Alpha Centauri Ab
Rigil Kentaurus b

The candidate planet imaged by JWST (third picture, circled with the label "S1")
Discovery[1]
Discovered by	K. Wagner, et al.
Discovery date	May–June 2019 (first imaging) February 2021 (reported)
Detection method	Direct imaging
Designations
Alternative names	Alpha Centauri Ab, Rigil Kentaurus b, S1+C1
Orbital characteristics[2]
Semi-major axis	1.64–2.23 AU
Eccentricity	~0.4
Orbital period (sidereal)	2 to 3 years
Inclination	~15 – 165°
Star	Alpha Centauri A (Rigil Kentaurus)
Physical characteristics[2]
Mean radius	1.0–1.1 RJ
Mass	90–150 M🜨
Temperature	225 K

Alpha Centauri Ab (also known asRigil Kentaurus b, or originally asCandidate 1) is a candidateexoplanet that was firstdirectly imaged aroundAlpha Centauri A in 2019 and reported in February 2021. If confirmed as an exoplanet, it would be the nearest, coldest, shortest-period and oldest directly imaged planet around a solar-type star,^[2] and Alpha Centauri would be the brightest planet-hosting star (seelist of brightest stars). The planet is expected to be agas giant based on physical properties.^[2] Additional observations are needed to confirm its existence.

Astronomers from theBreakthrough Watch Initiative directly imaged thehabitable zone candidate using theEuropean Southern Observatory'sVery Large Telescope in May and June 2019, using a newly developed system formid-infrared exoplanet imaging.^[3] The image suggested an apparent separation distance of approximately 1.1astronomical units (AU) from Alpha Centauri A, and previous observations from years before ruled out the possibility of it being a background star.^[1] The team presented the discovery of the exoplanet candidate in a publication inNature Communications titled "Imaging low-mass planets within the habitable zone of Alpha Centauri."^[4] However, theobservation arc, being only 100 hours long, was not enough to determine whether a signal was planetary in nature, and it was thought possible that it waszodiacal dust or an instrumental artifact.

In August 2024, theJames Webb Space Telescope'sMid-Infrared Instrument (MIRI) directly imaged a point-like source at a projected separation of2 AU from Alpha Centauri A. It is confirmed to be not a background or foreground source or a cloud of dust, and is unlikely to be an instrumental artifact. If it is an exoplanet, it should be the same candidate observed in 2021. The object was not recovered and will need more observations to be confirmed.^[5][2]

While little was known about the candidate planet, there were some characteristics that could be inferred based on its observations. It would have an orbital inclination of ~70° relative to Earth's point of view, consistent with the inclination of the Alpha Centauri system as a whole. Because of the detection algorithm, it would be somewhere around Neptune's mass, and would be no larger than 7 R_🜨 as its mass would exceed the radial-velocity threshold of ~50 M_🜨,^[6] but no smaller than 3.3 R_🜨 as that would not render the signature given in the paper. Due to this large size, it is highly unlikely to be rocky and is probably a Neptune-sized planet.^[1]

A 2025 study using observations from the James Webb Telescope derived a mass between 90 and150 M_🜨 and a radius of1.0–1.1 R_J. Combining non-detections and observations of the candidate in 2019 by VLT and JWST in 2024, the team estimated an orbital period between 2 and 3 years, anorbital eccentricity of 0.4 and an inclination relative to the Alpha Centauri AB orbital plane of approximately 50 to 130°.^[2]

• Lists of exoplanets
• List of directly imaged exoplanets
• List of largest exoplanets
• List of nearest exoplanets

• Alpha Centauri
• Exoplanet candidates

• Articles with short description
• Short description matches Wikidata