Investigating the Potentially Variable Surface of TNO 2015 RB281

The Kuiper Belt, located beyond Neptune, is home to remnants of early planet formation in the form of small planetesimals. Trans-Neptunian Objects (TNOs), which populate this region, are often non-spherical or part of binary systems, and typically rotate with lightcurve periods of ~6–15 hours (e.g. Trilling & Bernstein, 2006; Benecchi & Sheppard, 2013; Showalter et al., 2021). Spectral variability, observed in a very limited number of TNOs (e.g. Choi et al. 2003, Fraser et al. 2015), offers significant insights into their composition, history, and the broader processes that shaped the solar system. One such object, 2015 RB281, previously exhibited a notable optical colour variation between two epochs, as measured by the Colours of the Outer Solar System Origins Survey (Col-OSSOS, Schwamb et al. 2019, Fraser et al. 2023). This TNO is classified as a cold classical TNO, presumed undisturbed planetesimals which formed at roughly their current distances. Previously, this TNO showed >5σ variation in surface colour, as shown in Figure 1. Although the focus of this investigation was the large optical colour variation of 2015 RB281, within the 20 Col-OSSOS TNOs with repeat colour measurements we do see variation >3σ on a total of three TNOs, two varying in the optical and one in the NIR. This means that ~10% of the sample vary in optical wavelengths, and ~11% of the NIR repeat observations (note, only ~half the repeat colours TNOs got repeat observations at NIR wavelengths). Additionally, it is interesting to note that these TNOs vary in either optical colour or NIR colour, never both.

Figure 1: (g-r) versus (r-J) colors of TNOs observed by Col-OSSOS are shown in teal crosses for the BightIR class and orange crosses for the FaintIR class (where the FaintIR class is predominantly made up for cold classical TNOs). The two colors measured for 2015 RB281 are shown by the red diamonds, connected by a dashed black line. The yellow star shows solar colors. The red dashed line shows the solar reddening curve, along which TNOs have a single linear reflectance from the optical to NIR wavelengths. The blue dot-dashed line shows where the split in FaintIR/BrightIR surfaces is (Fraser et al., 2023). The vertical green dotted line shows the approximate split between red and neutral surfaces at (g-r) = 0.75. Note that although some other TNOs show some variation in colour, only 2015 RB15 shows such significant variation relative to its uncertainties. With the purple squares we show the additional TNOs with greater than 3σ variation.

Streaming instability, a key mechanism in planetesimal formation, suggests that planetesimals formed by this process should consist of a single, homogeneous composition. This also holds true for other prevailing theories of planetesimal formation, such as pebble collapse. As such, smaller TNOs like 2015 RB281 are expected to be made up of a single composition, unless influenced by external factors like collisions (which is predicted to be uncommon for small TNOs Batygin et al., 2011). While the observed colour variation in 2015 RB281 spans the entire optical range of the red/FaintIR compositional class, it does not shift between compositional classes. Similarly, the other two TNOs that show colour variation just shift within their colour class. 

To further investigate this variability, we conducted follow-up observations of 2015 RB281 in the g- and r-bands using the Gemini North telescope. We collected a sparse, multi-band lightcurve, with the goal of either identifying a contact binary lightcurve (where the sharp dips may have caused a false variation in the colour measurement) or confirming the optical colour variability, and investigating the associated lightcurve changes. Our follow-up observations, which spanned ~4 months and included 10 epochs each in a symmetrical rgrg sequence aiming for an SNR of ~20 per band per visit, confirm the previously observed colour variation, albeit at a slightly smaller magnitude. Reprocessing of the original Col-OSSOS photometry also reveals the same trend. Additionally, our analysis of the lightcurve indicates that the redder colour only appears in about 3 of the total ~12 observations, suggesting that the colour variation may be localised to a small area of the TNO’s surface such as the inferred red spot on Haumea’s surface (Lacerda 2009). We will present these findings and discuss their implications for theories of planetesimal formation, considering potential mechanisms that could explain the observed colour variability.