Recent research casts doubt on whether nearby Kapteyn b, a supposed super-Earth circling in its star’s habitable zone, is a planet at all.
If the nearby exoplanet Kapteyn b exists, it would be the oldest-known planet to orbit its star in the coveted habitable zone, where water could flow on the planet’s surface. But this super-Earth could turn out to be nothing more than a trick of starlight.
Kapteyn b is one of two recently-detected exoplanet candidates around Kapteyn’s star. Astronomers found the planets as part of the High Accuracy Radial Velocity Planetary Search (HARPS) project in the southern constellation Pictor. Their parent star is a cool, red dwarf and the nearest halo star to Earth at only 13 light-years away.
Both exoplanets appear to be super-Earths, but it’s Kapteyn b that had scientists in a tizzy. This exoplanet candidate had it all: a comfortable location in its star’s habitable zone, five Earths’ worth of mass, and an estimated age twice that of Earth — more than enough time for life to have evolved. Astronomers infer its age from the fact that the star probably originated in a dwarf galaxy that collided with the Milky Way eons ago, making it and its planets much older than their Milky Way counterparts.
But research from Paul Robertson (Pennsylvania State University) and colleagues posits that this goldilocks planet is actually an artificial signal created by stellar activity.
Seeing Planetary Ghosts?
Astronomers discovered Kapteyn b using the radial velocity method. This method relies on the fact that stars are not stationary — they orbit their system’s center of mass, responding to the tiny gravitational tugs of their planets, and that movement leaves a signature in the star’s light that astronomers can measure.
A star’s spectral lines move in the blueward direction if the star moves toward us, redward if the star’s moving away. Regular blue- and redshifts can indicate an exoplanet. But astronomers must take care. “Spots on the stellar surface and other magnetic phenomena can easily mimic a planetary signal as the stellar rotation brings these features in and out of sight,” says Tiago Campante (University of Birmingham, UK).
Usually, noise from starspots or other surface inconsistencies is small compared to the larger tug of an orbiting planet. But as technologies advance, scientists are able to look for finer details and smaller planets, so starspots become more relevant. In the case of Kapteyn b, the authors present a series of strong arguments that refute its existence, says Campante.
When Robertson and his team analyzed the Kapteyn b HARPS data, they were able to deduce the star’s rotation and found that Kapteyn b’s orbit was “worryingly close,” to an integer fraction of this rotation. At 48 days, the supposed exoplanet’s orbit is about a third of the star’s rotational period — consistent with a starspot being sampled at irregular intervals. The authors conclude that the exoplanet is nothing but a trick of starlight.
Is Kapteyn b No More?
Others disagree with this case of stellar mimicry.
If the signal that researchers originally attributed to Kapteyn b came from a starspot, that splotch should rotate with the star, assuming it lasts long enough. Along our line of sight, it would disappear behind the star as the star rotates, creating a signal with a period half that of the star’s rotation or some other fraction, says Xavier Dumusque (Harvard-Smithsonian Center for Astrophysics).
The authors argue that any signal at half-stellar rotation, as opposed to a third, is not seen in the data because of uneven sampling — telescopes simply couldn’t observe the star continuously. “It's very likely that there are signals at the [stellar] rotation period and other harmonics,” says Robertson.
Nevertheless, “a correlation between both signals is not a proof refuting the origin of the [planetary] signal,” Dumusque says, adding that the authors couldn’t show that the stellar activity caused the debated exoplanet signal.
So Kapteyn b’s existence is still up for debate. The only way forward is to keep observing, adding radial velocity measurements in order to get a better picture of Kapteyn’s star’s activity. The good news is that there are other similarly old halo and Galactic disk stars, should Kapteyn b be disproven conclusively. Kepler-10 and Kepler-444 are such stars, and even though they don’t host habitable-zone exoplanets, it “does not preclude the existence of habitable worlds around similarly old stars,” says Campante.
Robertson et al. Stellar activity mimics a habitable-zone planet around Kapteyn’s Star, Astrophysical Journal Letters, published online 11 May 2015.
Read more about the exoplanet revolution from Sara Seager in our classic August 2013 issue.