DISCOVERY OF A HYDRATED ASTEROID IN THE KUIPER BELT
Our team has discovered a Kuiper Belt Object which looks like an asteroid! 2004 EW95 is a small dynamically excited KBO in orbital resonance with Neptune. The orbital diagram displayed for 2004 EW95 below was created by NASA JPL’s Small Body Browser. In this diagram 2004 EW95’s orbit is plotted in white and the giant planets in colour (the terrestrial planets are not shown). Vertical lines show how 2004 EW95’s orbit is highly inclined to the ecliptic.
2004 EW95’s spectrum was first shown to be unusual compared to other KBOs by spectrophotometry gathered by Wes Fraser and collaborators with the Hubble Space Telescope (HST; published here and plotted above in yellow). Later spectroscopic observations with X-Shooter (black) and FORS2 (red) confirmed the HST observations and showed that 2004 EW95’s spectrum isn’t linear like those typical of objects that exist beyond Neptune. Instead it’s very similar to that of a primitive carbonaceous asteroid like those observed in the outer asteroid belt between Mars and Jupiter. The spectrum has two features, a near-UV drop in reflectance and a broad, shallow optical absorption feature centered at around 700 nm. In asteroid spectra these respective features are associated with ferric oxides and aqueously altered silicate material (for example see the SMASSII reflectance spectrum of C-type asteroid 38 Leda presented in blue above). Neither feature has been reliably confirmed in the spectrum of a KBO before. The presence of these features not only indicate that 2004 EW95 has likely been significantly heated at some point in its history, but suggests that it formed in a similar environment to other C-type asteroids. Dynamical simulations have shown that the C-type asteroids we observe in the asteroid belt may have formed among the gas giant planets in the early Solar System, before being scattered into the outer asteroid belt as the planets migrated. Those simulations also show that a few C-types could have been scattered out beyond Neptune as well, where they may later have been captured into Neptune’s mean motion resonances, likely in orbits very similar to that of 2004 EW95.