SEDNA:New Planet

orInterstellar Menace?

Steven GibsonThe 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 - 1000km)– 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

SednaOrbit

(fromBrown)

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 + 2002, 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: 11.927 deg (Pluto = 17; Mercury = 7)● Sednan year: 10,500 Earth years● Sednan day: ~ 40 Earth days (due to moon?)● Diameter: 1180-1800 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 - 2320 km (Charon - 1270 km)● Sedna - 1200-1800 km (detatched KBO / Oort?)● 2004 DW - 1600 km (plutino)● Quaoar - 1200 km (cubewano)● Varuna - 1060 km (cubewano)● Ixion - 1055 km (plutino)● (Ceres - 960; Triton - 2706; Ganymede - 5268)

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)