PLUTO PLANET – WHY PLUTO IS NOT PLANET ?
 |
| Pluto |
- Pluto was discovered in 1930 and assigned as the ninth planet in our solar system. However, in 2006, the status was withdrawn and reassigned as a dwarf planet.
- The name Pluto was proposed by Venetia Burney, an 11-year-old girl from England. The Student Dust Counter was named in her honour.
- By the time we discovered and stripped Pluto of its planet status, the icy object had not completed an entire orbit around the Sun.
- Pluto is the largest KBO and the second most massive.
- One-third of Pluto is water ice-almost equal to three times the water in all our oceans put together. The remaining two-thirds is rock.
- It takes 248 earth years for Pluto to orbit around the Sun.
- The sunlight on Pluto is as intense as moonlight on Earth.
- It takes five hours for the sunlight to reach Pluto (as against 8 mins to Earth).
- Pluto has a retrograde rotation (opposite to that of the Earth). So, on Pluto the Sun rises in the west and sets in the east.
- Pluto is a cold planet with an average temperature of about-230oC.
- The feeble light on Pluto makes the days appear dark, and one can see stars doing the day.
- on Pluto, sunrise to sunset is about a week long.
- Pluto has a thin atmosphere mostly of nitrogen ice, carbon monoxide and methane.
- The surface is rocky and believed to be having cryovolcanoes and geysers.
Why is Pluto not a planet & Where is
Pluto planet?
Pluto has only two of the three necessary characteristics to be called a planet. Pluto has not cleared its neighborhood or orbit. But, of course, there are additional details.
The simplest reason that Pluto is not a planet is that planet experts say so, and this is their job. But you may be looking for a more detailed explanation.
Let's look at what defines a planet. This could be a very long and tedious discussion, because "planet" is an ancient concept used long before scientists knew very much about them. Also, frankly, in many areas of science the definition of a thing, perhaps counter-intuitively to non-scientists, is often pretty irrelevant to its study.
Definitions that change over time that are never quite in line with the phenomenon being observed etc. may seem like an impediment to science, but they often are not. The definition of a "gene" has changed dramatically as we've learned more about them, but this shifting description has not hampered genetic research. To some extent, this may be the same with planets. A "planetologist" who studied Pluto back when it was still counted as a planet would not have to find a new job when our solar system went from 9 to 8.
The International Astronomical Union has settled on a set of definitions of solar system bodies, which includes planets, dwarf planets (which are mostly minor planets), small solar system bodies, trans-Neptunian objects which also might be called plutoids (those are also minor planets) and some small solar system bodies (including some comets) and satellites, and satellites are, of course, things that go around things that are not the Sun. Confused? Probably, but that is not a big problem because while these various identified flying objects have complex overlapping categorical status, one type of object does not. Planets are planets and they are not anything other than planets.
To be a planet, you have to be in orbit around the Sun. This would rule out the Moon, which, if it was in orbit around the Sun instead of the Earth, could well be a planet.
To be a planet the object has to have sufficient mass to have been shaped by gravity to be (mostly) a globe. This depends on various things so at the low end of the mass spectrum there will be different masses and different sizes of things that don't quite make it to globular status.
To be a planet the object has to have cleared its orbit. In other words, as an object orbits around the sun, it is likely to bump into other objects. Over a period of time, the object has finished bumping into everything it is likely to bump into and thereafter has only a low probability of bumping into something. That does not rule out something bumping into the object, of course.
A globe-shaped object that goes around the sun but that has not cleared its orbit is classified as a "dwarf planet." This is of course historically contingent. In the early days of a solar system, perhaps there would be large star-circling round things that have not yet cleared their orbit. This speaks to the strangeness of definitions alluded to above. The definition we use today to classify our solar system's objects applies to a solar system developed to the extent ours is developed. The IAU nomenclature would probably need revisions if applied to all planetary-star systems in the Universe.
This scheme is not without its critics and there is indeed debate. Some of that debate is a bit nitpicky but still interesting. For example, Alan Stern, with NASA, notes that many planets have not really cleared their orbit, noting in relation to the Pluto controversy, "If Neptune had cleared its zone, Pluto wouldn't be there." Yes, apparently heavenly bodies have irony.
Anyway, as implied, Pluto is not classed as a planet because it has not cleared its orbit. Therefore it is a Dwarf Planet. Since it is far away (farther than Neptune) it also gets classed as a Trans-Neptunian Object. Furthermore, it is a Plutoid. That is simply a newer term applied to Trans-Neptuina dwarf planets.
The term Plutoid, then, refers to a dwarf planet, which for various reasons is apparently always specifically an ice dwarf, which is a trans-Neptunian body (orbiting most of the time beyond Neptune) that is sufficiently massive to be shaped like a globe. This term, plutoid, is officially adopted A plutoid or ice dwarf is a trans-Neptunian dwarf planet, i.e. a body orbiting beyond Neptune that is large enough to be rounded in shape. The term plutoid was adopted by the International Astronomical Union's Committee on Small Bodies Nomenclature, but not by the working group on Planetary System Nomenclature. So you can use Plutoid or Dwarf Planet, or Ice Dwarf, depending on whom you wish to annoy.
Pluto, Eris, Haumea, and Makemake are the only known Plutoids. They are small enough and far enough away that more could be discovered.
I would say, "the only known Plutoids that are accepted as dwarf planets by the IAU". More already have been discovered, even recommended for classification as dwarf planets, but aren't officially there yet:
Orcus, Quaoar, Sedna, Salacia, Varuna, Ixion -- and those are just one that have non-numerical names...
Then there is the first dwarf planet to be discovered -- and it's not Pluto: Ceres, the planetino whose namesake gave us "cereal". It was discovered in 1801, before even Neptune, and was initially regarded as a planet; it's now officially just another dwarf planet. (And it was visited with a spacecraft ahead of Pluto.)
The practical problem that led to the need to come up with this definition of a planet is one of sheer number: If we were to accept Pluto as "The Ninth Planet", it would immediately have to take 10th place, as Ceres would be forced to return to planetary status as #5, displacing Jupiter. Then, joining Pluto as #10, would be the above-mentioned plutinos, plus, eventually, the other existing discoveries, and then future discoveries.
Anyone up to memorizing the names of the 537 "planets"?? No, me neither. We need a manageable number, and 8 is it.
The problem with poor Pluto is that it is indistinguishable from all its other Trans-Neptunian brethren. Sentimentality does not make it special, unfortunately. Nor would it in being the first discovered, which it also isn't. Too bad Clyde Tombaugh didn't keep looking, but he was simply lucky to have spotted Pluto in the first place...
In short, Pluto is not a planet because it never was. A case of mistaken identity. (Hello, Alan Stern?) Still, paying it a visit is very cool, and this issue doesn't detract from that.
