Kapteyn b

Kapteyn b is a proposed exoplanet candidate believed to orbit Kapteyn’s Star, located approximately 13 light-years from Earth in the direction of the constellation Pictor and recognized as one of the oldest known stars. However, its existence has been largely refuted in recent years.

When its discovery was announced in 2014, Kapteyn b was described as a potential super-Earth residing within the habitable zone of its host star and estimated to be about 11 billion years old. This would have made it one of the oldest known habitable planet candidates ever discovered, drawing significant attention from the scientific community.

Subsequent studies, however, strongly argued that the signal interpreted as evidence for this planet was in fact an observational artifact caused by the rotational period and magnetic activity of Kapteyn’s Star. Based on these findings, Kapteyn b’s status is now widely regarded in the scientific literature as “controversial” or “unconfirmed.”

^【1】 

Discovery Claim and Significance

The possible existence of Kapteyn b was announced in 2014 by an international team of astronomers led by Guillem Anglada-Escudé. The discovery was made using the radial velocity method. The research team combined data from the HARPS spectrograph at the La Silla Observatory in Chile, the HIRES instrument at the Keck Observatory in Hawaii, and the Planet Finder Spectrograph (PFS) at the Las Campanas Observatory in Chile to conduct a comprehensive analysis.

These analyses revealed two distinct periodic signals in the radial velocity data of Kapteyn’s Star. The shorter-period and stronger of these signals was attributed to a planet candidate named Kapteyn b. The proposed physical and orbital characteristics are as follows:

• Minimum mass: Approximately five times that of Earth
• Orbital period: About 48 days

This orbital period places Kapteyn b near the inner edge of its host star’s habitable zone. A planet located within the habitable zone suggests the potential for liquid water on its surface under suitable atmospheric conditions.

Moreover, considering Kapteyn’s Star’s estimated age of approximately 11 billion years, if Kapteyn b truly existed, it would have been the oldest known potentially habitable planet discovered to date. This would have offered important implications for understanding how early in the universe’s history life might have emerged.

Debates and Refutation of Its Existence

Immediately following the announcement of Kapteyn b’s discovery, scientific debate arose over the interpretation of the data. In 2015, a study by Paul Robertson and colleagues raised serious doubts about the planet’s existence. This study reevaluated the origin of the 48-day radial velocity signal attributed to Kapteyn b.

Robertson’s team compared the radial velocity data with photometric measurements of the star’s brightness variations. Their findings revealed that Kapteyn’s Star has a rotational period of approximately 143 days. However, active regions on the star’s surface—such as starspots—produce complex and periodic signals synchronized with this rotation. The 48-day signal was determined to be a harmonic component of the 143-day rotation, specifically close to one-third of the rotational period (143 / 3 ≈ 47.7 days). Consequently, the signal was concluded to be an artifact of stellar rotation and magnetic activity, not evidence of a planetary companion.

This study highlighted the challenge of distinguishing stellar-induced signals from genuine planetary signals, particularly in the search for exoplanets around active stars, and emphasized how critical and difficult such distinctions can be.

Today, the scientific consensus holds that Kapteyn b is very likely not a real planet and that the observed signal is an artifact of stellar activity. This case is now regarded as an instructive example of the importance of cautious interpretation and multi-faceted data analysis in exoplanet research.

Host Star: Kapteyn’s Star

The general properties of Kapteyn’s Star, around which Kapteyn b is claimed to orbit, are as follows:

• Type and Distance: Kapteyn’s Star is a red subdwarf of spectral type M1. It lies only 12.8 light-years from Earth, making it one of the closest stars to the Sun.

• Proper Motion: It possesses one of the highest proper motions in the sky, meaning it moves across the celestial sphere more rapidly than most other stars.

• Origin and Age: Kapteyn’s Star belongs to an ancient stellar population in the halo of the Milky Way. Its age is estimated at approximately 11 billion years. It is thought to have originated in a dwarf galaxy similar to Omega Centauri, which was consumed by the Milky Way billions of years ago. Due to this origin, the star orbits the galactic center in a retrograde direction—a key feature distinguishing it dynamically from typical Milky Way stars.

Another Candidate in the System: Kapteyn c

In 2014, a second planetary candidate, Kapteyn c, was also proposed to exist in the system. This candidate is estimated to have a minimum mass of about nine Earth masses and an orbital period of 121 days around Kapteyn’s Star. Based on these parameters, it would reside well beyond the outer edge of the habitable zone and be extremely cold.

However, since the signal attributed to Kapteyn b has been explained by stellar activity, similar doubts have been cast on the existence of Kapteyn c. As a result, Kapteyn c’s status remains uncertain and it is not recognized as a confirmed planet. This further illustrates the challenges involved in detecting low-mass planets around active stars.