Mike Brown, Emily Schaller, Henry Roe, David Rabinowitz, and Chad Trujillo released their Hubble measurements of the disk of 2003 UB313 and found it to be just 34 milliarcseconds in diameter--that's roughly a millionth of a degree on the sky (the moon is half a degree in diameter). That makes it 2400km +-100km in actual size, and makes it a brightly reflective object with an albedo of 0.86+-0.07 (86% reflectivity). Freshly fallen snow on Earth is at most 90% reflective, so the surface of 2003 UB313 has got to be covered in new ice of some sort--Brown et al. suggest methane ice, like Pluto.
Previous measurements using infrared had it at 3000km+-400km.
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