Kuiper Belt's Big, New, Far-Out Object

First spied last year, an object designated 2015 RR₂₄₅ turns out to be one of the largest and most distant objects yet found orbiting the Sun.

Planetary astronomers have been busy of late. The IAU's Minor Planet Center has tallied 1,491 objects in orbits more distant than Neptune ("transneptunians") and another 501 in the outer solar system that occupy odd, usually quite elliptical orbits ("Centaurs" and "scattered-disk objects"). On average, one more gets added to the list about every 5 days.

But a distant discovery announced two days ago has created heightened interest among Kuiper Belt cognoscenti. Designated 2015 RR₂₄₅, it's some 64 astronomical units (9½ billion km) from the Sun, more than twice Neptune's distance. And despite appearing just 22nd magnitude, it could be as large as 700 km (450 miles) across.

Observer JJ Kavelaars (National Research Council of Canada) first spotted it last September 9th using the Canada-France-Hawaii Telescope atop Mauna Kea. His observations were part of a larger search effort called the Outer Solar System Origins Survey (OSSOS), which since 2013 has paired CFHT's 3.6-m optics with a sensitive camera called MegaPrime that captures fields 1° square.

But it wasn't until Michele Bannister (University of Victoria) and others took follow-up images in February and again in June that the orbit could calculated with any certainty. It turns out that the mean solar distance (81.4 a.u.) and large orbital large eccentricity (0.59) of 2015 RR₂₄₅ carries it from Neptune's vicinity out to 129 a.u. (19.3 billion km). One trip around the Sun takes about 730 years. Dynamicists already suspect that the new find might be in an orbital resonance with Neptune.

Dwarf Planet or Not?

The large estimated size is just that — an estimate — based on its brightness and distance. For example, the 700-km diameter corresponds to having a surface that's 10% reflective, typical of Kuiper Belt objects. But if it's partly ice covered and, say, 25% reflective overall, then the corresponding diameter drops to about 450 km. In both of these examples, the same amount of sunlight gets reflected toward Earth. "It's either small and shiny, or large and dull," Bannister notes in a CFHT press release.

Size does matter, at least when considering whether 2015 RR₂₄₅ qualifies for "dwarf planet" status. We certainly know where CFHT management stands on this question: its announcement refers to the new discovery as a dwarf planet a half dozen times. But does it really qualify as such?

Technically, the International Astronomical Union — specifically its Committee on Small Body Nomenclature (CSBN) and Working Group for Planetary System Nomenclature (WGPSN) — unilaterally get to decide which objects belong to the Dwarf Planet Club. So far only five objects make the official cut: Ceres, Pluto, Eris, Makemake, and newcomer Haumea, which was approved almost a decade ago. "It has not happened for 2015 RR₂₄₅," says Lars Lindberg Christensen, the IAU's press officer.

However, by invoking the IAU's definition for a dwarf planet, quite a few more objects — perhaps dozens more — would qualify on the basis that they're likely massive enough to be round. Mike Brown (Caltech), who's found a healthy share of the largest KBOs (most notably Eris), calculates that a rocky object at least 900 km across, and an icy one of at least 400 km, probably meets the IAU's "roundness criterion."

Using that metric, Brown counts 10 objects (Ceres, Pluto, and 8 other KBOs) are "nearly certain" to be dwarf planets and another 27 are "highly likely". If 2015 RR₂₄₅ really is 700 km across, then it too is "highly likely" to qualify as a dwarf planet.

Regardless, the new discovery is yet another distant oddball — albeit a big one — that's giving planetary dynamicists lots of agita (as my Italian grandmother used to say) about how the Kuiper Belt formed and evolved.