Download Pluto, Charon & the Plutons

Module 17:
Ice Worlds
Activity 1:
Pluto, Charon and the Plutons
Summary:
In the current Activity we will investigate:
(a) Pluto & Charon - discovery & vital statistics,
(b) tidally locked - double planet or not planets at all?
(c) Triton and its similarities to Pluto,
(d) modelling the formation of Pluto & Charon; other
Plutons as relations; and
(e) ice dwarfs, the Kuiper Belt & NASA’s New Horizons
mission.
Discovery of Pluto and Charon
Pluto was discovered in in 1930 by Clyde Tombaugh, a young
astronomer working at the Flagstaff Observatory.
Using a technique called
“blinking”, Lowell examined
pairs of photgraphic plates in
search of any motion amongst
the background stars. He
searched hundreds of plates
and finally, on 18 February
1930, noticed a 15th magnitude
star-like object moving across
two images.
The observations were confirmed on 13 March and Flagstaff
Observatory propose the name Pluto in May 1930.
The newly discovered Pluto was observed to have a very
different orbit from the other planets of the Solar System.
Pluto’s average distance from the Sun is
39.54 AU, but its highly elliptic orbit (with an
eccentricity of 0.25) means that its orbital
distance varies by nearly 50%, from a
minimum of 29.7AU to a maximum of 49.4AU
- so that it crosses the orbit of Neptune. The
orbital period of Pluto is 247.7 years.
top down view
Pluto’s orbit is also highly inclined to the
ecliptic, with an inclination of 17 degrees.
Careful observations of Pluto’s brightness
showed that it rotated once every 6.4 days.
edge on view
Click here to see an
animation of Pluto’s orbit
The size and mass of Pluto remained uncertain for several
decades until the discovery of Charon (pronounced KAR-on).
James Christy and Robert Harrington discovered
Charon, which appeared as a “bump” on the side
of Pluto, from a series of photographic plates on
22 June 1978 at the US Naval Observatory
Flagstaff Station in Arizona.
The bump regularly revolved about Pluto in 6.39
days - same as Pluto’s rotation rate - strongly
indicating a satellite in synchronous rotation.
Charon’s orbit was soon confirmed, and showed something
never before seen in the Solar System….
The rotational period of Charon was the same as that of Pluto
(6 days 9 hours 21 minutes), and the period of Charon’s orbit
around Pluto was the same as its rotational period.
In other words Pluto and Charon rotate about their axes
synchronously with the period of their orbit about each other, and
so Pluto and Charon always show the same face toward each
other.
The two are “tidally locked” with Charon never rising or setting
when viewed from Pluto and Pluto never rising or setting in the
sky of Charon.
Of course if you are permanently situated on the other face of
Pluto you would never see Charon, and likewise if on the far face
of Charon you would never see Pluto.
Click here to see an animation of Pluto and Charon’s orbit.
Charon’s orbit is also inclined to the ecliptic, which means
that that Pluto and its companion undergo mutual eclipses,
which allowed their size and mass to be determined.
The first transit observation was made in February 1985,
while the Pluto/Charon orbital plane was edge on to the
Earth, an event which lasted 5 years. The mass of Pluto
was finally determined: a tiny 0.002 Earth masses, and a
radius of just 1137km, which is 65% of the size of the
Moon.
Charon is about half the size of Pluto - making it the
largest ratio of satellite:planet system in the Solar System.
Vital Statistics
Charon
Pluto
Average separation is 19,645km
which is ~8.5 times the diameter
of Pluto, or 5% the Earth-Moon
separation.
Image taken by Hubble Space Telescope
Charon has mass of about
0.0002 times the mass of
Earth or 0.15 times the mass
of Pluto.
Vital Statistics
Pluto has mass of 0.0022
times the mass of Earth, or
18% the mass of the Moon.
Pluto
Charon
The diameter of Charon is 0.5
times the diameter of Pluto.
The diameter of Pluto is 0.18
times the diameter of Earth or
65% the diameter of the Moon.
Pluto’s Properties
Pluto’s surface consists of low temperature volatile ices, and it
has an atmosphere of primarily nitrogen gas with minor amounts
of CO and NH3. The atmospheric pressure at the surface is only
about one millionth that at the Earth’s surface.
The surface of Pluto is dark reddish, probably cratered, and
covered with snow which consists of nitrogen, carbon monoxide
and methane.
HST images indicate that
ice caps exist at the poles.
The average surface
temperature is
approximately -223°C.
HST images of the two hemispheres of Pluto
Map of Pluto’s surface from HST images
Pluto has an average bulk density of 2.0g/cm3,
suggesting that its interior is a mixture of about
30% ice and 70% rock.
Charon’s Properties
Charon’s weaker surface gravity (due to its lower mass)
has allowed most of the nitrogen and carbon monoxide
to escape, leaving virtually no atmosphere.
Its surface consists of dirty water ice resulting in Charon’s
blue appearance.
With a density of 1.8 g/cm3 - somewhat lower than Pluto’s
- Charon’s interior probably contains more ice than Pluto.
Double-planet
System, or not Planetary?
Compared to other planet-satellite pairs,
Pluto and Charon are remarkably similar in
mass, size and density.
Average separation ~17 RPluto
Charon
Pluto
MCharon = 0.15 MPluto
RCharon = 0.52 RPluto
Because of their similar mass, Pluto and Charon orbit around a
common centre of mass, and thus the Pluto-Charon system is the
first example of a “double planet”.
Unlike the outer planets, Pluto has a solid surface. However,
unlike the inner planets, it contains a large amount of ice.
Pluto’s orbit is more elliptical and more steeply inclined to the
plane of the ecliptic than the orbit of any other planet. All
these properties highlight the unique nature of Pluto.
Pluto may have a near relation, which may indicate that it
should not be classified as a planet at all ...
Triton
Triton is the largest satellite of
Neptune and is of similar size, mass
and average density to Pluto:
RTriton = 1.17 RPluto
MTriton = 1.65 Mpluto
DTriton = 1.03 DPluto
Triton also has a thin atmosphere which consists largely of
nitrogen, a surface of nitrogen and methane snow with
active nitrogen geysers and craters.
Interestingly, Triton’s orbits
about Neptune is retrograde,
indicating that it was probably
formed elsewhere in the Solar
System, possibly collided with
another of Neptune’s
satellites, and was captured
by Neptune’s gravity.
Modelling the Formation of Pluto & Charon
Pluto, Charon and Triton may be “left over” material from
the formation of the Solar System - icy planetesimals.
Triton was probably captured by Neptune.
Neptune’s gravity probably influenced Pluto’s orbit into a
resonance with Neptune’s orbit resulting in Pluto’s period
being 3/2 times Neptune’s period.
Pluto may have captured Charon, or Pluto may have
collided with a similar-sized body and Charon could be
made of part of that object.
Collectively, these bodies are sometimes called “Plutons”
or “ice dwarfs”.
Other Plutons?
Click here for an animation of Chiron
©Michael Brown
Are there any other objects in the Solar System that we could put in
the same group as Pluto, Charon and Triton? There are a few other
strange objects out there, such as Chiron (not to be confused with
Charon!).
Discovered in 1977 by Charles Kowal, Chiron is interesting in that it
has asteroid and comet-like properties. It has an observable tail like a
comet, but is much larger (about 50,000 times larger) than typical
comets - probably about 180 km is size. Chiron’s orbit around the
Sun, between Jupiter and Neptune, is highly eccentric (e = 0.38) and
inclined (i = 6.9 degrees). Its orbit takes it as far as 19 AU from the
Sun and as close as 8.4 AU. Its surface is probably covered in highly
volatile ices like methane, CO and N2 and when close to the Sun
these frozen volatiles heat and result in the observed comet-like tail.
Since the discovery of Chiron over a hundred similar objects, now
called Centaurs, have been found on orbits from just beyond Jupiter to
as far as Neptune and Pluto.
Another unusual object out there that has some similar properties
to Pluto and Charon is 1992 QB1, discovered in 1992 (!) by David
Jewitt and Jane Luu. It has a diameter of about 200 km, was
found 41 AU from Sun, and is reddish colour - which, like Pluto,
indicates the existence of frozen methane on its surface.
Since the discovery of 1992 QB1, hundreds of other large icy
objects with sizes over 100 km have been detected orbiting
beyond Neptune by ground-based telescopes. It is estimated that
there are thousands of tiny worlds yet to be discovered in the
outer solar system.
Perhaps Pluto is better classified along with these objects rather
than as a planet...
Click here to see an animation
illustrating the discovery of 1992 QB
The Kuiper Belt
These large icy bodies in the outer Solar System are part of a
disk - not unlike the asteroid belt - that extends past the orbit
of Neptune out to perhaps 500 AU called the Kuiper Belt.
Since the the first sighting in 1992, searches have intensified
and by 1999 there were 200 Kuiper Belt Objects or “KBOs”
known and the number now exceeds 800.
So why do we think that these KBOs might be related to Pluto &
Charon? While Pluto and Charon are still the largest of the icy
bodies in the outer reaches of the Solar System, there are
currently six objects (including Pluto & Charon) with diameters
over 1000 km, there are many KBOs whose orbits are extremely
similar to that of Pluto & Charon, and there are even several
KBOs that are in binary systems like Pluto & Charon!
The Kuiper Belt has a rather complex structure - the KBOs are
not spread uniformly across the disk but cluster in subgroups:
• Classical KBOs: are on near circular orbits beyond the orbit of
Pluto (between 42 AU and 50 AU) and make up about 65% of
the KBO population.
• Resonant KBOs: are found in resonances with Neptune, mostly
at the 3:2 resonance - just like Pluto - and make up about 35%
of the KBOs population. They’re often called Plutinos.
• Scattered KBOs: are on large highly eccentric and inclined
orbits. They’re on very distant orbits - out as a few 100 AU. KBO
1996 TL66, discovered in 1996, has an
average distance of 85 AU, but its eccentric
orbit carries it out past 130 AU.
KBO Orbits
Plotting the semi-major axis
against eccentricity clearly
shows two quite distinct
populations:
• the classicial KBOs,
• and the resonant population.
KBO = Kuiper Belt Objects
SDO = Scattered Disk Objects
And if we plot the semi-major
axis out past 50 AU, we find the
scattered disk KBO population.
“q” denotes the perihelia of the KBO orbits
“a” denotes the semi-major axis
“e” denotes eccentricity
KBO Sizes
Determining the sizes of KBOs in not an easy task, but we
now have estimates for the larger KBOs and new
observing techniques allow the discovery of larger and
larger bodies.
The exciting announcement in April 2001 of Varuna, a KBO
with a diameter of about 900 km, has forced astronomers
to rethink Pluto & Charon’s place in the Solar System.
The June 2002 discovery of Quaoar (pronounced ‘Kwaaah’),
with a diameter of 1200 km suggests that one day we may
even find a KBO larger than Pluto...
The largest known KBO – besides
Pluto! – is Sedna. Discovered in
November 2003, Sedna may be as
large as 1500 km in size. The size
of all KBOs is uncertain to about
10-20% and is estimated by their
albedo and distance.
Sedna is a rather unique KBO with a semi-major axis of over 500 AU, making it is
the coldest and most distance known object in the Solar System. Its high
eccentricity (e=0.8) however brings it to 76 AU from the Sun at its perihelion.
KBO Masses
Masses of individual KBOs are extremely difficult to determine unless they have a companion (like Pluto & Charon). It is
thought that between 30 and 50 AU there any be as many as
100,000 KBOs with diameters larger than 100 km. Their
combined mass would be about 10% of the Earth’s mass. The
scattered disk KBOs, out to 150 AU, might add another 0.5
Earth masses.
While Pluto is unique amongst the major
planets as a double planetary system, this is
not the case for KBOs. To date there have
been 9* double systems found, including
1998 WW31.
Double KBO WW31
* including Pluto+Charon
The Kuiper Belt is believed to be the source of shortperiod comets and will be further explored in the
Activity “Where Do Comets Come From?”. Comets will
be seen to be dirty, icy objects whose outer layer
vaporises when they approach the Sun.
Visit the Kuiper Belt website at:
http://www.ifa.hawaii.edu/faculty/jewitt/kb.html
Unfortunately, the NASA project called the Pluto-Kuiper
Express, which was to fly by Pluto by 2013, has been put
on hold indefinitely.
However, this has now been replaced by the New
Horizons Mission, which includes the Pluto-Kuiper Belt
Mission due for launch in January 2006, arriving at Pluto
and Charon in 2015, and passing by the Kuiper Belt
Objects in 2026.
For a mission update, visit the New Horizons website:
http://pluto.jhuapl.edu/
In this Activity we have seen that objects in our Solar
System do not necessarily fall neatly within well
defined categories. Pluto is classified as a planet but
its nature is perhaps best explained by considering it
alongside Triton, Charon and other “Plutons” of the
outer Solar System.
Hubble:
Image Credits
Pluto and its satellite Charon as taken with ESA's Faint Object Camera on HST
http://oposite.stsci.edu/pubinfo/jpeg/PlutoCharon.jpg
Pluto as taken with ESA's Faint Object Camera on HST
http://nssdc.gsfc.nasa.gov/image/planetary/pluto/hst_pluto1.jpg
Map of Pluto derived from data from ESA's Faint Object Camera on HST
http://nssdc.gsfc.nasa.gov/image/planetary/pluto/hst_pluto2.jpg
Kuiper Belt Comets
http://oposite.stsci.edu/pubinfo/jpeg/KBComets.jpg
NASA:
Color image of Triton, Neptune's largest satellite
http://nssdc.gsfc.nasa.gov/image/planetary/neptune/triton_close.jpg
Computer rendering of Triton's surface
http://nssdc.gsfc.nasa.gov/image/planetary/neptune/triton_surface.jpg
Chiron Animation, © Michael Brown, used with permission
http://astro.ph.unimelb.edu/central/comets/other.html
IfA, University of Hawaii
Varuna, credit: David Jewitt
http://www.ifa.hawaii.edu/faculty/jewitt/varuna.html
Image Credits
Kuiper Belt orbits: e verus a, i verus a - Hal Levison (used with kind permission)
http://www.boulder.swri.edu/~hal/talks/KB/UM/kb000.html
Double KBO 1996 WW31 - Christian Veillet, CFHT (used with kind permission)
http://www.cfht.hawaii.edu/~veillet/1998WW31.html
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