Download Today in Astronomy 111: the Kuiper Belt and the Oort Cloud

Today in Astronomy 111: the Kuiper Belt and the
realm of the dwarf planets
Artist-astronomer’s conception
of Eris, with the distant Sun at
lower right (Robert Hurt,
SSC/Caltech).
22 November 2011
 Pluto: second discovery of a
ninth planet
• Relation to Neptune and
Uranus
• Charon and the new
moons
 The Kuiper Belt: undiscovery
of the latest ninth planet
 Distribution and composition
of KBOs
 Notable KBOs: Eris, Quaoar
and Haumea
 Past the Kuiper Belt: Sedna
Astronomy 111, Fall 2011
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Pluto and Neptune
After Neptune’s discovery, differences remained between the
observed and calculated orbits of both Uranus and Neptune,
that couldn’t easily be explained as perturbations by other
known planets. Thus a search for another perturber – a
massive trans-Neptunian planet – was launched.
 Percival Lowell – he of the Martian canals – was the most
ardent seeker of that planet, using telescopes at a site near
Flagstaff, AZ, now called Lowell Observatory .
 Years later Clyde Tombaugh was hired to carry on this
work at Lowell Observatory. He followed up many
dynamical-calculation predictions of the position of the
perturbing planet with photographic searches for faint
bodies that moved with respect to the fixed stars.
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Pluto and Neptune (continued)
 There were many such calculations over the years, with
results that varied widely.
 Eventually (on 18 February
U
1930), Tombaugh found
X
Pluto, near the position
N
S
predicted in one of these
calculations.
• This calculation, along with most of the others he
followed up through the years, was seriously in error,
as was immediately apparent by Pluto’s faintness.
• Thus the discovery was accidental, and mostly a result
of Tombaugh’s excellent observations.
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Pluto and Neptune (continued)
 The Roman god of the underworld provided an appropriate name for
such a distant, obscure solar system body (and also allowed
commemoration of Percival Lowell).
 Tombaugh continued searching, eventually concluding that there
were no other planets of Neptunian size in the ecliptic and closer to
the Sun than 100 AU, and thus ruling out perturbers such as were
theorized theretofore.
 Even more eventually, the methods with which to carry out the
difficult orbital-perturbation calculations improved, and the precision
with which we know the planetary orbits also improved.
 Result: essentially all of the anomalous differences between
observations and theory went away. Nevertheless interest in whether
there were massive trans-Neptunian planets remained, as
astronomers sought a perturber to knock comets out of the Oort
Cloud.
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Mass
1.25 × 10 25 gm (0.0021M⊕ )
Equatorial radius
1.195 × 108 cm (0.187R⊕ )
Average density
Bond albedo
1.75 gm cm -3
0.5
5.90638 × 1014 cm
Orbital semimajor axis
(39.482 AU)
Orbital eccentricity
0.2488
Obliquity
122.53°
Sidereal
247.68 years
revolution period
Sidereal
-6.38725 days
rotation period
Moons
4 so far
Surface temperature
~40 K
22 November 2011
Pluto’s vital
statistics
Pluto and Charon,
from HST (R.
Albrecht,
ESA/NASA)
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Pluto and Neptune (continued)
 Pluto’s orbit has larger semimajor axis, eccentricity and
inclination than the orbits of any of the eight planets.
 Its orbit takes it closer to the Sun than Neptune. In fact it
was the “eighth planet” in this regard fairly recently
(1979-1999).
 Although its orbit crosses Neptune’s in projection, the
orbits don’t intersect, and it never gets very close to
Neptune. In fact it gets closer to Uranus than it ever gets
to Neptune.
• This is due to orbital dynamics: Neptune and Pluto are
locked in a 3:2 mean-motion orbital resonance, as you
hopefully noticed in Homework #4, problem 5b.
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Pluto and Neptune (continued)
The orbits of Pluto and the giant planets, in an inertial frame of reference
(left), and in a frame rotating with the revolution of Neptune (right), for a
time span very long compared to Pluto’s orbital period. (Calculation by
Renu Malhotra and Jeff Williams.)
 Note that Pluto never gets nearly as close to Neptune as its orbit gets
to Neptune’s orbit, a result of their 3:2 resonance.
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Pluto and Charon
In 1978 Pluto was discovered, by Christy and Harrington, to
have a moon, dubbed Charon after the Styx boatman.
 Measurement of the revolution of the two, and Kepler’s
laws, provided the first good estimate of the mass of Pluto
(and Charon). Previous estimates were based upon
guesses for its albedo.
 From 1985 to 1990, Pluto and Charon eclipsed each other
frequently, from which their sizes (and thus densities)
could be deduced.
 Results: Pluto turned out to be smaller, and much higher
in albedo, than previously thought. Pluto is considerably
smaller than the Earth’s Moon, not to mention Titan,
Triton and the Galilean satellites of Jupiter.
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Pluto and Charon (continued)
 Charon turns out to be about half the diameter, and about
one-eighth the mass, of Pluto itself, and the two have a
relatively small separation (19,600 km).
• Their center of mass lies outside the bounds of both
bodies, the only major “double” in the Solar system
known to have this property.
• They are completely tidally locked, in the sense that both
Pluto and Charon have rotation periods equal to their
orbital period, 6.4 days; they always show each other
the same face.
• The two have similar densities. Both have large and
albedoes that vary over their surfaces, indicating some
differentiation and an icy composition.
• Obliquity >90°: the rotation is retrograde.
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Pluto and Charon (continued)
Above: views of Pluto centered on
longitudes 270°, 180° and 90° (left-right).
Left: full globe.
By Marc Buie, with the Hubble Space
Telescope (SwRI/ESA/NASA)
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Pluto and Charon (continued)
 Pluto has an atmosphere, revealed in the usual outerplanet manner by stellar occultation and spectroscopy.
• Composed mostly of nitrogen and methane, like the
atmosphere of Triton.
• Pretty thin (pressure 10 −6 standard Earth
atmospheres), also like the atmosphere of Triton.
 In its gross properties, Pluto seems a lot like Triton, which
we have of course seen at close range.
 Our first chance to see Pluto at close range will come with
the New Horizons (“Pluto-Kuiper Express”) mission,
which was launched early January 2006 and will reach
Pluto in 2015.
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Pluto and Charon (continued)
 Three additional moons, very small ones, have been
detected in the Pluto-Charon system: Nix and Hydra both
in 2005, and S/2011 P4 this past July. All revolve around
the system’s center of mass in the same direction Pluto
and Charon do.
Showalter et al. 2011,
SETI Institute/
STScI/NASA
22 November 2011
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The Kuiper Belt
 Speculation on the origin of short-period comets has
always focussed on the region just outside Neptune’s
orbit. In 1943 Edgeworth suggested – briefly, vaguely, and
mostly incorrectly – that comet-like bodies might be
concentrated there. Later this was also suggested,
similarly briefly and vaguely, by Kuiper (1951).
 The discovery of a belt of such objects, beginning in 1992
with observations by Dave Jewitt and Jane Luu, began
rather than completed the tale. Now called the Kuiper
Belt, there are currently 1585 members (called Kuiper Belt
Objects, or KBOs) known, mostly discovered recently as
large CCD detector arrays and large telescopes have
enabled searches far from the ecliptic plane.
 If life were fair, its name would be the Jewitt-Luu Belt.
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Characteristics of KBOs
 Most KBOs are found with a = 37-47 AU. There are
probably about 100,000 of them of substantial (> 100 km)
size.
 They have a wide range of colors, going all the way from
colors similar to very icy bodies (e.g. Chiron) to those of
very red, carbonaceous bodies (e.g. Pholus).
• Inside they’re still expected to be icy, but they can be
“carbonized” on the outside: covered with lots of dark,
probably organic, material.
• This material can be produced from carbon originally
in the ice (as CH4, CH3OH, H2CO,…) due to prolonged
exposure to cosmic rays, the solar wind, and solar
ultraviolet radiation.
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Colors of KBOs
From Jewitt and Luu
1996.
 Chiron’s color is like
that of Saturn’s iciest
moons.
 Pholus is the same
color as Mars and the
redder asteroids.
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Sorting the KBOs
Mean-motion
5:4
7:4
orbital resonances 4:3 5:3
with Neptune: 1:1
3:2 2:1
5:2
Curves of constant
perihelion distance,
=
rp a ( 1 − ε ) :
30 AU
35 AU
40 AU
45 AU
Figure by
Eugene Chiang
(see Chiang et al.
2007)
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Sorting the KBOs (continued)
 Classical KBOs (, 47%)
are rather red as KBOs
go, and live in loweccentricity, lowinclination orbits. The
edges of their
distribution are rather
sharp at a = 37 AU and
47 AU, the latter lining
up with the 2:1 Neptune
mean-motion resonance.
None have planetcrossing orbits.
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Sorting the KBOs (continued)
 Resonant KBOs (,
23%) stably occupy the
mean-motion orbital
resonances with
Neptune. The most
numerous of these, at
60% of the total, are the
plutinos (locked in the
3:2 resonance), which
include Pluto and Orcus.
There are even a few
Neptune Trojans (1:1).
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Sorting the KBOs (continued)
 Centaurs (×, 10%) lie
between the orbits of
Saturn and Neptune. They
have probably been
perturbed out of the other
three groups. Many have
orbits that cross those of
the giant planets, so none
will be in its present
situation very long,
probably not more than 10
Myr each. Chiron and
Pholus are both Centaurs.
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Sorting the KBOs (continued)
 Scattered KBOs (, 20%)
have perihelia near 35 AU,
high orbital inclination,
and high eccentricity. They
probably were perturbed
into their current elongated
orbits by the giant planets,
although for the ones with
the largest perihelia it is not
clear precisely how. These,
however, are probably the
parent population of shortperiod comets.
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Notable KBOs
Quite a few KBOs are larger than any asteroid, and similar in
size to the regular moons. They have escaped detection until
recently, partly by virtue of high inclination of some of their
orbits. Here are a few of the most famous ones.
Quaoar (KWA-wahr).
Discovered in 2002 by
Chad Trujillo and Mike
Brown (Caltech).
 Originally thought to be
1250 km in diameter, about
Charon’s size.
 Named after a California
Indian goddess involved
in creation stories.
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Notable KBOs (continued)
 Classical KBO: orbital
semimajor axis 42 AU,
eccentricity near zero;
inclination only eight
degrees.
 Quaoar’s orbit crosses
Pluto’s in projection, but
doesn’t intersect it.
 Crystalline water ice on
the surface of Quaoar is
seen very clearly in its
near-infrared reflectance
spectrum.
22 November 2011
Quaoar’s orbit (Trujillo/Brown,
Caltech).
Astronomy 111, Fall 2011
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Notable KBOs (continued)
Quaoar was recently discovered to have
a moon – Weywot – and thereby the
mass of the pair can be determined.
 This mass (~ Quaoar’s) winds up
unexpectedly large: 1.6×1024 gm,
about the same as Charon’s.
 Assuming a size derived from
Weywot’s orbit (Fraser
Quaoar’s brightness and the
& Brown 2010)
assumption that its albedo is about
the same as the average of Triton’s and those of the five
larger Uranian satellites, the diameter comes down to 890
km, and the density comes out rather large:
=
ρ 4.2 ± 1.3 gm cm −3 .
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Notable KBOs (continued)
Eris (goddess of discord, in
Greek mythology), neè
“Lilah,” “the Tenth Planet,”
and (my favorite) “Xena.”
Discovered in 2003 by Mike
Brown, Chad Trujillo and
David Rabinowitz, as
announced two years later.
 Its reflectance spectrum is
very similar to that of
Pluto, whence we can
infer a similar average
surface composition.
22 November 2011
Discovery images of Eris (Mike
Brown, Caltech).
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Visible-infrared spectra of Pluto and Eris
Eris
Mike Brown, Caltech
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Notable KBOs (continued)
 Eris also has a moon, which, when the name “Xena” was
current, was dubbed “Gabrielle.” The moon’s name has
since been changed to Dysnomia (= “lawlessness”,
daughter of Eris). In this way Xena, played on TV by Lucy
Lawless, is still honored.
 From Dysnomia’s orbit we get the mass of Eris: at
1.64 × 10 25 gm, it is 27% more massive than Pluto.
 Eris, a scattered KBO, is currently
the most distant Solar-system
Eris
object known, at 97 AU. Its
orbital semimajor axis is 68 AU,
eccentricity 0.4378, and
inclination 44° (which is
why it wasn’t seen earlier).
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Notable KBOs (continued)
Haumea (neè “Santa”). Discovered
between Christmas and New Year’s
Eve 2004 by Mike Brown et al., the
(currently) third-largest KBO is now
named for the patron goddess of
the Big Island. And it’s a weird one:
 Haumea is football-shaped,
Pluto-size in its longest
dimension and about half that in
perpendicular dimensions.
Animation by Mike Brown
 It rotates end-over-end with a
period of only 3.9 hours: the fastest large (>100 km)
rotator in the solar system.
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Notable KBOs (continued)
 It has two moons, Hi'iaka and Namaka, whose orbits
allow determination of the mass of the system:
4.2 × 10 24 gm, 29% that of the Pluto-Charon system
 Its density lies between 2.6 and 3.3 gm cm-3 – that is, much
larger than Pluto, Eris, or Triton, and extraordinarily large
for a sub-planetary body. Yet the spectrum and albedo
(0.6) show that it is coated with relatively pure water ice.
 Leading theory of its nature, from Mike Brown’s group:
early in the solar system’s life, Haumea (originally Pluto’s
size) was struck off-center by a Charon-size KBO. This
stripped off most of the ice and left a rapidly-spinning,
mostly rocky core. Some of the ice coalesced to form
Hi'iaka, Namaka, and a thin coating on Haumea itself.
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Notable KBOs (continued)
Pluto. This, the largest plutino and second largest of the
known KBOs, has been described briefly above.
It’s up to the International Astronomical Union (IAU) to
decide names and status as planets, and that august body
considered the following in 2006 while demoting Pluto to
“dwarf planet” status with Ceres, Eris, and some other KBOs.
 In 1801 Ceres was discovered, and hailed as the eighth
planet. In the ensuing decades, though, lots more bodies
of similar size and orbit were found, and eventually we
demoted Ceres to asteroid status.
 In 1930 Pluto was discovered, and hailed as the ninth
planet. In the ensuing decades, though, lots more bodies
of similar size and orbit were found, and eventually …
Now “pluto” is just a trendy verb.
22 November 2011
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Mike Brown’s new
book, available 7
December 2011
The Onion, 18 December 2006
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Which one is the planet?
The eight largest trans-Neptunian objects and a planet, suitably modified from Wikipedia
22 November 2011
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Beyond the Kuiper Belt: Sedna
In the astronomically-eventful year 2003 the first body to be
identified as a member of the Oort cloud was discovered.
 Also by the Brown-Trujillo-Rabinowitz team, with their
huge CCD camera on the Palomar 48” Schmidt.
 Named after the Inuit sea goddess.
 Larger than Quaoar, smaller
than Pluto, Eris or Haumea.
 Currently 90 AU from the Sun,
but near perihelion. (Eris is
currently near aphelion.)
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Sedna (continued)
 Orbital parameters: semimajor axis 480 AU, eccentricity
0.84, inclination 12°, period 10,500 years.
 Thus it doesn’t make it out
to 10,000 AU where most
of the Oort cloud is thought
to reside, and is not a
Planets
“classical” Oort-cloud
and
member, but is way past
KBOs
the limit for the Kuiper Belt,
and fits the description of
Sedna
a perturbed Oort-cloud
object.
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Sedna (continued)
 In contrast to such shiny bodies as Haumea and Pluto,
Sedna is about as red as Mars or Pholus. Detailed analysis
makes its surface composition out to be similar to Triton’s,
with a mixture of hydrocarbons and ice.
Artist-astronomer’s
(Robert Hurt’s)
conception of
Sedna (left), and
a Voyager 2
image of
Triton (right).
22 November 2011
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