Measuring the Shapes of Kuiper Belt Objects with New Horizons Photometry

NASA’s New Horizons spacecraft provided the first look at the full shape of a primordial Kuiper Belt Object (KBO) when it encountered the Cold Classical KBO (486958) Arrokoth on January 1, 2019. Arrokoth proved to be a contact binary, composed of two distinct lobes, with the larger lobe roughly twice the volume of the smaller. Examination of Arrokoth’s surface showed no tectonic evidence that lobes came together violently, implying that the present shapes of the lobes may be representative of the shapes of KBOs that formed directly from the protoplanetary disk. In particular, the shapes of the lobes of Arrokoth appear to be consistent with rapid formation from the disk through gravitational collapse triggered by the Streaming Instability (SI). SI also reliably creates large numbers of separated binaries that could evolve into contact binaries. The shapes of KBOs are thus important constraints on not only their own history, but also on the planet formation process in the Solar System as a whole.

 

Historically, the only practical way to estimate the shapes of small Solar System objects from Earth for objects beyond the reach of radar was through measuring photometry at various Sun-Target-Observer (STO) phase angles. However, this is difficult for KBOs, as their slow orbits (hundreds to thousands of years) and great distances (>35 AU) means that their apparent geometry as seen from Earth changes very slowly, even over many years of observation. Lightcurve studies of bright KBOs have been able to detect that some show evidence of being contact binaries. However, these studies are fundamentally limited by the bias of past surveys to detect KBOs with consistent brightness, which preferentially excludes KBOs that show high amplitude lightcurves. Since cold classical KBOs (like Arrokoth) are preferentially smaller and fainter than resonant KBOs, this bias is even stronger for Cold Classicals (CCs). A different method is thus required to properly understand how common or rare Arrokoth’s shape is among KBOs with a similar history.

 

Immediately after the Pluto flyby, New Horizons began observing distant KBOs with observations of (15810) Arawn at much higher STO angles than is possible from Earth. This was followed by high STO angle observations of >30 KBOs. Seven additional KBOs to the two identified separated binaries were observed by New Horizons at more than two STO angles, and with enough time coverage to determine their rotational periods. Here we use these unique observations in order to estimate the shapes of those KBOs, and therefore test how common Arrokoth-style contact binaries may be in the Kuiper Belt.

 

Initial results indicate 4/7 (57%) KBOs are best fit with a contact binary shape, including 3/5 (60%) Cold Classicals. Including Arrokoth in these statistics brings the fraction to 5/8 (63%) overall and 4/6 (67%) of CC KBOs that appear to be contact binaries, when discounting the two tight separated binaries (2011 JX31 and 2014 OS393). Previous studies estimated that 10-25% of CC KBOs were contact binaries based on ground-based lightcurves (in comparison to ~40% for 3:2 KBOs), but those results may be influenced by the fact that CCKBOs are on average fainter than known 3:2 KBOs, leading to a detection bias for CCKBOs with flat lightcurves.

 

While these are clearly small numbers to draw general conclusions from, it should also be clear that a very large fraction of CC KBOs are likely contact binaries. The upcoming Vera C. Rubin Observatory (Rubin) will be able to detect very large numbers of KBOs, and because of the nature of the survey, it will be substantially less biased towards KBOs with flat lightcurves, particularly after several years of observations. We thus predict that the long-term photometric statistics for CC KBOs from Rubin will be more similar to our results (~66% contact binaries) than past studies (~10-25% contact binaries). Rubin may therefore be able to both test SI as the formation method for the Kuiper Belt and place a constraint on the effectiveness of its creation of Arrokoth-like contact binaries.