Combined JWST and HST Deep Imaging to Characterize the Smallest Known Trans-Neptunian Objects

Abstract

We present optical-near-infrared (NIR) color measurements for small ($\leq 40$ km) Trans-Neptunian Objects (TNOs) using coordinated and nearly simultaneous observations from the James Webb Space Telescope (JWST) and the Hubble Space Telescope (HST). JWST/NIRCam data provided detections and NIR photometry for faint TNOs, while HST/ACS and WFC3 imaging enabled recovery in the optical, together yielding optical-NIR colors spanning 0.35-3.2 $\mu$m. Thirteen JWST-detected TNOs were recovered in the HST observations, and trailed PSF photometry was used to derive mean magnitudes, colors, and rotational lightcurves. The color distribution of our small cold classical TNO discoveries is narrow and consistent with the occupation of a single reflectance (color) sequence previously identified for larger cold classical TNOs. We find no evidence for a change in this sequence at smaller sizes. In contrast, the dynamically excited TNOs we discovered exhibit a broader range of colors consistent with multiple compositional classes seen at larger sizes. Lightcurve amplitudes were generally low for both dynamical groups in our sample. 2015 GK56, a previously known TNO in our field, displays a large-amplitude, structured lightcurve consistent with a contact binary. Our results indicate that the characteristic color distribution of TNOs extends to smaller sizes than previously studied, suggesting a primordial origin rather than size-dependent collisional processing.