Alpha Centauri Ab

Not to be confused with Alpha Centauri AB, Alpha Centauri Bb, or Alpha Centauri Cb.

Alpha Centauri Ab (also known as Rigil Kentaurus b, or originally as Candidate 1) is a candidate exoplanet directly imaged around Alpha Centauri A 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 (see list of brightest stars). The planet is expected to be a gas giant based on physical properties.^[2] Additional observations are needed to confirm its true nature.

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	2021
Detection method	Direct imaging
Designations
Alternative names	Alpha Centauri Ab, Rigil Kentaurus b, S1+C1
Orbital characteristics[2]
Semi-major axis	1.64–2.15 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

Discovery

The discovery image of Candidate 1, taken by the NEAR instrument at ESO's Very Large Telescope in 2021

Astronomers from the Breakthrough Watch Initiative directly imaged the habitable zone candidate using a newly developed system for mid-infrared exoplanet imaging.^[3] Previous observations from years before ruled out the possibility of it being a background star. The team presented the discovery of the exoplanet candidate in a publication in Nature Communications titled "Imaging low-mass planets within the habitable zone of Alpha Centauri."^[4] However, the observation arc, being only 100 hours long, is not enough to determine whether a signal is planetary in nature, and it may be zodiacal dust or an instrumental artifact.

James Webb Space Telescope observations

Artist's impression of Alpha Centauri A and its gas planet with Toliman (Alpha Centauri B) (right) on background

A point-like source at a separation of 2 astronomical units was detected by the James Webb Space Telescope in August 2024. 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]

Physical characteristics

Candidate 1

While little is known about the candidate planet, there are some characteristics that may 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]

New finding

Possible orbits of Alpha Centauri Ab based on 2021 and 2025 observations

A 2025 study using observations from the James Webb Telescope derived a mass between 90 and 150 M_🜨 and a radius of 1.0–1.1 R_J. Combining non-detections and observations of a candidate in 2019 by VLT/NEAR and JWST in 2024, the team estimated an orbital period between 2 and 3 years, an orbital eccentricity of 0.4 and an inclination relative to the Alpha Centauri AB orbital plane of approximately 50 to 130°.^[2]

See also

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