The Kuiper Belt as a Debris Disk Renu Malhotra University of Arizona a vast swarm of small bodies orbiting just beyond Neptune Art by Don Dixon (2000) 1 Dynamical classes Multi-opposition TNOs+SDOs+Centaurs: orbital distribution 3:2 5:3 2:1 Resonant KBOs • (e.g., 3:2, 2:1, 5:2) 0.4 Main Belt (40 º a º 47 AU, 0.2 • ie, between 3:2 and 2:1) 400 Scattered Disk • (a > 50 AU & 30 < q º 36 AU) 30 – Extended Scattered Disk 20 (a 50 AU & q ² 36 AU) 10 Centaurs (q < a ) • Neptune 0 30 35 40 45 50 55 a (AU) (data from MPC/05-july-2004) 2 The Hot and Cold Main Belt Multi-opposition TNOs+SDOs+Centaurs: orbital distribution 3:2 5:3 2:1 0.4 0.2 400 30 20 10 0 30 35 40 45 50 55 a (AU) (data from MPC/05-july-2004) 3 The Extended Scattered Disk Multi-opposition TNOs+SDOs+Centaurs: orbital distribution 0.8 0.6 0.4 0.2 400 30 20 10 0 100 200 300 400 500 a (AU) (data from MPC/05-july-2004) 4 The Edge of the Main Belt (Allen, Bernstein & Malhotra 2001, Trujillo & Brown 2001) KB Radial distribution (Trujillo & Brown, 2001). 5 Size Distribution Observed KBOs have radii • 10 º R º 1000 km 4 – Æ (Ê > 50 km) 5 ¢ 10 – main belt mass 0:01 M ⊕ – total mass (< 50AU) º 0 :03M ⊕ – 100¢ asteroid belt Size-class correlations • – “excited” KBOs contain more large objects & fewer small objects compared to the “classical” KBOs – largest CKBO is 1/60th mass of Pluto Collisional evolution models (Stern • 1995, Durda & Stern 2000) indicate that collisions are destructive for D º 100–300 km, in the present environment Bernstein et al (2004). The source of short period Jupiter-family (Red =“Classical disk”= i 5◦ and 38 <d<55 AU; • ≤ comets (JFCs): Uncertain. The scattered Green =“Excited” class = complement of “classical”) disk may be the more likely source... Accretion models (Stern 1995, Kenyon & Luu 1999) indicate that R ² 50 km KBOs must have • formed in a dynamically cold environment, ie, (e; i)initial º 0:001 – Some process has disturbed the Kuiper Belt & pumped up KBOs’ e’s and i’s 6 Resonant Kuiper Belt Objects 49 1 48.5 48 0 47.5 47 0.2 -1 0.15 0.1 -55 -50 -45 -40 -35 -30 0.05 X (AU) 0 10 5 40 P 0 20 300 200 0 J S U N 100 -20 0 0 0.2 0.4 0.6 0.8 1 -40 time (Gyr) -40 -20 0 20 40 X/AU Pluto’s 3:2 mean motion resonance with Neptune A weakly chaotic twotino 7 Resonance sweeping by a migrating Neptune 3:2 2:1 150 100 5:3 50 0 30 AU 3/2 2/1 Sun Neptune Pluto/ Plutino 8 Why would the giant planets migrate? cores of giant planets formed • within a planetesimal disk planet–formation was likely not • 100% efficient – residual planetesimal debris is left over recently–formed planets scatter • the planetesimal debris, exchange L with planetesimal disk Nbody simulations (Fernandez & Ip • 1984, Hahn & Malhotra 1999, Gomes, Morby, Levison 2004) show planets from Hahn & Malhotra (1999) evolve away from each other, ie, Jupiter inwards, Neptune outwards 9 How far did Neptune migrate? Neptune’s outward migration • causes its mean motion resonances (MMR’s) to sweep out across the Kuiper Belt KBOs get trapped at MMR’s, • are dragged outwards, and have their e pumped up Malhotra (1993) showed this • mechanism can account for Pluto’s orbit (in 3:2 with e =0:25, ∆a 5 AU) ≈ The e-pumping depends upon • Neptune’s ∆a A particle trapped at a j + k : j MMR has an adiabatic invariant, • 2 j+k√a =a B = a(√1 e2 j )2 e(a)2 = 1 initial if e = 0 − − j+k ⇒ − j+k init Plutinos (j = 2; k = 1)havee = 0:33, so they were dragged from • max ainitial = 27:3 to a = 39:5 AU, i.e., ∆a 12 AU ≈ 2=3 – hence, Neptune migrated ∆aNep =∆a=(3=2) 9AU ≈ 10 Challenges SIMULATION OF ADIABATIC RESONANCE SWEEPING OF THE KUIPER BELT (from Malhotra, 1995) Twotino population is too small • 4:3 3:2 5:3 2:1 – chaotic diffusion over 4 Gyr = p. 12 ⇒ 0.4 Observed inclinations are difficult to explain • 0.3 (Plutinos, hot main belt) 0.2 – non-adiabatic migration (Gomes, 2003) 0.1 0 Inclination–size correlation • – hot population formed closer to the Sun 20 The edge at 50 AU • ∼ 10 – stellar encounter = p. 13 ⇒ – primordial edge at 30 AU, KBOs pushe 0 ∼ out by the 2:1 (Levison & Morbidelli 2003 150 The extended scattered disk 100 • – lost/rogue planets = p. 14 50 ⇒ – long term chaotic diffusion = p. 15 0 ⇒ 30 35 40 45 50 – stellar encounter a (AU) Mass loss 99%?! • ∼ 11 Survival rate of Plutinos and Twotinos from Tiscareno (PhD thesis, 2004) 12 Stellar encounter – perturbations on Kuiper Belt from Ida, Larwood & Burkert (2000) 13 Origin of extended–SDOs Rogue planets? from Morbidelli & Levison (2004) 14 Origin of extended–SDOs Long term chaos – ‘Arnold diffusion’? from Malhotra (2004, in preparation) 15 Dust from the Kuiper Belt Present-day distribution Dust density measured to be nearly constant • in outer solar system (Pioneer 10,11; Voyagers 1,2) KB dust production rate (1µm <R<1mm) • 15 1 1 10 gy− (Landgraf et al 2002) ' × (eqv. D 1 km comet ground to dust every ≈ year) Small particles (R º 0:5µm) are blown out by • radiation pressure Bound dust grains spiral inward under • Poynting-Robertson (PR) drag – temporary trapping in Neptune’s MMRs produces azimuthal structure Gravitational scattering by Jupiter and Saturn • model from Moro-Martin & Malhotra (2003) ejects most particles – very small fraction of KB dust grains enter the inner solar system (“inner hole” in the KB debris disk) 16 A History of Kuiper Belt Dust Theoretical models of Uranus-Neptune • formation suggest a total mass 50M in ∼ ⊕ a dynamically cold planetesimal disk, and a planet formation timescale 107 yr (eg, ∼ Goldreich et al, 2004) – A collisional cascade in that disk would produce dust at a rate 4–5 orders of magnitude larger than present The planets were mobilized towards the • end of their formation, U-N undergoing outward migration fuelled by the outer planetesimal debris Shortly prior to the start of the migration, • some mechanism disturbed the outer planetesimal disk, exciting the e’s and i’s, perhaps stripping off the disk beyond Nep migration ∼ Planet accretion 50AU – this event would have also caused a ‘spike’ in the dust production rate Thereafter, a rapid depletion of • planetesimals led to a decline in the dust A schematic history of the outer solar system dust production production rate rate 17.