Ground observations of Varuna's thermal emission from 2000 to 2005 yielded large diameter estimates ranging from to , making it comparable to the size of Ceres. Contrary to the ground-based estimates, space-based thermal observations from the Spitzer Space Telescope provided a smaller diameter range of . The discrepancy between ground-based and space-based size estimates are due to the limited observable wavelengths for ground-based observations, as a result of absorption of Earth's atmosphere. Distant trans-Neptunian objects such as Varuna intrinsically emit thermal radiation at longer wavelengths due to their low temperatures. However, at long wavelengths, thermal radiation cannot pass through Earth's atmosphere and ground-based observations could only measure weak thermal emissions from Varuna at near-infrared and submillimeter wavelengths, hindering the accuracy of ground-based thermal measurements.