C/2014 UN271 (Bernardinelli–Bernstein)
Largest known Oort cloud comet, discovered at record distance.
C/2014 UN271 (Bernardinelli–Bernstein) is a large Oort cloud comet discovered by astronomers Pedro Bernardinelli and Gary Bernstein in archival images from the Dark Energy Survey. When first imaged in October 2014, it was 29 AU from the Sun, almost as far as Neptune's orbit, making it the most distant comet ever discovered at that time. With a nucleus diameter of at least 100 km, it is the largest known Oort cloud comet.
Quick Facts
- Discoverer
- Pedro Bernardinelli · Gary Bernstein · (Dark Energy Survey)
- Discovery Date
- 20 October 2014 / (first discovery image)
- Designations
- 2014 UN · 271 · Comet Bernardinelli–Bernstein · "BB"
- Barycentric
- yes
- Observation Arc
- 14.84 yr (5,419 days)
- Earliest Precovery Date
- 15 November 2010
- Orbit
- Oort cloud
- Aphelion
- ≈ 42,000 AU (inbound) / ≈ 59,000 AU (outbound)
- Semimajor
- ≈ 21,000 AU (inbound) / ≈ 30,000 AU (outbound)
- Eccentricity
- 0.99947 (inbound) / 0.99963 (outbound)
- Period
- ≈ 3.0 million yr (inbound) / ≈ 5.1 million yr (outbound)
- Inclination
- 95.466° (inbound) / 95.460° (outbound)
Facts from the source article.
Lore & Background
C/2014 UN271 was first detected in 42 images from the Dark Energy Survey taken between October 2014 and November 2018, appearing as a point-like object at 22nd magnitude. Its long observation arc revealed a near-parabolic inbound trajectory from the Oort cloud. Precovery observations from the VISTA survey date back to November 2010, when the object was 34.1 AU from the Sun. Cometary activity was confirmed in June 2021 when the comet was found to be one magnitude brighter than predicted, with a coma up to 15 arcseconds wide. Archival TESS images from September 2018 showed an extensive diffuse coma at least 43 arcseconds wide, and the comet's brightness increased by 1.5 magnitudes between 2018 and 2020. An outburst on 9 September 2021 brightened the comet by 0.65 magnitudes, ejecting an estimated 10 to 100 million kg of dust. The comet's nucleus has a very low visual geometric albedo of 4.4%±1.2% (for the minimum diameter), making its surface darker than coal, and exhibits a moderately red color similar to most long-period comets.
Reader's Guide
C/2014 UN271 holds significance as the largest Oort cloud comet ever discovered, with a nucleus more than 50 times larger than a typical comet. Its discovery at 29 AU—the greatest distance at which a comet has been found—and its sustained cometary activity beyond 20 AU place it among a rare group of comets active at extreme heliocentric distances. The comet's size and activity challenge models of cometary behavior in the outer Solar System. Observations by ALMA and the Hubble Space Telescope have provided constraints on its diameter and albedo, though uncertainties remain due to possible dust coma contamination. The comet will reach perihelion at 10.9 AU in January 2031, outside Saturn's orbit, and will not be visible to the naked eye. Its legacy includes providing a benchmark for understanding the physical properties of Oort cloud objects and the onset of cometary activity at large distances.
Did You Know?
- C/2014 UN271 was discovered in archival images from the Dark Energy Survey, not in real-time observations.
- The comet's nucleus has a diameter of at least 100 km, making it the largest known Oort cloud comet.
- It exhibited cometary activity at 20.2 AU from the Sun, one of only six comets known to be active beyond 20 AU as of 2025.
- The comet's surface reflects only about 4.4% of visible light, making it darker than coal.
Frequently Asked Questions
Who is C/2014 UN271 (Bernardinelli–Bernstein)?
It is a massive Oort cloud comet identified by Pedro Bernardinelli and Gary Bernstein from archived Dark Energy Survey plates. The '2014' in its designation refers to when the original images were taken, even though the pair didn't publicly announce the find until June 2021.
What makes C/2014 UN271 (Bernardinelli–Bernstein) special?
It was spotted at roughly 29 AU, close to Neptune's orbital distance, which set a record for the most distant comet ever identified at the time. Its nucleus, estimated at well over 100 km across, also makes it the largest known member of the Oort cloud.
How big is C/2014 UN271 (Bernardinelli–Bernstein)?
ALMA observations in 2021 suggested a diameter of about 137 ± 17 km, while more conservative estimates place it at a minimum of roughly 100 km. Either figure makes it dramatically larger than most other catalogued Oort cloud comets.
When will C/2014 UN271 (Bernardinelli–Bernstein) reach perihelion?
Its closest approach to the Sun is scheduled for 22 January 2031 at a distance of about 10.9 AU. Because it never ventures inside the asteroid belt, it will not produce a bright, easily visible show for Earth-based skywatchers.
How was C/2014 UN271 (Bernardinelli–Bernstein) discovered?
Bernardinelli and Bernstein found it by mining old Dark Energy Survey images from October 2014 and noticing a faint, uncatalogued object far out in the solar system. They confirmed its identity and made the discovery public on 19 June 2021.
More in Named Comets, Part 4 1-24
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