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[Chicago Astronomer - Astro Joe]

Definition of 'Planet' Expected in September

Historians and educators have joined astronomers in an effort to break a deadlock on contentious discussions over a definition for the word planet.

A decision is expected in September, but history suggests rewriting the textbooks could be more challenging than finding tiny new worlds at the edge of the solar system.

The International Astronomical Union (IAU) is expected to propose wording to delineate planets from other small, round objects at its 12-day General Assembly meeting in Prague this August. The proposal will be based on recommendations from a newly formed committee that includes experts outside the realm of astronomy tasked to break a deadlock in earlier committee discussions.

Depending on the outcome of a separate controversial procedural issue—whether IAU members should be allowed to vote on such things—astronomers might then have the chance to weigh in on the definition later.

Some might think it ironic that the world's governing body for astronomy does not have a definition for planet.

The problem stretches back to the late 1990s, when astronomers began discovering Pluto-like objects in the distant reaches of our solar system.

All the newfound worlds—there are several known now—were until recently smaller than Pluto, but they are round and orbit the Sun, two characteristics that had for centuries been sufficient for the implicit definition of planet. The hitch: These small objects are typically on wild, elongated orbits that stretch well above and below the main plane of the solar system where eight of the traditional planets travel (Pluto has a wild orbit, too, which is one reason many astronomers do not consider it a planet anymore).

So what to call them? Astronomers have been arguing about it in earnest since 1999.

The controversy came to a head with the July 2005 announcement of 2003 UB313, an object roughly the size of Pluto that orbits the Sun beyond Neptune. The object's discoverer, Mike Brown of Caltech, has argued it should be called a planet.

But other astronomers say that if planethood is bestowed upon 2003 UB313, then several similar way-out bodies should gain the same status, and the number of planets in our solar system could ultimately climb into the thousands as search technology improves.

Neither Bowell nor Boss knows what exactly might happen next, however. Nor does Brian Marsden, leader of the Minor Planet Center where newfound objects are catalogued. Marsden was also on the first definition committee.

"The new committee is supposed to recommend what 'should be done' about Pluto, 2003 UB313 and other 'largish' small bodies, but it is not clear that what they decide will depend on mass," Marsden told SPACE.com.

Marsden said it's also unclear how the IAU will reach an ultimate resolution.

"The IAU made the interesting policy decision in 2003 to disenfranchise its members, and they were therefore not allowed to vote on 'scientific matters' (such as what a planet is) at the last plenary General Assembly session at the Sydney meeting [in 2003]," Marsden said. "There are rumors that there may be an administrative decision to re-enfranchise us at the first of the upcoming plenary sessions this August in Prague—so that suggested vote might be possible at the second."

That is indeed the plan, IAU General Secretary Oddbjorn Engvold explained yesterday. The advisory Committee is scheduled to meet June 30, Engvold said by email.

Full story here from Space.com: www.space.com/scienceastronomy/060608_planet_definition.html

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Does Pluto get knocked down or do the many objects out there get elevated to planet status.

Quite a pondering...

[Maddad]

Pluto should not be a planet, but four generations have grown up used to thinking of it that way. Whatever the IAU decides, we are likely to continue thinking of Pluto as a planet for another seventy-five years.

[Chicago Astronomer - Astro Joe]

Dave,

Tell us your thoughts on why Pluto should not have planet status.

I think it should because:

* It's round
* It has satellites
* It orbits the Sun
* An atmosphere is detected
* It has a regular rotation period

Now any single one of these characteristics does not a planet make, but combined...I think it has merits.

[Centaur]

admin said:

Tell us your thoughts on why Pluto should not have planet status.

I think it should because:

* It's round
* It has satellites
* It orbits the Sun
* An atmosphere is detected
* It has a regular rotation period

Now any single one of these characteristics does not a planet make, but combined...I think it has merits.

All asteroids orbit the Sun and have regular rotation periods. Some asteroids are round, and some have satellites.

Pluto has a mass that is 1/500 of the Earth and 1/6 of the Moon, even though in 1930 it was thought to be 10 Earth masses. There are many trans-Neptunian objects besides Pluto, and it is not the largest one.

Its orbit is far more eccentric than any planet (occasionally bringing it closer to the Sun than Neptune.) Its orbit is far more inclined to the ecliptic than any planet. Its orbit is not stable enough to allow for the development of a long-term theory of Pluto’s position. It shares all of these orbital anomalies with asteroids and not planets.

For a many years after the first asteroids were discovered they were referred to as planets. Considering their small sizes and similar orbital characteristics, astronomers realized that it would make more sense to create a separate class for them. A rational classification system helps scientists as they seek to determine the origin, characteristics and future of natural objects and systems. Hence, it may make more sense to place all Kuiper Belt Objects, including Pluto, into a separate class from the major planets.

If the high school graduate who was assigned in 1930 the arduous task of comparing sky photographs and his mentors had not been so eager to discover a planet, they may have stopped to realize that they were seeing an example of a new class of objects that demanded further scientific investigation. The Kuiper Belt might then have been called the Tombaugh Belt - a far more impressive scientific accolade then having been the first to see a particular celestial body in a photograph.

Just because our third grade teachers had us learn by rote the multiplication tables and the names of the planets, does not imply that both accomplishments were equally valid. We’ll just have to accept that our teachers and their sources may have occasionally dispensed imperfect information. Acceptance of change and errors is an important aspect of living.

[Chicago Astronomer - Astro Joe]

I still think Pluto should be considered a planet.

*Asteroids do not have an atmosphere, but Pluto does...

Pluto's atmosphere is expanding, researchers say
web.mit.edu/newsoffice/2003/pluto.html

*I do not know of any spherical asteroids. Some come close, elliptical, oblong, but not a ball.


Perhaps Pluto got jostled out of a regular orbit by a passing star, comet etc.., and placed it in an eccentric orbit. The eccentric orbit should not automatically de-classify it.

And...

I think that a spherical form is the major consideration for an object in giving it planet designation status. In that the body had enough mass and gravity to form into a ball and not just random irregular space debris. Top it off with an atmosphere of any pressure and it's well on it's way. Pluto may be more of a planet than Mercury is by that measure.

The satellites around the gas giants in our solar system could also be classified as planets in their own right if they orbited the Sun instead of their host planet.

Smarter astronomers than us are arguing this matter, but perhaps the decision should wait until New Horizons does it's probing for more information in the area.

[Centaur]

admin said:

*Asteroids do not have an atmosphere, but Pluto does...

*I do not know of any spherical asteroids. Some come close, elliptical, oblong, but not a ball.

Perhaps Pluto got jostled out of a regular orbit by a passing star, comet etc.., and placed it in an eccentric orbit. The eccentric orbit should not automatically de-classify it.

I think that a spherical form is the major consideration for an object in giving it planet designation status. In that the body had enough mass and gravity to form into a ball and not just random irregular space debris. Top it off with an atmosphere of any pressure and it's well on it's way. Pluto may be more of a planet than Mercury is by that measure.

Ceres is a spherical asteroid. There are indications it may have a tenuous atmosphere. Let’s not forget that Jupiter and Saturn deviate rather noticeably from true spheres.

Pluto has an atmosphere because it is so far from the Sun that the extreme cold allows one to form. The same is likely true for the other KBO’s. Nearness to the Sun in combination with their sizes is the reason Mercury and most asteroids apparently do not have atmospheres. Comets often do have atmospheres, depending on their distances from the Sun.

Virtually all planetesimals began with highly eccentric and inclined orbits. As they collided and coalesced to form planets, their orbital characteristics became averaged resulting in nearly circular co-planar orbits. The asteroids, Pluto and the other KBO's never got big enough for this effect to take place. These dynamical considerations may be of greater significance in determining planetary status than some of the other qualitative and quantitative data.

The most meaningful classification system would be the one that is most helpful in directing the thinking of a scientist as he develops his theories. It should not be drawn from the opinion of pop culture. I suspect that many people simply do not want to abandon what they were told by their third grade teachers. I was taught in class that the lunar craters were volcanic, one side of Mercury perpetually faced the Sun, Pluto was 1 to 10 Earth masses, and the idea that the Earth's continents drift was a myth. I later acknowledged evidence that truth existed elsewhere. I started sensing the need for a reclassification of Pluto nearly 40 years ago.

Some say that Pluto should be grandfathered as a planet. Do they think it is a person (or dog, LOL) whose feelings might be hurt? Seventy-six years is not a long time in the history of science. Standards can still be revised and improved in the face of fresh evidence. It was nearly 50 years before astronomers stopped referring to asteroids as planets. The ultimate decision should have nothing to do with fairness or compassion for Pluto or its discoverers. Conservative attitudes regarding our grade school lessons may need to open up a little. I suggest that we bequeath to future astronomers a rational classification system that would allow them to continue the quest of determining the order of the universe, while not giving them reason to mock ancestors influenced by nostalgia.

Admittedly, all objects orbiting the Sun are types of planets including planetary satellites. Let's continue to keep natural satellites in a separate class. Let's accept that there are eight major planets. Then we can divide minor planets into several sub-classes: asteroids, KBO's and comets.

[Chicago Astronomer - Astro Joe]

Yes...I forgot about Ceres...and it is spherical - kinda...

Perhaps Ceres was a satellite at one time of a long lost planet between Mars and Jupiter, or jostled away from Jupiter itself. Either way, it has more in common with a planet than it does with an asteroid - with many of the attributes that a planet shares than just rocky space debris.

* The asteroid has a very primitive surface, say scientists. The asteroid, like a young planet, contains water-bearing minerals, and possibly a very weak atmosphere and frost. Infrared observations show that the surface is warm.

From Cornell University:

Asteroid or miniplanet? Cornell astronomer finds Ceres appears to have shape and interior similar to terrestrial planets

Ceres 1 was already holding the title of the solar system's largest asteroid. Now new observations show the space rock may be more worthy of the appellation "miniplanet."

NASA released photographs of Ceres that show the rock is a smooth ellipsoid, or oblong sphere, with an average diameter of approximately 590 miles -- about the size of Texas.

Being ellipsoid and smooth is special for a rock. It indicates that the body is heavy enough to possess gravity strong enough to suck its own surface smooth -- a process called "gravitational relaxing." Because the process typically requires a mass of many trillions of tons, depending on the temperature, the average pebble is not going to be gravitationally relaxed; even most asteroids aren't.

Based on their own models and observations, Thomas and his colleagues believe Ceres contains a rocky silicate core and icy mantel covered by a crust of carbon-rich compounds and clays. Furthermore, they predict that the icy mantel may contain more frozen water than all of the fresh water on Earth.

Whenever water is mentioned, people ask about life. But Thomas says the possibility is "very remote," noting that even if the interior of Ceres were warm enough for some of the water to liquefy, Ceres probably lacked a sufficient energy source for life to develop.

Ceres was discovered in 1801 by Sicilian astronomer Giuseppe Piazzi and declared to be the "missing planet" predicted between Mars and Jupiter. However, the title was revoked in 1802 when Ceres was found to be a member of the hundreds of thousands of other rocks and debris of the Asteroid Belt.

Since the discovery of 2003 UB313 -- which some have hailed as the tenth planet -- in July, some astronomers (and many non-astronomers) have begun to question whether objects such as Ceres should also be enshrined as planets.

Source: www.news.cornell.edu/stories/Sept05/Ceres.to.html

===============

It's certainly an area for further study, examination and definition.

[Maddad]

admin said:

Tell us your thoughts on why Pluto should not have planet status. I think it should because:

* It's round
* It has satellites
* It orbits the Sun
* An atmosphere is detected
* It has a regular rotation period

Joe, I've been away from here for far too long, and am trying to catch up. Lemme see if I can ruminate on this.

Yeah, Pluto's round, and we think of round things as being planets or stars. We're going to find though objects that are round but much smaller than Pluto. Roundness is something you have to have to be a planet, but also objects that are not planets will be round.

Satellites fal in much the same category as roundness. We've found asteroids with satellites.

About everything orbits the sun, down to a bazillion pepples in the asteroid belt.

Ah, the atmosphere. That's probably your strongest point.

Asteroids also have regular rotation periods.

The lable of planet is just that, a lable to give us a handle on the object. It helps us think about the critter as opposed to jamming it into a box from which it cannot escape. Hard and fast classifications will be less than satisfying because we will find some critters that have traits of more than one.

Ok, I'm reading down to Centaur's post, and he's got some pretty good information, as usual.

[Chicago Astronomer - Astro Joe]

Dave,

Each item on it's own, does not a planet make. But if a certain body is an aggregate of most of them, then I think it's a strong candidate for planet status.

Now, the question should be, what makes a body a planet, as we currently classify them as.

[Centaur]

Considering Pluto’s goofy characteristics, I would think it shares its name with the wrong one of Disney’s dogs.

[Maddad]

Hmmm. What characteristics ought a body to have to be called a planet. Ok, I'm going to assume that we are defining this for our solar system. It might not work in others.

I'd want any such definition to closely fit either terrestrials or jovians. Note that Pluto does not work well in either category. Yeah, if the IAU calls Pluto a planet, then we deal with it.

• I'd like to see a minimum mass definition that gives us Mercury, but excludes the Moon.
  
• I'd like the object to orbit the sun as opposed to another object within the solar system.
  

I can think of other traits for a planet, but I'm not thinking of any right now that I would use to exclude an object from the name. Can you think of anything?

[Chicago Astronomer - Astro Joe]

I think a planet has to be aware of what it is...even if we don't.

I'd like to see a minimum mass definition that gives us Mercury, but excludes the Moon.

I'd like the object to orbit the sun as opposed to another object within the solar system.

Along with those two, a planet must have enough organization to maintain an atmosphere...regardless of the density or composition. And this gives a foot in to the body's shape. It should have enough mass to pull it's self into a ball and a somewhat regular rotational period...to evenly heat and cool the surface.

And...I think it should show evidence of geological activity or interna/externall signs of being "alive".

Now, these characteristics apply to some satellites of Jupiter and Saturn, but since they orbit planets, these bodies will be considered satellites. If they ever break away and orbit the Sun someday, then I think they should be planets too. Titan certainly would a planet in it's own right.

But...

Let's consider Mercury.

It's a hot, dry rock and dead body...with no atmosphere to speak of. Is it still a planet?

From NASA: So far, observations of Mercury from ground-based observers and the Mariner 10 spacecraft have not shown evidence of a significant atmosphere compared to other planets. Mariner 10 did observe a tiny amount of helium 1000 km above the surface, but this is probably caused by the solar wind and the breakdown of Mercury's crust. Sodium, potassium and oxygen have also been detected in Mercury's very weak atmosphere.

If you drop a feather on your hand, it exerts a pressure of about 10-3 bars, which is one trillion times more than the atmospheric pressure on Mercury.

Pluto on the other hand has all the characteristics mentioned above...and a substancial atmosphere.

Here is an interesting link in the debate:

www.faqs.org/faqs/astronomy/faq/part5/section-13.html

Ceres seems to have all of the characteristics as well. The asteroid, like a young planet, contains water-bearing minerals, and possibly a very weak atmosphere and frost. Infrared observations show that the surface is warm. Ceres orbits the sun once every 4.6 years at a distance of 410 million kilometers, and it spins on its axis once every nine hours.

Could Ceres been a satellite of a planet long gone that once perhaps occupied the area between Mars and Jupiter?

It's a sphere, got air, got frost (water?), and temperate.

So, to wrap up...

If Pluto is downgraded, so must Mercury. And if Pluto keeps it's planet status, then Ceres must also be considered to be one as well.

If it's not a ball, it's space debris.

[Centaur]

I keep seeing the word status here, and now the word alive, as though Pluto were a person whose feelings could be hurt. The reason for calling Pluto a KBO would be to place it within a rational classification system that allows scientists to think more clearly about the nature, origin and future of various components of the solar system. After referring to asteroids as planets for nearly half a century, nineteenth century astronomers eventually agreed it would make more sense to place them in a class of their own. Similar logic would apply to the case of KBO's.

The word planet is derived from Latin, Greek and earlier Indo-European words meaning to wander. In an astronomical context it referred to one of the seven wandering stars that could be seen with naked-eyes. This included the Sun and Moon, but in a narrower sense applied only to Mercury, Venus, Mars, Jupiter and Saturn. Personally, I would restrict the use of the word to those five. But I realize that is a lost cause. I concede the word now applies to Earth, Uranus and Neptune, even though most people cannot casually witness them wandering across the sky.

Pluto’s size, composition and orbital characteristics make it clear that it is a member of the special class of bodies known as KBO’s. As was the case with asteroids, if we were to classify KBO’s as planets, the word planet would lose its usefulness. We might as well just call all of them things in outer space. The class needs to be kept to a reasonable size, especially if fourth grade teachers insist on having their pupils learn the list by rote. I’ll compromise and expand my list of planets to eight. But no more, please. Pluto will somehow have to learn to deal with the utter disgrace.

[Uranut]

This seems as good a place as any to for my first post on this forum.

(Centaur, you may recognize this from another forum we both visit - I just copied part it from my post there)

Here is a proposal for a quantified definition of a planet in terms of mass (orbital data would of course have to be included - I'm only referring to minimum mass here).

'The spherical region around a body in which that body dominates the motion of particles is called the Hill Sphere. Once the core of a body exceeds about 10*23 kg, it gravitationally attracts gas from the hill sphere and is able to form an atmosphere in this way. If we accept 10*23 kg as a minimum mass for a planet, then Pluto is too small to qualify'. (Pluto's atmosphere is caused by the sublimation of its surface and is probably a transient phenomenon present only near perihelion).

Interestingly, by this definition Mercury does (but barely) qualify. I have questioned Mercury's claim to planet status for some time due to it's diminutive size, eccentirc orbit and orbital inclination. So I was wrong!?!

(definition and discussion of Hill Sphere from: The Facts on File Dictionary of Astronomy - Checkmark Books)

[Chicago Astronomer - Astro Joe]

Michael,

Thank you for your membership and first post on the Chicago Astronomer.

Mass as a guideline...? Hmmmm.

Does Pluto sublimate it's gas, then loses it into space? If so, then it's not a planet...more like a sluggish comet. But I think it maintains a slight hold on it's nitrogen molecules, but Charon and the other two satellites pull away at the atmosphere. Observations from occulted stars show some air hugging Pluto.

It's now going out in it's eccentric orbit, into a deeper freeze, but here is another point. Suppose...Earth were in Plutos orbit. Would Earth be questioned if it were a planet or not? Gravity doesn't change with temp, but what about the atmosphere? Frozen liquid water would sublimate and try to form when it got closer to the Sun, but would fail again and again to stay afloat. New Horizons will drastically change how we view Pluto.

About the only thing going for Mercury is that it's round and we can see it naked eye. There's more sentiment there than actual membership.

In consideratio of previous posts, there are only 5 planets orbiting our Sun, and the others are along just for the ride.

[Centaur]

If Mercury is not a planet, what is it? Certainly not an asteroid. Asteroids exist because Jupiter's gravitational influence prevented bodies at their mean distance from the Sun from coalescing into a larger body. It is theorized (with wide acceptance) that Mercury was once far larger, but was hit by a quite large planetesimal that knocked away Mercury's crust and mantle, leaving it with little more than its iron core. That means it’s dense and more massive than many assume. It’s the heat of the nearby Sun that prevents the retention of an atmosphere. Being able to see Mercury with the naked eye is a huge factor in its classification as a planet. That is why the ancients included it on the list of wandering stars (planets.)

The hypothetical regarding Earth is not realistic. Planetesimals are too widely dispersed in the Kuiper Belt for them to have accumulated to Earth's size. If it were nevertheless true, I would have to consider that far out Earth to be a KBO because orbital dynamics trumps other considerations for membership in that group.

[Chicago Astronomer - Astro Joe]

Now, we are back to the basic fundimentals on what constitutes a planet.

The critiera must meet minimal statndards and so far no concensus has been agreed upon...here or at the IAU.

But...If Pluto goes down, so does Mercury...

[Uranut]

I didn't mean to imply that the presence or lack of an atmosphere should be a determining factor. I only proposed a figure for the minimum mass of a planet - that figure being based on the way a body could form an atmosphere.

As to what Mercury might be if not a planet, I'm glad it qualifies under this definition because, though I questioned its status, I never could find another reasonable explanation for it.

[Maddad]

uranut said:

'The spherical region around a body in which that body dominates the motion of particles is called the Hill Sphere. Once the core of a body exceeds about 10*23 kg, it gravitationally attracts gas from the hill sphere and is able to form an atmosphere in this way. If we accept 10*23 kg as a minimum mass for a planet, then Pluto is too small to qualify'. (Pluto's atmosphere is caused by the sublimation of its surface and is probably a transient phenomenon present only near perihelion).

That's an idea I'd not heard before. Would you have the link on it?

[Maddad]

centaur said:

If Mercury is not a planet, what is it? Certainly not an asteroid.

On NPR radia this morning they said they'd come to a decision. Interestingly, Ceres the asteroid is now a planet.

[Chicago Astronomer - Astro Joe]

Hail The goddess of agriculture...and Newest Member of the System of Sol

Well, now we are breaking things down to the common denominator. Round, Solar Orbit and got air. Ceres asteroid no more, never considered it one.

We still have the vote to go through yet, but I think Ceres is a lock in.

The Chicago Astronomer see's all...knows all...

[Chicago Astronomer - Astro Joe]

IAU Defining criteria for planet definition

So far, what the IAU has come up with is :

The part of “IAU Resolution 5 for GA-XXVI” that describes the planet definition, states “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) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.” Member of the Planet Definition Committee, Richard Binzel says: “Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet.”

According to the new draft definition, two conditions must be satisfied for an object to be called a “planet.” First, the object must be in orbit around a star, while not being itself a star. Second, the object must be large enough (or more technically correct, massive enough) for its own gravity to pull it into a nearly spherical shape. The shape of objects with mass

Source: www.physorg.com/news74917943.html

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Did I miss something here...I thought Pluto's Satellite Charon was just that...a satellite. But now it's up for planet consideration.

It looks like that there is minimal resistance on the criteria from the majority of the IAU astronomers who will vote on this soon.