A Thick Cloud of Neptune Trojans and Their

A Thick Cloud of Neptune Trojans and Their Colors Scott S. Sheppard Carnegie Institution of Washington Department of Terrestrial Magnetism and Chadwick Trujillo Gemini Observatory

Why Observe Asteroids? The stable asteroid reservoirs have a “fossilized” imprint from the formation and evolution of the Solar System. The effects of the solar nebula, growth of the planets, planetary migration, collisions, and resonances all potentially influence asteroid formation and evolution.

Trojan asteroids share a planet’s semi-major axis but lead (L 4) or follow (L 5) the planet by about 60 degrees near the two triangular Lagrangian points of equilibruim Minor Planet Brightness Albedo x radius Flux ~ Helio distance 2 4

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.

Numerical dynamical stability simulations show Neptune may retain up to 50% of its Trojan population over the age of the solar system after any significant planetary migration (Nesvorny et al. 2002, Marzari et al. 2003, Kortenkamp et al. 2004). No primordial Saturn or Uranus Trojans known or expected. Nesvorny and Dones 2002

Stability Regions for Trojan Asteroids of the Giant Planets Saturn Uranus Neptune Nesvorny and Dones 2002

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). Morbidelli et al. 2005 -Trojans would not be captured and are severely depleted during any migration (Gomes 1998; Kortenkamp et al. 2004). Nesvorny and Dones 2002

Neptune a i e mag r Trojan (AU) (deg) (km) 2001 QR 322 30. 14 1. 3 0. 03 22. 5 70 2004 UP 10 30. 08 1. 4 0. 03 23. 2 50 2005 TO 74 30. 05 5. 3 0. 06 23. 3 2005 TN 53 30. 05 25. 1 0. 07 23. 6 Median Jup 5. 2 50 40 11. 0 0. 07 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 (Horner et al. 2006). Capture of 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. Collisional interactions within the Lagrangian region (Chiang et al. 2005). -> Predicts low inclination Trojans In-situ accretion from a subdisk of debris formed from post-migration collisions (Chiang et al. 2005). -> Predicts low inclination Trojans Freeze-in Capture: The giant planets orbits become marginally excited 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). -> Predicts low and high inclination Trojans

The Trojans of the giant planets lie between the rocky main belt asteroids and volatile-rich Kuiper Belt. Parallax

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 4: 1 High i : Low i Sheppard and Trujillo 2006 +240 -180 400 with radii > 40 km Appear 5 to 25 times larger than the Jupiter Trojans and Main belt asteroid populations

Freeze-In Capture Tsigais et al. 2005 Gomes et al. 2005

Jupiter Trojan Capture Simulation Morbidelli et al. 2005

Physical Properties of the Trojans -Currently the space between the giant planets is mostly devoid of small stable objects. -Trojans were likely asteroids in heliocentric orbits which did not get ejected into the Oort cloud or incorporated in the planets. -> The 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

Comparison of Colors of Outer Solar System Objects Ultra Red Sheppard and Trujillo 2006 No ultra red material as seen In the Classical Kuiper Belt.

The Dispersed Populations Classical KBOs MBAs

Evolution of Asteroids in the Outer Solar System Morbidelli and Levison 2003

The End

Known 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. 6 m/Mega. Cam Palomar 1. 2 m/Quest Subaru 8. 3 m/Suprime. Cam Magellan 6. 5 m/IMACS