Two distinct populations of Kuiper-belt objects

Abstract

The discovery of the first member of the Kuiper belt 1 -a formerly hypothetical ancient reservoir of objects located beyond Neptune's orbit-started a revolution in our understanding of the outer Solar System: there is no longer a sharp edge at Pluto's orbit. About 60 Kuiper-belt objects, intermediate in size between comets and planets, are now known 2 to exist on stable circular orbits around the Sun, and no doubt many more objects await discovery. But owing to the recent discovery and intrinsic faintness of these objects, little has been done to explore their physical and chemical properties 3-8 . Here we report the results of a two-year survey of the broad-band optical colours of about one-quarter of the known Kuiper-belt objects. We find that their colours indicate the presence of two distinct populations: one consists of objects whose surface colours are only slightly redder than the colour of the Sun, while the other consists of the reddest objects known in the Solar System.Our survey uses Harris B (450 nm), V (550 nm) and R (650 nm) 5-inch ϫ 5-inch glass filters in front of a 1;200 ϫ 800 pixel CCD (charge coupled device) camera at the f/9 Cassegrain focus of the Steward Observatory 2.3-m telescope. The CCD has outstanding quantum efficiency, particularly in the blue where it exceeds 95%. This high quantum efficiency greatly aided our blue photometry of these faint objects. We used the CCD in a 2 ϫ 2 binning mode, yielding 600 ϫ 400 pixel images, covering 3 ϫ 2 arcmin on the sky at 0.30 arcsec per pixel. Here we will use ''pixel'' to refer to the binned 0.3-arcsec pixels. The typical full-width at half-maximum of the stellar point-spread function is 1.5 arcsec.Observations of Kuiper-belt objects (KBOs) present a formidable challenge for a photometrist. Their faintness requires us to image small areas of the sky to faint brightness levels; at such levels there are numerous faint stars and galaxies in the images. The Earth's revolution about the Sun results in the motion of KBOs relative to the ''fixed'' background of stars and galaxies in the images (ϳ3 arcsec h −1 ). Hence, KBOs sometimes pass near background stars and galaxies, and the stellar and galactic light sometimes contaminates the light from KBOs. Finally, some KBOs show brightness variations that are probably the result of the rotation and irregular shape of the KBOs or of albedo variations over their surfaces. Observations of these faint, moving, variable objects require great care.We have developed an observational and image processing procedure customized for photometry of KBOs 5-7 . The faintness and motion of these objects pushes the optimum exposure time in opposite directions. On the one hand, the faintness of the objects pushes us towards exposure times that are as long as possible, so that we can maximize the signal-to-noise ratio of the data. On the other hand, the motion of KBOs pushes us towards short exposure times, otherwise the light from a KBO is spread over many pixels, thereby degrading the signal-to-noise ratio. We have found tha...

Colors of Minor Bodies in the Outer Solar System

“…Recent papers have been published discussing fairly large samples of TNOs; while some authors see these objects evenly spread over the whole color range (Barucci et al 2000;Boehnhardt et al 2001;Delsanti et al 2001;Davies 2000), others have reported that the TNO population is distributed into two well separated color classes: one of solar color, the other very red (Tegler & Romanishin 1998). It is intriguing that different groups obtain such different results.…”

“…Tegler and Romanishin have repeatedly reported that their observations lead to a classification of the objects in 2 separate groups in the color-color diagrams (Tegler & Romanishin 1998;, one being of neutral-blue colors, while the other is very red. While this bimodality appears evident to the eye on their color-color diagrams, other authors (Barucci et al 2000;Davies 2000;Delsanti et al 2001) do not confirm it: their color-color diagrams show continuous distributions.…”

Multiple and Fast: The Accretion of Ordinary Chondrite Parent Bodies

“…This is not the first time that two spectrally distinct groups (i.e., a bimodality) are observed for relatively similar compositions among solar system small bodies. Emery et al (2011) reported the presence of two compositional groups among the Jupiter Trojans, while Tegler & Romanishin (1998) and later Peixinho et al (2012) reported the presence of two color groups among small TNOs and Centaurs.…”

“TNOs are Cool”: A survey of the trans-Neptunian region

“…comm. ); ( j) Grundy et al (2008); (k) Perna et al (2010); (m) Peixinho et al (2004); (n) Sheppard & Jewitt (2002); (o) Belskaya et al (2003); (p) Sheppard (2007); (q) Tegler & Romanishin (1998); (r) Jewitt & Luu (1998); (s) Barucci et al (1999); (t) Tegler et al (2003); (u) Trujillo & Brown (2002); (v) Boehnhardt et al (2002); (w) Tegler (2011, priv. comm.…”