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Report from the Oort Cloud. Simulations of the Formation of the Comet Reservoir Luke Dones Hal Levison Paul Weissman Martin Duncan. Scenario for Oort Cloud Formation. Icy planetesimals formed in the region of the giant planets

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report from the oort cloud

Report from the Oort Cloud

Simulations of the Formation of the Comet Reservoir

Luke Dones

Hal Levison

Paul Weissman

Martin Duncan

scenario for oort cloud formation
Scenario for Oort Cloud Formation

Icy planetesimals formed in the region of the giant planets

Some formed the cores of Jupiter, Saturn, Uranus, and Neptune

Some stayed small and were scattered into big orbits

Most were ejected from the Solar System, but the orbits of some

were lifted from the planetary region by galactic tides and

passing stars and reached the Oort Cloud

This scenario neglects gas, interactions between planetesimals

due to gravity or collisions, …

Tides and stars perturb some comets back into orbits that

approach the Sun within a few AU, making them sublime

KBO Workshop, 9/1/2003


Tremaine, in

Planets Around

Pulsars (1993)

KBO Workshop, 9/1/2003


To determine …

  • where Oort cloud [OC] comets originated

(Conventional wisdom: Uranus-Neptune region, but compositions are diverse)

  • the fraction of comets that end up in the Oort cloud 

the original mass in solids in the giant planets region

  • the spatial distribution of comets in the OC
    • Does a massive inner OC exist? 

If so, big comet showers are possible

  • the temperatures reached by OC comets as their orbits evolved

KBO Workshop, 9/1/2003


200,000 AU

Comets ejected

Outer Oort Cloud



Long period comets



Inner Oort cloud

Kuiper Belt, scattered disk

KBO Workshop, 9/1/2003


Wiegert and Tremaine (1999)

KBO Workshop, 9/1/2003

fading problem
Fading Problem
  • Observed
  • 109 new long-period comets (a > 10,000 AU)
  • 178 returning long-period comets (34 AU < a < 10,000 AU) 
  • There are 1.6 returning comets for each new comet.
  • Predicted
  • 57 returning comets for each new comet (gambler’s ruin model)
  • 34 returning comets for each new comet (Paul Wiegert’s thesis)
  •  We’re still off by a factor of 20.

KBO Workshop, 9/1/2003


Direct integrations of 3000 “comets” for up to 4 billion years

Integrations include


Jupiter, Saturn, Uranus, Neptune with current masses and orbits

Present-day Galactic tides (r and z components)

Passing stars

Two sets of runs, both with a0 between 4 and 40 AU

Cold: 2000 particles, root-mean-square e0 = 0.02, i0 = 0.01

Warm: 1000 particles, root-mean-square e0 = 0.2, i0 = 0.1

Integrate orbit of each comet until it:

is ejected from the Solar System

impacts the Sun or a planet

survives for 4 billion years

KBO Workshop, 9/1/2003






KBO Workshop, 9/1/2003




KBO Workshop, 9/1/2003

Everhart, Astron. J.72, 1002 (1967)

mass of the oort cloud
Mass of the Oort Cloud
  • Outer Oort cloud: 0.5 x 1012 comets

with absolute magnitude H10 < 11

  • Number in inner Oort cloud ~ Number in outer Oort cloud 

Total of 1012 comets in the entire Oort cloud

  • Calibrate to Halley, which has H10 ~ 6 and known size, assume density of 0.6 g cm-3 and Everhart’s size distribution 

mean mass of a comet ~ 4 x 1016g = 7 x 10-12Earth masses

  •  Present-day mass of Oort cloud Mnow~ 7 Earth masses
  • Fraction of comets in the Oort cloud at present: h = 0.046
  •  Original mass in planetesimals between 4 and 40 AU:

Mdisk= Mnow/h~ 150 Earth masses

KBO Workshop, 9/1/2003


Hahn & Malhotra (1999)

KBO Workshop, 9/1/2003


Science 280, 1250 (1998)

KBO Workshop, 9/1/2003

cometary impact rates
Cometary Impact Rates
  • Impact rates on Earth and galilean satellites due to active long-period comets are low at present (Zahnle et al. 1998)
  • Two uncertainties:
    • Inner Oort cloud: Many or few comets?
    • “Fading”

My my, hey hey,

Rock and roll is here to stay

It's better to burn out, than to fade away

My my, hey hey

KBO Workshop, 9/1/2003


57P/du Toit-Neujmin-Delporte

KBO Workshop, 9/1/2003

oort cloud to do list
Oort Cloud “To Do” List
  • Assume the Sun formed in a cluster
  • Let planets grow and migrate
  • Include other perturbers:
    • planet or brown dwarf in the Oort cloud
    • molecular clouds
    • residual gas in solar nebula
  • Model collisions
  • Include fading on the way out

KBO Workshop, 9/1/2003


Icarus 150, 151–162 2001)


Adams and Laughlin, Icarus 150, 151 (2001)

KBO Workshop, 9/1/2003


Ida et al.,Ap. J.528, 351 (2000)

KBO Workshop, 9/1/2003


First 50 Myr

First 5 Myr

Thommes et al., Astron. J.123, 2862 (2002)

KBO Workshop, 9/1/2003

KBO Workshop, 9/1/2003

Nature 409, 589 (2001)

  • The fraction of comets that reach the Oort cloud and survive to the present is small, < 5%.
  • Efficiency is low because most comets evolve inward and are ejected from the Solar System by Jupiter or Saturn.
  • Two-stage formation:
    • Early by Jupiter and Saturn
    • Late by Uranus and Neptune (peaks at 1 billion years)
  • Typical comet evolves inward to ~10 AU (Saturn’s distance from Sun) before being placed in the Oort cloud.
  • Inner Oort cloud does not appear to be the Phantom Menace
  • More observations and modeling are needed to work out the cosmogonic implications.

KBO Workshop, 9/1/2003