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Exoplanets thought to orbit Kapteyn’s star may not exist

The hypothetical super-Earth Kapteyn-b compared to Earth. The hypothetical super-Earth Kapteyn-b compared to Earth. Image credit: The Planetary Habitability Laboratory/University of Puerto Rico at Arecibo

Two “super Earth” exoplanets discovered orbiting a red dwarf known as Kapteyn’s Star last year may not exist, according to a paper just published in Astrophysical Journal Letters.

An M-class red dwarf, Kapteyn’s Star is located 13 light years away. At the 224th meeting of the American Astronomical Society in 2014, scientists announced the discovery of two planets orbiting it, one with five Earth masses and an orbit of 48 days and the other with seven Earth masses and an orbit of 122 days.

The planets were designated Kapteyn b and Kapteyn c respectively.

Their discovery was surprising because Kapteyn’s Star is a halo star and could be more than 11 billion years old.

Kapteyn b orbits in the star’s habitable zone, the region where surface temperatures make it possible for liquid water to exist.

Both planets were found with the radial velocity method, in which observers study a star’s spectra for signs of a planet tugging on the star.

The observation was made using the European Southern Observatory’s (ESO) High Accuracy Radial velocity Planetary research (HARP) instrument, along with assistance from the Keck and Las Campanas Observatories.

However, within the last year, Paul Robertson at Pennsylvania State University began to doubt the planets’ existence, positing that changes in Kapteyn’s Star’s stellar spectra are caused by star spots similar to sunspots.

Caused by stellar activity, star spots can mimic the spectral signs caused by an orbiting planet.

Kapteyn’s Star rotates once every 143 days. Robertson points out that the supposed orbital period of Kapteyn b, 48 days, comes close to an integer fraction (143/48= 2.979) of the star’s rotation period.

“Based on our analysis of the star’s magnetic activity, we determined the star has a rotation period that is three times that of the orbital period for ‘planet b,” Robertson said.

“Furthermore, the measurements of the star’s magnetic activity are correlated with the predicted Doppler shifts caused by planet b.  In such cases, the simplest explanation for the observations is that the Doppler periodicity is caused by the star’s activity, rather than a planet whose signal coincidentally matches the star’s activity.”

Robertson’s findings are disputed in another journal article published June 30, titled “No Evidence for Activity Correlations in the Radial Velocities of Kapteyn’s Star.”

He admits more data is required to determine whether the measured spectral changes are caused by the star itself or by orbiting planets.

Laurel Kornfeld

Laurel Kornfeld

Staff Writer
Laurel Kornfeld is a freelance writer and amateur astronomer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science in astronomy from Swinburne University’s Astronomy Online program.
About Laurel Kornfeld (1100 Articles)
Laurel Kornfeld is a freelance writer and amateur astronomer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science in astronomy from Swinburne University’s Astronomy Online program.