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
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
No ultra red material as seenIn the Classical Kuiper Belt.
Comparison of Colors of Outer Solar System Objects
Sheppard and Trujillo 2006
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.
-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