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Home , 2060 Chiron, 5145 Pholus, Kirk (crater), Oort cloud

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Chiron and the Centaurs: escapees from

the /

/ & Humberto Campins

The Centaurs--a group of objects orbiting chaotically among the giant of our --appear to be a population transitional in size between typical short-period and the large Kuiper-belt objects that beyond . They promise to reveal much about the origin of and interrelationships between the icy bodies of the outer Solar System.

THE outer Solar System has long appeared to be a largely empty candidate source locations for the JFC. One such region is the place, inhabited only by the four giant planets, and a zone beyond Neptune's orbit (at 30 Au, astronomical units) where transient population of comets. In 1977 however, a faint and nonlinear perturbations by Neptune and the other giant planets enigmatic object--2060 Chiton--was discovered _ moving on a can excite orbital eccentricities on timescales comparable to the moderately inclined, strongly chaotic 51-year orbit which takes it age of the Solar System. Once orbital eccentricities are excited from just inside 's orbit out almost as far as that of . sufficiently to cause objects to cross Neptune's orbit, a fraction of It was not initially clear from where Chiton originated. these objects become temporarily trapped on Centaur-like Following 's discovery, almost 15 years elapsed before among the giant planets. Other such objects are dynamically other similar objects were discovered; five more have now been transported by subsequent encounters with the other giant planets identifiedL Based on tile detection statistics implied by these onto orbits that pass within 1-2Au of the _, where they discoveries, it has become clear that these objects belong to a generate comae and become easily detectable. A second possible • significant population of several hundred (or possibly several source region for the JFC is the slowly dynamically evaporating thousand) large icy bodies moving on relatively short-lived jovian clouds, whose dynamics are controlled by the orbits between the giant planets. This new class of objects, stability of a narrow phase space surrounding the leading and known collectively as the Centaurs, are intermediate in diameter trailing lagrangian points of . However, recent dynamical between typical comets (1-20 km) and small icy planets such as simulations u' show that the jovian Trojan clouds are not as Pluto (_2,300 km) and (_2,700 km). Although the Cen- effective as the so-called trans-neptunian zone in populating the taurs are interesting in their own right, they have taken on added JFC and Centaur populations. Following historical suggestions significance following the recognition that they most probably dating back as far as the 1940s that a disk-shaped reservoir of originated in the ancient reservoir of comets and larger objects and other small objects might reside beyond the located beyond the orbit of Neptune known as the Kuiper belt. Neptune t7''8, the trans-neptunian region has been dubbed the Origin of the Centaurs Kuiper belt, or in analogy to debris belts around the other stars, the Kuiper disk. The first clue to the origin of the Centaurs came about as a result The pivotal breakthrough concerning the reality of the hypothe- of dynamical studies of Chiron's orbit. At first discovered in the sized Kuiper belt came in 1992, with the discovery of a faint (R- late 1970s 3, and forcefully reiterated in more modern calcula- band near 23), 180-km diameter object _9 designated tions d, Chiron's orbit is highly unstable to perturbations by the 1992QB_. 1992QBI orbits the Sun in a stable, nearly circular orbit giant planets. As a result, Chiron's orbital lifetime among the giant some 14 Au beyond Neptune. In the four years since 1992QBj was planets is short, leading to the conclusion that its origin was in a found, over three dozen similar objects have been discovered in more stable reservoir, either in the belt, or beyond the the trans-neptunian region =. Estimates z'm indicate that some of giant planets. The discovery of a around Chiron 5-7, in the these objects have diameters approaching 400 kin. Based on the late 1980s, indicated the presence of surface which could efficiency with which such objects are being detected and their not have survived the age of the Solar System in the comparatively surface density on the sky, it has been estimated that around warm asteroid beltX; such volatiles therefore strongly indicate that 7 x I04 objects with diameters greater than 100 km orbit between Chiron originated in a distant reservoir, beyond the giant planets. 30 and 50Au (ref. 2). Here we refer to these larger objects A second line of evidence relating to Chiron's origin came populating the Kuiper Belt as QB_s. about from simulations of cometary dynamics. These studies 9-1= Following the discoveries of numerous QB,s in the Kuiper belt, demonstrated that the dominant dynamical class of short-period the (HST) was used to conduct a search comets, called the Jupiter-family comets (JFC) cannot be derived for the much smaller, and much fainter, cometary nuclei which from the classical, Oorl-cloud cometary reservoir. The reason for must be present in this region if it is indeed a source of the JFCs. this is that their characteristically low orbital inclinations cannot Last year, Cochran et al. 2t reported exciting evidence, near the be efficiently produced by the action of planetary perturbations on limit of HST's capabilities, for numerous objects with V-band orbits initially in the inclination-randomized (that is, nearly magnitudes of +28.6, corresponding to -like diameters of a spherical) . few kilometres to +10 kin. This evidence corresponds to a popula- Instead, the JFC seem to derive fronl a dynamically stable tion of several hundred million cornel-sized objects. If this result is resm_oir conccntJaled heal Ihc plane of tile planeta]y system. coupled with models j2 that predict the ratio of the population Any such ]escl_,oil for lilt: .ll:(? must satisly the crite,ion thai tile detected in the region searched by Cochran to tile entire trans- loss i:lle of obiccts lltllll il bc h)w cntlttgh ihal the rcscivoir can ncpluni;ui zone, ;l Iotal population is calculated of seveial billion persist I(]l tim age ol thc S, lar Syslcnl. Because thc dynamical cornels in the 3t)-50 Au region. As such, it appca_ s thai Ihc Kuiper clealing linlc fill olhils I'lt+lwCt'lllilt' planets is chaiacleristically beh is in(Iced the sotucc region of Jut)st .IF(', as dynanlical one It] twt:, ordcls tlf nl;Lglltlt]dc shl)n IcJ Ihall Ihc age (If the Solar sinlulalions predicted TM. Syslenl '_ '+, Illcic :tic Icw lej_it)llS <)1 space thai pr_wide sl:lblc, Taken I()gelhel, Ihe discovery (11 both ()1], :llld ttilnel sized NAIIJf+! - VOI ;,/8[> • N AtJ(;U_;I l(t!3G 507

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i)biecls in IIw Kuipel belt ICltioil TABLE :1 Orbital characteristics of I1'1,;kiiown I;,-enlalirs indicates thai lhc Kuipei helt SUl)plies a wide sizc faille of Perihelion Present opposition objects (_llll) olhil_ ill lhe _ianl- Obiect Semi major axis distance EccentHcily lllchtilillon V magnitude planel ittgion, liu._c'd both on 13.70Au 8.46 ^u 0.38 25 _ 15.5 expectations resulling Item Ihc 20.30 ^u 8.68 Au 0.57 7° 3.7.9 planetesinlal acciclion codes, 1993HA2 24.73 ^u 11.84 Au 0.52 716< 21.0 and tile observational evidence 1994TA 16.82 AU 10.69 ^u 0.31 5° 23.8 1995DW2 25.03 Au 18.84 AU 0.25 4° 21.9 for many more comets than 1995G0 18.14 Au 6.79 ^u 0.62 18° 20.3 QB_s, it appears that a power- law-like source population his- These characteristics are taken from ref. 53. Although the basic orbital properties of the third to sixth objects togramm exists in the Kuiper are known, they have not yet been named because, by IAUconvention, objects must first be observed for long belt, with many more small enough to produce astrometrically reliable orbits. bodies than QB_s. Because the dynamical transport process that brings objects from the belt to Kuiper-belt objects, which enables more detailed studies of the -crossing orbits is essentially independent of the mass of Centaurs than are possible with the QBjs and Kuiper-belt comets. the object being transported 's, it is expected that the population of Additionally, being closer to the Sun, the Centaurs experience objects ranging in size from Centaurs down to JFC orbiting among greater heating, which generates characteristic perihelion surface the giant planets is representative of the population of objects in the 30-50 AU zone from which they are derived. temperatures in the range _120 to 150K (ref. 22); by contrast, Kuiper-belt objects probably neve_ experience surface tempera- Physical attributes of the Centaurs tures in excess of 60-70K. Therefore, because vapour pressure It is now established that the slow leakage of objects from the depends exponentially on the temperature of the , Centaurs are much more likely than Kuiper-belt objects to show sublimation- Kuiper belt due to planetary perturbations creates a population generated activity. Although such heating causes the surfaces of of objects on comparatively short-lived, planet-crossing orbits in the Centaurs to evolve chemically and physically over long time- the giant-planet region between 5 and 30AU from the Sun. scales _, it also causes the surface to sublime, and thus reveal Studies of the dynamical evolution of orbits dislodged from the valuable insights into the of these objects. Kuiner bel0 s predict a characteristic equilibrium population of Unfortunately, although the Centaurs are brighter than Kuiper- objects on planet-crossing orbits that is ,--,10-( of the population of belt objects, they are still faint in absolute terms, so considerable the Kuiper-belt reservoir from which they are derived. These dedication is required to obtain physical information on them. As studies also predict that the median lifetime of such orbits is of the a result, comparatively little work has been done to reveal their order of 5 x 107 years. Such findings imply that a population of compositions, colours, shapes, rotational properties and other _5 x 105 to perhaps 106 comets, and _30-300 Centaur objects of attributes (Table 2). Despite the great deficits in our knowledge diameter 100 km or larger, are orbiting between the giant planets. Chiron and its recently discovered cohort of Centaurs are thus about the physical and chemical characteristics of this unique population, several important pieces of information are emerging. now seen to be objects derived from the Kuiper belt. Table 1 summarizes the orbital attributes of the six known Centaurs; Fig. 1 First, with regard to the derived sizes of the Centaurs discovered to date, roughly half appear to be near 60 km in diameter, but 2060 depicts the orbits of these objects and their dynamical context in Chiron 22'24 and 5145 Pholus zs are much larger, with ,-,180-kin the outer Solar System. diameters that are comparable to typical QB_s being discovered As escapees from the Kuiper belt, the Centaurs are an impor- in the trans-neptunian zone. Second, infrared spectroscopy and tant population for study. Indeed, owing to their greater proximity colour have given the first clues about the surface to the Sun, the brightest centaurs are some 5-7 astronomical compositions of these objects. The first clearly detected spectral magnitudes (factors of _100 to _600) brighter than the brightest

FIG. 1 The orbits of tile giant planets (black lines), the six known Centaurs (red lines) and those Kuiper-belt objects with well established orbits (green lines). The dot on each orbit depicts the current location of the object. For scale, Jupiter's orbit is approximately 10 Auacross. Abbreviations on the figure as follows: GO, 1995G0; TA, 1994TA; DW2, 1995DW2; Chiron, 2060 Chiron; HA2, 1993HA2; Pholus, 5145 Pholus. (Figure courtesy of H. F. Levison) /

NAIIII.tf - v()l :'I;41> g AtJGIJS] 1996 508 r PROGRESS

TABIE 2 Physical charactermhc_; of lhe known Centaurs

[)trammeler' Geometnc Rotational Rotational Delecled Obff,(:t (kin) (in V) penod amplilude V J coh)ur_ achvwty 2060Chlron 182 t 10 0.11 0.20 5.9211 9% I 13 J O04 Yes 5145Pholus 1:55 J 22 004 ±O.02 9.98h 20% 253 _ OO6 No 1993HA 2 t52 kmt 0.05 2 07 i 0 40 No 19941A 2,q kml 0.05 No 1995DW2 68 kml 0.05 No 1995GO 60 kml 0.05 No

* The sizes of Chlron and Pholus were obtained 22_4_sfrom thermal fluxes, with computed based on their sizes and V magnitudes. Ch_ron's albedo is stnctly an upper hind owing a possible, small, residual coma contribution. I These sizes were computed by assuming a V-band geometnc albedo of 5%, a value whrch is commonly found for cometary surfaces. _tColour data are discussed Jnthe text"_7J_-_2.Note a V - J colour of .I_.I:1.6would be identical to the Sun "7,thereby indicating a neutrally coloured surface. Hfgher V J colours indicate red sudaces.

absorption feature among the Centaurs was a deep 2.25-1am uniqueness in showing activity is perhaps the most intriguing absorption on Pholus _. This feature has been associated with observable obtained on the Centaurs so far. light organic solids mixed with ices '7'2a. Importantly a 2.04-1am Chiron's activity was first recognized when it suffered an out- absorption features has also now been detected, both in Pholus burst that increased its brightness by a factor (in 1989) of just over and Chiron 29, which Cruikshank et al. identify _° in Chiron as an two _3"3_.In 1989-91 Chiron was also observed to show a highly absorption band of -ice. Third, although none of the three variable particulate coma and tail extending as far as 2 × 106 km Centaurs that have been explored in the infrared have displayed (refs 35, 36), and a cloud of CN gas 3_presumably derived from the any statistically significant colour variation with rotational phase 3_, photodissociation of some heavier, parent molecule. When these they do show striking colour differences between one another various observations were made, Chiton was still more than 10 AU (refs 27, 32 and Fig. 2). Indeed, whereas Chiron's intrinsic colour from the Sun, where the solar radiation field is too weak to is grey (that is, neutral) throughout the 0.3-2.5 lain band, Pholus, sublime water-ice, the common volatile that powers the cometary which lies in a similar orbit and is similar in size, is extremely red. activity close to the Sun. Although other mechanisms remain 1993HA v though smaller and in a more distant orbit, is also very plausible, the sublimation of highly volatile ices like CO, N2 or red compared to Chiron. How much of these differences between CH_ (buried a short distance below the surface) were therefore various Centaurs is due to evolutionary mechanisms (as opposed flavoured as the source of Chiron's activity. Further evidence for to intrinsic attributes) is not yet clear, but it is well established the sublimation of such volatiles was obtained through the dis- that long-term exposure to cosmic rays and solar ultraviolet covery of even more extreme activity on archival, pre-discovery radiation darkens (and initially reddens) surfaces containing images of Chiron obtained when it was near its aphelion at 19 nU, light-weight organics, in turn creating a more complex chemical and therefore far too cold to sublime anything but highly volatile m6lange. ices like those mentioned above _. The final confirmation of this Additionally, the determination of well constrained albedos for hypothesis came in 1995, though the discovery of CO gas itself in 2060 Chiron, and more particularly 5145 Pholus (because it is not Chiron's coma 39,4°. active), provides useful information for predicting the sizes of The fact that Chiron's activity was greater at its aphelion than it QB_s from their observed magnitudes. has been at any time since provides compelling evidence that its level of activity is not a simple function of heliocentric distance The particular value of Chiron alone. Instead, Chiron's activity probably involves a complex Chiron is uniquely valuable among the Centaurs because of the interaction between the level of insolation reaching its surface, long history of its sporadic outbursts. Why has such activity only the obliquity of its spin axis, the location of its near-surface been detected in Chiron? Possibly Chiron is dynamically younger, volatiles and extensive surface mantling by substances (possibly and therefore more active than the other objects. Alternatively the including silicates, water-ice and carbonaceous materials) which other objects may have thicker surface mantles, may be funda- do not strongly sublime that far from the Sun. mentally different in their composition or simply may have not Chiron's strong variability and the low gas-production rate been observed long enough to expect to detect activity. Chiron's inferred from CN and CO gas detections in its coma provide

1.00 i i n i i v i i

Pluto

FIG. 2 Colour-albedo diagram showing the visible-band albedos and visible-infrared (that is, V-J) colours for several Centaurs (Chiron, tJ "w-_ Pholus and 1993HA2), several cometary nuclei (Arend-Rigaux, Halley Chlron E 0.10 and Neujmin 1), Pluto and the one QB_ (1993FW) for which applicable Neuimi¢l 1 data are available. A neutrally coloured surface would have the V - J colour I_3FW 1993HA 2 of the Sun, 1./16 (ref. 54). Note that the Centaur Pholus is quite red; in ii fact, it is far redder than any other object in the Solar System. The V - +/ o colour for 1993FW is an upper limit. V - J for Pluto was obtained form D. P. Pholus Rlgaux Cruikshank (personal communication; the V-albedo error bars for Pluto Arid _ H_I_ represent itsintrinsic rotational lightcurve var_abon.

00! _ _ ___t_ ___l- L _ _j .... i ____L_ _ 0 12 14 16 I:1 2 0 2 2 2.4 2.6 2.8

V O colour

NATURF . VOL 382 • 8 AtJGtJSI 1996 509

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ou(m S_lal System is litleled wilh icy _blCCtS intermediate m size shong evidence that ('ltH(m's mtivily is generated by localized between cornels and Ihe j_l;In( p]allelS. sources covering<1%otthcsmlace Thccasesupportmghighly We have also learned lhal Ihc c;irly stages of p anelap,' fo_ma- localized vents _)z iels tm tile stHlacc ts further supported hy short- Into, with widespread grmvlh flora pianelesmlals Io objecls with term brightness [ItlClllalions ill ('hnl')n's con]a 41'42 that occul on dtamclers ol several hundlcds ol kih,uclres, provide concrete timescales consislenl wilh clouds ol material being ejected onto evidence fo( an ancient e,a of planet-building in the Kuiper-belt suborbital trajeclories, and by tile delection of complex opacity re e,ion 47'_. For some reason (t',robably involving the role of structures in Chiron's coma Lira mg two recently obsm_,ed stellar Neptune that excited orbital eccentricities thal were not condu- by Chiroll 2'_4_. ]1 has heen pointed out 44 that these cive to further growth), the era ot: ir_ the Kuiper belt was vei_ls or jets may resemble the geysers detected on the surface of plemalt,rely trunctated al a stage where intermediate-sized Neptune's large, captured satellite, Triton. objects had formed 4`J.The st,ong circumstantial evidence for the Chiron's low creates a situation in which its escape speed early formation of numerous objects in the 1,000-km class, of (_102m s -_) is comparable to both the thermal velocity of sub- which Pluto and Triton are apparently the sole extant remnants liming gas (_ 2 x 102 m s-' ), and the estimated muzzle velocity of within obsmwational reach _''5', further supports the case for Triton-like geysers _s (of the o(der of 40-300 m s-_). As a ,esult, initially strong but eventually arrested planetary accretion in the some of the gas and entrained particulates ejected from Chiron's Kuiper-belt region 4s's2.As such, the Kuiper belt has become one of surface would be deposited into high suborbital trajectories; much the most important regions in the Solar System for studies of would also escape. Modellers have only begun to explore the planetaly origins. The Centaurs and QB_s therefore represent a range of interesting physical phenomena likely to result in this valuable, relic population of icy objects whose growth was arrested intermediate regime between freely escaping cometary comae and at a fascinating, intermediate stage between comets and small strongly bound plane(my atmospher es4_- Among these is the phmets. distinct possibility that Chiron's neutral colour and comparatively The Centaurs also smwe as bright proxies for distant comets, as high albedo are the direct result of its activity, which probably laboratories for studying surface processes occurring on comets, causes a thin veneer of icy particulates to rain back onto the Triton and perhaps Pluto, and as nearby proxies for the QB_s and surface from suborbital trajectories. other intermediate-scale bodies that bridge the size gap between An emerging view comets, Pluto and Triton. As such, they hold special promise for understanding the origin and interrelationships among the icy We are witnessing a ;-evolution in our understanding of the bodies of the outer Solar System. [] content and architecture of the outer Solar System. Whereas a decade ago, the outer Solar System seemed to consist only of the A. Stem is at the Space Science Department, Southwest Reseamh Inst/tute, outer planets, the Oort-cloud comets and the then-rogue object Boulder, Colorado 80302, USA. H. Campins is at the Department of Chiron, we now see revealed both the teeming Kuiper belt and its , University of Flodda, Gainesville, Florida 32611, USA. progeny, the comets and Centaur-sized objects orbiting among the outer planets. As a result, we have come to recognize that the

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