Pluto orbits beyond the orbit of Neptune (usually). It is much smaller than any ofthe official planets and now classified as a "dwarf planet". Pluto is smaller than sevenof the solar systems moons(the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).orbit: 5,913,520,000 km (39.5 AU) from the Sun (average)diameter: 2274 kmmass: 1.27e22 kgIn Roman mythology, Pluto (Greek: Hades) is the god of the underworld. Theplanet received this name (after many other suggestions) perhaps because its so farfrom the Sun that it is in perpetual darkness and perhaps because "PL" are the initialsof Percival Lowell.Pluto was discovered in 1930 by a fortunate accident. Calculations which laterturned out to be in error had predicted a planet beyond Neptune, based on the motionsof Uranus and Neptune. Not knowing of the error, Clyde W.Tombaugh at Lowell Observatory in Arizona did a very careful sky survey whichturned up Pluto anyway.After the discovery of Pluto, it was quickly determined that Pluto was too small toaccount for the discrepancies in the orbits of the other planets. The search for PlanetX continued but nothing was found. Nor is it likely that it ever will be: thediscrepancies vanish if the mass of Neptune determined from the Voyager 2 encounterwith Neptune is used. There is no Planet X. But that doesnt mean there arent otherobjects out there, only that there isnt a relatively large and close one like Planet Xwas assumed to be. In fact, we now know that there are a very large number of smallobjects in the Kuiper Belt beyond the orbit of Neptune, some roughly the same size asPluto.Pluto has not yet been visited by a spacecraft. Even the Hubble SpaceTelescope can resolve only the largest features on its surface (left andabove). A spacecraft called New Horizons was launched in January2006. If all goes well it should reach Pluto in 2015.Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered(in 1978) just before its orbital plane moved edge-on toward the inner solar system. Itwas therefore possible to observe many transits of Pluto over Charon and vice versa.By carefully calculating which portions of which body would be covered at whattimes, and watching brightness curves, astronomers were able to construct a roughmap of light and dark areas on both bodies.
In late 2005, a team using the Hubble Space Telescopediscovered two additional tiny moons orbiting Pluto.Provisionally designated S/2005 P1 and S/2005 P2, they are nowknown as Nix and Hydra. They are estimated to be between 50and 60 kilometers in diameter.Plutos radius is not well known. JPLs value of 1137 is givenwith an error of +/-8, almost one percent.Though the sum of the masses of Pluto and Charon is known pretty well (it can bedetermined from careful measurements of the period and radius of Charons orbit andbasic physics) the individual masses of Pluto and Charon are difficult to determinebecause that requires determining their mutual motions around the center of mass ofthe system which requires much finer measurements -- theyre so small and far awaythat even HST has difficulty. The ratio of their masses is probably somewherebetween 0.084 and 0.157; more observations are underway but we wont get reallyaccurate data until a spacecraft is sent.Pluto is the second most contrasty body in the Solar System (after Iapetus).There has recently been considerable controversy about the classification of Pluto.It was classified as the ninth planet shortly after its discovery and remained so for 75years. But on 2006 Aug 24 the IAU decided on a new definition of "planet" whichdoes not include Pluto. Pluto is now classified as a "dwarf planet", a class distinctfrom "planet". While this may be controversial at first (and certainly causes confusionfor the name of this website) it is my hope that this ends the essentially empty debateabout Plutos status so that we can get on with the real science of figuring out itsphysical nature and history.Pluto has been assigned number 134340 in the minor planet catalog.Plutos orbit is highly eccentric. At times it is closer to the Sun than Neptune (as itwas from January 1979 thru February 11 1999). Pluto rotates in the opposite directionfrom most of the other planets.Pluto is locked in a 3:2 resonance with Neptune; i.e. Plutos orbital period is exactly1.5 times longer than Neptunes. Its orbital inclination is also much higher than theother planets. Thus though it appears that Plutos orbit crosses Neptunes, it reallydoesnt and they will never collide. (Here is a more detailed explanation.)
Like Uranus, the plane of Plutos equator is at almost right angles to theplane of its orbit.The surface temperature on Pluto varies between about -235 and -210C (38 to 63 K). The "warmer" regions roughly correspond to the regionsthat appear darker in optical wavelengths.Plutos composition is unknown, but its density (about 2 gm/cm3) indicates that it isprobably a mixture of 70% rock and 30% water ice much like Triton. The bright areasof the surface seem to be covered with ices of nitrogen with smaller amounts of(solid) methane, ethane and carbon monoxide. The composition of the darker areas ofPlutos surface is unknown but may be due to primordial organic material orphotochemical reactions driven by cosmic rays.Little is known about Plutos atmosphere, but it probably consists primarily ofnitrogen with some carbon monoxide and methane. It is extremely tenuous, thesurface pressure being only a few microbars. Plutos atmosphere may exist as a gasonly when Pluto is near its perihelion; for the majority of Plutos long year, theatmospheric gases are frozen into ice. Near perihelion, it is likely that some of theatmosphere escapes to space perhaps even interacting with Charon. NASA missionplanners want to arrive at Pluto while the atmosphere is still unfrozen.The unusual nature of the orbits of Pluto and of Triton and the similarity of bulkproperties between Pluto and Triton suggest some historical connection betweenthem. It was once thought that Pluto may have once been a satellite of Neptunes, butthis now seems unlikely. A more popular idea is that Triton, like Pluto, once moved inan independent orbit around the Sun and was later captured by Neptune. PerhapsTriton, Pluto and Charon are the only remaining members of a large class of similarobjects the rest of which were ejected into the Oort cloud. Like the Earths Moon,Charon may be the result of a collision between Pluto and another body.Pluto can be seen with an amateur telescope but it is not easy. There areseveral Web sites that show the current position of Pluto (and the other planets) in thesky, but much more detailed charts and careful observations over several days will berequired to reliably find it. Suitable charts can be created with many planetariumprograms.Charon
Charon ( "KAIR en" ) is Plutos largest satellite:orbit: 19,640 km from Plutodiameter: 1206 kmmass: 1.52e21 kgCharon is named for the mythological figure who ferried the dead across the RiverAcheron into Hades (the underworld).(Though officially named for the mythological figure, Charons discoverer was alsonaming it in honor of his wife, Charlene. Thus, those in the know pronounce it withthe first syllable sounding like shard ("SHAHR en").Charon was discovered in 1978 by Jim Christy. Prior to that it was thought thatPluto was much larger since the images of Charon and Pluto were blurred together.Charon is unusual in that it is the largest moon with respect to its primary planet inthe Solar System (a distinction once held by Earths Moon). Some prefer to think ofPluto/Charon as a double planet rather than a planet and a moon.Charons radius is not well known. JPLs value of 586 has an error margin of +/-13,more than two percent. Its mass and density are also poorly known.Pluto and Charon are also unique in that not only does Charonrotate synchronously but Pluto does, too: they both keep thesame face toward one another. (This makes the phases ofCharon as seen from Pluto very interesting.)Charons composition is unknown, but its low density (about 2gm/cm3) indicates that it may be similar to Saturns icy moons(i.e. Rhea). Its surface seems to be covered with water ice. Interestingly, this is quitedifferent from Pluto.Unlike Pluto, Charon does not have large albedo features, though it may havesmaller ones that have not been resolved.
It has been proposed that Charon was formed by a giant impact similar to the onethat formed Earths Moon.It is doubtful that Charon has a significant atmosphere.Read more about Pluto l Pluto facts, pictures and information. by nineplanets.orgWhat are the requirements for being a planet?I was always under the impression that for an object to be a planet it had to have a satelliteorbiting around it, a moon, that is why Pluto can be called a planet even though it is sosmall. My question then is, why are Mercury and Venus planets and what are theparameters required for planet status.Not all planets have moons (youve pointed out that Venus and Mercury do not), and its not arequirement.The definition of planet is mostly a historical distinction. Planets must be orbiting the Sun (oranother star), and must be "large," whatever that means. Beyond that, there are no parameters --after all, there are only 9! Historically, things have been called planets, and we stick to that.Some astronomers think Pluto should not be called a planet because it doesnt qualify in theirminds as "large" and it has a weird orbit. Really, though, its just a label.August 2006 Update by KLM: this month the International Astronomical Union (IAU) voted toupdate the definition of what makes a planet. According to their decision a planet must satisfythe following three criteria:It must be an object which independently orbits the SunIt must have enough mass so that gravity pulls it into a roughly speroidalshapeIt must be large enough to "dominate" its orbit (ie. its mass must be muchlarger than anything else which crosses its orbitPart 3 – The Ongoing DebateAnd third, it must have cleared other objects out of the way in its orbital neighborhood.To clear an orbit, a planet must be big enough to pull neighboring objects into theplanet itself or sling-shot them around the planet and shoot them off into outer space.According to the IAU, Pluto does not meet this third requirement but is now in a newclass of objects called "dwarf planets." It is this third part of the definition that hassparked debate.
To distinguish a planet from a round asteroid in the asteroid belt, a planet must be massive enough toclear smaller objects – like asteroids – from their own orbit. The gravity of the planet would pull insmaller objects which would become part of the new planet.The problem for PlutoThe problem for Pluto is the fact that its orbit is in the Kuiper Belt along with 43 otherknown Kuiper Belt Objects (KBOs). There are possibly billions of objects in the KuiperBelt that have not been cataloged yet. Scientists have even found 8 KBOs betweenNeptune and Pluto. Some scientists view the new definition as unclear. Exactly howmuch does Pluto have to "clear" from its neighborhood to be considered a planet? Andhow much has Pluto already influenced its own neighborhood since the planet formed?These and other questions have been raised in response to the IAUs definition of aplanet.Consider this: Pluto crosses into Neptunes orbit, but Neptune is still classified a planet.This is because of the orbits of Pluto and Neptune and that they never get closer toeach other than 17AU (AU=distance from Earth to the Sun). Pluto may cross orbits withmany other Kuiper Belt Objects, but how close do these objects get to Pluto? How closeto objects have to get to Pluto to be considered "in" Plutos neighborhood?Diagram of the planet orbits in our solar system, including Pluto, distinctly shows the cross over ofNeptunes and Plutos orbits.Journey to the edgeNASAs New Horizon spacecraft is speeding toward the edge of the solar system on itsmission to Pluto. Launched in January 2006, it will not be until July 2015 that we willreach Pluto. It will swing past Jupiter for a gravity boost and scientific studies inFebruary 2007, and reach Pluto and its moon, Charon, in July 2015. Then, as part of anextended mission, the spacecraft would head deeper into the Kuiper Belt to study oneor more of the icy mini-worlds in that vast region, at least a billion miles beyondNeptunes orbit. Sending a spacecraft on this long journey will help us answer basic
questions about the surface properties, geology, interior makeup and atmospheres onthese bodies.The Planets of the Solar SystemClick here to go to The Nine Planets home pageIncluding...The Inner PlanetsMercuryVenusThe Earth/Moon system (a separate file)MarsThe Outer PlanetsJupiterSaturnUranusNeptunePlutoThe Inner (Terrestrial) PlanetsMercuryThe innermost planet
One of the brightest objects in the skyBut rarely seen - WHY? (Never more than 28 deg. from the sun)Fastest moving of the planets (named for the Greek & Roman God of Speed)Orbits sun in 88 earth days at 48 km/sOrbit is most varied of all planets (except Pluto)Highly eccentric (0.206)Perihelion: 46 million kilometersAphelion: 70 million kilometersAlso, inclined 7 deg. to plane of eclipticVery small, but with a similar density to earth (5.5)Seems to be similar in composition to earth, but different proportionsCore accounts for 60% of its total mass (fig 13.22, pg. 230)Basically a small metal ball with a thin silicate crustSurface featuresQuite a range of temperaturesUp to 400 deg. C at noonProximity to sun supplies the daytime heatDrops to -175 deg. C just before dawnLack of atmosphere allows the heat to escape at nightThis would give your heat pump a real workoutHeavily cratered like the moonWith areas that have been flooded by basalt
Volcanics very early in planets history (4 b.y.)No evidence of plate tectonicsIsolated scarps indicate shrinkage during cooling (fig. 13.25, pg. 232)General tectonic history indicates:Early expansion while hotReleasing basaltic flowsLater shrinkage during coolingCausing scarps due to compression/contractionVenusVery similar to earth on overall featuresPhysical features nearly identical (Table 15.1; pg. 258)Surface is only lightly crateredDominated by volcanic activityDefinite "continents" as on earth (2 of them)Indirect evidence for surface water in the pastOne difference is its retrograde rotation"One of the most beautiful objects in the night sky"Named for the Goddess of Love and BeautyBeauty is clearly "only skin deep" because its a rather ugly place at the surfaceExtremely harsh surface conditionsTemperature well above 400 deg. C
Atmospheric pressure 90X that of earth (we would implode!)Both the result of the extremely dense atmosphere (96% CO2)Thick cloud cover is the result of H2SO4 droplets in the atmosphereProbably derived from extensive volcanic activityGreenhouse effectDIGRESS TO: runaway greenhouse effectVenus used to be more like EarthAlmost certainly had large amounts of surface waterInitial slow surface heating due to small increase in atmospheric CO2Leads to increased evaporation and H2O content in airLeads to more heat retention, and the "Runaway Greenhouse Effect" cycleCarried to its logical conclusion...Leads to evaporation of any surface waters and a "hot water" atmosphereWater vapor is not stable in UV light and breaks down into atomic formHydrogen escapes into spaceOxygen combines with iron, etc. at the surfaceTherefore, the loss of surface water is permanentCould this happen on the earth?MarsThe "red planet" named for the God of WarMuch smaller than the earth
Approx. 11% earths massAtmosphere similar to Venus in composition (95% CO2)But not in density - .006 bar (Mars) to 1 bar (Earth) to 90 bar (Venus)Surface similar to earth 200 mya when Pangea was completeSouthern highland (continent) which is heavily cratered (probably older)Surrounded by younger volcanic plains (not covered by water)Several kilometers lower in elevation than the "continent"Extensive tectonic and volcanic activityNo direct evidence of plate tectonic activitySeveral features indicating tectonic/volcanic activityMost 1-3 billion years oldTharsis Bulge - active region the size of North AmericaConcentration of "recent" volcanic activityOlympus Mons (fig. 14.8, pg. 241)Probably largest volcano in solar system (fig. 15.14, pg. 268)Possibly still intermittently active!Valles Marineris (fig. 14.10, pg. 243)A tectonic feature so not really a "valley"Basically tension cracks on the edge of the Tharsis BulgeSimilar to tensional features in AfricaBig! 5000 km X 100 km X 7 km deepPossibly plate-style activity may have started long ago
Did not develop like on earth due to smaller mass, quicker overall coolingMuch evidence for surface water (See: photo pg. 234; fig. 14.8b, pg. 241)Most drainage features limited to older cratered highland areasTwo kinds of drainage patternsNormal dendritic patterns (fig. 14.12, pg. 244)Developed on the older cratered upland areasEvidence for catastrophic floods (fig. 14.21, pg. 250)From the upland onto the lava plainsLike the Channeled Scablands of Eastern WashingtonEvidence for glacial ice ages in the Martian past?Supports theory that surface water was present during 2 periods in the pastThe first 4 billion years ago related to "normal" rainfall/runoffThen later a sudden release of frozen water by volcanic heating, or?All surface water now frozen into polar ice caps (fig. 14.2, pg. 236)Possibility for life in the past (maybe now?)Mars has been the origin of some of earths most fearsome alien creaturesInvaders From Mars, War of the Worlds, My Favorite MartianNone identified by any of the Martian probesIntense UV from sun would make "life as we know it" unlikelyHowever, "water is life" and Mars has waterStay tuned for an update
The Outer (Jovian) PlanetsGeneral features of Jupiter and SaturnLargest planets in our solar systemJupiter is almost a binary partner to the sunEach has a system of orbiting satellites (moons) and ringsCan be quite extensiveSaturn has an impressive ring structure and 19 moonsBasically mini-suns - Composed of hydrogen and heliumTheoretical internal structure - fig. 16.3, pg. 277Pressure at depth compresses the hydrogen into liquid, then "metallic" formWith a small "ice and rock" coreCentral cores may represent the original rock/ice bodies which accumulated from thenebulaWith the hydrogen being "captured" at a later timeNot "rock" as we recognize it due to extreme pressure and temperatureSome form of iron, silica, and oxygenThe "ice" is also probably different from what goes into a glass of PepsiAny combination of hydrogen with carbon, nitrogen, or oxygenBoth radiate impressive amounts of heat from 2 sourcesResidual heat from the initial condensation of the nebulaNewly generated heat from continued contraction of the gasWell developed "atmospheres"
Hydrogen and helium, with methane (CH4) and ammonia (NH3)JupiterIn orbit: 16 moons, faint ringHas an extensive cloud coverVivid colors (white, orange, red, brown)Essentially condensed ammonia (anyone for a walk in the rain?)Great Red Spot (fig. 16.2, pg. 276) and (fig. 16.12, pg. 284)A large "storm" in the atmosphere - almost 30,000 km across!Has been "stable" for at least 300 yearsHow can it last for so long?Nothing solid to interfere with the circulation of the gasSaturnIn orbit: 19 moons. extensive ringsUranusIn orbit: 15 moons, intricate system of dark ringsNeptuneIn orbit: 8 moons, faint ringsGreat Dark Spot: similar to GRS on Jupiter
Atmospheric storm 10,000 km acrossPlutoDiscovered after a systematic searchIts presence was indicated by "wobbles" in Neptunes orbitIn orbit: a single large moon (Charon)Essentially a binary systemLet’s find out why Pluto is no longer considered a planet.Pluto was first discovered in 1930 by Clyde W. Tombaugh at the Lowell Observatory in Flagstaff Arizona. Astronomers had long predictedthat there would be a ninth planet in the Solar System, which they called Planet X. Only 22 at the time, Tombaugh was given the laborioustask of comparing photographic plates. These were two images of a region of the sky, taken two weeks apart. Any moving object, like anasteroid, comet or planet, would appear to jump from one photograph to the next.After a year of observations, Tombaugh finally discovered an object in the right orbit, and declared that he had discovered Planet X. Becausethey had discovered it, the Lowell team were allowed to name it. They settled on Pluto, a name suggested by an 11-year old school girl inOxford, England (no, it wasn’t named after the Disney character, but the Roman god of the underworld).The Solar System now had 9 planets.Astronomers weren’t sure about Pluto’s mass until the discovery of its largest Moon, Charon, in 1978. And by knowing its mass (0.0021Earths), they could more accurately gauge its size. The most accurate measurement currently gives the size of Pluto at 2,400 km (1,500miles) across. Although this is small, Mercury is only 4,880 km (3,032 miles) across. Pluto is tiny, but it was considered larger than anythingelse past the orbit of Neptune.Over the last few decades, powerful new ground and space-based observatories have completely changed previous understanding of theouter Solar System. Instead of being the only planet in its region, like the rest of the Solar System, Pluto and its moons are now known to bejust a large example of a collection of objects called the Kuiper Belt. This region extends from the orbit of Neptune out to 55 astronomicalunits (55 times the distance of the Earth to the Sun).Astronomers estimate that there are at least 70,000 icy objects, with the same composition as Pluto, that measure 100 km across or more inthe Kuiper Belt. And according to the new rules, Pluto is not a planet. It’s just another Kuiper Belt object.
Here’s the problem. Astronomers had been turning up larger and larger objects in the Kuiper Belt. 2005 FY9, discovered by Caltechastronomer Mike Brown and his team is only a little smaller than Pluto. And there are several other Kuiper Belt objects in that sameclassification.Astronomers realized that it was only a matter of time before an object larger than Pluto was discovered in the Kuiper Belt.And in 2005, Mike Brown and his team dropped the bombshell. They haddiscovered an object, further out than the orbit of Pluto that was probably the same size, or even larger. Officially named 2003 UB313, theobject was later designated as Eris. Since its discovery, astronomers have determined that Eris’ size is approximately 2,600 km (1,600 miles)across. It also has approximately 25% more mass than Pluto.With Eris being larger, made of the same ice/rock mixture, and more massive than Pluto, the concept that we have nine planets in the SolarSystem began to fall apart. What is Eris, planet or Kuiper Belt Object; what is Pluto, for that matter? Astronomers decided they would make afinal decision about the definition of a planet at the XXVIth General Assembly of the International Astronomical Union, which was held fromAugust 14 to August 25, 2006 in Prague, Czech Republic.Astronomers from the association were given the opportunity to vote on the definition of planets. One version of the definition would haveactually boosted the number of planets to 12; Pluto was still a planet, and so were Eris and even Ceres, which had been thought of as thelargest asteroid. A different proposal kept the total at 9, defining the planets as just the familiar ones we know without any scientific rationale,and a third would drop the number of planets down to 8, and Pluto would be out of the planet club. But, then… what is Pluto?In the end, astronomers voted for the controversial decision of demoting Pluto (and Eris) down to the newly created classification of “dwarfplanet”.Is Pluto a planet? Does it qualify? For an object to be a planet, it needs to meet these three requirements defined by the IAU:It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet.It needs to have enough gravity to pull itself into a spherical shape – Pluto…checkIt needs to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule breaker. According to this, Pluto is not a planet.
What does “cleared its neighborhood” mean? As planets form, they become the dominant gravitational body in their orbit in the SolarSystem. As they interact with other, smaller objects, they either consume them, or sling them away with their gravity. Pluto is only 0.07 timesthe mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.Any object that doesn’t meet this 3rd criteria is considered a dwarf planet. And so, Pluto is a dwarf planet. There are still many objects withsimilar size and mass to Pluto jostling around in its orbit. And until Pluto crashes into many of them and gains mass, it will remain a dwarfplanet. Eris suffers from the same problem.It’s not impossible to imagine a future, though, where astronomers discover a large enough object in the distant Solar System that couldqualify for planethood status. Then our Solar System would have 9 planets again.Even though Pluto is a dwarf planet, and no longer officially a planet, it’ll still be a fascinating target for study. And that’s why NASA has senttheir New Horizons spacecraft off to visit it. New Horizons will reach Pluto in July 2015, and capture the first close-up images of the (dwarf)planet’s surface.Space enthusiasts will marvel at the beauty and remoteness of Pluto, and the painful deplaneting memories will fade. We’ll just be able toappreciate it as Pluto, and not worry how to categorize it. At least now you know why Pluto was demoted.Read more: http://www.universetoday.com/13573/why-pluto-is-no-longer-a-planet/#ixzz2WHovp9OyThe definition of planet set in 2006 by the International Astronomical Union (IAU) states that, inthe Solar System, a planet is a celestial body which:1. is in orbit around the Sun,2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and3. has "cleared the neighbourhood" around its orbit.A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet". Accordingto the IAU, "planets and dwarf planets are two distinct classes of objects". A non-satellite body fulfillingonly the first criterion is termed a "small Solar System body" (SSSB). Initial drafts planned to includedwarf planets as a subcategory of planets, but because this could potentially have led to the addition ofseveral dozens of planets into the Solar System, this draft was eventually dropped. The definition was acontroversial one and has drawn both support and criticism from different astronomers, but has remainedin use.According to the definition, there are currently eight planets and five dwarf planets known in the SolarSystem. The definition distinguishes planets from smaller bodies and is not useful outside the SolarSystem, where smaller bodies cannot be found yet. Extrasolar planets, or exoplanets, are coveredseparately under a complementary 2003 draft guideline for the definition of planets, which distinguishesthem from dwarf stars, which are larger.