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Transcript
Planet Searching
• After Uranus and Neptune, some began searching for yet another
planet, among them was Percival Lowell from 1905-1916
• Search resumed in 1929 by Clyde Tombaugh, who identified a
candidate on 2/18/1930
• The name “Pluto” was suggested by an English girl of age 11.
Discoveries: Pluto at left and moon Charon
at right
Clyde
Tombaugh
James
Christy
Hubble Image of Pluto & Charon
Pluto
Charon
a
39.5 AU
19600 km
P
248 yr
6.39 d
i (orb)
17o
99o
e
0.25
0
Prot
6.39 d
6.39 d
i (rot)
123o
0o
R
0.67 RM
0.34 RM
M
0.18 MM
0.02 MM
r
2.0 g/cc
2.0 g/cc
vesc
1.1 km/s
0.6 km/s
A
0.5
0.4
Pluto and Charon to Scale
Mapping Pluto
Charon’s orbit is such as to eclipse Pluto at times, allowing us to
map its surface
(Last occurrence was in 1985)
HST mapped Pluto to a resolution of 150 km, revealing a strong
variation of reflectivity
Pluto-Charon Eclipses
Charon-Pluto Eclipse: Visual Aid
Surface Mapping with HST
Pluto is ODD!
•
•
•
•
•
Similar to Triton
Cold at T=50K
Has frozen CH4
Mostly rock and ice
Largest eccentricity, with D ~ 30-50 AU
(sometimes closer than Neptune)
• Large orbital inclination
• Smaller than planets
• Large moon in relative size
Origin of Pluto-Charon:
• Pluto ejected from Neptune –
problem is Charon
• Pluto and Charon formed together (?)
• Pluto captured Charon (?)
• Giant impact like Earth-Moon system
(?)
Orbits for Pluto and Neptune
Atmosphere of Pluto
It is possible to study
Pluto’s atmosphere from
Earth using occultation of
starlight.
The light passes through
the tenuous atmosphere.
In this way we can “see”
the atmosphere.
Spectroscopy reveals composition of Pluto and
Charon
Charon
•
•
•
•
•
•
•
Discovered in 1978 by J. Christy
Has an icy surface of frozen N and a tenuous CH4 atmosphere
Orbital plane is nearly perpendicular to the ecliptic
Charon is in synchronous orbit with Pluto (and vice versa)
Ratio of masses: MC/MP=0.13
Ratio of sizes: RC/RP=0.51
As seen from Pluto, Charon subtends an angle of 4o, but the Sun is only
1’ !
Synchronous Orbits for Pluto and Charon
Structure of Pluto and Charon
New Moons: Nix,
Hydra,
and “P4”
New Horizons
(July 2015!)
Dwarf Planets
A "dwarf planet" is a celestial body that, within the Solar System:
1.
2.
3.
4.
is in orbit around the Sun
has sufficient mass for its self-gravity to overcome rigid body forces so that it
assumes a (near-spherical) shape
has not cleared the neighborhood around its orbit
is not a satellite
The term "dwarf planet" applies only to objects in the Solar System and is distinct
from "planet" and "small solar system body".
Eris and its moon Dysnmonia
(goddess of discord and daughter of lawlessness)
Xena
Gabrielle
Orbit of Eris
Comparison of Pluto with Eris
• Ceres is now a dwarf planet but was
previously the largest of the asteroids.
• It was discovered on Jan 1, 1801, by
Giuseppe Piazzi. With a diameter of
about 950 km.
• Ceres is in the asteroid belt, and it was
the largest and most massive body there,
but now is the 3rd of the dwarf planets.
• Interestingly, it contains approximately a
third of the belt's total mass.
Ceres
Dawn Visits Ceres
• Dawn visited Ceres in
spring 2015
• The white splotch is a
“cryovolcano”: a
salty-mud volcano
that spews salt water
• Ceres is about 70%
rock and 30% ice
• Surface gravity is
1/34 of Earth’s
• Dawn remains in
“perpetual” orbit
about Ceres
Two Other Dwarf Planets
Haumea
Makemake
Future Dwarf Planets?
2015 RR245
81 AU, 733 year orbit, highly elliptical, about 30% as large as Pluto