Alpha Centauri Ab

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 orbit at approximately 1.1 AU away from Alpha Centauri A with a period of about a year and would have a mass between that of Neptune and one-half that of Saturn and would therefore likely be a giant planet.^[1] The planet candidate is yet to be confirmed as an exoplanetary signal with additional observations needed to confirm its true nature.

Alpha Centauri Ab

The candidate planet imaged by JWST (third picture, circled with the label "S1")
Discovery[1][2][3]
Discovered by	K. Wagner, et al., Beichman & Sanghi et al., Sanghi & Beichman et al.
Discovery date	2021, 2024
Detection method	Direct imaging
Designations
Alternative names	Alpha Centauri Ab, Rigil Kentaurus b, S1+C1
Orbital characteristics
Semi-major axis	1.1 AU
Eccentricity	0.4[2]
Orbital period (sidereal)	2 to 3 years
Inclination	~15, ~165[2]
Star	Alpha Centauri A
Physical characteristics[2]
Mean radius	1.0–1.1 RJ
Mass	90–150 M🜨
Temperature	225 K

History

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.^[4] 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.”^[5] 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.

Physical characteristics

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. Follow-up observations will be needed to determine whether it is a planet, cloud of dust, or simply an artifact due to its short observation arc.

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, which is confirmed to be neither a background or foreground source and is unlikely to be an instrumental artifact. If it is an exoplanet, it is likely the candidate observed in 2021. The object was not recovered and will need more observations to be confirmed.^[3][2] Assuming the point source to be a planet, its mass and radius are estimated at 90–150 M_🜨 and 1.0–1.1 R_J. The observations and non-detections of this source suggest 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