Is Kapteyn B Not to Be?

The hypothetical super-Earth Kapteyn-b compared to Earth. Image credit:

Are the ancient planets discovered around Kapteyn’s Star for real?

As the saying goes, all that glitters isn’t gold, and the same could be said in the fast-paced hunt for exoplanets. In 2014, we reported on an exciting new discovery of two new exoplanets orbiting Kapteyn’s Star. The news came out of the American Astronomical Society’s 224th Meeting held in Boston Massachusetts, and immediately grabbed our attention. The current number of exoplanet discoveries as of July 2015 sits at 1,932 and counting.

An M-class red dwarf, Kapteyn’s Star is relatively nearby at only 13 light years distant. The planetary discovery consisted of a  world five times the mass of the Earth  in a 48 day orbit (Kapteyn b), and a world seven times the mass of the Earth  in a 122 day orbit (Kapteyn c). The discovery was hailed as an example of an ancient—possibly over 11 billion years old—system with its innermost world cast as a ‘super-Earth’ in the habitable zone…

But is Kapteyn-b not to be?

An interesting paper came up in the Astrophysical Journal Letters recently that suggests the exoplanets discovered orbiting Kapteyn’s Star in 2014 may in fact be spurious detections.

Image credit: Jcpag2012 under a Wikimedia Commons 4.0 International license
Kapteyn’s Star versus the Sun, Jupiter and the Earth. Image credit: Jcpag2012 under a Wikimedia Commons 4.0 International license

The idea of a planetary system around Kapteyn’s Star, real or not, is an interesting tale of exoplanet science.  The original discovery was made using the High Accuracy Radial velocity Planetary research (HARP) instrument at the European Southern Observatory, with supporting observations from the Las Campanas and Keck Observatory. You’d think that would make the discoveries pretty air-tight. The planets discovered orbiting Kapteyn’s Star were discerned using the radial velocity method, looking at the spectra of the star for the characteristic tugging of an unseen companion.

Recent research led by Paul Robertson of Pennsylvania State University suggests that the signal for the discovery of Kapteyn B may in fact be the result of stellar activity. Starspots—think sunspots on our own host star—can mimic the spectral signal of an unseen planet. Analyzing the HARPS data, we know that Kapteyn’s Star rotates once every 143 days. Kapteyn-b’s orbit of 48 days is very close to an integer fraction (143/48= 2.979) making it extremely suspicious.

Universe Today recently caught up with Paul Robertson, who had this to say about exoplanets around Kapteyn’s Star:

Q-How does this put the existence of a planet around Kapteyn’s Star in jeopardy?

“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.’  Theoretical simulations have predicted—and subsequent observations have proven—that a star can create Doppler signals at integer fractions of its rotation period (that is, one half, one third, etc).  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.”

Q-Is it possible to discern the starspot cycle that we’re seeing on Kapteyn’s Star?

“We infer the presence of active magnetic regions—possibly starspots—on the stellar surface through the variability of certain magnetically-sensitive absorption lines in the star’s spectrum. Previous observations suggest that the star’s brightness is relatively constant, so any starspots must be fairly small or not especially dark. It is possible that a space-based photometer such as K2 or TESS might see starspots.”

Q-Are future observations planned?

“Honestly, I don’t know.  My paper used data from previous observing programs that are now available in public archives.  I certainly think additional data would be quite valuable for Kapteyn’s Star.  Given that Kapteyn’s Star is somewhat special, being the closest halo star and one of the oldest nearby stars, I suspect someone will take more observations.”

This discovery is significant either way. An ancient super-Earth orbiting in the habitable zone of a nearby star has had lots of time to get the engine of evolution underway, more than twice the span of the history of life on Earth. But if Kapteyn-b is merely a transitory flicker in the data, it also serves as a good case study in perils of exoplanet hunting as well.

There’s still a good deal of controversy, however, surrounding the existence of planets orbiting Kapteyn’s Star. One very recent paper released just last week on June 30th titled No Evidence for Activity Correlations in the Radial Velocities of Kapteyn’s Star is safely in the ‘pro- Kapteyn-b’ camp.

Discovered due to its high (8 arc seconds per year) proper motion by Dutch astronomer Jacobus Kapteyn in 1898, Kapteyn’s Star is the closest known halo star to our solar system. It’s thought that Kapteyn’s Star might be associated with the large globular cluster Omega Centauri, which itself is thought to be the remnant of a dwarf galaxy gobbled up by our own Milky Way in the distant past.

The location of Kapteyn's Star in the constellation Pictor. Image credit: Starry Night Education software
The location of Kapteyn’s Star in the constellation Pictor. Image credit: Starry Night Education software

Kapteyn-b also made our list of red dwarf stars visible in backyard telescopes.

And Kapteyn-b wouldn’t be the first exoplanet detection that turned out to be spurious, as the existence of the exoplanet Alpha Centuari Bb announced in 2012 has been called into question as well.

It’s a brave new world on exoplanet science out there for sure, and for now, the worlds of Kapteyn’s Star will remain a mystery.

Discovered: Two New Planets for Kapteyn’s Star

An artist's conception of the planets orbiting Kapteyn's Star (inset) and the stream of stars associated with an ancient galaxy merger. Credit: image courtesy of Victor Robles, James Bullock, and Miguel Rocha at University of California Irvine and Joel Primack at University of California Santa Cruz.

The exoplanet discoveries have been coming fast and furious this week, as astronomers announced a new set of curious worlds this past Monday at the ongoing American Astronomical Society’s 224th Meeting being held in Boston, Massachusetts.

Now, chalk up two more worlds for a famous red dwarf star in our own galactic neck of the woods. An international team of astronomers including five researchers from the Carnegie Institution announced the discovery this week of two exoplanets orbiting Kapteyn’s Star, about 13 light years distant. The discovery was made utilizing data from the HIRES spectrometer at the Keck Observatory in Hawaii, as well as the Planet Finding Spectrometer at the Magellan/Las Campanas Observatory and the European Southern Observatory’s La Silla facility, both located in Chile.

The Carnegie Institution astronomers involved in the discovery were Pamela Arriagada, Ian Thompson, Jeff Crane, Steve Shectman, and Paul Butler. The planets were discerned using radial velocity measurements, a planet-hunting technique which looks for tiny periodic changes in the motion of a star caused by the gravitational tugging of an unseen companion.

“That we can make such precise measurements of such subtle effects is a real technological marvel,” said Jeff Crane of the Carnegie Observatories.

Kapteyn’s Star (pronounced Kapt-I-ne’s Star) was discovered by Dutch astronomer Jacobus Kapteyn during a photographic survey of the southern hemisphere sky in 1898. At the time, it had the highest proper motion of any star known at over 8” arc seconds a year — Kapteyn’s Star moves the diameter of a Full Moon across the sky every 225 years — and held this distinction until the discovery of Barnard’s Star in 1916. About a third the mass of our Sun, Kapteyn’s Star is an M-type red dwarf and is the closest halo star to our own solar system. Such stars are thought to be remnants of an ancient elliptical galaxy that was shredded and subsequently absorbed by our own Milky Way galaxy early on in its history. Its high relative velocity and retrograde orbit identify Kapteyn’s Star as a member of a remnant moving group of stars, the core of which may have been the glorious Omega Centauri star cluster.

The worlds of Kapteyn’s Star are proving to be curious in their own right as well.

“We were surprised to find planets orbiting Kapteyn’s Star,” said lead author Dr. Guillem Anglada-Escude, a former Carnegie post-doc now with the Queen Mary University at London. “Previous data showed some irregular motion, so we were looking for very short period planets when the new signals showed up loud and clear.”

The location of Kapteyn's Star in teh constellation Pictor. Created using Stellarium.
The location of Kapteyn’s Star in the constellation Pictor. Created using Stellarium.

It’s curious that nearby stars such as Kapteyn’s, Teegarden’s and Barnard’s star, though the site of many early controversial claims of exoplanets pre-1990’s, have never joined the ranks of known worlds which currently sits at 1,794 and counting until the discoveries of Kapteyn B and C. Kapteyn’s star is the 25th closest to our own and is located in the southern constellation Pictor. And if the name sounds familiar, that’s because it made our recent list of red dwarf stars for backyard telescopes. Shining at magnitude +8.9, Kapteyn’s star is visible from latitude 40 degrees north southward.

Kapteyn B and C are both suspected to be rocky super-Earths, at a minimum mass of 4.5 and 7 times that of Earth respectively. Kapteyn B orbits its primary once every 48.6 days at 0.168 A.U.s distant (about 40% of Mercury’s distance from our Sun) and Kapteyn C orbits once every 122 days at 0.3 A.U.s distant.

This is really intriguing, as Kapteyn B sits in the habitable zone of its host star. Though cooler than our Sun, the habitable zone of a red dwarf sits much closer in than what we enjoy in our own solar system. And although such worlds may have to contend with world-sterilizing flares, recent studies suggest that atmospheric convection coupled with tidal locking may allow for liquid water to exist on such worlds inside the “snow line”.

And add to this the fact that Kapteyn’s Star is estimated to be 11.5 billion years old, compared with the age of the universe at 13.7 billion years and our own Sun at 4.6 billion years. Miserly red dwarfs measure their future life spans in the trillions of years, far older than the present age of the universe.

A comparison of habitable zones of Sol-like versus Red dwarf stars. Credit: Chewie/Ignacio Javier under a Wikimedia Commons 3.0 license).
A comparison of habitable zones of Sol-like versus red dwarf stars. Credit: Chewie/Ignacio Javier under a Wikimedia Commons 3.0 license).

“Finding a stable planetary system with a potentially habitable planet orbiting one of the very nearest stars in the sky is mind blowing,” said second author and Carnegie postdoctoral researcher Pamela Arriagada. “This is one more piece of evidence that nearly all stars have planets, and that potentially habitable planets in our galaxy are as common as grains of sand on the beach.”

Of course, radial velocity measurements only give you lower mass constraints, as we don’t know the inclination of the orbits of the planets with respect to our line of sight. Still, this exciting discovery could potentially rank as the oldest habitable super-Earth yet discovered, and would make a great follow-up target for the direct imaging efforts or the TESS space telescope set to launch in 2017.

“It does make you wonder what kind of life could have evolved on those planets over such a long time,” added Dr Anglada-Escude. And certainly, the worlds of Kapteyn’s Star have had a much longer span of time for evolution to have taken hold than Earth… an exciting prospect, indeed!

-Read author Alastair Reynolds’ short science fiction piece Sad Kapteyn accompanying this week’s announcement.