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: 4, Insightful) by jdavidb on Wednesday February 22 2017, @02:04PM
ⓋⒶ☮✝🕊 Secession is the right of all sentient beings
(Score: 0) by Anonymous Coward on Wednesday February 22 2017, @02:44PM
I don't know why this was modded "Touche".
It doesn't contradict my point about the definition being somewhat arbitrary and historical, starting as those objects that were visible (and BTW, that moved, unlike the stars). Certainly the moon (and the sun) are considered differently because they are so close to us.
The point is that there -is- no objective criteria for what is a "planet." We tend to redefine it time to time so it just so happens to include all the historical planets as a requirement.
(Score: 0) by Anonymous Coward on Wednesday February 22 2017, @02:49PM
To clarify, I meant the historical planets to be those we considered planets in the 1700's and 1800's when the generally agreed definition was basically a large body that orbited the sun.
(Score: 0) by Anonymous Coward on Wednesday February 22 2017, @02:52PM
Also to clarify, the sun and moon might be considered planets in ancient times, but I meant "considered differently" in a *qualitative* sense.
The sun lights up the sky and gives life and days, the moon has phases and gives tides, etc.
(Score: 2) by bzipitidoo on Wednesday February 22 2017, @03:35PM
"Planetes" means "wanderer". "Planetes asteres", planets for short, are stars that do not stay fixed relative to other stars. but which wander across the sky. While the sun and the moon both "wander", those are in categories of their own. Note also the original definition fails to include Earth itself.
(Score: 2) by jdavidb on Wednesday February 22 2017, @03:17PM
I was shooting for "informative" or "interesting" or something.
If I understand correctly the original word for planet basically meant "traveler" and meant the celestial objects that move to different locations in the sky each night as opposed to the stars which move but much more slowly. I remember at some point being surprised to learn that also included the moon and sun, although I'm not sure if every culture classified them that way.
ⓋⒶ☮✝🕊 Secession is the right of all sentient beings