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Planemos, the reigniton of semantics arguments June 5, 2006

Posted by dr. gonzo in Space.

There is a number of things I could say, Pennsylvania was excellent, Iraq is screwed, Bush is an idiot but instead I think I will talk about planets.

The recent discovery of planetary-like systems not bound to stars has further blurred the definition of planet. The International Astronomical Union promises to work on a definition in August at their 26th General Assembly in Prague. It should be published in September.

In the meantime the debate heats up. What astronomers are calling “planemos” exist on their own, have clouds of dust and gas around them likely to form planets or moons and range in size from 5 to 15 times the mass of Jupiter.

Are they planets? Are they stars? What is going on here?

Personally, I believe any defintion will be arbitrary at best. Our knowledge of the Cosmos is extremely limited, new discoveries change the shape and extent of our knowledge and theories regularly.

If we choose to define planet by mass or size that could end up leaving out a number of objects that are planet like (such as the array of newly discovered Kupier Belt objects).

A simple way to define planet would be: “any object that orbits a star.” Of course, that would make the thousands of asteroids and comets in our star system planets. Far too many to know through pneumonic devices and other memorization techniques. But it seems, again, to be arbitrary because people will always see the Sun’s system as having the nine planets they grew up knowing.

Instead, if the IAU is smart they will lay down certain requirements for planetary status instead of publishing a binding and static definition.

Requirements such as:

1) Must orbit a star
2) Must have enough mass to acrete into a spherical shape under its own gravity
3) Must not orbit another planet which orbits a star
4) Must not exist as a member of a larger group of very similar objects (i.e. asteroid belt or Kupier belt)

Requirements such as those above would leave the defintion up in the air for objects such as “planemos” but it would narrow down the realm of what constitutes a planet in the scientific community. It would also eliminate Trans Neptunian Object 2003 UB313 from consideration as the 10th planet in our system.

While this may irk the discovery team of 2003 UB313 it would not reduce the scientific value of such discoveries only the glamour attached to them. And instead of arguing incessantly over semantics scientists could get down to science.


1. Matt - June 7, 2006

I like your requirements.

One of the things I love about science (of which astronomy is a branch of) is its constant ability to change with new discoveries. Then there are the seemingly endless possibilities still out there to discover and learn from.

Fascinating stuff. And good post.

2. dr. gonzo - June 15, 2006

I appreciate the comment. I spend a lot of time pondering this stuff.

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