Completing the Inventory of theCompleting the Inventory of theOuter Solar System Outer Solar System

Scott S. SheppardCarnegie Institution of WashingtonDepartment of Terrestrial Magnetism

The Dynamical and Physical Properties of asteroids offer one of the few constraints on the origin and migration of the planets.

The effects of nebular gas drag, collisions, planetary migration, overlapping resonances, and mass growth of the planets all potentially influence the asteroids formation and evolution.

In particular, the currently Stable Reservoirs in our Solar System have a “fossilized” imprint from the evolution of the Solar System.

Why Observe Asteroids?

Main Asteroid Belt25 > 200 km

Trojans5 ~ 200 km

Kuiper Belt10,000 > 200 km

Irregular Satellites5 ~ 200 km

ObservedStable

Reservoirs

Wide-Field CCDs on Small/Medium/Large Telescopes

Power of a Survey

A x OmegaA = Area of TelescopeOmega = Solid Angle Observed

CFHT 3.6m/MegaCam

Magellan 6.5m/IMACS

Subaru 8.3m/SuprimeCam

Palomar 1.2m/Quest

Helio distance

Albedo x radius2

Flux ~4

Minor Planet Brightness

Parallax of Asteroids and Satellites

JupiterSatellite

Asteroid

Asteroid

Dynamically Disturbed and Collisionally Processed

Trans-Neptunian Objects

The Largest Minor Planets

Orcus1400 km

2003 EL611600 km

2005 FY91800 km

Planets?

2003UB313

2003 EL612005 FY9

Pluto

Sedna

How did the extremly red object Sedna come to be inits currently highly eccentric distant orbit?

- If formed in current location must have initially been on circular orbit (Stern 2005).

- If interacted with currently known giant planets its perihelion must have been raised some how.

Theories on Sedna’s History

1. Scattering by Unseen Planet in the Solar System -Neptune only to ~36 AU (Gladman et al. 2002) -Including complicated planet migration ~50 AU (Gomes 2003)

2. Single Stellar Encounter -Galactic tides too weak (only good for Oort cloud ~10,000 AU) -Needs to be very close encounter for Sedna to be excited (~500 AU) -May hint that our Sun formed in a very dense stellar environment. -May cause edge in Kuiper Belt -Too early and Sedna not formed in outer KB, too late disrupts Oort Cloud

3. Highly Eccentric Neptune4. Massive Scattered Planetary Embryos5. Massive Trans-Neptunian Disk6. Capture of Extrasolar Planetesimals

Neptune TrojansThe first Neptune Trojan was serendipitously discovered in 2001 by Chiang et al. (2003). Our ongoing Neptune Trojan survey has quadrupled the known population.

Neptune Trojans (1:1) are distinctly different from other known Neptune resonance populations. -Kuiper Belt resonances may be from sweeping resonance capture of the migrating planets (Hahn and Malhotra 2005). -Trojans would not be captured and are severely depleted during any migration (Gomes 1998; Kortenkamp et al. 2004).

Trojan asteroids share a planet’s semi-major axis but lead (L4) or follow (L5) the planet by about 60 degrees near the two triangular Lagrangian points of equilibruim

Like the irregular satellites the Trojans of the giant planets lie between the rocky main belt asteroids and volatile-rich Kuiper Belt.

No primordial Saturn or Uranus Trojans known or expected (Nesvorny et al. 002).

The four known Neptune Trojans appear stable over the age of the solar system (Sheppard and Trujillo 2006).

Neptune can not currently efficiently capture Trojans. Capture or Formation of the Neptune Trojans likely occurred during or just after the planet formation epoch.

Gas Drag not efficient at Neptune.

No rapid mass growth of the planet.

Freeze-in capture: Giant planets migrate across a mutual 2:1 resonance. Their orbits become marginally unstable perturbing many minor planets. Once the planets stabilize any objects in the Lagrangian regions will also become stable and thus trapped (Morbidelli et al. 2005).

Collisional interactions within the Lagrangian region (Chiang et al. 2005).

In-situ accretion from a subdisk of debris formed from post-migration collisions (Chiang et al. 2005).

Neptune Trojan Formation Scenarios

Neptune Trojan Inclinations

Can test formation theories on the inclination distribution of Neptune Trojans.

Magellan-Baade 6.5 meterWith the 0.2 square degreeIMACS imager.

50 : 12+75-35

+10-7

Assuming low albedos the known Neptune Trojans are between 40 to 70 km.

375+240-180

Maybe 3 to 20 times larger than the Jupiter Trojans and Main belt asteroid populations

with radii > 40 km

High i : Low i

Sheppard and Trujillo 2006

Gomes et al. 2005

Tsigais et al. 2005

Freeze-In Capture

No ultra red material as seenIn the Classical Kuiper Belt.

Comparison of Colors of Outer Solar System Objects

Sheppard and Trujillo 2006

The Dispersed Populations

Classical KBOs

MBAs

Kuiper Belt Formation

The End

Mercury = 0 Venus = 0 Earth = 1 Mars = 2 Jupiter > 8Saturn > 21Uranus > 18Neptune > 7 Pluto = 1

Irregular

00005526960

Regular Satellites

1. “e” is small2. “i” is small3. “a” is small4. Prograde only

-> Formed by Circumplanetaryaccretion

Irregular “outer” Satellites

1. “e” is big2. “i” is big3. “a” is big4. Prograde or Retrograde

-> Captured from heliocentricorbits

acrit = (2 J 2 r2p a3

pmp / M

sun)1/5

Other

(Burns 1986)

Satellites

Comet Shoemaker-Levy 9

Reversibility ofNewton’s Equations

Energy dissipationNeeded forPermanent capture

Capture?

-Collide with planet-Ejected from system

1. Gas Drag (Pollack et al. 1979; Cuk and Burns 2004) - Extended atmosphere or circumplanetary disk of gas and dust surrounding the planet (is dependent on satellite size).

2. Hill Sphere Englargement (Heppenheimer and Porco 1977) - Mass growth of the planet

3. Collisional or collisionless interactions (Colombo and Franklin 1971; Tsui 2000; Funato et al. 2004; Agnor and Hamilton 2004) - More probable during the heavy bombardment epoch.

(During the Planet Formation Epoch)

Irregular Satellites provide a unique window on processes operating in the young Solar System

Hartman

HartmanCapture Mechanisms

Jupiter

Saturn

Uranus

Neptune

Kozai Effect Carruba et al. 2002 Nesvorny et al. 2003

Retrograde vs Prograde Henon 1970 Hamilton & Krivov 1997

Resonances Saha & Tremaine 2003 Whipple & Shelus 1993 Nesvorny et al. 2003 Cuk & Burns 2004

Sheppard et al. 2005

Stable over age of theSolar System. Henon 1970 Carruba et al. 2002 Nesvorny et al. 2003

Dynamical Families -> Collisions with Comets or Defunct Gladman et al. 2001 Satellites After capture Sheppard and Jewitt 2003 Nesvorny et al. 2004

All giant planets have similar outer satellite systems!

Collisionless Three Body Interactions as a Capture Mechanism

KBO 1999 TC36Viewed by Hubble

Funato et al. 2004

-> Recently described by Agnor and Hamilton 2004

Preferred Because LessDependent on planetFormation scenario.

Each giant Planet may haveHad a similar number ofSmall body encounters. -Less objects further out But bigger Hill spheres

Captured just after thePlanet formation epoch.

Where Did Triton Come From?

-Currently the space between the giant planets is devoid of small stable objects.

-Irregular satellites and Trojans were likely asteroids in heliocentric orbits which did not get ejected into the Oort cloud or incorporated in the planets.

-> The irregular satellites and Trojans may be the key needed to showing us the complextransition between rocky objects which formed in the Main asteroid belt and the volatile rich objects which formed in the Kuiper Belt.

Brown 2000

Physical Properties of the Irregular Satellites and Trojans

Produced by NASA/JPL/University Arizona/LPLFrom Cassini imager and VIMS data.Porco et al. 2005; Clark et al. 2005

Volatiles Observed on Phoebe

No VolatilesOn Jupiter’sOuter Satellites!