has also been used to study objects in the outer reaches of the Solar System, including the dwarf planets Pluto, Eris, and Sedna. During June and July 2012, U.S. astronomers using Hubble discovered Styx, a tiny fifth moon orbiting Pluto. From June to August 2015, Hubble was used to search for a Kuiper belt object (KBO) target for the New Horizons Kuiper Belt Extended Mission (KEM) when similar searches with ground telescopes failed to find a suitable target. This resulted in the discovery of at least five new KBOs, including the eventual KEM target, 486958 Arrokoth, that New Horizons performed a close fly-by of on January 1, 2019. In April 2022 NASA announced that astronomers were able to use images from HST to determine the size of the nucleus of comet C/2014 UN271 (Bernardinelli–Bernstein), which is the largest icy comet nucleus ever seen by astronomers. The nucleus of C/2014 UN271 has an estimated mass of 50 trillion tons which is 50 times the mass of other known comets in our solar system. Supernova reappearance On December 11, 2015, Hubble captured an image of the first-ever predicted reappearance of a supernova, dubbed "Refsdal", which was calculated using different mass models of a galaxy cluster whose gravity is warping the supernova's light. The supernova was previously seen in November 2014 behind galaxy cluster MACS J1149.5+2223 as part of Hubble's Frontier Fields program. The light from the cluster took roughly five billion years to reach Earth, while the light from the supernova behind it took five billion more years than that, as measured by their respective redshifts. Because of the gravitational effect of the galaxy cluster, four images of the supernova appeared instead of one, an example of an Einstein cross. Based on early lens models, a fifth image was predicted to reappear by the end of 2015. Refsdal reappeared as predicted in 2015. Mass and size of Milky Way In March 2019, observations from Hubble and data from the European Space Agency's Gaia space observatory were combined to determine that the mass of the Milky Way Galaxy is approximately 1.5 trillion times the mass of the Sun, a value intermediate between prior estimates. Other discoveries Other discoveries made with Hubble data include proto-planetary disks (proplyds) in the Orion Nebula; evidence for the presence of extrasolar planets around Sun-like stars; and the optical counterparts of the still-mysterious gamma-ray bursts. Using gravitational lensing, Hubble