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Dr. Christine M. Rodrigue

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1 Dr. Christine M. Rodrigue
Pluto Geography 441/541 S/16 Dr. Christine M. Rodrigue C.M. Rodrigue, 2016 Geography, CSULB

2 Why Isn't Pluto a Planet Anymore?
International Astronomical Union 2006 Facing a tangle of controversies: Huge number of Trans Neptunian Objects Kuiper Belt, mostly ~30-50 AU out there Mostly icy debris from early solar system First Trans Plutonian Object found in 1992 (QB1) >1,000 of them found since then! Some are pretty large So, do you keeping adding planets? How small do you go? C.M. Rodrigue, 2016 Geography, CSULB

3 Why Isn't Pluto a Planet Anymore?
International Astronomical Union 2006 Definition of a planet: Has to be in independent orbit around the sun Has to have mass sufficient to achieve hydrostatic equilibrium Self-gravity to balance pressure gradient force Usually results in a spherical or even quite ellipsoid shape Has to have cleared its orbit of most other objects This can be by impact It can also be by exchanges of angular momentum that fling the smaller orbiting objects off the orbital path Pluto satisfies first two but not the third, so it was voted off the planet island and put among the new dwarf planet class C.M. Rodrigue, 2016 Geography, CSULB

4 Why Isn't Pluto a Planet Anymore?
International Astronomical Union 2006 Decision cleared up a lot of other “plot complications”: The eight remaining planets share a common orbital plane Their orbital inclinations are ≤ ~7° of Earth's plane of ecliptic Most dwarf planets's orbits are markedly inclined, from 17° to 44° Pluto's is tilted about 17° (but Ceres' is only ~3°) Planets generally have eccentricities <0.10 Mercury is pretty eccentric, though, at 0.21 The dwarf planets range from 0.15 to 0.44, except Ceres at 0.07 Pluto's is 0.25, placing its perihelion inside the orbit of Neptune! No other planets' orbits cross one another Pluto's icy composition is typical of most Kuiper Belt Objects Inner solar system planets are rocky Outer giants are largely gas Pluto doesn't really fit among them, but it does fit KBOs C.M. Rodrigue, 2016 Geography, CSULB

5 Why Isn't Pluto a Planet Anymore?
Kuiper Belt 7 AU thick doughnut of mostly icy objects in solar orbit Estimates are of several hundred thousand >20 m in diameter >1,000 have been found since 1992 check out: So many that the KBO needed to be subdivided: Classical KBOs or “cubewanos” (after 1992QB1) “Cold”: stable orbits ~42-48 AU out there, mostly <5° inclination “Hot”: unstable orbits, same distance but very eccentric orbits and often markedly tilted orbits C.M. Rodrigue, 2016 Geography, CSULB

6 Why Isn't Pluto a Planet Anymore?
Kuiper Belt KBO subdivisions: Resonant KBOs (Pluto is one of these): Previous interaction with Neptune sculpted their orbits into resonances Pluto is in 3:2 resonance with Neptune, which orbits 3 times for every 2 orbits of Pluto. 3:2 objects are called “plutinos” There are 2:1 resonant objects (“twotinos”) farther out Scattered disk objects scrambled by Neptune: Wildly elliptical orbits Aphelia can get up to several hundred AU out Detached KBOs Extremely far out, just tenuously connected to the sun Orbits can take up to 12,000 years! C.M. Rodrigue, 2016 Geography, CSULB

7 Why Isn't Pluto a Planet Anymore?
Other officially recognized dwarf planets Compared with Pluto: Pluto 2,374 km in diameter (our moon is 3,474 km) it has 5 moons (Charon, Styx, Nix, Kerberos, Hydra) 0.25 eccentricity; 17° orbital inclination extreme obliquity: axis is tilted 122.5° from ecliptic (57.5° R) albedo variable: to 0.66 39.5 AU out there, 3:2 resonance with Neptune year takes Earth years Ceres Not a KBO, but the largest object in asteroid belt 945 km in diameter; no moons; dark: albedo = 0.09 2.77 AU; eccentricity; 11° orbital inclination; 3° obliquity year takes 4.6 Earth years C.M. Rodrigue, 2016 Geography, CSULB

8 Why Isn't Pluto a Planet Anymore?
Ceres NASA Dawn 2015 C.M. Rodrigue, 2016 Geography, CSULB

9 Why Isn't Pluto a Planet Anymore?
Other officially recognized dwarf planets Compared with Pluto: Pluto 2,374 km in diameter (our moon is 3,474 km) 0.25 eccentricity; 17° orbital inclination; 122.5° obliquity; 5 moons 39.5 AU out there, 3:2 resonance with Neptune year takes Earth years; rotation ~ 6.9 Earth days Haumea a resonant KBO, 12:7 with Neptune fastest rotating object in solar system: ~3.9 hours! very ellipsoidal shape: 1,940 x 1,530 x 993 km! a rocky world in the ice-dominated Kuiper Belt albedo variable: an old impact may have blasted off the ice and imparted rapid spin it has 2 moons: Hi'iaka and Namaka 43.2 AU out; 0.19 eccentricity; 28° inclination; 284 year orbit C.M. Rodrigue, 2016 Geography, CSULB

10 Why Isn't Pluto a Planet Anymore?
Haumea (artists' images) C.M. Rodrigue, 2016 Geography, CSULB

11 Why Isn't Pluto a Planet Anymore?
Other officially recognized dwarf planets Compared with Pluto: Pluto 2,374 km in diameter (our moon is 3,474 km) 0.25 eccentricity; 17° orbital inclination; 122.5° obliquity; 5 moons 39.5 AU out there, 3:2 resonance with Neptune year takes Earth years Makemake 1,428 km in diameter, 45.7 AU out there 0.16 eccentricity; 29° inclination; obliquity unknown; 1 moon “hot” classical KBO bright (0.8 albedo), reddish, uneven in color atmosphere frozen onto surface: N2 and CH4 tholins (UV acting on CH4 and other ices) may impart red color year takes Earth years; rotation ~ 7.8 hours! C.M. Rodrigue, 2016 Geography, CSULB

12 Why Isn't Pluto a Planet Anymore?
Makemake (artist's image) C.M. Rodrigue, 2016 Geography, CSULB

13 Why Isn't Pluto a Planet Anymore?
Other officially recognized dwarf planets Compared with Pluto: Pluto 2,374 km in diameter (our moon is 3,474 km) 0.25 eccentricity; 17° orbital inclination; 122.5° obliquity; 5 moons 39.5 AU out there, 3:2 resonance with Neptune year takes Earth years Eris ~2,326 km in diameter, but may be a bit larger 67.8 AU out there, scattered disk object 0.44 eccentricity ( AU!); 44° inclination; obliquity? 1 moon: Dynomia this was the TNO that forced the IAU to address planets very bright: 0.96 C.M. Rodrigue, 2016 Geography, CSULB

14 Why Isn't Pluto a Planet Anymore?
Eris Artist L; Hubble R C.M. Rodrigue, 2016 Geography, CSULB

15 Why Isn't Pluto a Planet Anymore?
And these are just the IAU-recognized dwarf planets Besides Pluto, Ceres, Haumea, Makemake, and Eris: Quaoar 1,110 km in diameter and its moon, Weywot classical cold KBO, 43.3 AU out, dark reddish Varuna ~668+ km in diameter, ~43 AU out there with Quaoar 17° inclination but still considered a cold KBO Orcus (aka “anti-Pluto”) 917 km in diameter same orbital distance, same eccentricity, similar inclination as Pluto, but its perihelion is during Pluto's aphelion Sedna .... C.M. Rodrigue, 2016 Geography, CSULB

16 Why Isn't Pluto a Planet Anymore?
And these are just the IAU-recognized dwarf planets Besides Pluto, Ceres, Haumea, Makemake, and Eris: Sedna 995 km - 1,060 km in diameter one of the reddest TNOs (H2O, CH4, N ices + tholins) semi-major axis is WAY out there: 506 AU - perihelion: 76 AU (way past KBOs, even SDOs) - aphelion: 936 AU (!!!) it takes ~11,400 Earth years to orbit the sun! maybe the first Oört Cloud object to be detected? or the first detached KBO to be detected? - too far from Neptune to be scattered by it - perhaps tugged out by a passing star? - maybe unseen culprit is a binary star? (Nemesis) - perhaps culprit is a big undiscovered planet? C.M. Rodrigue, 2016 Geography, CSULB

17 Why Isn't Pluto a Planet Anymore?
Sedna C.M. Rodrigue, 2016 Geography, CSULB

18 Why Isn't Pluto a Planet Anymore?
Planets, Kuiper Belt, Oört Cloud C.M. Rodrigue, 2016 Geography, CSULB

19 Why Isn't Pluto a Planet Anymore?
Speaking of big undiscovered planets Planet Nine Evidence Sedna and 2012VP113 share similarly eccentric, far out orbits 13 TNOs with semi-major axes > 150 AU, high eccentricities, and high inclinations all have their perihelia bunched together in the same general area about 30 AU out this bunching suggests something massive is gathering them together like so many petals in a flower could be about 10 x the mass of Earth and 2-4 x its diameter - “super-Earth” - the fifth giant planet, ejected by movement of others to their present orbits about 4 Ga when Jupiter and Saturn achieved a 2:1 resonance, pushing Neptune past Uranus where it then messed with a lot of KBOs search is underway in locations that could plausibly create the bunching of perihelia and the extreme eccentricity of the involved TNOs C.M. Rodrigue, 2016 Geography, CSULB

20 Why Isn't Pluto a Planet Anymore?
Planet Nine C.M. Rodrigue, 2016 Geography, CSULB

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