Today in Astronomy 111: the Kuiper Belt and the Oort Clouddmw/ast111/Lectures/Lect_22b.pdf · Today in Astronomy 111: the Kuiper Belt and the realm of the dwarf planets Pluto: second

22 November 2011 Astronomy 111, Fall 2011 1

Today in Astronomy 111: the Kuiper Belt and the realm of the dwarf planets

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 KBOsNotable KBOs: Eris, Quaoar

and Haumea Past the Kuiper Belt: Sedna

Artist-astronomer’s conception of Eris, with the distant Sun at lower right (Robert Hurt, SSC/Caltech).

http://www.gps.caltech.edu/~mbrown/planetlila/�http://www.gps.caltech.edu/~mbrown/planetlila/�http://www.gps.caltech.edu/~mbrown/planetlila/�


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.


Pluto and Neptune (continued)

There were many such calculations over the years, with results that varied widely.

Eventually (on 18 February 1930), Tombaugh found Pluto, near the position 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.

N

U

S

X



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.

http://adsabs.harvard.edu/abs/1961plsa.book...12T�


Pluto’s vital statistics

⊕

⊕

×

×

×

°

25

8

-3

14

Mass 1.25 10 gm (0.0021 )

Equatorial radius 1.195 10 cm (0.187 )

Average density 1.75 gm cmBond albedo 0.5

5.90638 10 cmOrbital semimajor axis(39.482 AU)

Orbital eccentricity 0.2488Obliquity 122.53Sid

M

R

ereal 247.68 years

revolution periodSidereal

-6.38725 daysrotation periodMoons 4 so farSurface temperature ~40 K

Pluto and Charon, from HST (R. Albrecht, ESA/NASA)

http://hubblesite.org/newscenter/newsdesk/archive/releases/1994/17/�http://photojournal.jpl.nasa.gov/catalog/PIA06193�



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.

http://www.pas.rochester.edu/~dmw/ast111/Homework/ps4s.pdf�



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.

http://pluto.jhuapl.edu/science/everything_pluto/16_plutoOrbit.html�


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.

http://adsabs.harvard.edu/abs/1978AJ.....83.1005C�


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 bothPluto 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.



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)

http://hubblesite.org/newscenter/archive/releases/2010/06/text/�http://hubblesite.org/newscenter/archive/releases/2010/06/text/�



Pluto has an atmosphere, revealed in the usual outer-planet manner by stellar occultation and spectroscopy.• Composed mostly of nitrogen and methane, like the

atmosphere of Triton. • Pretty thin (pressure 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.

610−

http://pluto.jhuapl.edu/�


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

http://www.cbat.eps.harvard.edu/cbet/cbet002769.txt�


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 KuiperBelt, 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.

http://adsabs.harvard.edu/abs/1943JBAA...53..181E�http://adsabs.harvard.edu/abs/1951PNAS...37....1K�http://adsabs.harvard.edu/abs/1993Natur.362..730J�http://www.minorplanetcenter.net/db_search�


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.


Colors of KBOs

From Jewitt and Luu1996. 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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Mean-motion orbital resonances with Neptune: 1:1

5:44:3

3:25:37:4

2:1 5:2

Curves of constant perihelion distance,

30 AU35 AU40 AU45 AU

( )1 :pr a ε= −

Figure by Eugene Chiang (see Chiang et al. 2007)

Sorting the KBOs

http://adsabs.harvard.edu/abs/2007prpl.conf..895C�http://adsabs.harvard.edu/abs/2007prpl.conf..895C�

Sorting the KBOs (continued)

Classical KBOs (, 47%) are rather red as KBOs go, and live in low-eccentricity, low-inclination 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 planet-crossing orbits.



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).



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.




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 short-period comets.


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 involvedin creation stories.

http://www.gps.caltech.edu/~chad/quaoar/�http://www.gps.caltech.edu/~chad/quaoar/�http://www.gps.caltech.edu/~chad/quaoar/�


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. Quaoar’s orbit (Trujillo/Brown,

Caltech).

http://www.gps.caltech.edu/~chad/quaoar/�http://www.gps.caltech.edu/~chad/quaoar/�http://www.gps.caltech.edu/~chad/quaoar/�


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 Quaoar’s brightness and the 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:


Weywot’s orbit (Fraser & Brown 2010)

34.2 1.3 gm cm .ρ −= ±

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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.

Discovery images of Eris (Mike Brown, Caltech).

http://imdb.com/title/tt0112230/�http://www.gps.caltech.edu/~mbrown/planetlila/�


Visible-infrared spectra of Pluto and Eris

Mike Brown, Caltech

Eris

http://www.gps.caltech.edu/~mbrown/planetlila/�http://www.gps.caltech.edu/~mbrown/planetlila/�http://www.gps.caltech.edu/~mbrown/planetlila/�


Eris

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: atit is 27% more massive than Pluto.

Eris, a scattered KBO, is currently the most distant Solar-system 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).


251.64 10 gm,×

http://movies.netflix.com/WiMovie/Xena_Warrior_Princess/70140464?trkid=2361637�http://www.lucylawlessfanclub.com/�http://www.lucylawlessfanclub.com/�


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.

It rotates end-over-end with a period of only 3.9 hours: the fastest large (>100 km) rotator in the solar system.


Animation by Mike Brown

http://web.gps.caltech.edu/~mbrown/2003EL61/�


It has two moons, Hi'iaka and Namaka, whose orbits allow determination of the mass of the system:

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.


× 244.2 10 gm, 29%

http://www.gps.caltech.edu/~mbrown/papers/ps/EL61.pdf�http://arxiv.org/abs/astro-ph/0509401�http://arxiv.org/abs/astro-ph/0509401�http://arxiv.org/abs/astro-ph/0509401�



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.

http://www.iau.org/fileadmin/content/pdfs/Resolution_GA26-5-6.pdf�http://www.iau.org/fileadmin/content/pdfs/Resolution_GA26-5-6.pdf�http://www.macmillandictionary.com/New-Words/070312-pluto.htm�


The Onion, 18 December 2006

Mike Brown’s new book, available 7 December 2011

http://www.theonion.com/articles/nasa-launches-probe-to-inform-pluto-of-demotion,2116/�http://www.theonion.com/articles/nasa-launches-probe-to-inform-pluto-of-demotion,2116/�http://www.gps.caltech.edu/~mbrown/howikilled.html�http://www.gps.caltech.edu/~mbrown/howikilled.html�http://www.gps.caltech.edu/~mbrown/howikilled.html�

Which one is the planet?


The eight largest trans-Neptunian objects and a planet, suitably modified from Wikipedia

http://en.wikipedia.org/wiki/Image:EightTNOs.png�


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.)

http://www.gps.caltech.edu/~mbrown/sedna/sedna-comp-big.tif.tiff�


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 “classical” Oort-cloud member, but is way past the limit for the Kuiper Belt,and fits the description of a perturbed Oort-cloudobject.

Planets and

KBOs

Sedna

Sedna (continued)

In contrast to such shiny bodies as Haumea and Pluto, Sedna is about as red as Mars or Pholus. Detailed analysismakes 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 2image of Triton (right).


http://adsabs.harvard.edu/abs/2005A&A...439L...1B�http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/ssc2004-05b.shtml�http://photojournal.jpl.nasa.gov/target/Triton�

Today in Astronomy 111: the Kuiper Belt and the realm of the dwarf planetsPluto and NeptunePluto and Neptune (continued)Pluto and Neptune (continued)Pluto’s vital �statisticsPluto and Neptune (continued)Pluto and Neptune (continued)Pluto and CharonPluto and Charon (continued)Pluto and Charon (continued)Pluto and Charon (continued)Pluto and Charon (continued)The Kuiper BeltCharacteristics of KBOsColors of KBOsSorting the KBOsSorting the KBOs (continued)Sorting the KBOs (continued)Sorting the KBOs (continued)Sorting the KBOs (continued)Notable KBOsNotable KBOs (continued)Notable KBOs (continued)Notable KBOs (continued)Visible-infrared spectra of Pluto and ErisNotable KBOs (continued)Notable KBOs (continued)Notable KBOs (continued)Notable KBOs (continued)Slide Number 30Which one is the planet?Beyond the Kuiper Belt: SednaSedna (continued)Sedna (continued)

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Today in Astronomy 111: the Kuiper Belt and the Oort Clouddmw/ast111/Lectures/Lect_22b.pdf · Today in Astronomy 111: the Kuiper Belt and the realm of the dwarf planets Pluto: second