We present a catalog of 316 trans-Neptunian bodies detected by the Dark Energy Survey (DES ). These objects include 245 discoveries by DES (139 not previously published) detected in ≈ 60, 000 exposures from the first four seasons of the survey ("Y4" data). The survey covers a contiguous 5000 deg 2 of the southern sky in the grizY optical/NIR filter set, with a typical TNO in this part of the sky being targeted by 25 − 30 Y4 exposures. We describe the processes for detection of transient sources and the linkage into TNO orbits, which are made challenging by the absence of the fewhour repeat observations employed by TNO-optimized surveys. We also describe the procedures for determining detection efficiencies vs. magnitude and estimating rates of false-positive linkages. This work presents all TNOs which were detected on ≥ 6 unique nights in the Y4 data and pass a "subthreshold confirmation" test wherein we demand the the object be detectable in a stack of the individual images in which the orbit indicates an object should be present, but was not detected. This eliminates false positives and yields TNO detections complete to r 23.3 mag with virtually no dependence on orbital properties for bound TNOs at distance 30 AU < d < 2500 AU. The final DES TNO catalog is expected to yield > 0.3 mag more depth, and arcs of > 4 years for nearly all detections.

We evaluate the dynamical stability of a selection of outer solar system objects in the presence of the proposed new Solar System member Planet Nine. We use a Monte Carlo suite of numerical N-body integrations to construct a variety of orbital elements of the new planet and evaluate the dynamical stability of eight Trans-Neptunian objects (TNOs) in the presence of Planet Nine. These simulations show that some combinations of orbital elements (a, e) result in Planet Nine acting as a stabilizing influence on the TNOs, which can otherwise be destabilized by interactions with Neptune. These simulations also suggest that some TNOs transition between several different mean-motion resonances during their lifetimes while still retaining approximate apsidal anti-alignment with Planet Nine. This behavior suggests that remaining in one particular orbit is not a requirement for orbital stability. As one product of our simulations, we present an a posteriori probability distribution for the semi-major axis and eccentricity of the proposed Planet Nine based on TNO stability. This result thus provides additional evidence that supports the existence of this proposed planet. We also predict that TNOs can be grouped into multiple populations of objects that interact with Planet Nine in different ways: one population may contain objects like Sedna and 2012 VP 113 , which do not migrate significantly in semi-major axis in the presence of Planet Nine and tend to stay in the same resonance; another population may contain objects like 2007 TG 422 and 2013 RF 98 , which may both migrate and transition between different resonances.

We report the discovery of a compact multi-planet system orbiting the relatively nearby (78pc) and bright (K = 8.9) K-star, K2-266 (EPIC248435473). We identify up to six possible planets orbiting K2-266 with estimated periods of P b = 0.66, P .02 = 6.1, P c = 7.8, P d = 14.7, P e = 19.5, and P .06 = 56.7 days and radii of R P = 3.3 R ⊕ , 0.646 R ⊕ , 0.705 R ⊕ , 2.93 R ⊕ , 2.73 R ⊕ , and 0.90 R ⊕ , respectively. We are able to confirm the planetary nature of two of these planets (d & e) from analyzing their transit timing variations (m d = 8.9 +5.7 −3.8 M ⊕ and m e = 14.3 +6.4 −5.0 M ⊕ ), confidently validate the planetary nature of two other planets (b & c), and classify the last two as planetary candidates (K2-266.02 & .06). From a simultaneous fit of all 6 possible planets, we find that K2-266 b's orbit has an inclination of 75.32 • while the other five planets have inclinations of 87-90 • .This observed mutual misalignment may indicate that K2-266 b formed differently from the other planets in the system. The brightness of the host star and the relatively large size of the sub-Neptune sized planets d and e make them well-suited for atmospheric characterization efforts with facilities like the Hubble Space Telescope and upcoming James Webb Space Telescope. We also identify an 8.5-day transiting planet candidate orbiting EPIC248435395, a co-moving companion to K2-266. Subject headings: planetary systems, planets and satellites: detection, stars: individual (K2-266), stars: individual (EPIC248435395)

We report the observation and physical characterization of the possible dwarf planet 2014 UZ 224 ("DeeDee"), a dynamically detached trans-Neptunian object discovered at 92 AU. This object is currently the second-most distant known trans-Neptunian object with reported orbital elements, surpassed in distance only by the dwarf planet Eris. The object was discovered with an r-band magnitude of 23.0 in data collected by the Dark Energy Survey between 2014 and 2016. Its 1140year orbit has (a, e, i) = (109 AU, 0.65, 26.8 • ). It will reach its perihelion distance of 38 AU in the year 2142. Integrations of its orbit show it to be dynamically stable on Gyr timescales, with only weak interactions with Neptune. We have performed followup observations with ALMA, using 3 hours of on-source integration time to measure the object's thermal emission in the Rayleigh-Jeans tail. The signal is detected at 7σ significance, from which we determine a V -band albedo of 13.1 +3.3 −2.4 (stat) +2.0 −1.4 (sys) percent and a diameter of 635 +57 −61 (stat) +32 −39 (sys) km, assuming a spherical body with uniform surface properties.

We report the discovery and dynamical analysis of 2015 BP 519 , an extreme Trans-Neptunian Object detected by the Dark Energy Survey at a heliocentric distance of 55 AU, perihelion of ∼36 AU, and absolute magnitude H r = 4.3. The current orbit, determined from an 1110-day observational arc, has semi-major axis a ≈ 450 AU, eccentricity e ≈ 0.92, and inclination i ≈ 54 degrees. With these orbital elements, 2015 BP 519 is the most extreme TNO discovered to date, as quantified by the reduced Kozai action, η 0 = (1 − e 2 ) 1/2 cos i, which is a conserved quantity at fixed semi-major axis a for axisymmetric perturbations. We discuss the orbital stability and evolution of this object, and find that under the influence of the four known giant planets 2015 BP 519 displays rich dynamical behavior, including rapid diffusion in semi-major axis and more constrained variations in eccentricity and inclination. We also consider the long term orbital stability and evolutionary behavior within the context of the Planet Nine hypothesis, and find that 2015 BP 519 adds to the circumstantial evidence for the existence of this proposed new member of the solar system, as it would represent the first member of the population of high-i, -shepherded TNOs.