SEDNA: New Planet or Interstellar Menace? Steven Gibson The University of Calgary March 30, 2004

Sedna, Innuit Goddess of the Sea

Discovery of 2003 VB12 Sedna (“Mickey”)

Why Study the Outer Solar System? ● Structure and formation history of solar system – coalescence, collisions, scattering, migration ● Composition of Solar Nebula material – Are objects primordial or evolved? ● Probe for gravitational evidence of large bodies – Planet X, Nemesis,... ? ● Near-interstellar environment beyond heliopause ● Because it's there! (What else do you need?)

Methods of Exploration ● Direct (difficult and still untried) – Voyager 1 (opted for Titan pass instead) – Pluto-Kuiper Express (cancelled) – New Horizons (launch 2006, flyby 2015) – TAU (1000 AU; 50 yr mission; concept only) ● Observation – Early discoveries came very slowly ● Uranus (1791), Neptune (1846), Pluto (1930) ● 2060 Chiron (1977), Charon (Pluto moon; 1978) – CCDs and computers have helped enormously ● 1992 QB1 until today : 831 TNOs!

Taxa of Sun-Orbiting Bodies, Part 1 ● (Major) Planets - large substellar objects orbiting the sun – terrestrial (Mercury, Venus, Earth, Mars) – jovian (Jupiter, Saturn, Uranus, Neptune) – ice dwarfs (Pluto et al.) - not planets by some definitions ● Planet definitions vary – historical (classical 9 with no future changes) – by mass or size (Pluto may be outclassed in future) – by shape (rounded by gravity - includes many moons) – relative (dominates mass in orbital domain)

Taxa of Sun-Orbiting Bodies, Part 2 ● Minor Planets (Planetoids) - smaller than planets ● Composition Categories – asteroids (rocks, no volatiles; 10m km) – comets (“primitive” composition; <~ a few km) ● long-P (>200yr) and intermed (20-200yr) from Oort Cloud ● Jupiter-family (<20yr) from Kuiper Belt ● Location Categories – Near-Earth (NEOs) - Atens, Apollos, Amors – Trojans (Mars, Jupiter, and Neptune to date) – Trans-Jovian - Centaurs (KB?), Damocloids (OC?) – Trans-Neptunian (TNOs)

Taxa of Sun-Orbiting Bodies, Part 3 ● Trans-Neptunian Objects – Kuiper Belt (KBOs) (30-50 AU) - nebular remnant? ● “Classical” KBOs, a.k.a. “Cubewanos” (after 1992 QB1) ● Resonance KBOs – “Plutinos” (“little Plutos” in 3:2 resonance with Neptune) – “Twotinos” (2:1) and others (1:1, 2:5, 4:5, 4:7, 3:5, 3:4) ● Scattered Disk Objects ● “Detatched” KBOs (Jewitt includes Sedna here) – Oort Cloud (OCOs) (50 AU - 50,000 AU) - scattered? ● “Classical” (Outer?) OCOs - aphelia ~ a few x 10,000 AU ● “Inner” OCOs may exist (Brown includes Sedna here)

TNO Motions over 100 Years (MPC) Comets High-e Objects Centaurs Plutinos Classical KBOs Scattered Disk Objects

TNO Orbits (from Jewett) Planets: Jupiter Neptune TNOs: Classical Plutinos Scattered

Sedna Orbit (from Brown)

Sedna Observations (the paper!) ● 48” Palomar Schmidt w/ 172 Mpix CCD array, drift-scanning 9.5 deg^2 area to 21 mag in R ● Detected Sedna 2003 Nov 14 w/ 1.5”/hr => r ~ 100 AU (apparent drift mostly Earth motion) ● Found prior detections 2003 Aug , 2001 data from NEAT survey; 1991 PDSS match poor ● From these data, refined orbital parameters derived: r=90.32+/-0.02 AU, a=480+/-40 AU, etc. ● This orbit is unprecedented. What is it?

Sedna Data ● RA+DEC for March 30: ~ 3h14m32s +5d49.0m ● Current Distance from Sun: 90 AU (12.5 light hr) ● Perihelion / Aphelion: 76 / 850 AU (a ~ 480 AU) ● Inclination: deg (Pluto = 17; Mercury = 7) ● Sednan year: 10,500 Earth years ● Sednan day: ~ 40 Earth days (due to moon?) ● Diameter: km (51%-78% of Pluto) (upper limit from Spitzer & IRAM nondetection) ● Albedo, Color: > ~ 0.2?, very red (like Mars)

How Did It Get There? ● Orbit resembles Scattered KBO, but perihelion too big for Neptune to get it out there. ● Scattering by unseen planet near 70 AU – no detections of such an object to date ● Galactic tidal perturbation too soft ● Stellar encounter – very unlikely at short enough range, but who knows? ● Formation in a stellar cluster – denser than present stellar neighborhood; statistics more favorable; requires large new inner Oort pop ● All mechanisms more likely w/ richer population

Biggest Known TNOs ● Pluto km (Charon km) ● Sedna km (detatched KBO / Oort?) ● 2004 DW km (plutino) ● Quaoar km (cubewano) ● Varuna km (cubewano) ● Ixion km (plutino) ● (Ceres - 960; Triton ; Ganymede )

Texas

What Else is Out There? ● Other Sednas (“Sedninos”?) - time will tell ● Oort Cloud Objects with much larger aphelia? ● “Planet X”? (Lowell's term) – no Neptunes unless d > 160 AU; Tombaugh looked (and Voyager 2 showed no gravitational evidence) ● Nemesis?? (Whitmore & Matese 1985) – hard to disprove object w/ d=20,000-90,000 AU and period ~ 32 Myr, but no real evidence, even in geological record; not useful for Sedna orbit either

(R mag ~ 20.5)