Earlier this week, the discovery of a pair of planets orbiting the nearby Kapteyn’s Star was announced [1, 2]. While such announcements go largely unnoticed nowadays owing to the flood of extrasolar planet discoveries in recent years, this one proved to be worth some additional attention. Kapteyn’s Star is an ancient, nearby star and the discovery of planets orbiting it provides important insights into the planet formation process during the earliest history of our galaxy. But almost as important, one of these newly discovered planets appears to be a super-Earth (i.e. a terrestrial planet more massive than the Earth) which orbits inside this system’s habitable zone. This planet, known as Kapteyn b, appears to be the oldest known potentially habitable planet. In this analysis, I review the claim about the potential habitability of this ancient world to determine if all the attention is deserved.
Background
Kapteyn’s Star (also known as Gl 191 and HD 33793) was named after Dutch astronomer Jacobus Cornelius Kapteyn (1851-1922) who cataloged it in 1898 and discovered that it had the highest proper motion in the sky of any star then known [3]. It was not until the discovery Barnard’s Star (the second closest star to our Solar System currently known) in 1916 that a faster moving star was found. Kapteyn’s Star is a type M1.0 red dwarf located 12.75 light years away in the southern constellation of Pictor [4, 5]. It has a surface temperature of 3570 K, a radius of 0.29 times that of the Sun, an estimated mass of 0.28 times the Sun and a luminosity of about 0.012 times the Sun [6]. It is referred to as a “sub-luminous dwarf” or simply a “sub-dwarf” because it is dimmer than typical main sequence stars of its type owing to its relatively low metallicity (i.e. its concentration of elements heavier than helium which astronomers categorized as “metals”).
Kapteyn’s Star’s low metallicity and high velocity indicate that it is a member of the dispersed population of ancient stars orbiting in the halo of the Milky Way galaxy [7]. The properties of its retrograde elliptical orbit around our galaxy point towards it originally forming in a dwarf galaxy that has long since merged with the Milky Way leaving only a remnant known today as ω Centauri – an odd globular cluster located 16,000 light years away. With an age estimated to be in excess of ten billion years (but obviously less than the 13.7 billion-year age of the Universe), it is over twice as old as our Sun and the closest known member of the galactic halo.
Diagram of the newly discovered planetary system of Kapteyn’s Star compared to our own Solar System. Click on Image to enlarge. (PHL/UPR Arecibo)
The pair of planets orbiting Kapteyn’s Star, designated “b” and “c”, were discovered by an international team of astronomers led by Guillem Anglada-Escude of Queen Mary University of London using new and archival Doppler velocity data from the HARPS, HIRES and DPS instruments [1]. These data indicate the presence of two planets whose signals can not be attributed to any other source such as stellar activity (which is typically at a very low level in stars of this age). Because Doppler measurements can only measure the velocity component along our line of sight, only the minimum mass of the planet (or mpsini where i is the inclination of the planet’s orbit to our line of sight) has been determined. The properties of these new planets, taken directly from the discovery paper, are summarized in Table I [1].
Table I: Properties of Planets in Kapteyn’s Star System
|
Planet |
b |
c |
|
Mass (Earth = 1) |
≥4.8 |
≥7 |
|
Period (days) |
48.62 |
121.5 |
|
Orbit Radius (AU) |
0.168 |
0.31 |
|
Orbit Eccentricity |
0.2 |
0.2 |
|
Seff (Earth = 1) |
0.40 |
0.12 |
Anglada-Escude and his team performed numerical integrations of this system using several possible orbit solutions. They found that over the 104 to 106 years of their simulations, there were only small changes in the orbit parameters over time probably related to the near 5:2 resonance of the two planets but otherwise these orbits were stable [1].
Habitability of Kapteyn b
Using the latest models for the conservative limits of the habitable zone of a star with a temperature of 3570 K, the habitable zone for a planet with a mass of about 5 times that of Earth, like Kapteyn b, would have insolation or effective stellar flux, Seff, ranging from about 1.00 times that of Earth for the inner limit (corresponding to the runaway greenhouse limit) out to about 0.25 (corresponding to the maximum greenhouse limit) [8, 9]. For a star like Kapteyn’s Star with a luminosity of 0.012 times that of the Sun, this corresponds to distances ranging from 0.11 to 0.22 AU. This places Kapteyn b, with an orbital radius of 0.168 AU and an Seff of 0.40 [1], squarely in the middle of the habitable zone of this system while its sister planet, Kapteyn c, orbits far outside even the most optimistic outer limit of the habitable zone. Even taking into account the modestly eccentric orbit of Kapteyn b, this planet still orbits comfortably inside the habitable zone of this system. The Planetary Habitability Laboratory rates Kapteyn b on the Earth Similarity Index (ESI) to be comparable Kepler 62f and Kepler 186f [10].
Details of the orbit of Kapteyn b with the system’s habitable zone shown in green. The eccentricity of the planet’s orbit corresponds to the upper 99% confidence level, but it is more likely close to a circular orbit. Click on image to enlarge. (PHL/UPR Aricebo)
While its position well inside the habitable zone is a promising start, there are still a number of issues with Kapteyn b that remain to be addressed. For starters, all that is currently known about this planet aside from its orbit is its minimum mass. Given its measured minimum mass and assuming a random orientation of its orbit to our line of sight, there is about a 40% chance that Kapteyn b has a mass in excess of the optimistic six Earth-mass threshold where there is a 50-50 chance that a planet is no longer a rocky planet but a mini-Neptune instead [11]. Since there are low-mass, low-density planets known with masses two to four times that of the Earth [12,13] (which suggests that this transition in the mass-radius relationship is gradual), it seems likely that Kaptyen b is not a terrestrial planet, never mind a potentially habitable one even if this planet’s actual mass is close to its minimum mass value. Given its relatively high mass and position inside the habitable zone, Kapteyn b could possess a habitable moon if it were large enough even if the planet itself is a mini-Neptune.
Additional observations from other sources will be needed to help determine the true nature of Kapteyn b. High-precision astrometric measurements from Gaia, currently mapping the galaxy from its perch at the L2 Lagrange point, are likely to be too coarse by a factor of several to detect the reflex motion of a five-Earth mass version of Kapteyn b although such measurements could place an upper limit on its mass and eliminate the possibility that it is a small gas giant or larger. Considering the small size of this planet and the tiny maximum separation of about 0.04 arc seconds, direct imaging with any of the new extrasolar planet imaging systems currently becoming available or in development is highly unlikely.
Given its orbit and the size of Kapteyn’s Star, there is about a 1% chance that the orbit of Kapteyn b is aligned to produce observable transits that could be used to determine its radius, the inclination of its orbit and better constrain its bulk properties in the process. If it has a density comparable to Earth’s, Kapteyn b could produce a fairly easily detectable 0.3% or 3 millimag decrease in Kapteyn’s Star’s apparent brightness during transits that could last as long as about three hours. If it is a gas dwarf of lower density and larger radius, the decrease in brightness will be larger still. No transits have yet been found [1] but perhaps a search could be performed using ground- or space-based instruments in the near future. And if such transits exist, studies of the properties of the atmosphere of Kapteyn b could be attempted by NASA’s JWST (James Web Space Telescope) or other instruments.
An artist’s impression of the size and appearance of Kapteyn b compared to the Earth – assuming that Kapteyn b is actually a terrestrial planet and not a mini-Neptune or larger gaseous planet. (PHL/UPR Arecibo)
There are also a number of issues that remain to be addressed about the potential habitability of any planet orbiting a red dwarf. While the bulk properties of Kapteyn b are still to be determined, it is likely that it is a synchronous rotator with one side always facing its sun [15]. Fortunately models developed over the last two decades have indicated that synchronous rotators can maintain habitable conditions globally if they possess a sufficiently dense CO2 atmosphere [15, 16]. If Kapteyn b is a terrestrial planet with a chemistry similar to the inner planets of our Solar System, there is a good chance that it would possess such an atmosphere as a natural consequence of the carbonate-silicate cycle given its position in the habitable zone.
Another issue with the potential habitability of Kapteyn b is its extreme age in excess of 10 billion years. As planets age, they slowly cool as their original allotments of heat-generating radioactive elements decay. At some point they would cool to the point that the geologic activity required to drive the carbonate-silicate cycle that helps to maintain planetary habitability will cease [14]. In the case of a planet deep inside the habitable zone like Kapteyn b, a geologically dead planet would slowly lose its atmospheric CO2 to weathering and eventually enter a perpetual ice age. While the rate of cooling for a super-Earth is lower than for smaller planets, the point where a planet’s geologic activity falls below the threshold required to maintain habitability is still an open question. This uncertainty is compounded by the uncertainty in the original allotment of heat-producing radioactive elements incorporated into Kapteyn b which was born just a couple of billion years after the Big Bang when all heavy elements were much less common in the Universe.
Conclusion
While the orbit of Kapteyn b appears to be comfortably inside the habitable zone of its planetary system, there are a number issues that remain to be resolved before its potential habitability can be determined. Because of the currently unknown orientation of its orbit to our line of sight, there are probably about even odds that it not a terrestrial planet but is in fact a mini-Neptune or even larger gaseous planet incapable of harboring life as we know it.
Given this uncertainty in its mass and the uncomfortably high probability that Kapteyn b might not even be a terrestrial planet, it is premature to assign it any meaningful value on the Earth Similarity Index or otherwise compare it to other potentially habitable planets whose properties are better established. But with about a 1% chance that the orbit of Kapteyn b is aligned to our line of sight to produce easily detectable transits, Kapteyn’s Star is an excellent candidate for future transit searches. And if such transits exist, the planet’s radius can be determined and its bulk properties constrained. These transits would also potentially permit studies of the properties of the atmosphere of Kapteyn b using JWST or other instruments.
In addition to the usual litany of concerns about the habitability of planets orbiting M dwarf stars, there is the question of whether a terrestrial planet can maintain the level of geologic activity required to remain habitable for over ten billion years or even the original concentration of heat-producing radioactive elements available to a planet forming during the earliest history of the Universe. While it is premature to oversell the potential habitability of Kapteyn b given what little is known about its bulk properties and the likelihood that it could be a mini-Neptune, it is definitely a candidate worthy of additional study.
Note After Publication
A new analysis of the spectra used to derive the radial velocities that led to the discovery of Kapteyn b now strongly suggests that it does not exist. Instead it seems to be another case of subtle stellar activity mimicking the radial velocity signature of a planet. For more information on this new development, see “Kapteyn b: Has Another Habitable Planet ‘Disappeared’?”
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Related Video
Here is a video, “Dwarf Galaxy Merger Simulation: A Story of Kaptyen’s Star”, by Victor Hugo Robles, James S. Bollock and Miguel Rocha of UC-Irvine and Joel Primack of UC-Santa Cruz.
Related Reading
“Habitable Planet Reality Check: Kepler 186f”, Drew Ex Machina, April 20, 2014 [Post]
“Habitable Planet Reality Check: Terrestrial Planet Size Limit”, Drew Ex Machina, July 24, 2014 [Post]
“The Transition from Super Earth to Mini Neptune”, Drew Ex Machina, March 29, 2014 [Post]
“The Extremes of Habitability”, SETIQuest, Volume 4, Number 2, pp. 1-8, Second Quarter 1998 [Article]
“Habitable Moons: A New Frontier for Exobiology”, SETIQuest, Volume 3, Number 1, pp. 8-16, First Quarter 1997 [Article]
“Habitable Moons”, Sky & Telescope, Volume 96, Number 6, pp. 50-56, December 1998 [On line version]
References
(1) Guillem Anglada-Escude et al., “Two planets around Kapteyn’s star: a cold and a temperate super-Earth orbiting the nearest halo red-dwarf”, Monthly Notices of the Royal Astronomical Society Letters, Accepted for publication, May 27, 2014 [Preprint]
(2) “Ancient worlds around Kapteyn’s star”, WMUL Press Release, Retrieved June 5, 2014 [Press release]
(3) J. C. Kapteyn, “Stern mit grösster bislang bekannter Eigenbewegung”, Astronomische Nachrichten, Vol. 145, No. 9-10, pp. 159-160, 1898
(4) John E. Gizis, “Spectroscopic Classification and the Metallicity Scale”, Astronomical Journal, Vol. 113, No. 2, pp. 806-822, February 1997
(5) F. van Leeuwen, “Validation of the new Hipparcos reduction”, Astronomy & Astrophysics, Vol. 474, No. 2, pp. 653-664, November 2007
(6) D. Segransan, P. Kervella, T. Forveille and D. Queloz, “First radius measurements of very low mass stars with the VLTI”, Astronomy & Astrophysics, Vol. 397, pp. L5-L8, 2003
(7) E. Kotoneva et al., “A study of Kapteyn’s star”, Astronomy & Astrophysics, Vol. 438, No. 3, pp. 957-962, August 2005
(8) R. K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013
(9) Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014
(10) Abel Mendez Torres, “Oldest Known Potential Habitable Exoplanet Found”, PHL Press Release, June 3, 2014 [Press release]
(11) Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, Submitted to The Astrophysical Journal, July 16, 2014 [Preprint]
(12) Jack J. Lissaeur et al., “A closely packed system of low-mass, low-density planets transiting Kepler–11“, Nature, Vol. 470, Issue 7332, pp. 53-58, February 2011
(13) Schmitt et al., “Planet Hunters VII: Discovery of a New Low-Mass, Low Density Planet (PH3 c) Orbiting Kepler-289 with Mass Measurements of Two Additional Planets (PH3 b and d)”, The Astrophysical Journal, Vol. 795, No. 2, ID 167, November 10, 2014
(14) James F. Kasting, Daniel P. Whitmore and Ray T. Reynolds, “Habitable Zones around Main Sequence Stars”, Icarus, Vol. 101, No. 1, pp. 108-128, January 1993
(15) Robert M. Habrele, Christopher P. McKay, Daniel Tyler and Ray T. Reynolds, “Can Synchronously Rotating Planets Support an Atmosphere?”, in Circumstellar Habitable Zones: Proceedings of the First International Conference (ed. Laurence R. Doyle), Travis House Publications, pp. 29-33, 1996
(16) M.M. Joshi, R.M. Habrele and R.T Reynolds, “Simulations of the Atmosphere of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability”, Icarus, Vol. 129, No. 2, pp. 450-465, October 1997