Kapetyn b – Another One Bites the Interstellar Dust

A new analysis of Kapetyn’s Star by Paul Robertson at Penn State University suggests that Kapetyn b, the innermost and most Earth-like of two planets detected in 2013, is not a planet but rather an artefact of sunspots on the star’s surface.

Kapetyn’s star compared with Earth. Credit: Habitable Exoplanet Catalogue
The two planets were detected by Anglada-Escude using the radial velocity technique. This involves tracing the spectrum of the star, the light from which is imprinted with a barcode of absorption lines, to detect minute changes in the velocity of the star. The team used this to spot the to-and-fro (Doppler) motion of the star due to gravitational pull of two unseen planets.

This also allowed Anglada-Escude to place the innermost planet in the Habitable or “Goldilocks Zone”, the region around the star where temperatures might be just right for liquid water to exist on the planet’s surface.

Activity Tracer showing Rotation Signal & alias
Activity Tracer showing Rotation Signal & 48-day alias

But planets are not the only thing that can influence a star’s spectra – Robertson’s reanalysis of the spectra found tracers for starspot activity which varied on a 143 day period. This caused an artefact signal at 143/3 days, or 48 days: precisely the supposed orbital period of Kapetyn b.

This latest result is the third skirmish in a bitter war between the two teams with three habitable-zone planets detected by Anglada-Escude all now refuted by Robertson.

(paper: Stellar activity mimics a Habitable-Zone planet around Kapteyn’s Star )

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