Kuiper Gürtel

1. Kuiper Gürtel

2. Rückblick • 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

3. 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“).

4. 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)

5. CLASSICAL KUIPER BELT OBJECTS (CKBOs) • 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.

6. CLASSICAL KUIPER BELT OBJECTS (CKBOs) • . 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.

7. CLASSICAL KUIPER BELT OBJECTS (CKBOs) • 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:

8. CLASSICAL KUIPER BELT OBJECTS (CKBOs) • 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.

9. CLASSICAL KUIPER BELT OBJECTS (CKBOs) • 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.

10. CLASSICAL KUIPER BELT OBJECTS (CKBOs) • 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.

11. Plutinos • 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).

12. Plutinos • 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.

13. Plutinos • 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.

14. Plutinos • 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.

15. Plutinos • 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).

16. Plutinos • 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).

17. Plutinos • 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.

18. SCATTERED KUIPER BELT OBJECTS (SKBOs) • 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-France-Hawaii 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.

19. SCATTERED KUIPER BELT OBJECTS (SKBOs) 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.

20. SCATTERED KUIPER BELT OBJECTS (SKBOs) 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.

21. 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.

22. SEDNA • 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.

23. Why is Sedna interesting? • 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.

24. Why is Sedna interesting? • 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.

25. Is Sedna an Oort Cloud Comet? • 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.

26. 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.

27. 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.

28. 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.