What is a planet?, Part II

[Dr Mordrid]

http://cosmiclog.msnbc.msn.com/archive/2008/02/20/684017.aspx

The next great planet debate

Posted: Wednesday, February 20, 2008 3:30 PM by Alan Boyle

How do you define a planet? Officials at the International Astronomical Union thought the matter was settled more than a year ago when it drew up a definition of planethood that separated little Pluto from its eight bigger siblings and put it in the dwarf-planet category. Boy, were they wrong.

Many astronomers say the definition that the IAU came up doesn't adequately reflect the diversity of worlds we see even in our own solar system - and arguably, might even exclude Jupiter as an official planet. Now a replay of the "Great Planet Debate" has been scheduled for August. Pluto may remain in the pint-size pigeonhole - but the other planets, in our solar system and beyond, would get their own pigeonholes as well.

The "Great Planet Debate" is due to begin on Aug. 14 at Johns Hopkins University's Applied Physics Laboratory in Laurel, Md. Here's how the conference is described on APL's Web site:

"During the first two days of the conference, we will present what we have learned about planetary bodies over more than 40 years of robotic exploration of the Solar System and what we are learning about planets around other stars. The IAU’s dynamical definition of a planet will be presented, as well as an alternative geophysical definition. The utility of each will be debated, along with other potential planet definitions.

"A public lecture and panel discussion, featuring scientists who are prominent in the debate on planet definitions, is planned for the evening of the second day, following a reception that concludes the scientific portion of the conference.

"The third day of the meeting will be an Educator Workshop to discuss how the question of 'The Great Planet Debate' should be treated in schools and how that can be used as a springboard to discuss science as a process, as well as other topics in planetary science."

So what's being proposed as an alternative to the IAU's definition? The answer comes in a paper prepared last year by one of the conference's organizers, Mark Sykes of the Planetary Science Institute:

"'A planet is an object orbiting a star that has mass sufficient to maintain a gravity-determined (hydrostatic equilibrium) shape.' More simply put, planets are 'round' objects that orbit stars. Spacecraft imagery reveals that it is at this point of 'roundness' that solar system bodies begin to exhibit geology - reflecting interior processes, not just impact history. Smaller bodies (e.g., asteroids) are irregular 'inactive' objects. This definition is easily extensible to objects around other stars, unlike the [IAU's] Prague definition. ..."

The idea of revisiting the definition of planethood was a lively topic in Boston earlier this week during the American Association for the Advancement of Science's annual meeting - and it was in that context that Alan Stern, NASA's associate administrator for science, mentioned the August event.
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[cjolley]

I like that definition.
It manages to find an intersection between a physical property and a more general intuitive understanding of what one is.

What about multiple star systems though (the majority)?
The members fit that narrow definition: "an object orbiting a star that has mass sufficient to maintain a gravity-determined (hydrostatic equilibrium) shape."

Are the members stars, planets, or both?

[Dr Mordrid]

The number of multiple planet systems are common is getting very strong. In fact our solar system but in miniature was recently discovered. The star is smaller, as well as the planets and their distances smaller in a similar scale. Given the ratio of the stars luminosity to these distances it's very possible that a rocky planet could exist within its habitable zone.

Another interesting case is the red dwarf 40 Eridani A, a member of a trinary (3 star) system.

Red dwarfs are very long lived, perhaps a trillion years, and it's been found to have a planetary system that includes at least one (possibly two) within its habitable zone (about .6 AU). Such a planet would have a year of about 200 days.

Interestingly enough 40 Erandi A was the home star for the Star Trek planet Vulcan, which has caused no small amount of pressure to name the first rocky planet (if any) found in the habitable zone "Vulcan".

[cjolley]

Recent modeling says that stable planets are possible, and in some cases probable, in multiple star systems.

No, I mean the stars themselves. They are shaped by gravity and orbit (other) stars so they would seem to qualify by the definition's literal wording.

[Dr Mordrid]

I believe thermonuclear fusion occurring in an orbiting objects core excludes it from consideration as a "planet". The rub is what are called brown dwarfs. These are star like objects that are hot and usually classified as "stars", but many do not have fusion occurring in their cores. At some point they'll have to resolve that issue.

[NetSnake]

how is this different from the prague one???

From the IAU meeting in Prague:

"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."