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Scientists against the demotion of objects like Pluto, Eris, Sedna, etc. to "dwarf planet" status have crafted a new definition:
It's no secret that Alan Stern and other scientists who led the New Horizons mission were extremely displeased by Pluto's demotion from planet status in 2006 during a general assembly of the International Astronomical Union. They felt the IAU decision undermined the scientific and public value of their dramatic flyby mission to the former ninth planet of the Solar System.
But now the positively peeved Pluto people have a plan. Stern and several colleagues have proposed a new definition for planethood. In technical terms, the proposal redefines planethood by saying, "A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters." More simply, the definition can be stated as, "round objects in space that are smaller than stars."
From the proposal:
The eight planets recognized by the IAU are often modified by the adjectives "terrestrial," "giant," and "ice giant," yet no one would state that a giant planet is not a planet. Yet, the IAU does not consider dwarf planets to be planets. We eschew this inconsistency. Thus, dwarf planets and moon planets such as Ceres, Pluto, Charon, and Earth's Moon are "fullfledged" planets. This seems especially true in light of these planets' complex geology and geophysics. While the degree of internal differentiation of a given world is geologically interesting, we do not use it as a criterion for planethood in the spirit of having an expansive rather than a narrow definition.
Here's another article about the significance of the New Horizons mission. New Horizons will fly by 2014 MU69 on January 1, 2019.
(Score: 3, Insightful) by JoeMerchant on Wednesday February 22 2017, @04:48PM
How many comets meet this definition?
Without a size or orbital shape cutoff, any icy comets that significantly melt and then reform into spheroidal shapes would qualify as planets.
I'm all for returning Pluto to planethood, I don't mind the idea of having 29785 planets in the outer reaches of the solar system. But, at some point, an elliptical orbit becomes a very distinguishing feature. Just like the spheroid has to have limits (earth is not a perfect sphere), it would seem reasonable to impose something like a maximum 2:1 ratio on the shape of planet's orbits.
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(Score: 0) by Anonymous Coward on Wednesday February 22 2017, @08:42PM
That has a snowball's chance in hell of happening ... oh, wait
(Score: 2) by RedBear on Wednesday February 22 2017, @08:46PM
It's unscientifically arbitrary, just like the current definition of "planet" where it has to have "cleared its orbit", which was so arbitrary that it seemed to have been designed specifically to exclude Pluto. (I've often wondered if someone has defined the precise moment when a young planet in a young solar system still full of debris will have "cleared its orbit" sufficiently to allow it to be called a planet. Seems like an awfully fuzzy concept. What about every time Earth flies through a meteor swarm?)
Why not make your arbitrary elliptical orbit ratio 2.2:1 or 3.14153265898:1? Why is it a problem if certain comets fit the definition of "planet" by forming themselves into a sphere under their own gravity (plus the heat of the sun)? Surely we can find some less arbitrary way to exclude comets if that is really deemed scientifically necessary. You're fine with having tens of thousands of planets in the outer solar system, but you're not fine of some of them are comets? We already have huge planets that are mostly gas and ice.
Then there's the problem that orbits can change drastically when two objects in space interact with one another. Ganymede, Io, Titan, Europa, many of the objects in our own solar system that we refer to as "moons" are nearly as large as Earth. The only reason we don't call them "planets" is because they've been gravitationally captured by another planet. If they were freely orbiting the Sun on their own they would be clearly recognized as planets. If a rogue micro-blackhole zipped through our inner solar system and grabbed the planet Venus (gently) and pulled it out into the outer solar system where it got captured by Jupiter or kept on going out to the Kuiper Belt, the current definition of "planet" says we couldn't call it a planet anymore. Even though it would be the same physical object with the same physical attributes of size, shape, physical composition, etc. This is the equivalent of stating that a bicycle can only be called a bicycle if it's being ridden on a paved road, but not if it's being ridden on a dirt path in the forest. Most people would consider this a ludicrous argument when talking about bicycles, but are somehow OK with having to call a planet something else if it moves to a different place or follows the "wrong" orbital path. There is nothing scientific or logical about confusing an object's basic physical properties with its location or movement.
We should be perfectly capable of saying "this is a planet that orbits the Sun" or "this is a planet that orbits another planet" or "this is a planet that is also a Kuiper Belt Object". There is nothing confusing about this.
Something to think about: You have two objects in space, both the size of Earth. They are orbiting each other. They are precisely the same size and mass, thus they both orbit a point in space precisely in the middle of the line connecting their two centers of mass. Which of these two objects is a planet? Which is a moon? Are they both planets? If they're physically identical to Earth, why aren't they planets? How would it make any scientific sense for them not to be called planets? And what damn difference does it make whether they're orbiting a star, another larger planet, or just traveling together through the empty space between galaxies?
¯\_ʕ◔.◔ʔ_/¯ LOL. I dunno. I'm just a bear.
... Peace out. Got bear stuff to do. 彡ʕ⌐■.■ʔ
(Score: 2) by JoeMerchant on Wednesday February 22 2017, @09:49PM
It makes a difference to the solar energy influx pattern whether or not a "planet" is in a relatively circular, or highly elliptical, or hyperbolic escape trajectory. Of course, rotation relative to the Sun is another thing.
It is all rather arbitrary, I believe the thinking on calling "large enough to assume a spheroid shape" is that it is an object which one might stand on, or at least float on if it has a gassy surface. It's kinda hard to stand on the smaller comets. And, who says that a view from the scale of a 180cm tall biped is the correct universal definition?
Your binary planet pair could easily be defined as such by an arbitrary ratio (2:1, pi:1, whatever you like) - some have referred to Earth-Moon as a binary pair due to the Moon's rather strong gravitational influence on Earth as compared to most moons around their ersatz planets.
No matter what we define from our obscure corner of an average galaxy, it will not be in alignment with the greater pan-galactic consensus. Of course, we may never know unless we manage to communicate with them before self-extinguishing our civilization.
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