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The recently announced Oort-cloud comet C/2014 UN271(Bernardinelli-Bernstein) is remarkable in at least three respects: (i) it wasdiscovered inbound as far as ~29 au from the Sun (with prediscovery images upto ~34 au); (ii) it showed cometary activity already at almost 24 au; and (iii)its nuclear magnitude (Hr ~ 8.0) indicates an exceptionally large object.Detection of gases is expected in the upcoming years as the comet is headingtowards a ~11 au perihelion in 2031. We used ALMA in extended configuration(resolution ~0.064") to measure the 1287 um (233 GHz) continuum flux of thecomet. Observations were performed on August 8, 2021 at a 20.0 au distance fromthe Sun. The high spatial resolution was chosen in order to filter out any dustcontribution. We detected the thermal emission of the object at ~10 sigma, witha flux of 0.128+/-0.012 mJy. Based on observational constraints and ourtheoretical estimates of the dust contribution, the entirety of the measuredflux can be attributed to the nucleus. From NEATM modelling combined with theHr magnitude, we determine a surface-equivalent diameter of 137+/-17 km and ared geometric albedo of 5.3+/-1.2 %. This confirms that 2014 UN271 is by farthe largest Oort-cloud object ever found (almost twice as large as comet C/1995O1 Hale-Bopp), and except for the Centaur 95P/Chiron which shows outburst-likeactivity, the largest known comet in the Solar System. On the other hand, theobject albedo is typical of comets, adding credence for a "universal" cometnucleus albedo. With its distant perihelion and uniquely large size, 2014 UN271is the prominent archetype of distant comets, whose activity is driven byhypervolatiles. Post-perihelion thermal measurements will permit to studypossible albedo changes, such as a surface brightening compared topre-perihelion, as was observed for Hale-Bopp.

Size and albedo of the largest detected Oort-cloud object: comet C/2014 UN 271 (Bernardinelli-Bernstein)