Statistics of Optical Colors of
KBOs and Centaurs
W. Romanishin – U. of Oklahoma
S. C. Tegler – Northern Arizona U.
Solar System pre-1992
4 inner (rocky) planets – M,V,E,M
Main Asteroid belt (rocks: <1 – 1000 km,
between Mars and Jupiter)
4 outer (giant – gas/liquid) planets- J,S,U,N
Pluto (small – 1/7 mass of Moon!)
Comets (long and short period)
Oort Cloud (large spherical comet
reservoir)
Brief history of Kuiper Belt idea
• 1930 – Pluto discovered
• ~1950 Kuiper and Edgeworth- Solar nebula out past
Neptune – No major planet formed- What kind of objects
out there, if any survive?? Kuiper Belt= remains of solar
nebula out past Neptune
• 1970s – search for objects (besides Pluto) past Neptuneno luck
• Objects in KB probably icy, much like comets
• Comets = “dirty snowball” – need a few subtleties of
comets to follow Kuiper Belt history
Comet= Icy nucleus + active gas/dust tail when near Sun
When inactive, very hard to see
Comets – Oort Cloud vs. Ecliptic
Oort Cloud comets- long periods, random
inclinations – Originate from Oort cloud
reservoir (“deep freeze” for comets)
Occasionally get knocked into inner
Solar System, are heated by Sun and we
see them as comets
Ecliptic comets- short orbital periods
( < 200 years), orbital inclinations near
plane of Solar System
Oort Cloud- Large “deep freeze” for comets
~spherical = random orbital inclinations
Kuiper Belt – found at last!
• 1980s- dynamicists said “Ecliptic comets
NOT from Oort cloud”= need a flattened
comet reservoir
• Inspired further observational searches
with better telescopes and detectors
(CCDs)
• 1992 Jewitt and Luu found first object past
Neptune = 1992 QB1 , about 44 AU from
Sun
Sky motion – KBO and asteroid
Jewitt and Luu looked for “slow moving
objects” = distant solar system bodies
Angular motion primarily reflex motion
of Earth going around Sun
Why so hard to find?
• Over 1200 outer solar system
objects now known, some
larger than largest asteroid
• Why so long to find? Largest
asteroid can be seen with
binoculars!
• Objects seen (in visible light)
only by reflected sunlight
• Typical KBO a million times
fainter than same size MB
asteroid
Kuiper belt objects provide an opportunity to study the preserved building
blocks of a large outer solar system planet
Dynamical Zoo of Outer Solar System Objects
With ~1200 objects discovered (about ½ with good orbits), we see a number of
different dynamical classes:
Classical
(e.g. 1992 QB1)
= KBO “main belt” r~ 42-46AU, stable orbits,
near-circular orbits, low inclination
Plutinos (little Plutos)
a~39.5AU, significant eccentricity
and inclination
For stability, orbits in 3:2 resonance with Neptune- even
though Pluto gets closer to Sun than Neptune, Pluto orbit
stable
Stability of Pluto’s (and Plutino’s)
orbits
Neptune orbits Sun 3 times for
every 2 times Pluto does
This resonance insures Pluto
doesn’t come close to Neptune
(inclination helps also)
Outer Planets/ Plutinos
Scattered disk objects (SDOs)
Objects with large orbits (Q> 60AU), presumably gravitationally
scattered outwards from origin radius
Eccentric orbits
Centaurs
Objects presumably scattered inwards towards Sun- have low
lifetime (<10**7 year) orbits that cross orbits of giant planets
“refugees from the Kuiper Belt”
Ecliptic comets
Objects close enough to Sun to be active, easily visible comet,
perhaps evolving dynamically through Centaur stage
SDO and Classical orbits
Note scale of box
Outer Planets / Classical KBOs + Centaurs
Centaurs and Ecliptic comets
Kuiper Belt and related objects
Qualitatively, at least, a nice story:
Outer solar nebula disk formed into some objects in 1- 2000 km size
range, but not dense enough for a planet to form
These objects formed the Primordial Kuiper Belt (PKB)
Classical and Plutinos surviving members of PKB
Centaurs, SDOs and Ecliptic comets show continuing loss of objects
from PKB by gravitational scattering
Optical colors of KBOs
In 1995, we started a program to measure optical colors of KBOs
CCD imaging on 2 – 10 meter telescopes through 3 filters, B (400500nm), V (500-600nm), R(600-700nm)
Measure brightness in each filter- ratio gives us colors (measure of
slope of spectral energy distribution)
Why not spectroscopy? Objects very faint, and colors much easier to
obtain than spectra
Idea is to get colors of significant samples of objects in different
dynamical classes to look for patterns (color vs. dynamical class) as
possible clues to object’s origin and history
To date, we have colors for ~120 objects / ~150 scheduled nights on
2-10 meter telescopes
• We began our CCD imaging survey of KBOs in 1995
• 162 nights on 2 m to 10 m class telescopes
• 12 nights of NASA Keck time
• Colors & magnitudes for 120 of ~ 500 objects with good orbits
• Surface colors, densities, & albedos
Color distributions
From our first color measurements, it was clear that objects not all
same color. Some had ~solar colors (gray), some were much
redder than solar color. (Why? Different surface compositionswater ice vs. methane?? Still trying to figure out!)
In 1998, we claimed that KBOs/Centaurs showed a bimodal color
distribution
Peixinho etal showed this was partially due to our lumping Centaurs
with KBOs- we and they agree Centaurs show bimodal color
distribution. Peixinho etal claimed Plutinos not bimodal
We have sought maximum size sample for different classes by
combining our colors with those of other groups
Surface Colors
Two distinct color populations
(Tegler and Romanishin
NATURE 1998)
Our B-R colors vs. Europeans
Solid line is 1:1 – Good agreement of colors for objects in common,
so combination of samples is valid.
Centaur Color Distribution
Diptest shows significant bimodality (99.5% SL for combined
sample)
Plutino Color Distribution
Diptest does not indicate significant bimodality in any sample
B-R Colors of Classical KBOs
vs. Plutinos
Nonparametric Wilcoxon rank-sum test shows Classical and Plutino
color distribution differ at 99.98% SL
Conclusions
Wilcoxon rank-sum test shows classical KBO color distribution different
from Plutinos
Diptest shows Centaurs have a bimodal B-R color distribution, while
Plutinos do not
Bottom line:
Using nonparametric statistical tests, we HAVE found statistically
different color distributions for different dynamical classes of objects.
We hope these distributions will help elucidate the origin and history of
these objects.