2019 UO14: A Transient Trojan of Saturn

Saturn has long been the only giant planet in our solar system without any known Trojan members. In this Letter, with serendipitous archival observations and refined orbit determination, we report that 2019 UO14 is a Trojan of the gas giant. However, the object is only a transient Trojan currently librating around the leading Lagrange point L4 of the Sun–Saturn system in a period of ∼0.7 kyr. Our N-body numerical simulation shows that 2019 UO14 was likely captured as a Centaur and became trapped around L4 ∼ 2 kyr ago from a horseshoe co-orbital. The current Trojan state will be maintained for another millennium or thereabouts before transitioning back to a horseshoe state. Additionally, we characterize the physical properties of 2019 UO14. Assuming a linear phase slope of 0.06 ± 0.01 mag deg−1, the mean r-band absolute magnitude of the object was determined to be Hr = 13.11 ± 0.07, with its color measured to be consistent with that of Jupiter and Neptune Trojans and not statistically different from Centaurs. Although the short-lived Saturn Trojan exhibited no compelling evidence of activity in the observations, we favor the possibility that it could be an active Trojan. If confirmed, 2019 UO14 would be marked as the first active Trojan in our solar system. We conservatively determine the optical depth of dust within our photometric aperture to be ≲10−7, corresponding to a dust mass-loss rate to be ≲1 kg s−1, provided that the physical properties of dust grains resemble Centaur 29P/Schwassmann–Wachmann 1.