Download Why was Pluto once considered a planet?

Lecture #32
The Main Point
• The Trans-Neptunian Region
– (134340) Pluto:
The trans-Neptunian region contains billions of icy
bodies orbiting the Sun beyond the orbit of
Neptune. (134340) Pluto is a large member of the
trans-Neptunian region (or Kuiper Belt) and is
officially classified as a dwarf planet.
• Discovery.
• Classification.
Cl ifi i
• Physical characteristics.
• Charon.
– Other Kuiper Belt Objects.
• Reading: Chapter 12.3.
Astro 102/104
1
4
Why was Pluto once
considered a planet?
Discovery of (134340) Pluto
• Pluto was discovered in 1930 by a fortunate accident.
• Percival Lowell made calculations which later turned out
to be in error that predicted a planet beyond Neptune,
based on the motions of Uranus and Neptune.
• Not
N t kknowing
i off the
th error, Clyde
Cl d W.
W Tombaugh
T b h att Lowell
L
ll
Observatory in Arizona did a very careful sky survey. For
14 years Tombaugh scanned the skies, and discovered:
More than a dozen asteroids, including
(1) Ceres and (2) Pallas, seemed unique
at the time of their discovery. They
were classified as planets before their
demotion some 50 yyears later.
(134340) Pluto seemed unique at the
time of its discovery, and it was hailed
as a new planet.
• 1 globular star cluster
• 1 comet
• 1 supercluster of galaxies
• 5 open star clusters
• 775 asteroids
• 1 trans-Neptunian object
(Pluto)
Astro 102/104
Astro 102/104
6
Orbits of outer solar system objects known at the time of
Pluto’s discovery. J, S, U, N in blue. Pluto in red.
Astro 102/104
Later it was found that Pluto is quite
small. Pluto's diameter is only 2320 km
(smaller than the Moon, Io, Europa,
Ganymede, Callisto, Titan, & Triton).
7
1
Why did Pluto lose its planet status?
Astro 102/104
8
Orbits of objects known at the time of Pluto’s discovery.
This diagram shows that there is a
difference of five orders of
magnitude (100,000) in the ability of
planets and dwarf planets to
gravitationally control their orbital
zone.
This difference is the result of the
planet formation process. Objects
above and below the dotted line
underwent different formation
scenarios.
Astro 102/104
Astro 102/104
9
Orbits of objects known today.
What makes a planet?
S. Soter, Astronomical Journal, 132, 2006.
S. Soter, Scientific Am
merican, Jan 07.
Starting in 1992, astronomers began to discover bodies in the trans-Neptunian
region. They gradually realized that Pluto is not unique but is embedded in a vast
swarm of objects sharing similar orbital and physical properties. Just like Ceres
and Pallas, Pluto lost its planet status when new discoveries prompted a revision
to the classification.
The “scattering parameter” is
relatively easy to quantify and makes
classification of newly discovered10
objects practical.
Decision by the International
Astronomical Union
The IAU is an organization
of over 9,000 professional
astronomers. It has been the
arbiter of planetary and
satellite nomenclature since
its inception in 1919. It is
the only administrative body
with the jurisdiction to
define a planet. The 2006
resolution was approved by
an overwhelming majority.
Astro 102/104
11
2
Basic physical properties:
Pluto is far away, small, and icy
•
Orbit: a ≈ 6 billion km (39.5 AU); orbital period = 248.6 years.
–
–
–
–
•
•
•
•
Astro 102/104
12
e = 0.25, so perihelion = 29.6 AU and aphelion = 49.4 AU.
Orbit crosses Neptune's, but 3:2 resonance prevents close encounters.
Orbit is inclined by 17
17° to the ecliptic
ecliptic.
Pluto's tilt (inclination) is 118°: Pluto "rolls" on its side like Uranus!
Pluto's diameter is only 2320 km (smaller than the Moon, Io, Europa,
Ganymede, Callisto, Titan, & Triton).
Pluto's mass is 1.3x1022 kg (0.0022 ME); density is about 2 g/cm3.
Surface temperature ranges from 40K to 60K (-235°C to -215°C).
Surface consists of N2, CH4, C2H2, and CO ices (like a comet!)
Astro 102/104
Pluto has satellites
Charon
Pluto (D = 2320 km): Named after
the Greek god of the underworld who
was able to render himself invisible.
• Discovered in 1978.
• Charon is large relative to Pluto:
–
–
–
–
Charon (D = 1270 km): Named after
the mythological boatman who
y to
ferried souls across the river Styx
Pluto for judgment.
Groundbased, 1978
1270 km diameter (more than half Pluto's size!).
Orbital separation ~20,000 km.
Revolves around Pluto in 6.4 days.
Pluto's spin period is 6.4 days.
• Charon is bright: icy.
• But spectra show H2O ice, not
CO, N2, or CH4 like Pluto.
• Density about 1.6 g/cm3.
• Probably formed by a giant impact.
Nix (D = 40 – 160 km): Goddess of
darkness and night, mother of
Charon.
Hydra (D = 40 – 160 km): Terrifying
monster with the body of a serpent
and nine heads that guarded the
Underworld.
Astro 102/104
13
14
Astro 102/104
15
3
Surface Composition
Pluto/Charon Eclipses!
•
•
•
•
Spectroscopy.
Phase Diagrams.
Modeling.
Methane ice (CH4) was
discovered first in 1976.
• Surface is dominated by Nitrogen ice (N2) with
traces of CH4 and CO.
• Very low pressures and temperatures:
• Earth passed through the plane
of the Pluto/Charon orbit in the
late 1980s.
• Resulting eclipses provided a
way to study & map both
objects
bj t in
i greatt detail.
d t il
• Won't happen again until 2110!
118° inclination
– Comparable to Triton.
Astro 102/104
17
Astro 102/104
Pluto has a thin atmosphere
Pluto and the Kuiper Belt
• Between 1979 and 1999, Pluto was
near perihelion and closer to the
Sun than Neptune.
• Surface temperatures warm enough
for some CO, CH4, and N2 ices to
sublimate
sublimate.
• Atmosphere discovered by stellar
occultations by Pluto.
• Surface pressure is only a few
microbars (10-6 bar).
• As Pluto recedes from the Sun, the
atmosphere may freeze out back
onto the surface.
Astro 102/104
18
• Pluto is a large member of
a class of small bodies
known as Kuiper Belt
Objects (KBOs) or transNeptunian objects (TNOs)
(TNOs).
• The discovery of the
second TNO was in 1992!
• Currently more than 1000
TNOs known.
• Most known TNOs have
sizes above 100 km.
• Icy compositions.
19
Astro 102/104
Pluto
Neptune
20
4
Trans-Neptunian Objects
Classes of TNOs
• Primitive objects with semi-major axis between
30 AU and hundreds of AU.
• Many protected from encounters with the planets
by orbital resonances (like Pluto).
Pluto)
• TNOs are the source of short period comets.
• There may be 100,000 TNOs larger than 100 km,
and the total mass may be ~0.01 ME.
• Still much debate about differences in color and
composition--they are hard to measure.
• But there are classes based on orbit parameters:
– "Classical"
Classical TNOs: Objects with circular orbits never too
close to Neptune; formed by "quiet" slow accretion of
primordial outer solar nebula materials?
– "Scattered" TNOs: Objects on eccentric, inclined orbits;
these objects have once encountered a giant planet?
– “Resonant" TNOs: Objects that occupy an orbital
resonance with Neptune (like Pluto).
– This is more than the number and mass of asteroids in
the main asteroid belt!
Astro 102/104
22
Overhead view of the known
trans-Neptunian region as of
2000. (Blue=classical,
Red=resonant, Black=scattered).
Astro 102/104
23
Mission to the trans-Neptunian region
NASA selected the “New Horizons” mission to fly by Pluto
and another TNO. Launch in 2006, arrival at Pluto in 2015.
Mission cost ~$650 million.
Plot of TNO semi-major axis
(x-axis) vs. eccentricity (y-axis),
showing classical & resonant TNOs.
Astro 102/104
24
Astro 102/104
26
5
Summary
• (134340) Pluto is a dwarf planet. It is part of the Kuiper belt or
trans-Neptunian region.
• Pluto is a small icy outer solar system world:
– Surface composed of N2, CO,
CO CH4 ices.
ices
– Thin atmosphere formed by ice sublimation.
• It has an eccentric and inclined orbit similar to that of many
TNOs. Pluto is "protected" in a 3:2 resonance with Neptune.
• Pluto has a moon (Charon) half its size, and two smaller moons
(Nix and Hydra).
• It remains a fascinating object worthy of scientific inquiry.
Astro 102/104
29
6