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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: 2) by melikamp on Wednesday February 22 2017, @03:34PM
I think a lot of hubbub is coming from NASA and other US research places, where certain people felt that the funding is often conditioned by the nomenclature. They get their money from the US congress, so I cannot totally dismiss their concerns, since most congresscritters seem to be more familiar with Disney zoo than with the basic scientific facts and concepts. Of course, and it's been pointed out by eminent scientists, the planetary status of Pluto is a question of nomenclature: it settles no scientific question about any of the celestial bodies, and has virtually no impact on the advancement of science.
The official definition came in the wake of finding out new facts about Pluto and other KB objects. The union took a conservative approach, shaping the new definition to include precisely the same bodies which were traditionally considered the planets. Unfortunately, they failed to come up with objective criteria that would distinguish Pluto from other KBO, so they had to toss it. Pluto, as most of you know, has a couple of highly distinguishing features: it is a component of a binary, with barycernter located outside of the body, its orbit is more elliptical and is at 17 deg to the ecliptic, and its location and low mass prevent it from dominating (clearing, as they said) its orbital neighborhood in the way the big 8 have done.
In contrast, Alan Stern seems to think that tradition is irrelevant, unless it's the specific 100 years tradition of calling Pluto a planet. In the white paper linked from TFA he proposes a definition which would expand the number of planets to dozens or may be hundreds, and would include all large moons, including the one you can see with the naked eye. For my money, this effort is precipitated by Plutofilia and nothing else.
Alan Stern brings up a very good point about dwarf planet not being a planet, how silly that is; so silly in fact, even Mike Brown agrees. But Stern fails IMO to make a strong case for why this should be fixed by expanding the number of planets. It would be just as easy to officially rename dwarf planets into "planetoids" or some such, which is a name already used in practice.
(Score: 1) by WillR on Wednesday February 22 2017, @05:23PM
The union took a conservative approach, shaping the new definition to include precisely the same bodies which were traditionally considered the planets. Unfortunately, they failed to come up with objective criteria that would distinguish Pluto from other KBO, so they had to toss it.
Except they didn't, because Neptune hasn't cleared it's orbit of Pluto/Charon. Setting a minimum size/mass for "planethood" that keeps Mercury but tosses Pluto and the rest of the inconvenient KBOs would still be obvious appeal to tradition, but it feels more honest to me.
(Score: 2) by melikamp on Wednesday February 22 2017, @05:56PM
(Score: 1) by khallow on Wednesday February 22 2017, @07:05PM
and their orbital neighborhoods meet
Neptune never gets closer to Pluto than the Sun. Also, "orbital neighborhood" remains solidly undefined.
(Score: 2) by melikamp on Wednesday February 22 2017, @09:10PM
Orbital neighborhood is a neighborhood of an elliptical orbit, an elongated donut, at least that's one obvious way to define it. Just because you do not have a definition, doesn't mean it's undefined. Also, no one says there has to be one true definition. Neptune would have "cleared" its orbit, with Pluto accounted for, by many similar definitions. Likewise, Pluto would have failed to do the same using the same definitions. I am sure if you go through the relevant discussions by astronomical union members, you will see the definition examined, picked apart, and fleshed out in detail.
Once again, this classification in no way extends our knowledge of the natural world, so going along with the traditional use is the non-controversial solution. Pluto used to be a planet until astronomers decided to define the word planet for the first time, and then they realized Pluto shares so many parameters with KBOs, that any objective, not-entirely-ad-hoc definition including Pluto would sweep them in as well. The new definition from TFA is exactly what they already rejected: extending the notion of a planet to dozens, and most likely hundreds of icy snowballs in the KB, for no reason other than keeping Pluto a planet. To see how invasive and arbitrary Stern's definition is, consider this: the official definition reduced the number of planets by 11%; Stern's definition increases the number of planets by hundreds, and likely thousands of percent. It makes a mockery of the traditions which go back thousands of years (e.g. how we call a certain black rock the Moon, and not a planet) in order to rescue one recent tradition because the latter has some reputations riding on it.
(Score: 1) by khallow on Wednesday February 22 2017, @09:49PM
Orbital neighborhood is a neighborhood of an elliptical orbit, an elongated donut, at least that's one obvious way to define it. Just because you do not have a definition, doesn't mean it's undefined.
I did have an implied definition, you just missed it. A natural neighborhood of an orbit or other trajectory is a locus of points near the object at a given time. Neptune never gets close to Pluto (I was incorrect earlier, it does get closer to Pluto than the Sun at some points of the pair's orbits (around 17 AU). Uranus at times is closer than Neptune can be. That's pretty distant for a body that supposedly is in the neighborhood of Pluto.
And if you look at the official IAU definition, they don't actually define an orbital neighborhood.
The new definition from TFA is exactly what they already rejected: extending the notion of a planet to dozens, and most likely hundreds of icy snowballs in the KB, for no reason other than keeping Pluto a planet.
Which let us note would be just fine. There's no actual problem here.
To see how invasive and arbitrary Stern's definition is, consider this: the official definition reduced the number of planets by 11%; Stern's definition increases the number of planets by hundreds, and likely thousands of percent.
So what?
It makes a mockery of the traditions which go back thousands of years (e.g. how we call a certain black rock the Moon, and not a planet) in order to rescue one recent tradition because the latter has some reputations riding on it.
It doesn't. This is absurd claim to make particularly since we know more than those people thousands of years ago, and we'd have to exclude Neptune and Uranus as well because they weren't known for thousands of years.
(Score: 2) by melikamp on Thursday February 23 2017, @12:51AM
I don't think it's absurd. None of the science matter is affected by classifying Pluto one way or the other, so the state of knowledge is irrelevant for the purposes of evaluating how traditional something is. Seniority, on the other hand, as well as the actual scope of the definition do seem to be relevant. Pluto has the lowest seniority among the planets, with Neptune having 84 years on it. The decision to dump it was not taken lightly, either, but was a result of a prolonged discussion. And increasing the number of planets 10- or 100-fold by admitting a bunch of snowballs with absolutely no seniority seems much less of a break with tradition than dumping 11% of least senior planets.
(Score: 2) by melikamp on Thursday February 23 2017, @01:20AM
s/less/more
(Score: 2) by melikamp on Thursday February 23 2017, @01:24AM
(Score: 1) by khallow on Thursday February 23 2017, @04:21AM
None of the science matter is affected by classifying Pluto one way or the other, so the state of knowledge is irrelevant for the purposes of evaluating how traditional something is.
And as I noted, planets which can't be seen with the naked eye aren't traditional in the very sense you invoked at the time.
The decision to dump it was not taken lightly, either, but was a result of a prolonged discussion. And increasing the number of planets 10- or 100-fold by admitting a bunch of snowballs with absolutely no seniority seems much less of a break with tradition than dumping 11% of least senior planets.
Sure it was a prolonged discussion. Sounded more like an afternoon vote packed by the side that wanted to throw out Pluto. Second, how traditional again is it to call something a "dwarf planet" and then say it's not a real "planet"? Potentially hundreds of objects just fell into that silly semantics game.
(Score: 2) by FatPhil on Wednesday February 22 2017, @11:36PM
There are thousands of objects that even jupiter hasn't cleared from its orbit, they're stuck in synchronised orbits, sometimes even simply phase-shifted identical orbits - that's right, it hasn't even cleared things from its own path - because of the balance between the sun's gravity and jupiter's gravity creating islands, "lagrange points", where small objects will, for want of a better word, gravitate. Those 5 points are just for starters. There's a great number of other objects in synchronised orbits with jupiter. It gives them a kick one way, and then gives them a kick the other way, alternately, and they never get the chance to deviate from this pattern as jupiter's almost-clockwork orbitting and gravitational attraction is in full control of their orbits. All kinds of ratios of periods are possible, but they're all synchronised. Like a juggler who will throw one of the balls really high, continuing for any number of cycles with the rest of the moons, sorry, balls before catching and rethrowing the wild one, they're all being juggled, even if the periods are different.
And neptune is juggling pluto. That's factored into their definition, even if perhaps as an explanatory note.
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