View Full Version : Two planets at Kapteyn's Star

2014-Jun-04, 06:53 AM
Two planets reported discovered around Kapteyn's Star (12.8 ly away). arXiv link (http://arxiv-web3.library.cornell.edu/abs/1406.0818) press release (http://www.eurekalert.org/pub_releases/2014-06/ci-tpo060214.php)

This is one of the closest stars to have planets. Epsilon Eridani apparently has at least one, although it's not without controversy. And there was an unconfirmed planet around Alpha Cent B. Otherwise this is closest.

Also not expected, since Kapteyn's Star is very old and has a low metallicity.

2014-Jun-04, 12:12 PM
Cool, I almost selected that paper for the Fun Papers, but glad to see someone did.

2014-Jun-04, 06:21 PM
I was just about to post about this. I think it's the most exciting planetary discovery in years, even more than Alpha Centauri Bb, both for the system's age and remarkably low metallicity. A double bonus is the fact that this star may be from Omega Centauri, which I think will prompt more astronomers to look for more planets around metal-rich globular clusters (especially ones that may be stripped cores).

2014-Jun-05, 06:28 PM
Also the host start is likely between 10 - 12 billion years old, showing metal rich planets were capable of being hatched back then. Furthermore, these planets were likely formed outside of the Milky Way galaxy as Kapteyn was part of a smaller galaxy that since merged with the MW.

2014-Jun-06, 03:29 AM
Do we actually know they are metal rich?

2014-Jun-06, 04:17 AM
The star itself has a much lower metallicity than the Sun. The planets are probably metal-rich inasmuch as they are low-mass objects too small to be gas giants or stellar companions. That said, there's an outside chance that the planets are in fact brown dwarfs or stars *if* their orbits are very nearly pole-on to the Sun; however, that's statistically unlikely, and astrometry (when and if it's carried out) should be able to settle the issue once and for all.

2014-Jun-07, 06:21 AM
OK, that's what I thought. But note their orbits do not have to be nearly pole-on in order for them to be gas giants. In fact, the odds are fairly good the smaller one is at least as massive as Neptune. This page: (Habitable Planet Reality Check: Kapteyn b (http://www.drewexmachina.com/2014/06/06/habitable-planet-reality-check-kapteyn-b/)) calculates that Kapteyn b has a 50% chance of that.

Roger E. Moore
2017-Feb-15, 02:43 AM
Have begun to look at this planetary system as my poking around at Proxima b has begun to wind down for now. A lot was learned from doing Proxima b tables, such as how not to screw it up. I hope to use that knowledge to post some things here soon.

Extremely interesting system, tiny but cool.

Roger E. Moore
2017-Feb-15, 03:11 AM
Kapteyn's Star is also called GJ 191 (Gliese catalog), and searches for information on this system require the use of multiple terms.

Here are the major papers I can find so far on this system. Note the controversy over whether Kapteyn-b exists.

The composition of Kapteyn's star and the M subdwarfs.
Mould, J. R., 1976

Coronal properties of nearby old disk and halo dM stars.
Micela, G. et al, 1997
ROSAT X-ray (PSPC) and EUV (WFC) observations of a sample of 12 nearby Halo and Old-Disk low mass stars have been analyzed to determine their emission levels, properties of their coronal spectra and to characterize their temporal variations on time scales from hours to ten years. The light curves of the old-disk stars of our sample with the highest count statistics, show that variability of a factor two on a time scale of a few thousands of seconds is a common property of old-disk stars. The coronal emission of GJ 191, the only halo star with enough counts to make the time analysis feasible, shows significant variations on a time scale of six months. The time resolved spectral analysis of the most intense stars shows that their coronal emission is consistent with a two temperature model with (approximately) constant values of the temperatures along the entire observation, while the observed variability can be explained by changes in their emission measure distributions. We find that the emission measures of the two components change coherently during the observations. For all ten detected stars we have computed hardness ratio and X-ray luminosity. While our old-disk stars fit well in the body of the distribution outlined by a complete volume limited sample of dM and dK stars within 7pc (Schmitt et al. 1995), the halo stars tend to occupy the softest and weakest envelope defined by population I low-mass stars.

The HARPS-TERRA project I. Description of the algorithms, performance and new measurements on a few remarkable stars observed by HARPS
Guillem Anglada-Escudé et al., 2012
Doppler spectroscopy has uncovered or confirmed all the known planets orbiting nearby stars. Two main techniques are used to obtain precision Doppler measurements at optical wavelengths. The first approach is the gas cell method, which consists on the least-squares matching of the spectrum of Iodine imprinted on the spectrum of the star. The second method relies on the construction of a stabilized spectrograph externally calibrated in wavelength. The most precise stabilized spectrometer in operation is HARPS, operated by ESO in La Silla Observatory/Chile. The Doppler measurements obtained with HARPS are typically obtained using the Cross-Correlation Function technique (CCF). It consists of multiplying the stellar spectrum with a weighted binary mask and finding the minimum of such product as a function of the Doppler shift. It is known that CCF is suboptimal in exploiting the Doppler information in the stellar spectrum. Here, we describe an algorithm to obtain precision RV measurements using least-squares matching of each observed spectrum to a high signal-to-noise ratio template derived from the same observations. Such algorithm is implemented in our software called HARPS-TERRA (Template Enhanced Radial velocity Re-analysis Application). New radial velocity measurements on a representative sample of stars observed by HARPS is used to illustrate the benefits of the proposed method. We show that, compared to CCF, template matching provides a significant improvement in accuracy, specially when applied to M dwarfs.

Two planets around Kapteyn's star: a cold and a temperate super-Earth orbiting the nearest halo red-dwarf
Guillem Anglada-Escudé et al., 2014
Exoplanets of a few Earth masses can be now detected around nearby low-mass stars using Doppler spectroscopy. In this paper, we investigate the radial velocity variations of Kapteyn's star, which is both a sub-dwarf M-star and the nearest halo object to the Sun. The observations comprise archival and new HARPS, HIRES and PFS Doppler measurements. Two Doppler signals are detected at periods of 48 and 120 days using likelihood periodograms and a Bayesian analysis of the data. Using the same techniques, the activity indices and archival ASAS-3 photometry show evidence for low-level activity periodicities of the order of several hundred days. However, there are no significant correlations with the radial velocity variations on the same time-scales. The inclusion of planetary Keplerian signals in the model results in levels of correlated and excess white noise that are remarkably low compared to younger G, K and M dwarfs. We conclude that Kapteyn's star is most probably orbited by two super-Earth mass planets, one of which is orbiting in its circumstellar habitable zone, becoming the oldest potentially habitable planet known to date. The presence and long-term survival of a planetary system seems a remarkable feat given the peculiar origin and kinematic history of Kapteyn's star. The detection of super-Earth mass planets around halo stars provides important insights into planet-formation processes in the early days of the Milky Way.

X-ray and Hubble/COS UV Measures of Kapteyn's Star: A Crucial Proxy of X-UV Irradiances for Old Red Dwarf Stars that May Host Habitable Zone Planets
Durbin, Allyn J. et al, 2014
Red dwarfs (dM) stars make up over 80% of the local stellar population and a significant fraction of them are old (age > 4 Gyr). Because of the high frequency of red dwarfs and their longevity, there is a greater possibility of more advanced life in red dwarf planet systems. MEarths, UVES, SDSS-III, and the upcoming TESS mission are some surveys that are targeting these objects. As part of Villanova’s Living with a Red Dwarf program, we have obtained HST/COS spectra and Chandra X-ray observations of Kapteyn's star (M1V, V = 8.853, d = 12.76 +/- 0.05 ly, P_rot = 195 days). This star is crucial to the study of old red dwarfs as it is the nearest halo star with a radial velocity of +245.2 km/s and an estimated age of 10-12 Gyr. In our program, Kapteyn's star is the oldest red dwarf and as such serves as an anchor for our age, rotation, and activity relations. The spectra obtained from HST/COS provide one of the cleanest measurements of Lyman-alpha emission for red dwarfs. This is due to Doppler shift from the high radial velocity, separating the Lyman-alpha line from emission produced by the ISM and geocoronal sources. These observations further provide calibration at the old age/low rotation/low activity extremes for our relations. They also provide insights into the magnetic properties as investigating coronal x-ray and UV emission in very old, slowly rotating dM stars. Kapteyn’s star also serves as a proxy for metal-poor old disk/Pop II M dwarfs by providing information about X-UV emissions. This information is crucial for determining X-UV irradiances of possible habitable zone planets hosted by old red dwarfs.

Stellar Activity Mimics a Habitable-zone Planet around Kapteyn's Star
Paul Robertson et al., 2015
Kapteyn’s star is an old M subdwarf believed to be a member of the Galactic halo population of stars. A recent study has claimed the existence of two super-Earth planets around the star based on radial velocity (RV) observations. The innermost of these candidate planets—Kapteyn b (P = 48 days)—resides within the circumstellar habitable zone (HZ). Given recent progress in understanding the impact of stellar activity in detecting planetary signals, we have analyzed the observed HARPS data for signatures of stellar activity. We find that while Kapteyn’s star is photometrically very stable, a suite of spectral activity indices reveal a large-amplitude rotation signal, and we determine the stellar rotation period to be 143 days. The spectral activity tracers are strongly correlated with the purported RV signal of “planet b,” and the 48-day period is an integer fraction (1/3) of the stellar rotation period. We conclude that Kapteyn b is not a planet in the HZ, but an artifact of stellar activity.

No Evidence for Activity Correlations in the Radial Velocities of Kapteyn’s Star
Anglada-Escudé, G. et al., 2015
Stellar activity may induce Doppler variability at the level of a few m s-1 which can then be confused by the Doppler signal of an exoplanet orbiting the star. To first order, linear correlations between radial velocity measurements and activity indices have been proposed to account for any such correlation. The likely presence of two super-Earths orbiting Kapteyn’s star was reported in Anglada-Escudé et al., but this claim was recently challenged by Robertson et al., who argued for evidence of a rotation period (143 days) at three times the orbital period of one of the proposed planets (Kapteyn’s b, P = 48.6 days) and the existence of strong linear correlations between its Doppler signal and activity data. By re-analyzing the data using global statistics and model comparison, we show that such a claim is incorrect given that (1) the choice of a rotation period at 143 days is unjustified, and (2) the presence of linear correlations is not supported by the data. We conclude that the radial velocity signals of Kapteyn’s star remain more simply explained by the presence of two super-Earth candidates orbiting it. We note that analysis of time series of activity indices must be executed with the same care as Doppler time series. We also advocate for the use of global optimization procedures and objective arguments, instead of claims based on residual analyses which are prone to biases and incorrect interpretations.

Roger E. Moore
2017-Feb-15, 03:11 AM
And a few more.

Living with an Old Red Dwarf: X-ray-UV Emissions of Kapteyn’s Star - Effects of X-UV radiation on Habitable Zone Planets hosted by old Red Dwarf Stars
Guinan, Edward F. et al., 2015
Red dwarfs (dM) stars make up over 75% of the local stellar population and a significant fraction (~40-50%) are older than the Sun. Because of the high frequency of red dwarfs and their longevity (> 50 Gyr), there is a greater possibility of more advanced life in red dwarf-exoplanet systems. MEarths, UVES, SDSS-III, and the upcoming TESS mission are some surveys that are targeting red dwarfs in the search for hosted potentially habitalble planets. As part of Villanova's 'Living with a Red Dwarf' program, we have obtained HST-COS Ultraviolet spectra (1150-3000A) and Chandra X-ray observations of Kapteyn's star (GJ 191; M1 V, V = 8.85 mag , d = 12.76 +/- 0.05 ly). Kapteyn's Star is important for the study of old red dwarfs because it is the nearest (Pop II) halo star with a radial velocity of +245.2 km/s and an estimated age of 11.2 +/-0.9 Gyrs. Recently Kapteyn's Star was found to host two super-Earth mass planets - one of these is orbiting inside the star's Habitable Zone (Anglada-Escude' 2014: MNRAS 443, L89). In our program, Kapteyn's star is the oldest red dwarf and as such serves as an anchor for our age, rotation, and activity relations. The spectra obtained from HST/COS provide one of the cleanest measurements of the important HI Lyman-alpha 1215.6 A emission flux for red dwarfs. This is due to the large Doppler shift from the high radial velocity, separating the stellar Ly-alpha emission from by the Ly-alpha ISM and local geo-coronal sources. These observations further provide calibrations at the old age/low rotation/low activity extremes for our relations. As the nearest and brightest old red dwarf star, Kapteyn's Star also provides insights into its magnetic properties to investigate coronal x-ray and UV emission for the large population of old, slowly rotating red dwarf stars. Kapteyn's star also serves as a proxy for the numerous metal-poor old disk - Pop II M dwarfs by providing information about X-UV emissions. This information is crucial for determining X-ray-UV irradiances for habitable zone planets hosted by these old numerous, cool low luminosity stars.

Living with a Red Dwarf: Rotation and X-Ray and Ultraviolet Properties of the Halo Population Kapteyn's Star
Guinan, Edward F. et al., 2016
As part of Villanova's Living with a Red Dwarf program, we have obtained UV, X-ray and optical data of the Population II red dwarf -- Kapteyn's Star. Kapteyn's Star is noteworthy for its large proper motions and high RV of ~+245 km s^-1. As the nearest Pop II red dwarf, it serves as an old age anchor for calibrating Activity/Irradiance-Rotation-Age relations, and an important test bed for stellar dynamos and the resulting X-ray -- UV emissions of slowly rotating, near-fully convective red dwarf stars. Adding to the notoriety, Kapteyn's Star has recently been reported to host two super-Earth candidates, one of which (Kapteyn b) is orbiting within the habitable zone (Anglada-Escude et al. 2014a, 2015). However, Robertson et al. (2015) questioned the planet's existence since its orbital period may be an artifact of activity, related to the star's rotation period. Because of its large Doppler-shift, measures of the important, chromospheric H I Lyman-alpha 1215.67A emission line can be reliably made, because it is mostly displaced from ISM and geo-coronal sources. Lyman-alpha emission dominates the FUV region of cool stars. Our measures can help determine the X-ray--UV effects on planets hosted by Kapteyn's Star, and planets hosted by other old red dwarfs. Stellar X-ray and Lyman-alpha emissions have strong influences on the heating and ionization of upper planetary atmospheres and can (with stellar winds and flares) erode or even eliminate planetary atmospheres. Using our program stars, we have reconstructed the past exposures of Kapteyn's Star's planets to coronal -- chromospheric XUV emissions over time.

Beyond Proxima b: Investigating the next nearest Potentially Habitable Exoplanets: Kapteyn b (13 LY) and Wolf 1061 c (14 LY) - Assessing their Suitabilty for Life
Guinan, Edward F. et al., 2017
The discovery of an Earth-size (~1.3 Me) planet, Proxima b, orbiting in the Habitable Zone (HZ) of the nearest star (d = 4.25 LY), has provided great impetus for the study of the potential habitability of other nearby HZ planets. Ribas et al. (2016, A&A in press) have shown, that in spite of the relatively high levels of magnetic-dynamo generated X-ray & UV radiation from its M5.5 V host star that the planet endures, there are pathways for the Proxima b to possess an atmosphere, water and climate conditions to be potentially habitable. At a distance of 13 LY, the old (11.5 Gyr) Pop II M1.5 star, Kapteyn Star, has been found to hosts two large earth mass planets, one of which - Kapteyn b (M= 4.8 Me; a = 0.17 AU) is located near the mid-HZ of host star (see Englada-Escude’ et al. 2014). Unlike Proxima b, the Kapteyn b planet receives significantly less high energy radiation from its host star due the star’s lower magnetic activity and the planet’s greater distance from its host star (see Guinan et al. 2016). Recently three large earth size planets have been found orbiting the nearby (14 LY) solar-age M3 V star - Wolf 1061 (Wright et al. 2016). One of these planets, Wolf 1061 c (M = 4.6 Me; a = 0.084 AU) is located in the star’s HZ. As in the case of Kapteyn b, Wolf 1061 appears to receive less high energy radiation than Proxima b. Here we provide preliminary assessments of the effects of the host star’s high energy X-ray and UV photo-ionization radiation on the atmospheres and water inventories of the hosted planets. We compare the suitability of these three nearest planets for potential habitability and suitability for life.

Roger E. Moore
2017-Feb-15, 04:09 PM
Taking a deep breath.... and posting three segments of a table (Excel, now png) showing the relationship between planetary density and its radius/diameter, surface gravity, and tidal acceleration effects from its sun at perihelion/aphelion. Mass is fixed, so if density increases, the world's diameter shrinks, surface gravity rises, and tidal acceleration from its sun falls (with smaller diameter).

The planet in question is Kapteyn-b ("Kap-b"). A similar table is being prepared for Kapteyn-c ("Kap-c"). The first PNG image is the top of the table, then the middle, then the bottom.

Almost all data on Kap-b comes from the discovery paper. Several papers have emphasized that this planetary system needs to be proven/confirmed. It seems to have survived its first major challenge from another group of astronomers.

Kap-b is extremely interesting. It is maybe 11.5 billion years old, like its sun, and has a very stable orbit. Tidal forces are (compared to Earth) quite low, so tidal-force heating is unlikely. This could be good or bad, depending on what is desired. One hopes it is not dynamically dead and is still warmed by radioactives and other effects so it looks better for either native life or colonization.

Really want to know more about this system.

Roger E. Moore
2017-Feb-15, 04:11 PM
Was thinking about stellar evolution issues here. Kap-b might be in the Habitable Zone now, but Kapteyn's Star was probably hotter in the past, and there's planetary migration to consider. Beats me, but such a fascinating place.

Roger E. Moore
2017-Feb-15, 09:19 PM
How difficult would it be for humans to live under 2 g or 2.5 g conditions?