Using Known Long-Period Comets to Constrain the Inner Oort Cloud and Comet Shower Bombardment

Nathan Kaib & Tom QuinnUniversity of Washington

Outline

Long-Period Comet Production

Inner Oort Cloud Comet Production

Constraints from Known LPCs

Comet Shower Significance

Long-Period Comets

X

Jupiter-Saturn Barrier• Comets must have large

perihelion shift to make it past Jupiter/Saturn in one orbital period

• Only weakly bound comets will have large perihelion changes

• Jupiter/Saturn shield inner solar system from inner 20,000 AU of Oort Cloud

25000 AU

Simulations

• Initial cloud orbits (~106) drawn from recent OC formation simulation results (Kaib & Quinn, 2008)

• Oort Cloud model has 1.5:1 inner-to-outer population ratio

• Modify SWIFT (Levison & Duncan, 1994) with time-reversible adaptive timestepping routine (Kaib & Quinn, 2008)

• Evolved under influence of Sun, 4 giant planets, Milky Way tide and passing stars for 1.2 Gyrs

• Analyze LPCs from last 200 Myrs

OC Objects Fatesa = 500,000 AU

Start

Start

x

NOC

a (AU

)

Constrained

Unconstrained

Inner OC LPCs

Similar evolution in SDO simulations (Levison et al., 2006)

Start

q = 1 AUa = 28,000 AU

Inner OC LPCs

NOC

a (AU

)

Constrained

Unconstrained

Original Orbits

qLPC < 5 AU

Incoming Orbits

qLPC < 5 AU

Start

Inner OC LPCs

NOC

a (AU

)

Population Constraints

• Predicted LPC rate: 1/116,000 per Myr

• Max Observed rate: 10 per yr (everhart, 1967)

• Predicted population: ~1012 km-sized bodies between 3,000 and 20,000 AU (assuming nOC ~ r -3.2)

• Not much larger than current outer Oort Cloud population estimates (3-5 x 1011)

• Modest inner Oort Cloud can produce observed LPC flux

Comet Showers

25000 AU

• Rare close stellar encounters (< 5000 AU) are able to perturb more tightly bound orbits

• The Earth is temporarily exposed to the entire Oort Cloud

• Possible source of mass extinctions seen in fossil record (Hut et al., 1987; Farley et al., 1998)

Comet Shower LPCs

NOC

a (AU

)

M* = MSun v = 20 km/s,

Dmin = 3000 AU t = 105 yrs

25,000 AU 4 AU

vSun = (2GM*)/(bv)

Rel

ativ

e S

ho

wer

Str

eng

th

1/ ~ (vSun)-2 (Rickman, 2008)

1.5:1 3:1 10:1R

elat

ive

Sh

ow

er S

tren

gth

Constrained Shower Curve

most powerful shower yields 2-3 km-sized impactors

(We

issm

an

, 2

00

7)

Rohde & Muller (2005)

3 impacts2 Myr He3 spike(Farley et al., 1998)

Conclusions

• Inner Oort Cloud is a significant and perhaps dominant source of LPCs

• Current LPC flux gives an estimate of the total Oort Cloud population, not just outer

• Implies comet showers are not responsible for more than ~1 extinction event

Oort Cloud Mass Problem

• Outer Oort Cloud traps 1-2% of planetesimals during formation

• Previous outer Oort Cloud mass estimates implied > 200 MEarth planetesimal disk between 4 and 40 AU (Dones et al., 2004)

• Inner Oort Cloud can trap more efficiently (5-15%) and still produce observed LPCs (Brasser et al., 2006; Kaib & Quinn, 2008)

Disk Mass Requirements

Minimum mass solar nebula 40 MEarth (Dones et al., 2004)

LPC Inclinations

55% Retrograde

LPC semimajor axes

Inner amed = 26,000 AU; Outer amed = 35,000 AUObserved amed = 27,000 AU

Compact Inner Clouds

~103 MSun/pc3 produces Sedna and LPCs

(Brasser et al., 2006)

Showers from Alternative Clouds

A < 3,000 AU case requires 1200 MEarth disk

Quantifying Shower Strength

LPC defined as q < 5 AU

M* = 0.8 MSun

v = 20 km/sDmin = 1300 AU

Jupiter-Saturn Barrier Edge

Duncan et al. (1987)

log

(1

/a)

q (AU)