arXiv:1101.1035v3 [astro-ph.EP] 15 Mar 2011

o.Nt .Ato.Soc. Astron. R. Not. Mon. Received egiNayakshin Sergei comets. System and Solar formation of composition for the hypothesis Downsizing Tidal The togysget Woe ta.20)arda rnpr of transport radial a 2005) al. et (Wooden suggests strongly lntfraini o eylkl stesldcr forma- The 2010). core Rafikov solid 1969; outside (Safronov the temperature long as are likely scales where very time place tion not cold and is uninteresting formation 2008), rather Lin planet a & is Ida disc 1996; outer al. the et Pollack 1972; (Safronov mation 2007), tempera al. high et at (Wooden form make silicates to ture. crystalline K all 1000 tempe not require of although olivine excess as in such atures silicates crystalline some as oesaeiybodies icy are Comets INTRODUCTION 1 anblw10K hrfr,i h otx fC model, at CA made of materials context the of in presence Therefore, the K. 100 below main stnaiigyhg Zlnk ta.20) ehp shig as 2009) perhaps al. 2006), 1 et al. 9P/Tempel (Westphal et comet as perio (Zolensky of short ejecta high the the tantalisingly in of is and comae 2, the frac- 81P/Wild in mass comet silicates present the However, crystalline the Neptune. of around and tion Uranus probably of out, orbits far day very 2004) formation al. their et above (Kawakita amorphous temperatures never e.g., have experienced nuclei, ices, cometary ammonia in and water found Som materials pro- diverse. formation the confusingly is its of comets about of clues composition The vital the cess. of offer dawn and the from System, materials Solar pristine con most Comets the vaporised. of are some ices tain when (dust) material of tails c ∗ 2 1 E-mail: pc eerhCnr,Dprmn fPyis&Atooy U Astronomy, & Physics of Department Centre, Research Space eateto hsc srnm,Uiest fLeicester of University Astronomy, & Physics of Department 08RAS 2008 nte“oeAcein C)prdg fpae for- planet of paradigm (CA) Accretion” “Core the In [email protected] R ∼ ∼ > 000 0A sgnrlyepce ore- to expected generally is AU 10 ψ 1 mars htlaespectacular leave that across km , ∗ ∼ – 20)Pitd2Nvme 08(NL (MN 2018 November 2 Printed (2008) 1–5 , en-onCha Seung-Hoon , 0 . 5 oesaeblee ob oni h ue oa ytmweetet the where System Solar outer the in exceeded never born have be to to assumed believed are Comets ABSTRACT 2 high-tempera at forged materials of also are but they expected, that showed as Earth ices, to of returned particles dust cometary ntbeadfrsmsiegatpae mro Gs.Teehot d These proto-planetary (GEs). outer embryos planet the giant high-tempe latter, massive Downsizing” the forms the “Tidal of and recent In unstable origin the formation. the on planet based regarding for comets, view in new materials radically cessed a propose We bevtoso h oa ytmadexoplanets. and b System bef may Solar ices picture toget containing the this off disc of of peeled cold observations predictions ambient Several are the comets. and with into gas mixed corporated into then “frozen-in” are separate hig solids are GEs the small latter of and Disruption the GEs. as the solids both inside that synthesised are propose comets We disrupted. eventually are ∼ , − 0 )addnergos mesdi h akrudcl n o de low and cold background the in immersed regions, dense and K) 000 30 T 0 . 5 hsi surprising is This 65. ∼ > − 00Ki comets in K 1000 5 ,confirming K, 150 1 r- h d e - - ecse,L17H UK 7RH, LE1 Leicester, , n onBridges John and iest fLietr ecse,L17H UK 7RH, LE1 Leicester, Leicester, of niversity aeil.Isd fteehtgsos“vn” hmclco chemical “ovens”, gaseous hot solid these of of Inside processing materials. thermal and 2010d,c) (Nayakshin growth as . hot parent as become to manage no ∼ clumps thus these should that It (Nayakshin surprising disc. star be protostellar parent the the star anchors mass of that low presence 2010b) a un- imposing into develop the fact to to (Larson destined in due not are are of due that embryos clumps – – simply 1969) The close cores” but “first clumps. being heating, isolated the to disc dermassive due of viscous not contraction or dense star to and parent hot the very to are clumps gas ih- risfo h inner the from grains hight-T outer, omto,a ti h itpaeo h in lntem- planet giant the of birthplace planet ( for the massive region These is important bryos. it most the as is formation, disc outer the ture, non- disc. the the CA is in concerned, the relation is from radius-temperature formation of model comet unique as the composition far of may as puzzling difference scenario, by-product defining a otherwise The as the comets. formation, explain planet to for alternative naturally an model as CA proposed the 2010d,c,b), Nayakshin 2010; solids. signific of a yield transfer numerous to outward show order enough in 2010) satisfy Armitage to & necessary constraints Hughes 2001; (Gail cess T 00Kalo hi w,a rirr itne rmthe from distances arbitrary at own, their on all K 1000 h rtcrs(a lms r xeln ie o grain for sites excellent are clumps) (gas cores first The nteT oe,i tr otatt h Apic- CA the to contrast stark a in model, TD the In al. et (Boley ideas recent of set a that show we Here ∼ R 0 .Srrsnl,osrain n ape of samples and observations Surprisingly, K. 100 ∼ > 10 A T − E tl l v2.2) file style X 0A ein.Dtie oeso h pro- the of models Detailed regions. AU 30 2 ∼ 0Jptrmse)planet-forming masses) 10 R ∼ < nfc aeo mix a of made fact in etbewt future with testable e Urgosit the into regions AU 1 s sgravitationally is isc ue ( tures lnt n small and planets s e ihi.These it. with her ( T T)hypothesis (TD) - aeil in materials h-T ∼ meaueis emperature T r en in- being ore udesto hundreds auepro- rature ∼ st disc, nsity 50K). 1500 ant m- t 2 S. Nayakshin, S.-H. Cha and J. Bridges pounds can be baked into materials not normally expected mainly coherent and prograde rotation of planets, which to form at tens to hundreds of AU. Furthermore, a vital is unexpected in the CA framework since the planets are part of the TD model is the eventual disruption of gas em- built by randomly oriented impacts. Note, however, that bryos which release the planets back into the “ambient” disc. Johansen & Lacerda (2010) shows that accretion of pebble- This disruption process, as we argue below, also releases the sized grains onto a could provide another smaller thermally reprocessed solids back into the disc. The explanation for the observed planetary spins. thermally reprocessed materials can then be rapidly mixed It is also not impossible (Nayakshin 2010b) that both with the cold materials. As this is an in situ model, no out- the TD and the CA processes operate to sculpture the plan- ward transport of solids is required. etary systems we observe: the first in the early, gas-rich 5 but short (t ∼< 10 yrs) embedded period (Vorobyov & Basu 2010), and the second in the latter, much more quiescent 2 THE TIDAL DOWNSIZING HYPOTHESIS phase t ∼< a few Myrs. In such a hybrid model the CA would kick-start with the benefit of the massive terrestrial cores The TD hypothesis is a new combination of earlier well pre-assembled in the early TD phase. known ideas and contains four important stages (as illus- trated in Figure 1 of Nayakshin 2010a): (1) Formation of gas clumps (which we also call giant planet embryos; GEs). As the protoplanetary disc cannot fragment inside R ∼ 50 AU (Rafikov 2005; Boley et al. 2006), GEs are formed at somewhat larger radii. The mass of the clumps is estimated at MGE ∼ 10MJ (10 Jupiter masses) (Boley et al. 2010; Nayakshin 2010d); they are intially fluffy and cool 3 RETAINING SMALL SOLIDS (T ∼ 100 K), but contract with time and become much hotter (Nayakshin 2010d). Although our arguments can be made completely analytical, simulations of Cha & Nayakshin (2010) illustrate our model (2) Inward radial migration of the clumps due to here. In the simulations, evolution of a massive 0.4 M⊙ gas gravitational interactions with the surrounding gas disc around a 0.6 M⊙ proto-star was followed for about 6000 disc (Goldreich & Tremaine 1980; Lin et al. 1996; years. The massive disc becomes gravitationally unstable, Vorobyov & Basu 2010; Boley et al. 2010; Cha & Nayakshin develops spiral arms, which then fragment into clumps. The 2010). black solid curve in Figure 3a shows the annuli-averaged (3) Grain growth and sedimentation inside the clumps and density weighted gas temperature from the simulation, (McCrea 1960; McCrea & Williams 1965; Boss 1998; defined as <ρT >/<ρ>, whereas the solid curve in Figure Boss et al. 2002). If the clump temperature remains below 3b shows the corresponding density profile, < ρ>, as a 1400 − 2000K, massive cores may form function of radius R. The temperature and the density spikes (Nayakshin 2010c), with masses up to the total high Z ele- correspond to the GEs in the simulation. To emphasise that ment content of the clump (e.g., ∼ 60 M⊕ for a Solar met- even higher temperatures are present in the centres of the alicity clump of total mass 10MJ ). gas clumps, the red dashed curve in Figure 3a shows the same as the black curve except for regions where density (4) A disruption of GEs in the inner few AU due to exceeds ρ > 10−10 g cm−3, where the red curve shows the tidal forces (McCrea & Williams 1965; Boley et al. 2010; maximum temperature of the gas inside those regions. Nayakshin 2010b) or due to irradiation from the star The black dashed curve in Figure 3b shows the tidal (Nayakshin 2010b) can result in (a) a smallish solid core 3 density of the disc, ρt = M∗/(2πR ). Density of a disc and a complete gas envelope removal – a terrestrial planet; marginally stable to the gravitational instability would fol- (b) a massive solid core, with most of the gas removed – a low the dashed curve. The “ambient” disc, i.e., the disc be- Uranus-like planet; (c) a partial envelope removal leaves a tween the gas clumps, has a density lower than ρt and is also planet like Jupiter or Saturn. For (b), an internal very cold, as expected. This confirms the two-phase division energy release due to a massive core formation removes the of the outer disc suggested above. envelope (Handbury & Williams 1975; Nayakshin 2010c). A GE disruption should release gas and small solids In contrast to the CA model, the TD scheme can- with it back into the cold disc. Figure 2 shows the dust sed- not work without a massive outer R ∼> tens to a hun- imentation time scale as a function of dust particle radius, dred AU region of the disc. The elements (3,4) from a, for GEs at three different ages from birth, 103, 104 and an earlier 1960s scenario for terrestrial planet formation 105 years (dotted, solid and dash-triple-dotted, respectively) (McCrea 1960; McCrea & Williams 1965) were rejected by (Nayakshin 2010b). The grains are assumed to be located at Donnison & Williams (1975) because step (1) is not possible rd, set to equal exactly the half radius of the embryo, RGE, in the inner Solar System. Similarly, the giant disc instabil- from the embryo centre, but the results are almost indepen- ity (Kuiper 1951; Boss 1998) cannot operate at R ∼ 5 AU dent of rd. For example, the dashed curve is same as the to make Jupiter (Rafikov 2005). It is therefore the proper solid one but calculated for rd = 0.02RGE. As the embryos placement of step (1) into the outer reaches of the Solar are disrupted in ∼ 104 to 105 years (Nayakshin 2010b), small System and then the introduction of the radial migration a ≪ 1 cm grains should be abundant in the gas envelope at (step 2) that makes this model physically viable. the moment of disruption. Rapid radiative cooling and mix- Nayakshin (2011) suggested that, as a bonus, the new ing with the cold background naturally deposits the high-T hypothesis resolves an old mystery of the Solar System: the processed materials into the cold disc.

c 2008 RAS, MNRAS 000, 1–5 Tidal Downsizing and comet composition 3

To appreciate the argument, compare the binding en- ergy of the solid core with that of the GE. We expect the 100 −3 core of high-Z elements to have a density ρc ∼ a few g cm . ∼ 1/3 (a) The radial size of the solid core, Rcore (3Mcore/4πρc) . The binding energy of the solid core is 1000 1000 1000 2 5/3 3 GMcore 41 Mc Ebind,c ∼ ≈ 10 erg . (1) 5 Rcore  10 M⊕  Let us now look at the gas clump itself. The clump radius 4 100 100 100 RGE ≈ 0.8 AU at the age of t = 10 years, independently of its mass (Nayakshin 2010b), MGE. Thus, the GE binding

Temperature [K] energy at that age is 2 2 3 GMGE 41 MGE Ebind,GE ∼ ≈ 10 erg . (2) 10-9 10-9 10-9 10 RGE  3MJ  (b) The two are comparable for Mcore ∼ 10 M⊕. Radiation hy- 10-10 10-10 10-10

] drodynamics simulations confirm such internal disruption

-3 events: the run labelled M0α3 in Nayakshin (2010c) made

-11 10-11 -11 10 10 a ∼ 20 M⊕ solid core that unbound all but 0.03 M⊕ of the gaseous material of the original 10MJ gas clump.

-12 10-12 -12 10 10 If our model is right, then the outer ∼ tens of AU Solar System must have produced at least one “naked” or almost

-13 10-13 -13 10 10 so core as massive as 10 M⊕. There are actually two – Uranus Density [g cm and Neptune with core masses of ∼ 13 and 15 M⊕, respec-

-14 10-14 -14 10 10 tively. The self-disruption of GEs at tens or hundreds of AU is 100 potentially observationally testable in exoplanetary systems. The Core Accretion model (Pollack et al. 1996) is unlikely to Radius [AU] > produce ∼ 10 M⊕ solid cores that far out; the conventional disc instability model (Boss 1998; Boley et al. 2006) would Figure 1. Solid curves: gas density (b) and temperature (a) av- result in massive gas-dominated giant planets. Thus Super- eraged on annuli for numerical simulation of a gas disc presented Earth to Saturn mass planets on orbits of moderate eccen- in Cha & Nayakshin (2010). The red curve shows the maximum > temperature found inside the clumps. The blue dotted curves tricity, if found at semi-major distances of ∼ tens of AU, show the corresponding temperature and density profiles in the may be sign-posts of the gas clump self-destruction events. standard picture of planet formation (Chiang & Goldreich 1997). Another implication of our picture is that the composi- The need for radial transfer of solids by factors of ten or more is tion of comets and Neptune/Uranus may be related to some obvious (Gail 2001; Wooden et al. 2007) in that theory. In con- degree. The crystalline materials found in comets are mate- trast, in the TD hypothesis, the outer disc has both hot and cold rials that did not contribute to the building of the gas giant regions. Mixing of solids produced in these two components may planets. Estimates above show that forming solid cores is yield a better explanation for the observed composition of comets. absolutely essential to the release of the high-T processed materials back into the cold disc. For the release to occur, the cores must be as massive as these outer planets, and so 4 HOW ARE THE GAS CLUMPS DISRUPTED may have used up a significant fraction of the solids orig- AT LARGE RADII? inally present in the GEs. Therefore materials in comets that came from the same GE may be deficient in materi- In the “bare-bone” version of the TD model, step (4) als/elements abundant in Uranus and Neptune. disrupts the envelope by tidal forces (Boley et al. 2010; Nayakshin 2010b) or due to stellar irradiation (Nayakshin 2010b). Both of these effects are weak in the outer disc, e.g., beyond ∼ 10 AU. What could disrupt the GE there, and is 5 IMPLICATIONS FOR THE ORIGIN OF there any evidence for such disruption(s) in the Solar Sys- OTHER HIGH-TEMPERATURE MINERALS tem? IN THE SOLAR SYSTEM As early as 35 years ago, Handbury & Williams (1975) suggested that the massive core formation in Uranus and Other types of materials requiring high-temperature pro- Neptune evaporated most of their hydrogen envelopes. The cesses are chondrules and Calcium Aluminium-rich Inclu- idea here is that the energy due to core formation is trapped sions (CAIs). Chondrules are igneous-textured, mm-size par- inside the optically thick embryo, making it expand to sizes ticles, composed mainly of olivine and low-Ca pyroxene set much larger than is expected in the simple analytical model in a feldspathic or glassy matrix (e.g., Scott & Krot 2005). of the gas clumps that do not take into account the energy They are a major constituent of most chondrite groups (e.g., release by core formation (Nayakshin 2010b). A more ex- ∼ 80% of ordinary chondrites). The origin of chondrules is tended gas envelope is then much easier to disrupt, even at controversial, but in general they are believed to have formed tens of AU. as rapidly cooling molten silicate droplets. The maximum

c 2008 RAS, MNRAS 000, 1–5 4 S. Nayakshin, S.-H. Cha and J. Bridges temperatures are taken to be approximately around the liq- uidus temperatures of 1200-1500 K. The cooling rates re- main uncertain for the range of different textural types but 105 Non-rotating embryo it is thought that if chondrules had been molten for more than a few minutes they would not have preserved the sort of 4 volatile abundances that they often contain (Yu & Hewins 10 1998).

103 CAIs are the light-coloured inclusions commonly found in carbonaceous chondrites. CAIs are more refractory- 3 temb=10 yrs rich than chondrules. Their shapes are less regular, while 102 common chondrules are more uniformly spherical. Radio- metric dating using the 26Al − 26Mg chronometer sug- ≈ 1 gests that chondrules started to form 2 Ma after CAIs Sedimentation time [years] 10 (McKeegan & Davis 2005). A Pb-Pb absolute age for CAI 5 formation is 4,567 Ma (Amelin et al. 2002). temb=10 yrs 100 Thus CAIs are considered to predate chondrules. Nu- 1 10 100 1000 merous chondrule and CAI formation models include nebu- a [cm] lar shocks (Cassen 1996), lightning (Desch & Cuzzi 2000), jets from near the proto-Sun (Shu et al. 2001) and im- Figure 2. pacts (Bridges et al. 1998). No one model is universally Sedimentation time scales for grains versus their size, a. All the curves are calculated assuming the MGE = 10M gas accepted but the impact models have the advantage of J embryo according to the model of Nayakshin (2010b), but at three producing abundant chondrules (e.g., Scott & Krot 2005). t 3 4 5 26 different embryo ages: = 10 , 10 and 10 years, for the dot- Hevey & Sanders (2006) used the likely abundance of Al ted, the solid and the triple-dot-dashed curves, respectively. For shortly after CAI formation in the early Solar System to these curves, the grains are located at half the gas clump ra- show that nebular dust which rapidly accreted into ∼ 60 km, dius, whereas the dashed curve shows same as the solid curve or larger, planetesimals would start melting. Disruption of but the grains are located at 0.02 the clump radius. The results these planetesimals by impact would cause the sprays of are thus largely independent of the grain location inside the em- melt droplets now seen preserved in chondrites. bryo. Grains smaller than a few mm to a few cm may remain suspended inside the embryo for a long time due to the gas drag forces. If the embryo is disrupted, these grains are released into The GEs are present only in the early “embedded” the surrounding cold disc. stage of star formation(Vorobyov & Basu 2010; Nayakshin 5 2010b), which is likely to last t ∼< 10 yrs. If this is true, and if the inferred age difference between the CAIs and chon- 6 CONCLUSIONS drules is real, then GEs are likely to be dispersed or become very massive giant planets by the time of chondrule forma- We have argued for an entirely different origin of the puz- tion. zling comet compositions. Instead of assuming that hot T ∼ 1500 K regions needed for thermal processing of hot If we assume that formation of CAIs is co-eval with minerals are located in the inner (R ∼ 1 AU) Solar Sys- the early GE-rich epoch of star and planet formation, then tem, we identify them with the massive and appropriately one may question whether GEs have anything to do with hot gas clumps inside of which planets are born in the Tidal CAIs. We believe such a view is attractive because the tem- Downsizing hypothesis for planet formation. In the latter, peratures near the solid core inside the gas embryos may the clumps are born at radii of many tens to hundreds of AU, (Nayakshin 2010d) reach 1500-2000 K, e.g., high enough and migrate inwards due to disc torques. The clumps are hot formation of CAIs. Vigorous convection near the solid core due to their self-gravity, and due to contraction caused by ra- (Helled & Schubert 2008; Nayakshin 2010c) probably drive diative cooling. We showed that small ≪ cm sized solids are strong shocks, which might be one way of producing CAIs. suspended inside the gas clumps and ought to be released One-dimensional simulations of Nayakshin (2010c) also show back into surrounding cold disc if the clump is disrupted. melting/re-forming cycles for grains in some cases, e.g., see Disruption of the clumps in the outer Solar System requires the right panel of his Fig. 8, the simulation M2α4. Physi- a rapid formation of massive M > 10 M⊕ cores inside the cally, the cycles result from a negative feedback loop. The gas clumps, which puffs up the gaseous envelope to the point accretion luminosity of the solid core increases as grains in- of its removal. We tentatively identify Uranus and Neptune crease in size. However, this causes the inner GE regions to as two such cores that could have disrupted their gas em- heat up, melting the grains. As grains become smaller, the bryos and contributed to building the comets in the Solar rate of their accretion onto the solid core drops, and hence System. the luminosity of the core drops as well. The inner region There may be chemical signatures of a casual link be- cools down and the grains start growing again, repeating the tween compositions of comets and the cores of the icy giants cycle. Thus, in the TD model, this might explain the pres- confirming (or rejecting) our model, perhaps testable with ence of CAIs with original sizes up to ∼< cm being found future results from the Rosetta mission. We also note that in comets. There is evidence for this from the Comet Wild2 Tidal Downsizing hypothesis predicts massive solid cores analyses and from IDPs (interplanetary dust particles). (tens of Earth masses, e.g., planets like Uranus and Nep-

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c 2008 RAS, MNRAS 000, 1–5