UCLA professor proposes simpler way to define what makes a planet

November 10, 2015
Ceres. Credit: NASA/Jet Propulsion Laboratory

Since the late 1980s, scientists have discovered nearly 5,000 planetary bodies orbiting stars other than the sun. But astronomers are still working on what exactly we should call them.

Today at an American Astronomical Society meeting, UCLA professor Jean-Luc Margot described a simple test that can be used to clearly separate planets from other bodies like dwarf planets and minor planets.

The current official definition of a planet, which was issued by the International Astronomical Union in 2006, applies only to bodies in our solar system, which Margot said has created a "definitional limbo" for the newly discovered bodies. A paper by Margot that has been accepted for publication in the Astronomical Journal proposes to extend the planet definition to all planetary systems.

The new approach would require only estimates of the star's mass and the planet's mass and orbital period—all of which can be easily obtained with Earth- or space-based telescopes. According to Margot's criteria, all eight planets in our solar system and all classifiable exoplanets—the large bodies that orbit stars other than our sun—would be confirmed as planets.

Margot, a professor of planetary astronomy, wanted to ensure that the new system would be easy to follow. "One should not need a teleportation device to decide whether a newly discovered object is a planet."

Some of the current confusion he's seeking to eliminate stems from the fact that the IAU's definition did not address how to classify exoplanets—that question was left for future consideration. Now, Margot argues, the recent flood of exoplanet discoveries should encourage the IAU to refine and extend the definition.

The IAU's definition is based primarily on the ability of a planet to "clear its orbit," meaning whether it can evacuate, accumulate or dominate small bodies in its orbital neighborhood. Margot's test can be used to determine whether a body can clear a specific region around its orbit within a specific time frame, such as the lifetime of its host star. The test is easy to implement and it could immediately classify 99 percent of all known exoplanets. (Scientists do not currently have enough data to apply the test to the other 1 percent.)

When applied to our own , the test clearly places the eight planets into one distinct category and the dwarf planets—Ceres, Pluto and Eris—into another.

"The disparity between planets and non-planets is striking," Margot said. "The sharp distinction suggests that there is a fundamental difference in how these bodies formed, and the mere act of classifying them reveals something profound about nature."

Margot also found that bodies that can clear their orbits, and therefore qualify as , are typically spherical. "When a body has sufficient mass to clear its orbital neighborhood, it also has sufficient mass to overcome material strength and pull itself into a nearly round shape," he said.

This is important because astronomers can't always see exoplanets well enough to accurately determine their shape, but they can almost always measure the parameters needed to apply Margot's criteria—and because the current IAU definition requires that a planet be nearly round.

Margot presented his proposal at the annual meeting of the AAS's Division for Planetary Sciences.

It is not known whether the new approach will be considered by the IAU, whose resolutions are typically crafted and reviewed by committees before members vote on them during a general assembly. The next IAU general assembly is scheduled for 2018.

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12 comments

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TechnoCreed
5 / 5 (7) Nov 10, 2015
For those who are curious about prof. Margot proposition http://arxiv.org/...00v4.pdf
wduckss
1 / 5 (4) Nov 10, 2015
The proposal is a step backwards. General understanding of star systems today is wrong.
Depending on the distance the body of the stars, we have bodies with and without self-rotation about an axis, with the temperatures we have the body, with and without the molten core and the body completely molten which are the stars ...
The proposed suggestion is like that the variety of woods you see just a little and larger trees without naming species etc.
Captain Stumpy
5 / 5 (6) Nov 10, 2015

For those who are curious about prof. Margot proposition http://arxiv.org/...00v4.pdf

@Techno
thanks!

.

.

General understanding of star systems today is wrong
@wduck
and you know this how?
this is based upon what evidence?
this is called an opinion
http://www.auburn...ion.html
JustAnotherGuy
not rated yet Nov 10, 2015
UCLA professor proposes simpler way to define what makes a planet"

Simpler? Way for presenting it... That was the problem after all. Letting aside that categorization worked, its definition was too simple and ambiguous: http://www.iau.or...-5-6.pdf

From @TechnoCreed's link it can be seen that a better work is being done. Maybe an update to the definition, next year, at the 10 anniversary of the resolution B5, could be presented... just saying...

By the way, I'm not talking about "promoting/demoting" objects again. Although, a category for 'Gas Giants' could be nice...
wduckss
1 / 5 (2) Nov 11, 2015

For those who are curious about prof. Margot proposition http://arxiv.org/...00v4.pdf


I have no intention to impose his opinion.
The bodies in the universe I share only according to the degree of development and the conditions their existence (there is no division into small, large, planetew, satellites, stars etc. It is only history, nostalgia ..).
Mark Thomas
3.5 / 5 (2) Nov 11, 2015
Dr. Margot is attempting to put mathematical precision to the abiguous "cleared its orbit" standard currently favored by the IAU. However, putting a band aid on a fundamentally flawed standard will not solve the problem. Furthermore, Dr. Margot's standard makes the planet determination dependent on its star's mass. So an object may or may not be a "planet" depending on the star it orbits. This is what happens when Star Trek is off the air for too long. Star Trek fans might know about rogue planets. The galaxy probably has billions of rogue planets that aren't orbiting any star (Star Trek: Enterprise, Rogue Planet episode). The easiest way to define a planet is with a minimum diameter of say 2,000 kilometers and up to a size that does not include deuterium fusion at about 12-13 Jupiter masses.

After we get this definition fixed maybe we can start working on a more precise one for "M class planets." :-)
ogg_ogg
3 / 5 (2) Nov 11, 2015
Why isn't (planetary) mass sufficient to categorize objects? I wonder if we'll ever discover evidence for a "rocky" planet with the mass of a Gas Giant? Should be possible, right? Seems like this focus on the term "planet" is as silly as trying to define what "love" is, and just as useful. The first definition the astronomical community should be worrying about is the definition of a star. I don't know any authoritative definition, do you? ...At least one which isn't ridiculous and which also discriminates between stars, brown dwarves, and planets...Issue I have with defining star by energy output mechanism is that "fusion" may include any element up to (approximately) iron & mechanism changes with time - does a star stop being a star at some point in its existence as a compact object?
ogg_ogg
2.3 / 5 (3) Nov 11, 2015
Maybe we should admit that we don't understand the possible mechanisms (of stellar formation) well enough (yet) to be able to define the objects they produce. Critical parameters, it seems to me, are composition (by isotope, about a hundred with adequate half-lives), density, temperature and mass.(of course, in the real world all of these variables change with time, so we'd need some sort of probability distribution over the set of possible density, composition, temperature, and mass distributions (I wonder if velocity, relative to center of mass, also has to be included?).... I think a similar argument should be made for planets - we don't understand their formation mechanisms well enough to define them.
wduckss
1 / 5 (4) Nov 11, 2015
If there is a fusion of the stars why no radiation? Earth also has melted (except crust) also no radiation.
A higher level of radiation emitted our nuclear power plants in the environment of molten body.
The body of 10% of the mass of the Sun due power compressive forces begin light or if the effect of gravity higher limit is reduced.
SuperThunder
not rated yet Nov 11, 2015
Can we stick a flag on it and kill others to defend it?

Planet enough for us apes!
jsdarkdestruction
5 / 5 (5) Nov 11, 2015
If there is a fusion of the stars why no radiation? Earth also has melted (except crust) also no radiation.
A higher level of radiation emitted our nuclear power plants in the environment of molten body.
The body of 10% of the mass of the Sun due power compressive forces begin light or if the effect of gravity higher limit is reduced.

Are you asking why the solar radiation and that from the earths core is less than that of a nuclear reactor? Its very hard to understand what you are trying to say or ask.
FineStructureConstant
5 / 5 (5) Nov 14, 2015
Its very hard to understand what you are trying to say or ask.
- don't worry about that - it's hard for him to understand himself.

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