Download Neptune Trojans

Completing the Inventory of the
Outer Solar System
Scott S. Sheppard
Carnegie Institution of Washington
Department of Terrestrial Magnetism
Why Observe Asteroids?
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.
Observed
Stable
Reservoirs
Main Asteroid Belt
25 > 200 km
Trojans
5 ~ 200 km
Irregular Satellites
5 ~ 200 km
Kuiper Belt
10,000 > 200 km
Wide-Field CCDs on Small/Medium/Large Telescopes
Power of a Survey
A x Omega
A = Area of Telescope
Omega = Solid Angle Observed
CFHT 3.6m/MegaCam
Palomar 1.2m/Quest
Subaru 8.3m/SuprimeCam
Magellan 6.5m/IMACS
Minor Planet Brightness
Albedo x radius
Flux ~
Helio distance
2
4
Parallax of Asteroids and
Satellites
Asteroid
Asteroid
Jupiter
Satellite
Trans-Neptunian Objects
Dynamically Disturbed and Collisionally Processed
The Largest Minor Planets
Orcus
2003 EL61
2005 FY9
1400 km
1600 km
1800 km
Planets?
2003UB313
2003 EL61
2005 FY9
Pluto
Sedna
How did the extremly red object Sedna come to be in
its 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 Neptune
4. Massive Scattered Planetary Embryos
5. Massive Trans-Neptunian Disk
6. Capture of Extrasolar Planetesimals
Neptune Trojans
The 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 Trojan Formation Scenarios
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 Inclinations
Can test formation
theories on the inclination
distribution of Neptune
Trojans.
Magellan-Baade 6.5 meter
With the 0.2 square degree
IMACS imager.
+75
-35
50
+10
-7
: 12
High i : Low i
Sheppard and Trujillo 2006
Assuming low albedos the
known Neptune Trojans
are between 40 to 70 km.
+240
-180
375
with radii > 40 km
Maybe 3 to 20 times larger than the
Jupiter Trojans and Main belt asteroid
populations
Freeze-In Capture
Tsigais et al. 2005
Gomes et al. 2005
Comparison of Colors of Outer Solar System Objects
No ultra red
material as
seen
In the Classical
Kuiper Belt.
Sheppard and Trujillo 2006
The Dispersed Populations
Classical KBOs
MBAs
Kuiper Belt Formation
The End
Regular Satellites
1. “e” is small
2. “i” is small
3. “a” is small
4. Prograde only
-> Formed by Circumplanetary
accretion
Satellites
Other
Mercury =
Venus =
Earth
=
Mars
=
Jupiter >
Saturn >
Uranus >
Neptune >
Pluto
=
0
0
1
2
8
21
18
7
1
Irregular
0
0
0
0
55
26
9
6
0
Irregular “outer” Satellites
1. “e” is big
2. “i” is big
3. “a” is big
4. Prograde or Retrograde
-> Captured from heliocentric
orbits
a crit = (2 J
2
r
2 p
a 3p
mp / M
sun
)
1/5
(Burns 1986)
Capture?
Reversibility of
Newton’s Equations
Energy dissipation
Needed for
Permanent capture
-Collide with planet
-Ejected from system
Comet Shoemaker-Levy 9
Hartman
Capture Mechanisms
(During the Planet Formation Epoch)
Hartman
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.
 Irregular Satellites provide a unique window on
processes operating in the young Solar System
Jupiter have similar outer satelliteUranus
All giant planets
systems!
Stable over age of the
Solar System.
Henon 1970
Carruba et al. 2002
Nesvorny et al. 2003
Kozai Effect
Carruba et al. 2002
Nesvorny et al. 2003
Neptune
Saturn
Retrograde
vs Prograde
Henon 1970
Hamilton & Krivov 1997
Sheppard et al. 2005
Dynamical Families -> Collisions with Comets or Defunct
Gladman et al. 2001
Satellites After capture
Sheppard and Jewitt 2003
Nesvorny et al. 2004
Resonances
Saha & Tremaine 2003
Whipple & Shelus 1993
Nesvorny et al. 2003
Cuk & Burns 2004
Collisionless Three Body Interactions as a Capture Mechanism
-> Recently described by Agnor and Hamilton 2004
Preferred Because Less
Dependent on planet
Formation scenario.
KBO 1999 TC36
Viewed by Hubble
Each giant Planet may have
Had a similar number of
Small body encounters.
-Less objects further out
But bigger Hill spheres
Captured just after the
Planet formation epoch.
Funato et al. 2004
Where Did Triton Come From?
Physical Properties of the
Irregular Satellites and Trojans
-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 complex
transition between rocky objects
which formed in the Main asteroid
belt and the volatile rich objects
which formed in the Kuiper Belt.
Brown 2000
Volatiles Observed on Phoebe
No Volatiles
On Jupiter’s
Outer Satellites!
Produced by NASA/JPL/University Arizona/LPL
From Cassini imager and VIMS data.
Porco et al. 2005; Clark et al. 2005