Discovery of a satellite around the transneptunian object 1998 WW31

Alain Doressoundiram (Observatoire de Paris) and Christian Veillet (CFH Institute) have just discovered that the transneptunian object 1998 WW31 is in fact a double object. It is during their multi-color photometry and recovery of transneptunian objects program that they made this discovery. This program is carried out on the Canada-France-Hawaii Telescope of 3.6 m located at 4200m on Mauna Kea, Hawaii. In December 2000, they pointed the telescope at 1998 WW31, a transneptunian object which required additional observation, without which they would have lost its position. Its double nature or its elongated shape did not appear to them immediately at the time of the observations, but only later during the meticulous analysis of the data. The transneptunian objects are small bodies of the Solar system located beyond the orbit of Neptune, at more than 30 astronomical units from the Sun (1 astronomical unit = distance Sun-Earth). They are icy bodies, very primitive as fossil remnants of the formation of the Solar system, 4.6 billion years ago. The study of these objects, discovered for the first time less than ten years ago, is currently in full development. It can provide invaluable indices on the composition of the primitive nebula and on the processes prevailing at the beginning of the Solar system. Moreover, the knowledge of the physical properties of the transneptunian objects could be a significant step in the study of circumstellar discs and planet formation around other stars that the Sun. Figure 1 : 1998 WW31 in the night of 22 DEC 2000 UT (top) and 23 DEC 2000 UT (medium and bottom) On the chronological sequence of the 3 reduced images of figure 1, 1998 WW31 (red square), like all the objects of the Solar system moves relative to fixed stars of the field. It is a weak object of 23.6 in magnitude and distant by 45.6 astronomical units from the Earth. 1998 WW31 appears undoubtedly double. The exceptional quality of the Mauna Kea site allowed the resolution of the pair (1.2 seconds of arc). With only these images, two alternative interpretations were possible : They are two transneptunian objects on distinct orbits but having by pure coincidence a common apparent motion over two nights ! 1998 WW31 is undoubtedly a twin (like Pluto-Charon) Image of the discovery (23/12/2000 UT) Archive image (7/01/2000 UT) Angular separation : 1.2 " Direction of the pair : +45 deg. Angular separation : 0.8 " Direction of the pair : +25 deg. Figure 2 It is the second alternative, much more probable than the first, which proved to be right. Indeed, the CFHT archive images of January 7 2000, made it possible to find 1998 WW31. But the orientation of the pair, as well as the separation between the two components had changed : 1998 WW31 is actually a twin ! 1998 WW31 is thus the second transneptunian object (after Pluton) to have a satellite. The combined diameter of the pair is estimated at 150 km. The angular separation of 1.2 " observed in December corresponds to an effective distance between the two components of approximately 40000 km (Pluto and Charon are distant by 19636 km). The difference in magnitude between 1998 WW31 and its satellite is estimated at 0.4, which thus gives a mass ratio of approximately 1.7 (ratio of 8 for Pluto-Charon) the system 1998 WW31 is more a double " planet " than a planet-satellite system. With all the archive images available and new observations expected in the future, it will be possible to get a full orbit determination. This will make it possible to determine for the first time the mass and the density of a transneptunian object, capital information on the intimate nature of these primitive objects, playing the role of reservoir for short period comets.