The end result was demotion of Pluto and the redesignation of it and other Kuiper-belt objects as dwarf planets. Schaller says that the new results, obtained with Hubble Space Telescope and Keck Observatory data, indicate that the density of the material making up Eris is about two grams per cubic centimeter.
This means that Eris very likely is made up of ice and rock, and thus is very similar in composition to Pluto. Past results from the Hubble Space Telescope had already allowed planetary scientists to determine that its diameter is 2, kilometers, also larger than Pluto's. The reason for Eris's blustery surface conditions is its sheer distance from the sun. Currently 97 astronomical units from the sun an astronomical unit being the distance between the sun and Earth , Eris hovers at temperatures well below degrees Fahrenheit and is pretty dark.
What difference does it make which one is bigger? I remember reading in an astronomy book from the s which speculated that Pluto, due to its eccentric orbit, may be just an escaped moon of Neptune, so even back then it was not considered a proper planet.
That was speculation, and it is probably wrong. But back then, no one knew that our solar system has many dwarf planets, not just Pluto. Yes, Pluto DOES fit in with the rest of the solar system if you recognize that the solar system has three, not two classes of planets—terrestrials, jovians, and dwarf planets.
Dwarf planets are a subclass of planets because they are much more akin to the bigger planets than to asteroids. They are shaped by their own gravity, are geologically differentiated, have atmospheres and weather, and are complex bodies—unlike asteroids and tiny KBO iceballs. In fact, Earth has a lot more in common with dwarf planets like Pluto than with jovians like Jupiter. Both Earth and Pluto have large moons formed via giant impact; both have solid surfaces and are differentiated into core, mantle, and crust; and both have nitrogen in their atmospheres.
Pluto may even have a subsurface ocean. Unlike Earth, it has not solid surface. To put Earth and Jupiter in the same category while excluding Pluto and all dwarf planets amounts to cherry picking characteristics that Earth and Jupiter have in common while ignoring the differences between them as well as the characteristics Earth and Pluto have in common.
Interesting viewpoint. Past the ice giants you have dusty, icy objects in the region beyond Neptune, all the way out to the Oort cloud. When you look at the physics of solar system formation, this layout makes sense. You say Earth has more in common with Pluto than Jupiter? Apples and Oranges…. So we have terrestrial, gas giant, ice giant, and dwarf planet categories. Seems logical. Ever watch sesame street?
Pluto is far more similar to Eris and the other Dwarf planets than it is to say, Earth or Venus. Sure, Pluto likely has a crust, mantle and core. Pluto is pretty much at hydrostatic equilibrium too. It takes far less ice, mass wise, to reach hydrostatic equilibrium than rock or metal.
A more likely candidate for subsurface water is Ceres. New Horizons will give us more info in a few years, though so anything at this point is speculation. So when everyone classifies Pluto as a dwarf planet, that is because its composition, orbital distance, orbital parameters, and other characteristics are far more similar to the rest of the dwarf planets, than say Earth, Saturn or Uranus.
Where does it best fit in our current planetary classification system? Pluto now has a family of dozens, possibly hundreds of similar objects. Everything you say makes sense except for one thing: the IAU decision specifically states that dwarf planets are NOT planets at all a position you reinforce with that mnemonic. In astronomy, dwarf stars are still stars, and dwarf galaxies are still galaxies. Tonight's Sky — Change location. US state, Canadian province, or country.
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