbit".

Pluto has not cleared the neighborhood of its orbit of Kuiper Belt Objects, so it is no longer considered a planet.

posted on Thu, 08/31/2006 - 5:05pm
Robbie's picture
Robbie says:

I think that this is very weird... Doesn't the length of time that something has been considered a planet count for something?

Isn't there also some other thing that was/is (being) considered for the tenth planet. And didn't they nickname it Sedna?

posted on Fri, 08/25/2006 - 9:46am
James Satter's picture

Scientists reclassify things all of the time, as new discoveries lead to more information and new definitions. If astronomers used tradition or length of time as a compelling argument, then Earth's sun and moon would still be planets. Early astronomers classifed them as planets (along with Mercury, Venus, Mars, Jupiter, and Saturn) since they were among the seven objects that seemed to "wander" independently above the clouds.

Today, Pluto remains important as the first of numerous "ice drawfs," which include Sedna and that object nicknamed "Xena."

In many respects, Ceres experienced a similar fate in the 19th century as Pluto is today. Although astronomers called spherical Ceres a planet soon after its discovery, they later reclassifed it as the first in a long series of asteroids between Mars and Jupiter.

This means that, officially speaking, Uranus and Neptune are the only planets in our solar system not visible from Earth with the naked eye.

posted on Fri, 08/25/2006 - 8:43pm
Abhimanyu PS's picture

Pluto IS a planet! It orbits a star, has sufficient mass to have a round shape. I'm from PLUTO. Anyway, these rules (in this website) are cranky! It's evil to have clear the neighborhood around its orbit.

posted on Sat, 05/10/2008 - 9:56pm

Post new comment

The content of this field is kept private and will not be shown publicly.
  • Allowed HTML tags: <a> <h3> <h4> <em> <i> <strong> <b> <span> <ul> <ol> <li> <blockquote> <object> <embed> <param> <sub> <sup>
  • Lines and paragraphs break automatically.
  • You may embed videos from the following providers vimeo, youtube. Just add the video URL to your textarea in the place where you would like the video to appear, i.e. http://www.youtube.com/watch?v=pw0jmvdh.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Images can be added to this post.

More information about formatting options