Transcript CLASSICAL KUIPER BELT OBJECTS (CKBOs)

Kuiper Gürtel
Rückblick
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1949 K.E. Edgeworth – Materiescheibe außerhalb
Plutobahn
1950 J. Oort: gr. sphärische Wolke von Kometen
(10^12)
1951 G. Kuiper: Ring von Planetesimalen außerhalb
Plutos (da kurzperiodische Kometen zeigten eine
Konzentration zur Ekliptik)
1973 P.C. Joss: alle kurzperiodischen Kometen können
nicht von der Oortschen Wolke kommen
OORT‘SCHE WOLKE:
Sie beginnt jenseits der KBOs – und ist die (noch hypothetische) Herkunftsregion
der langperiodischen Kometen. Die einige km bis hunderte km großen Kometen
bestehen fast ausschließlich aus H2O-Schnee („schmutzige Schneebälle“).
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1980 Fernandez: zeigte, dass eine Materiescheibe
außerhalb der Plutobahn 300x effizienter ist für
kurzp.Kometen
1983 -84 IRAS Beobachtungen – Staubringe um
Hauptreihensterne (Bsp. Beta Pictoris)
1988 Duncan et al. Bestätigen das Resultat von
Fernandez
1992 D.C. Jewitt and J.X. Luu entdecken mit
dem 2.2 m Teleskop das erste KBO (1992 QB1)
CLASSICAL KUIPER BELT
OBJECTS (CKBOs)
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A majority of the observed Kuiper Belt Objects
maintain large separations from Neptune even
when at perihelion. The archetypal "Classical
KBO" is 1992 QB1. Such objects are able to
survive for the age of the solar system without
the special protection offered by resonances to
the Plutinos, simply because they are already
Neptune-avoiding.
CLASSICAL KUIPER BELT
OBJECTS (CKBOs)
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. The CKBOs are found mostly with semi-major axes
between about 42 and 48 AU. The deficiency of more
distant CKBOs is real: the Classical Belt has an outer
edge at about 50 AU (Jewitt et al. 1998)
The CKBOs are "classical" in the sense that their
orbits tend to have small eccentricities as is expected of
bodies formed by quiet agglomeration in a dynamically
cool disk.
CLASSICAL KUIPER BELT
OBJECTS (CKBOs)
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The inclinations of the Classical KBOs range
up to very high values (1996 RQ20 and 1997
RX9 have i > 30 degrees). This suggests that the
inclinations have been excited by some agency
yet to be identified. Two ideas have been
suggested for the excitation mechanism:
CLASSICAL KUIPER BELT
OBJECTS (CKBOs)
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i) A few massive planetesimals might have been
scattered into the Kuiper Belt in the early days by
Neptune. These objects could excite the inclinations of
the CKBOs. One problem with this hypothesis is that
massive planetesimals (they would have to approach
Earth mass in order to be effective) would also disturb
and depopulate the resonances. That we see many
Plutinos is evidence against the action of massive
planetesimals.
CLASSICAL KUIPER BELT
OBJECTS (CKBOs)
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ii) A passing star might have stirred up the CKBOs. Proponents
of this idea claim, based on numerical simulations, that the
Classical objects can be excited while the Plutinos remain
relatively undisturbed. One obvious problem with the external
perturbation hypothesis is that passing stars rarely pass close
enough to the sun (a miss distance of a few 100 AU is required).
However, it is possible (likely?) that the sun formed with other
stars in a cluster that might have been initially very dense. In this
case, the early rate of close stellar passages might have been
much higher than at present.
CLASSICAL KUIPER BELT
OBJECTS (CKBOs)
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The outer edge of the Classical Kuiper Belt, near 50
AU, could also be a result of distrurbance by a close
encounter with a passing star. This scenario has been
explored by Ida et al. (2000)
It is worth noting that stellar close approaches and
resulting tidal truncation have been suggested as the
cause of the sharp edged and small disk-like structures
known as Proplyds. Some proplyds are only 50 AU to
100 AU across, similar to the diameter of the known
portion of the Classical Kuiper Belt.
Plutinos
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A surprising result of the new observational
work is that many of the distant objects are in or
near the 3:2 mean motion resonance with
Neptune. This means that they complete 2
orbits around the sun in the time it takes
Neptune to complete 3 orbits. The same
resonance is also occupied by Pluto. To mark the
dynamical similarity with Pluto, we have
christened these objects as "Plutinos" (little
Plutos).
Plutinos
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Probably, the 3:2 resonance acts to stabilize the Plutinos
against gravitational perturbations by Neptune.
Resonant objects in elliptical orbits can approach the
orbit of Neptune without ever coming close to the
planet itself, because their perihelia (smallest distance
from the sun) preferentially avoid Neptune. In fact, it is
well known that Pluto's orbit crosses inside that of
Neptune, but close encounters are always avoided. This
property is also shared by a number of the known
Plutinos (e.g. 1993 SB, 1994 TB, 1995 QY9), further
enhancing the dynamical similarity with Pluto.
Plutinos
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Approximately 1/4 of the known trans-Neptunian objects are
Plutinos. A few more are suspected residents of other
resonances (e.g. 1995 DA2 is probably in the 4:3). By
extrapolating from the limited area of the sky so far examined,
we have estimated that the number of Plutinos larger than 100
km diameter is 1400, to within a factor of a few, corresponding
to a few % of the total. The number is uncertain for several
reasons. First, the Plutinos are observationally over-assessed due
to their being closer (brighter), on average, than the Classical
KBOs giving rise to an observational bias in favor of the
Plutinos. The intrinsic fraction is smaller than the actual fraction.
Second, the initial orbits published by the IAU are little more
than guesses, only weakly constrained by the limited orbital arcs.
Pluto is distinguished from the Plutinos by its size: it is the
largest object identified to date in the 3:2 resonance.
Plutinos
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How did the 3:2 resonance come to be so full? An exciting idea
has been explored by Renu Malhotra. Building on earlier work by
Julio Fernandez, she supposes that, as a result of angular
momentum exchange with planetesimals in the accretional stage
of the solar system, the planets underwent radial migration with
respect to the sun. Uranus and Neptune, in particular, ejected
many comets towards the Oort Cloud, and as a result the sizes
of their orbits changed. As Neptune moved outwards, its mean
motion resonances were pushed through the surrounding
planetesimal disk. They swept up objects in much the same way
that a snow plough sweeps up snow. Malhotra has examined this
process numerically, and finds that objects can indeed be trapped
in resonances as Neptune moves, and that their eccentricities and
inclinations are pumped during the process.
Plutinos
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This scenario has the merit of being a natural
consequence of angular momentum exchange with the
planetesimals: there is really no doubt that angular
momentum exchange took place. However, some
researchers are unsure whether Neptune moved out as
opposed to in, and question the distance this planet
might have moved. They also assert that the inclination
of Pluto is larger than typical of the objects in
Malhotra's simulations (and notice that the inclination
of 1995 QZ9 is still larger than that of Pluto).
Plutinos
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The dynamical situation is presently unclear, but the "moving planets"
hypothesis appears as good as any, and better than most.
A plot of the semi-major axes of the KBOs versus their orbital eccentricities
clearly shows a non-random distribution. The Plutinos lie in a band at 39 AU,
while most of the other KBOs are further from the sun. Solid blue points in
this plot mark KBOs observed on 2 or more years. Their orbits are thought
to be reasonably well determined. Unfilled circles mark KBOs observed only
in one year. In some cases, these objects were recently discovered and we
expect that they will be re-observed next year. In other cases, the KBOs have
been lost. The upper diagonal line in the figure separates objects with
perihelion inside Neptune's orbit (above the line) from the others. Note that
Pluto (marked with an X) falls above the line. The lower diagonal line shows
where objects have perihelion at 35 AU (i.e. 5 AU from Neptune's orbit).
Note also that 1996 TL66 and the other scattered KBOs are so far off scale
that we have not included them in this plot. This plot is updated from a paper
describing our 8k CCD observations of the Kuiper Belt (Jewitt, Luu &
Trujillo, 1998).
Plutinos
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The inclinations of the well observed Plutinos range up to about
20 degrees (see also PS version, PDF version). This is in
reasonable agreement with the inclinations expected from the
migration hypothesis under plausible assumptions about the
motion of Neptune. Some non-resonant KBOs have inclinations
much higher than the Plutinos and this is a dynamical surprise,
for which no clear explanation currently exists. We expect that
resonance trapping should excite the inclinations of Plutinos, but
there are no self-evident mechanisms by which the inclinations
of Classical KBOs should be pumped.
Dan Green has written a detailed opinion about the perceived
status of Pluto in the era of the Kuiper Belt. It's worth a look.
SCATTERED KUIPER BELT
OBJECTS (SKBOs)
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Some KBOs possess large, eccentric, inclined orbits that have
perihelion distances near q = 35 AU. The archetypal "Scattered
Kuiper Belt Object" is 1996 TL66 , discovered as part of a 50
square degree survey using the University of Hawaii 2.2-m
telescope on Mauna Kea. In February 1999, we discovered 3
more examples of SKBOs (1999 CV118, CY118 and CF119) in
a deeper wide field survey undertaken with the Canada-FranceHawaii Telescope and a 12288x8192 pixel CCD. As our survey
has progressed the number of SKBOs has risen dramatically, so
that now we clearly see that that the SKBOs are a distinct
dynamical population in the Kuiper Belt, separate from the
Classical and Resonant objects. We expect that more SKBOs will
be discovered as improved technology allows us to probe larger
areas of the ecliptic sky to deeper limiting magnitudes.
SCATTERED KUIPER BELT
OBJECTS (SKBOs)
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Population
The 35 AU perihelion distances allow Neptune to exert weak dynamical
control over the SKBOs. On billion year timescales, perihelic perturbations by
Neptune will change the orbit parameters from their present values. The
SKBOs form a fat doughnut around the Classical and Resonant KBOs,
extending to large distances. 1999 CF119 has an aphelion distance near 200
AU, showing that the SKBO doughnut extends to at least this distance.
Eventually, much larger orbits will be found. There is, however, an important
bias against finding SKBOs with very large aphelion distances. Such objects
spend only a small fraction of each orbit close enough to the sun to be
detected in ground-based observational surveys. 1999 CF119, for example,
would be undetectable in the survey in which it was discovered for more than
90% of each orbit. This is why large sky areas must be studied in order to
find SKBOs. In fact, SKBOs account for only 3 to 4% of the known Kuiper
Belt Objects but, because of observational bias, this is a strong lower limit to
the abundance of these objects. A list of SKBOs (unfortunately mixed in
with the Centaurs) is maintained by the Minor Planet Center.
SCATTERED KUIPER BELT
OBJECTS (SKBOs)
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Origin
How did the SKBOs get their eccentric, looping orbits? Fernandez (1980)
suggested that planetesimals might be scattered into this type of orbit in the
early days of the solar system. KBOs that approach Neptune closely are
generally scattered away on short (million year) timescales. Many are passed to
the dynamical control of other planets, ultimately to be lost from the solar
system by ejection or by absorption (collision with a planet or the sun).
Planetesimals ejected into very large orbits either escape from the
gravitational influence of the sun (and then enter the realm of interstellar
space) or may be perturbed by the galactic tidal field and by passing stars into
orbits in the Oort Cloud. Objects scattered to the few 100 AU aphelion
distances seen in the SKBOs are immune to galactic and stellar tides, and so
remain in a tightly bound swarm (the fat doughnut) surrounding the solar
system. Numerical simulations of this process by Duncan and Levison (1997)
show this process in operation.
SCATTERED KUIPER BELT
OBJECTS (SKBOs)
Source of Short-Period Comets
 The dynamical involvement with Neptune means that
the SKBOs are a potential source of short-period
comets. Occasional Neptune perturbations can deflect
SKBOs to planet-crossing orbits. Some of these bodies
may find their way to the inner solar system, where
sublimation of embedded ices will lead to their
classification as comets. In part because the SKBO
population is very uncertain, the ratio of short-period
comets delivered from the resonances to those from
the scattered disk is highly uncertain.
SEDNA
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Sedna was discovered as part of a continuing
and highly productive survey lead by Mike
Brown and Chad Trujillo, of Caltech and
Gemini Observatory, respectively. The survey
uses a wide-field telescope on Palomar Mountain
to hunt for bright Kuiper Belt Objects (KBOs).
The orbit has semimajor
axis/eccentricity/inclination = a/e/i =
532AU/0.857/11.9.
Why is Sedna interesting?
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Its perihelion (closest approach to the Sun) is at 76 AU. This
means that it is effectively beyond the scattering influence of
Neptune. This is unlike the Classical KBOs, and unlike the
Scattered KBOs. It is similar, dynamically, to 2000 CR105 (for
which a/e/i = 227AU/0.805/22.7) which has perihelion at 44
AU, also outside Neptune's reach, and which has been discussed
in papers by Gladman et al (Icarus 157, 269, 2002) and
Emelyanenko et al (Monthly Notices RAS, 338, 443, 2003).
Other objects have larger aphelia than Sedna's 990 AU (e.g.
Kuiper Belt Object 2000 OO67, with aphelion at 1010 AU) and
many comets travel to larger distances. Sedna is interesting
because of its perihelion distance.
Why is Sedna interesting?
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Sedna is large (1000 - 1500 km). An object this large
cannot have formed by accretion in the tenuous regions
of the protoplanetary disk corresponding to its current
location. Sedna must have formed elsewhere,
presumably amongst the planets or in the Kuiper Belt,
and been ejected outwards. Lastly, its perihelion was
lifted out of the range of Neptune.
The orbit and the size attest to an early epoch in which
strong gravitational scattering events rearranged the
small bodies of the solar system.
Is Sedna an Oort Cloud Comet?
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From the Classical Oort Cloud - no. The latter consists
of objects whose orbits are so large (50,000 AU) that
passing stars and galactic tides can alter their properties.
Sedna doesn't travel very far out (1000 AU) and is
effectively immune to external forces. Also, the
inclinations of both Sedna and 2000 CR105 are small
(12 and 23 degrees, respectively). These objects know
where the plane of the solar system lies. Oort Cloud
orbits are random with inclinations all the way up to
180 degrees.
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Sedna could be a member of a substantial
population of bodies trapped between the
Kuiper Belt and Oort Cloud. These would have
been emplaced at early times and unseen until
recently. 2000 CR105 and Sedna are "just the tip
of the iceberg", as they say. The scientific
interest lies in how these objects had their
perihelia lifted out of the planetary region.
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Is Sedna = Planet X?
No. Planet X is a term invoked by Percival
Lowell in the beginning of the 20th Century,
when he thought that a planet massive enough
to perturb Neptune might exist at large
distances. Sedna, although big relative to most
other KBOs, is too puny to measurably perturb
Neptune (or anything else for that matter). Its
mass is roughly one thousandth that of the
Earth.
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Sedna = 2003 VB12 is an exciting new object
whose large perihelion distance - beyond the
reach of Neptune - is nearly unique amongst
Kuiper Belt Objects. It has probably followed a
dynamical path different from those of most
KBOs and different from the Classical Oort
Cloud comets. Its large size indicates that it was
formed closer to the Sun and scattered
outwards, probably at early times.