Discovery

C/2014 UN₂₇₁ was discovered by astronomers Pedro Bernardinelli and Gary Bernstein in an algorithm-assisted search for slowly-moving trans-Neptunian objects, in archival images from the Dark Energy Survey (DES) at Cerro Tololo Inter-American Observatory.^[13] It was detected at the 22nd apparent magnitude in 42 DES images spanning 10 October 2014 to 26 November 2018.^[3] The long observation arc by the DES images revealed that the object was on a near-parabolic trajectory inbound towards the Solar System, implying a cometary origin from the Oort cloud, despite the object's apparently asteroidal (point-like) appearance in the images.^[14][15] When first imaged by the DES, the object was located in the southern constellation Sculptor, inside the orbit of Neptune at a distance of 29.0 AU (4.3 billion km; 2.7 billion mi) from the Sun.^[16][2] The object's relatively high brightness from its distance indicated that its diameter must be on the order of 100 km (62 mi)—an exceptionally large size for an object of cometary origin.^[3]

The discovery was announced by the Minor Planet Center on 19 June 2021, and the object was given the minor planet provisional designation 2014 UN₂₇₁.^{[11][lower-alpha 8]} The object attracted significant attention from astronomers worldwide: astronomers made follow-up observations and found several precoveries within days of the announcement.^[1][14] The earliest precovery observations of 2014 UN₂₇₁ were obtained from Paranal Observatory's VISTA survey images taken on 15 November 2010, when the object was 34.1 AU (5.1 billion km; 3.2 billion mi) from the Sun.^[3]

Cometary activity

Cometary activity in 2014 UN₂₇₁ was first reported on 22 June 2021, by Tim Lister at Las Cumbres Observatory's telescope in Sutherland, South Africa and by Luca Buzzi at the SkyGems Remote Telescope in Namibia. The comet was found to be one magnitude brighter than predicted in their observations, with a slightly asymmetric coma up to 15 arcseconds in width.^[18][1] At that time, the comet's distance from the Sun was 20.2 AU (3.0 billion km; 1.9 billion mi).^[18] The detection of cometary activity was confirmed by the Minor Planet Center and the comet was formally named C/2014 UN₂₇₁ (Bernardinelli–Bernstein) on 24 June 2021.^{[1][lower-alpha 9]}

Analysis of archival images from NASA's Transiting Exoplanet Survey Satellite (TESS) show that C/2014 UN₂₇₁ had an extensive, diffuse coma at least 43 arcseconds wide in observations as early as September 2018, when it was 23.8 AU (3.6 billion km) from the Sun.^[3][20] Between the 2018 and 2020 TESS observation epochs, the comet's brightness had significantly increased by 1.5 magnitudes, likely as a result of continuous activity rather than a spontaneous outburst.^[3][21]

Reexamination of other telescope datasets have also identified a diffuse and distinctly asymmetric coma in DES images beginning from 2017 (at 25.1 AU) and Pan-STARRS 1 images beginning from 2019 (at 22.6 AU). C/2014 UN₂₇₁'s coma brightness has been growing exponentially since 2017, while the comet's overall brightness had remained steady in 2014–2018, hinting that activity may have well begun prior to the comet's discovery at 29.0 AU.^[3][20] The observation of cometary activity from such large heliocentric distances is rare: only three other comets, Comet Hale–Bopp (27.2 AU outbound),^[22] C/2010 U3 (Boattini) (25.8 AU inbound), and C/2017 K2 (PanSTARRS) (24.0 AU inbound), have been observed to exhibit activity at heliocentric distances greater than 20 AU.^[3][21] As of 2022^[update], C/2014 UN₂₇₁ holds the record for the greatest distance at which a comet has been discovered in the Solar System.^[12]

C/2014 UN₂₇₁ was observed by the Hubble Space Telescope in January,^[23][9] March, July, August, and October 2022.^[24]

Occultations

Rigorous computations of C/2014 UN₂₇₁'s orbit and ephemeris have identified few potential occultation events by the comet from 2021–2025, during which the comet would pass in front of a bright star and briefly block out the star's light.^[28] Observing these occultation events would allow for opportunities to make precise measurements of the comet's size and position, as well as search for surrounding dust and possible satellites.^[29] The first attempt at observing one of those occultations was made from Australia and New Zealand on 19 September 2021, but was unsuccessful due to poor weather conditions.^[28]

Size and mass

Radio thermal emission measurements by the Atacama Large Millimeter Array (ALMA) in 2021 estimate a maximum diameter of 137 ± 17 km (85 ± 11 mi) for C/2014 UN₂₇₁'s nucleus, assuming negligible contamination of the nucleus's thermal emission by an unseen dust coma.^[10] The ALMA measurements have not ruled out the possibility of a dust coma contaminating up to 24% of the nucleus's thermal emission, so the actual diameter may well be smaller.^[9] Hubble Space Telescope observations confirmed C/2014 UN₂₇₁'s large size in 2022, placing a lower limit diameter of 119 ± 15 km (74 ± 9 mi) for the maximum possible dust coma contamination.^[9]

Even at its minimum estimated diameter, C/2014 UN₂₇₁ is the largest Oort cloud comet discovered, being more than 50 times larger than a typical comet which is less than 2 km (1.2 mi) in diameter. The previous largest known long-period comet was C/2002 VQ₉₄ (LINEAR) with a diameter of 96 km (60 mi),^[35] followed by Comet Hale–Bopp at 74 km (46 mi).^{[10][30][lower-alpha 10]} The only known comet larger than C/2014 UN₂₇₁ is the active centaur 95P/Chiron, which has a diameter of approximately 215 km (134 mi).^[10]

While the mass and density C/2014 UN₂₇₁ have not yet been measured,^[3] a rough estimate by NASA places its mass at 450 quadrillion kg (500 trillion short tons), about 100,000 times greater than that of a typical comet.^[37]

Albedo and color

Without its coma, the nucleus of C/2014 UN₂₇₁ has a visual (V-band) absolute magnitude of 8.63±0.11, which is calculated from its distance and apparent magnitude.^[9] Given the minimum estimated diameter (119 km) and absolute magnitude, the nucleus is calculated to have a very low visual geometric albedo up to 4.4%±1.2%, meaning that it reflects only 4.4%±1.2% of visible light—making its surface darker than coal.^[37] For the maximum estimated diameter (137 km), the minimum albedo of the nucleus would be 3.3%±0.9%.^[9] C/2014 UN₂₇₁'s low albedo is characteristic of small comet nuclei from both short- and long-period populations, suggesting a lack of correlation between albedo, nucleus size, and orbit type in Solar System comets.^[10] The low albedos of cometary nuclei are generally attributed to the deposition of carbon, organic compounds, and sulfides produced by cosmic rays dissociating molecules on the nucleus's surface.^[10][38]

Optical observations of C/2014 UN₂₇₁ during its inbound passage show that its nucleus appears more reflective at longer wavelengths, indicating a moderately red color similar to (albeit slightly less red than) most long-period comets.^[3][10] The albedo and color of C/2014 UN₂₇₁'s nucleus are expected to change over time due to cometary activity, especially after perihelion passage when temperatures decrease; its nucleus is massive enough to gravitationally recapture deposited icy ejecta back onto its surface, similar to what has been observed on Comet Hale–Bopp after its perihelion.^[10]

Rotation

The rotation period of 2014 UN₂₇₁'s nucleus is disputed, as some studies found no significant rotational periodicity in its light curve. Continuous observations by TESS in 2018 and 2020 did not detect any periodicity, placing an upper limit of 0.3 magnitudes for the nucleus's amplitude of variability.^[39][21] In 2021, Bernardinelli and collaborators from the DES analyzed various ground-based telescope datasets from 2018 and earlier, finding an apparent nucleus variability of 0.2 magnitudes, but no periodicity due to sparse data. Bernardinelli et al. do not rule out other factors such as small dust outbursts that may contribute to this apparent variability, leaving room for the possibility that the nucleus's true rotational variability may be even less than 0.2 magnitudes.^[3] In April 2022, astronomers Ignacio Ferrin and A. Ferrero reported a nucleus rotation period of 20.6±0.2 days, based on an analysis of 2014 UN₂₇₁'s long-term light curve behavior in observations from the Minor Planet Center's database.^[8] Ferrin and Ferrero found a nucleus light curve amplitude of 0.5±0.1 magnitudes, incongruous with findings by TESS and Bernardinelli et al.^[8]