The methane ice rich surface of large TNO 2005 FY9: a Pluto-twin in the trans-neptunian belt?

Context.The population of known large trans-neptunian objects (TNOs) is growing very fast and the knowledge of their physical properties is a key issue to understand the origin and evolution of the Solar System. Aims.In this paper we studied the surface composition of the recently discovered TNO 2005 FY9, one of the largest known TNOs (~0.7 times the diameter of Pluto, i.e. 1600 km, if the albedo is similar, or 3100–1550 km in diameter assuming an albedo range 0.2 < pV < 0.8). Methods.We report visible and near infrared spectra covering the 0.35–2.5 μm spectral range, obtained with the 4.2 m William Herschel Telescope and the Italian 3.58 m Telescopio Nazionale Galileo at “El Roque de los Muchachos” Observatory (La Palma, Spain). Results.The spectrum of this large TNO is similar to that of Pluto, with an infrared region dominated by very prominent absorptions bands formed in solid CH4. At wavelengths shorter than 0.6 μm, the spectrum is almost featureless and red. The red color most likely indicates the presence of complex organics, as has been hypothesized for Pluto and many other TNOs. The icy-CH4 bands in this new giant TNO are significantly stronger than those of Pluto, implying that methane could be even more abundant on its surface. The existence of a volatile such as methane on the surface of 2005 FY9, likely accompanied by N2 and CO ices, coupled with its large size, make this Pluto-like TNO an excellent candidate to have an atmosphere comparable to Pluto's. Conclusions.