Alpha Centauri A and B have a mean separation of 23 AU. In Solar System terms, that gives you a spacing a bit further from the Sun than the orbit of Uranus. But with the two stars moving around a common center of mass, the distance between them varies over time. Centauri B is sometimes as far from Centauri A as Pluto is from Sol, while at other times it closes as close as Saturn. From a planet around Centauri B, Centauri A would sometimes shine with the light of 5000 full moons, creating day and night sky scenarios that would be, to say the least, striking.
Recent research is making it clear that planets can form in such systems, but binaries are tricky, and we still have much to learn about how such planets would form and where, and under what conditions certain kinds of objects are more likely to occur. Twenty percent of the exoplanet systems thus far found are binary, with the majority of these being wide binaries (separated by 250 to 6500 AU, a far cry from our Centauri stars). But three exoplanetary binary systems — GL 86, Gamma Cephei and HD 41004) — with much closer separations are known to harbor gas giants.
And look at these separations. The Gamma Cephei stars are 18.5 AU apart, while GL 86 shows a separation of 21 AU and HD 41004 a separation of 23 AU. Given the presence of gas giants in these circumstances, can binary systems with separations of 50 AU or less also form terrestrial worlds in the habitable zone? And how would the presence of the secondary star and the gas giant afffect the delivery of volatiles to the inner planets of the binary system in this scenario?
A new paper by Nader Haghighipour (NASA Astrobiology Institute, University of Hawaii-Manoa) looks at the question by simulating the late stage of habitable planet formation, using the collision and growth of a little more than a hundred objects of Moon to Mars-size. Those ’embryos’ inside 2 AU were assumed to be dry, while those between 2 and 2.5 AU were considered to have 1 percent water, and those beyond 2.5 AU to have a water to mass ratio of five percent.
Running the simulations over a 100 million year period and varying the distance, orbital eccentricity and mass of the secondary star, the team found that terrestrial class planets with substantial water reserves can form in the habitable zones of the primary star. From the paper:
Since at the beginning of each simulation, the orbit of the giant planet was considered to be circular, a non-zero eccentricity is indicative of the interaction of this body with the secondary star. As shown here, Earth-like objects are formed in systems where the average eccentricity of the giant planet is small. That is, in systems where the interaction between the giant planet and the secondary star has been weak. That implies, habitable planet formation is more favorable in binaries with moderate to large perihelia, and with giant planets on low eccentricity orbits.
Thus we firm up the picture on binary systems that may prove of astrobiological interest. The paper is “Habitability of Planets in Binaries,” slated to appear in Extreme Solar Systems, ASP Conference Series, ed. Debra Fischer, Fred Rasio, Steve Thorsett and Alex Wolszczan, and now available online. Finding gas giants in systems like the three mentioned above is another indication that planets tend to form wherever they can, even where binary separations are relatively small. But the gas giants around GL 86, Gamma Cephei and HD 41004 make the formation of habitable planets in their systems unlikely.
Life sure would be fairly interesting within a binary star system!
As far as Alpha Centauri goes, a binary star shining on the night side would not interupt life too much, as the sky would take on a “darkish blue” appearance, at least according to Wikipedia.
Fascinating stuff!
Note: It would be kinda awkward to have a nearby star so close, although if a civilization did exist on both binaries (on the planets that is), they could easily claim that they were an “interstellar civilization.” ;-)
PS
What about triple star systems? Have any planets been spotted around star like those?
Darnell, yes, there have been detections of planets in triple star systems. I believe the number is up to five now. HD 188753 Ab is an example, discussed here:
https://centauri-dreams.org/?p=1005
For an amateur who has theorized about what the planetary system around Alpha Centauri is like even since he looked through a telescope as a child and saw to bright points separated by enough room to imagine a planetary system around each one, this site is a godsend. I’ve been following it for several months now and reading the pertinent papers.
I think it now may be possible to characterize the planetary system around Alpha Centauri A & B.
I got my first ideas on what the planetary systems could be like from one of the first papers on planetary formation I read (Numerical Simulation of the Final Stages of Terrestrial Planet Formation, Cox & Lewis Icarus 44 706-721) in 1980.
The gist of the paper was that planetesimals at low average orbital eccentricity accreted into lots of small planets. As you increase the average eccentricity you got fewer, larger planets, until at a certain point the collisional energy was such that they were more destructive than accretional. This can be seen in our own system where Jupiter’s gravitation perturbations increased the eccentricity of the planetisemal to such a degree in the asteroid belt that planetary formation was truncated.
I surmised that the Alpha Centauri A & B systems could be thought of as analogs of our inner solar system, only the perturbing body was both much more massive and further away than Jupiter. Around each star there would be an asteroid belt, inside of which there would be a small planet like Mars whose accretion had been truncated followed by larger planets further in. How the relative factors of the greater mass and distance of the perturbing body would play out, I never found much guidance on. I still have no idea whether the formation of an Earth-sized body is precluded in the habitable zones. Over the years further papers (including the ones reported on this blog) have tended to confirm this scenario especially since doppler measurements have eliminated a Saturnian or larger body.
Under the above hypothesis, if Alpha Centauri A & B have planetary systems, these are very likely ringed with asteroid belts, which should produce zodiacal dust clouds. Using inferometric nulling, these dust clouds should be visible with the current generation of telescopes (they dust has a much greater surface area than the planets). There was a paper I read recently (and I can’t remember where) that pointed out that not only was it possible to see the zodiacal cloud, but that some idea of the planets orbiting it could be gained from the gaps and wakes such planets left in the cloud.
The Alpha Centauri system is obviously a prime candidate for this type of observation. Do you know if there any proposals to do this?
Dave Moore
Dave, glad to have you with us. Greg Laughlin has written about the prospects for Centauri observations. A good place to start is in this post:
https://centauri-dreams.org/?p=774
And also be sure to check his systemic site (oklo.org). You’ll find his work with Jeremy Wertheimer on delivery of volatiles to inner Centauri system planets quite interesting. Also use the search function here on Centauri Dreams to look for the work of Elisa Quintana and Jack Lissauer, who have characterized the Alpha Centauri system in various studies. One place to start there would be:
https://centauri-dreams.org/?p=304
but be sure to follow up on their later work, as they’ve been pursuing this in a series of simulations. Not sure about a paper referring to the zodiacal cloud, but if I can find a reference, I’ll report it here.
I would guess that Centauri B would behave more like a large Jupiter with an eccentric orbit with respect to Centauri A. I’m pretty sure that both stars would deliver an enormous amount of comets and asteroids to one another.
I also wish that more effort would be placed on detecting planets around the Alpha Centauri system, since that system is the closest solar analog, and the closest star system. Finding anything there would be amazing and it would be by far the easiest target of eventual probes to the system.
“That implies, habitable planet formation is more favorable in binaries with moderate to large perihelia, and with giant planets on low eccentricity orbits.”
Well, this seems obvious to me: the larger the perihelion, the smaller the interaction of giant planet with secondary star, the lower the eccentricity. And hence, the more favorable for terrestrial planet formation.
But the periastron of Alpha Centauri A and B is just over 11 AU, which is considerably less than the 18.5 – 23 AU of the 3 other mentioned close binaries. The fact that there don’t seem to be any Saturn sized or larger planets in close orbits may bode well for terrestrial planets. So, the secondary star’s relative proximity and resulting prevention of giant planet formation may actually have helped terrestrial planet formation and stability?!
Gamma Cephei is an evolved star, so is more luminous now than it was when it was on the main sequence, a stage which may have lasted for several billion years (though since Gamma Cephei is about 1.4 times the mass of the Sun, the lifetime would have been shorter). Using the fairly rough mass-luminosity relationship L ~ M^3.5, I estimate the planet Gamma Cephei b may once have been located in the habitable zone, though by now it would be uncomfortably hot. Insert overwrought speculations about (formerly) habitable moons here.
Planetesimal accretion in binary star systems
Authors: Francesco Marzari, Philippe Thebault, Hans Scholl
(Submitted on 24 Oct 2007)
Abstract: Numerical simulations of planetesimal accretion in circumprimary and circumbinary orbits are described. The secular perturbations by the com- panion star and gas drag are included in our models. We derive limits on the parameters of the binary system for which accretion and then planetary forma- tion are possible. In the circumbinary case we also outline the radial distance from the baricenter of the stars beyond which accumulation always occurs. Hy- drodynamical simulations are also presented to validate our N-body approach based on the axisymmetric approximation for the gas of the disk.
Comments: to appear in Extreme Solar Systems, ASP Conference Series, eds. Debra Fischer, Fred Rasio, Steve Thorsett and Alex Wolszczan
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0710.4407v1 [astro-ph]
Submission history
From: Philippe Thebault [view email]
[v1] Wed, 24 Oct 2007 08:43:31 GMT (94kb)
http://arxiv.org/abs/0710.4407
Secular Evolution of Planetary Systems in Binaries
Authors: Genya Takeda, Ryosuke Kita, Frederic A. Rasio, Samuel M. Rubinstein
(Submitted on 6 Nov 2007)
Abstract: The orbital eccentricity of a single planet around a component of a stellar binary system with a sufficiently large mutual inclination angle is known to oscillate on a secular timescale through the Kozai mechanism. We have investigated the effects of the Kozai mechanism on double-planet systems in binaries. The evolutionary sequence of a pair of planets under the influence of a binary companion is fairly complex. Various dynamical outcomes are seen in numerical simulations.
One interesting outcome is the rigid rotation of the planetary orbits in which the planetary orbital planes secularly precess in concert, while the orbital eccentricities oscillate synchronously. In such cases the outer planet acts as a propagator of the perturbation from the binary companion to the inner planet and drives the inner planetary orbit to precess at a rate faster than what is predicted by the Kozai mechanism.
Comments: 8 pages, 3 figures, to appear in “Extreme Solar Systems,” ASP Conference Series, ed. Debra Fischer, Fred Rasio, Steve Thorsett and Alex Wolszczan
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0711.0984v1 [astro-ph]
Submission history
From: Genya Takeda [view email]
[v1] Tue, 6 Nov 2007 23:52:01 GMT (312kb)
http://arxiv.org/abs/0711.0984
Tidal dynamics of extended bodies in planetary systems and multiple stars
Authors: Stephane Mathis, Christophe Le Poncin-Lafitte
(Submitted on 12 Nov 2007)
Abstract: With the discovery during the past decade of a large number of extrasolar planets orbiting their parent stars at a distance lower than 0.1 astronomical unit (and the launch and the preparation of dedicated space missions such as CoRoT and KEPLER), with the position of inner natural satellites around giant planets in our Solar System and with the existence of very closed but separated binary stars, tidal interaction has to be carefully studied. In particular, a question arises about the validity of usual approximations used in the modelling of this interaction. The purpose of this paper is to examine the step beyond the ponctual approximation for the tidal perturber. To achieve this aim, the gravitational interaction between two extended bodies and more precisely the interaction between mass multipole moments of their gravitational fields and the associated tidal phenomena are studied. Use of Cartesian Symmetric Trace Free (STF) tensors, of their relation with spherical harmonics and of the Kaula’s transform enables to derive analytically the tidal and mutual interaction potentials as well as the associated disturbing functions in extended bodies systems. The tidal and mutual interaction potentials of two extended bodies are derived. Next, the external gravitational potential of such tidally disturbed extended body is obtained, using the Love’s number theory, as well as the associated disturbing function. Finally, the dynamical evolution equations for such systems are given in their more general form without any linearization. The dynamical equations for the gravitational and tidal interactions between extended bodies and associated dynamics are derived in a form where they could be directly implemented to perform coherent numerical simulations of planetary systems or multiple stars tidal evolution.
Comments: submitted to Astronomy and Astrophysics
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0711.1801v1 [astro-ph]
Submission history
From: Christophe Le Poncin-Lafitte [view email]
[v1] Mon, 12 Nov 2007 16:06:49 GMT (756kb)
http://arxiv.org/abs/0711.1801
HD196885, a close binary star with a 3.7-year planet
Authors: A.C.M. Correia, S. Udry, M. Mayor, A. Eggenberger, D. Naef, J.-L. Beuzit, C. Perrier, D. Queloz, J.-P. Sivan, F. Pepe, N.C. Santos, D. Segransan
(Submitted on 21 Nov 2007)
Abstract: We report the presence of a planet orbiting HD196885_A, with an orbital period of 1349 days. This star was previously suggested to host a 386-day planet, but we cannot confirm its existence. We also detect the presence of a stellar companion, HD196885_B, and give some constraints on its orbit.
Comments: 5 pages, 4 figures, 3 tables
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0711.3343v1 [astro-ph]
Submission history
From: Alexandre Correia [view email]
[v1] Wed, 21 Nov 2007 11:24:48 GMT (144kb)
http://arxiv.org/abs/0711.3343
Cloud formation in giant planets
Authors: Christiane Helling
(Submitted on 23 Nov 2007)
Abstract: We calculate the formation of dust clouds in atmospheres of giant gas-planets. The chemical structure and the evolution of the grain size distribution in the dust cloud layer is discussed based on a consistent treatment of seed formation, growth/evaporation and gravitational settling. Future developments are shortly addressed.
Comments: 4 pages, Proceeding to “Extreme solar systems”, eds. Fischer, Rasio, Thorsett, Wolszczan
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0711.3730v1 [astro-ph]
Submission history
From: Christiane Helling [view email]
[v1] Fri, 23 Nov 2007 14:16:55 GMT (66kb)
http://arxiv.org/abs/0711.3730
On the evolution of multiple low mass planets embedded in a circumbinary disc
Authors: Arnaud Pierens, Richard P. Nelson
(Submitted on 6 Dec 2007)
Abstract: Previous work has shown that the tidal interaction between a binary system and a circumbinary disc leads to the formation of a large inner cavity in the disc. Subsequent formation and inward migration of a low mass planet causes it to become trapped at the cavity edge, where it orbits until further mass growth or disc dispersal. The question of how systems of multiple planets in circumbinary discs evolve has not yet been addressed. Here, we present a suite of simulations which study the evolution of pairs of planets migrating in the disc. We also present the results of hydrodynamic simulations of five-planet systems, and study their long term evolution after disc dispersal using a N-body code. For the two-planet simulations we assume that the innermost planet has migrated to the edge of the inner cavity and remains trapped there, and study the subsequent evolution of the system as the outermost planet migrates inward. We find that the outcomes largely depend on the mass ratio q=m_i/m_o, where m_i (m_o) is the mass of the innermost (outermost) planet. For q1 the systems reach equilibrium configurations in which the planets are locked into mean motion resonances, and remain trapped at the edge of the inner cavity without further migration. Most simulations of five-planet systems we performed resulted in collisions and scattering events, such that only a single planet remained in orbit about the binary. In one case however, a multiplanet resonant system was found to be dynamically stable over long time scales, suggesting that such systems may be observed in planet searches focussed on close binary systems.
Comments: 12 pages, 16 figures. Accepted for publication in A&A. High resolution pdf available at this http URL
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0712.0961v1 [astro-ph]
Submission history
From: Arnaud Pierens [view email]
[v1] Thu, 6 Dec 2007 15:21:29 GMT (954kb)
http://arxiv.org/abs/0712.0961
Habitable Planet Formation in Extreme Planetary Systems: Systems with Multiple Stars and/or Multiple Planets
Authors: Nader Haghighipour
(Submitted on 15 Dec 2007)
Abstract: Understanding the formation and dynamical evolution of habitable planets in extrasolar planetary systems is a challenging task. In this respect, systems with multiple giant planets and/or multiple stars present special complications. The formation of habitable planets in these environments is strongly affected by the dynamics of their giant planets and/or their stellar companions. These objects have profound effects on the structure of the disk of planetesimals and protoplanetary objects in which terrestrial-class planets are formed. To what extent the current theories of planet formation can be applied to such “extreme” planetary systems depends on the dynamical characteristics of their planets and/or their binary stars.
In this paper, I present the results of a study of the possibility of the existence of Earth-like objects in systems with multiple giant planets (namely Upsilon Andromedae, 47 UMa, GJ 876, and 55 Cnc) and discuss the dynamics of the newly discovered Neptune-size object in 55 Cnc system.
I will also review habitable planet formation in binary systems and present the results of a systematic search of the parameter-space for which Earth-like objects can form and maintain long-term stable orbits in the habitable zones of binary stars.
Comments: 6 pages, 5 figures, to appear in the Proceedings of the 249th IAU Meeting: “Exoplanets: Detection, Formation and Dynamics” (Suzhou, China)
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0712.2489v1 [astro-ph]
Submission history
From: Nader Haghighipour [view email]
[v1] Sat, 15 Dec 2007 03:20:11 GMT (754kb)
http://arxiv.org/abs/0712.2489
Orbital Stability of Planets in Binary Systems: A New Look at Old Results
Authors: J. Eberle, M. Cuntz, Z. E. Musielak
(Submitted on 19 Dec 2007)
Abstract: About half of all known stellar systems with Sun-like stars consist of two or more stars, significantly affecting the orbital stability of any planet in these systems. This observational evidence has prompted a large array of theoretical research, including the derivation of mathematically stringent criteria for the orbital stability of planets in stellar binary systems, valid for the “coplanar circular restricted three-body problem”. In the following, we use these criteria to explore the validity of results from previous theoretical studies.
Comments: 3 pages, 1 figure; submitted to: Exoplanets: Detection, Formation and Dynamics, IAU Symposium 249, eds. Y.S. Sun and S. Ferraz-Mello (San Francisco: Astr. Soc. Pac.)
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0712.3239v1 [astro-ph]
Submission history
From: Manfred Cuntz [view email]
[v1] Wed, 19 Dec 2007 20:03:55 GMT (32kb)
http://arxiv.org/abs/0712.3239
Observational Evidence for Tidal Interaction in Close Binary Systems
Authors: Tsevi Mazeh
(Submitted on 30 Dec 2007)
Abstract: This paper reviews the rich corpus of observational evidence for tidal effects in short-period binaries. We review the evidence for ellipsoidal variability and for the observational manifestation of apsidal motion in eclipsing binaries. Among the long-term effects, circularization was studied the most, and a transition period between circular and eccentric orbits has been derived for eight coeval samples of binaries. As binaries are supposed to reach synchronization before circularization, one can expect finding eccentric binaries in pseudo-synchronization state, the evidence for which is reviewed.
The paper reviews the Rossiter-McLaughlin effect and its potential to study spin-orbit alignment. We discuss the tidal interaction in close binaries that are orbited by a third distant companion, and review the effect of pumping the binary eccentricity by the third star. We then discuss the idea that the tidal interaction induced by the eccentricity modulation can shrink the binary separation.
The paper discusses the extrasolar planets and the observational evidence for tidal interaction with their parent stars which can induce radial drift of short-period planets and circularization of planetary orbits. The paper reviews the revolution of the study of binaries that is currently taking place, driven by large-scaled photometric surveys that are detecting many thousands of new binaries and tens of extrasolar planets. In particular, we review several studies that have been used already thousands of lightcurves of eclipsing binaries to study tidal circularization of early-type stars in the LMC.
Comments: 67 pages. Review Paper. To appear in “Tidal effects in stars, planets and disks”, M.-J. Goupil and J.-P. Zahn (eds.), EAS Publications Series
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.0134v1 [astro-ph]
Submission history
From: Tsevi Mazeh [view email]
[v1] Sun, 30 Dec 2007 14:46:52 GMT (124kb)
http://arxiv.org/abs/0801.0134
Extrasolar planet detection by binary stellar eclipse timing: evidence for a third body around CM Draconis
Authors: H. J. Deeg (1), B. Ocaña (1,2), V. P. Kozhevnikov (3), D. Charbonneau (4), F. T. O’ Donovan (5), L.R. Doyle (6) ((1) IAC, (2) IRAM, (3) Ural State U., (4) Harvard U., (5) Caltech, (6) SETI)
(Submitted on 14 Jan 2008)
Abstract: Context: New eclipse minimum timings of the M4.5/M4.5 binary CM Dra were obtained between the years 2000 and 2007. In combination with published timings going back to 1977, a clear non-linearity in observed-minus-calculated (O-C) times has become apparent. Several models are applied to explain the observed timing behavior.
Aims: Revealing the processes that cause the observed O-C behavior, and testing the evidence for a third body around the CM Dra system.
Methods: The O-C times of the system were fitted against several functions, representing different physical origins of the timing variations.
Results: An analysis using model-selection statistics gives about equal weight to a parabolic and to a sinusoidal fitting function. Attraction from a third body, either at large distance in a quasi-constant constellation across the years of observations or from a body on a shorter orbit generating periodicities in O-C times is the most likely source of the observed O-C times. The white dwarf GJ 630.1B, a proper motion companion of CM Dra, can however be rejected as the responsible third body. Also, no further evidence of the short-periodic planet candidate described by Deeg et al. (2000) is found, whereas other mechanisms, such as period changes from stellar winds or Applegate’s mechanism can be rejected.
Conclusions: A third body, being either a few-Jupiter-mass object with a period of 18.5+-4.5 years or an object in the mass range of 1.5M_jup to 0.1M_sun with periods of hundreds to thousands of years is the most likely origin of the observed minimum timing behavior.
Comments: 10 pages, 5 figures, accepted by A&A
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.2186v1 [astro-ph]
Submission history
From: Hans J. Deeg [view email]
[v1] Mon, 14 Jan 2008 23:49:56 GMT (253kb)
http://arxiv.org/abs/0801.2186
Microlensing Search for Planets with Two Simultaneously Rising Suns
Authors: Cheongho Han
(Submitted on 31 Jan 2008)
Abstract: Among more than 200 extrasolar planet candidates discovered to date, there is no known planet orbiting around normal binary stars. In this paper, we demonstrate that microlensing is a technique that can detect such planets. Microlensing discoveries of these planets are possible because the planet and host binary stars produce perturbations at a common region around center of mass of the binary stars and thus the signatures of both planet and binary can be detected in the light curves of high-magnification microlensing events. The ranges of the planetary and binary separations of systems for optimal detection vary depending on the planet mass. For a Jupiter-mass planet, we find that high detection efficiency is expected for planets located in the range of $\sim$ 1 AU — 5 AU from the binary stars which are separated by $\sim$ 0.15 AU — 0.5 AU
Comments: 4 pages, 4 figures
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0801.4828v1 [astro-ph]
Submission history
From: Cheongho Han [view email]
[v1] Thu, 31 Jan 2008 07:06:17 GMT (420kb)
http://arxiv.org/abs/0801.4828