The Extraordinary Colors of Trans-Neptunian Objects

In 1992, Jewitt and Luu (1993) carried out a survey of 0.7 square degrees of the sky to a limiting magnitude of We present B–V and V–R colors for trans-neptunian objects (TNOs) 1994 TB and 1993 SC. We have found that these TNOs R p 25 and discovered the first TNO (1992 QB1). Thirtyhave colors almost as red as 5145 Pholus which are therefore six TNOs have been discovered as of June, 1996. The consistent with surfaces rich in complex organic molecules. We TNOs have effective diameters ranging from 100 to 400 have measured B–V colors of 1.10 6 0.15 and 1.27 6 0.11 for km (Jewitt and Luu 1995), making them significantly larger 1994 TB and 1993 SC, respectively, as well as V–R colors of than typical comets such as 1P/Halley (15 3 8 km; Keller 0.68 6 0.06 and 0.70 6 0.04 for 1994 TB and 1993 SC, respecet al. 1987). Jewitt and Luu have estimated that there are tively. Currently, there are only four TNOs with published p35,000 TNOs larger than 100 km in the 30 to 50 AU broadband colors (1992 QB1, 1993 FW, 1993 SC, and 1994 heliocentric distance range. TB). TNO colors range from solar (a single V–R measurement of 1993 FW in the literature) to nearly the reddest objects in Almost nothing is known about the chemical nature the solar system (our measurements of 1993 SC and 1994 TB). of the TNOs that have been discovered so far. It is Although we have concluded 1993 SC is a very red TNO, a possible, perhaps likely, that they are mostly similar in measurement in the literature has suggested that there is nothcomposition to the short-period comets that we believe ing special about the color of 1993 SC. Because there are so are derived from the Kuiper Belt, though cosmic-ray few TNOs with color measurements and because colors for processing may have greatly altered their surfaces. How1993 SC are inconsistent, we cannot say yet with any certainty ever, TNOs span wide ranges in size and heliocentric whether the surfaces of TNOs exhibit a diversity of colors and distance and some or all of them may have compositions hence a diversity of surface compositions.  1997 Academic Press very different from those of short-period comets. Composition may also be a function of heliocentric distance, thereby providing a signature of formation mechanisms,