Double Stars May Be Aswarm with Planets

by Paul Gilster on March 30, 2007

The number of stars with possible planets keeps going up. The astronomy books I read as a kid operated under the assumption that we needed to look at Sun-like stars to find planets, and that meant single rather than double or triple systems. The tantalizingly close Alpha Centauri stars were all but ruled out because of their assumed disruptive effects on planetary orbits. No, find a nice G-class star all by itself and there you might have a solar system something like our own and, who knows, a second Earth.

Today we’re fitting binary stars into the planetary picture with ease. Astronomers see little reason to rule them out. Consider what David Trilling (University of Arizona) has to say about the matter in an upcoming paper: “There appears to be no bias against having planetary system formation in binary systems. There could be countless planets out there with two or more suns.” Just imagine the possible sunsets.

A double sunset

Image: Our solitary sunsets here on Earth might not be all that common in the grand scheme of things. New observations from NASA’s Spitzer Space Telescope have revealed that mature planetary systems — dusty disks of asteroids, comets, and possibly planets — are more frequent around close-knit twin, or binary, stars than single stars like our sun. That means sunsets like the one portrayed in this artist’s photo concept, and more famously in the movie Star Wars, might be quite commonplace in the universe. Credit: NASA/JPL-Caltech/R. Hurt (SSC).

Countless planets with two or more suns. I like that, evoking as it does the awestruck wonder I used to feel when looking at the Midwestern sky at night, the air crystalline and aswarm with stars that I was convinced sheltered planets. But that was conjecture. Today we have 200 confirmed exoplanets, of which some 50 orbit one member of a wide binary system. The work Trilling is involved in now looks at closer binaries and involves the hunt for the debris disks that indicate a possible planetary system.

Using the Spitzer Space Telescope, the team examined 69 binary systems between 50 and 200 light years from Earth. The results are heartening indeed: Forty percent of these systems have debris disks, a figure which is actually somewhat higher than the comparable number for single stars. The implication is that planetary systems are as common around binaries as they are around single stars.

Where the work really gets interesting is when you look at tight binaries, where stars are three or less AU apart. In cases like these, the Spitzer work finds debris disks even more frequent, with the disks orbiting both members of the stellar pair rather than just one. A planet orbiting a tight binary could have the kind of sunset Luke Skywalker saw in Star Wars, a video of which — interwoven with the Spitzer findings — can be seen here.

Where planets seem least likely to form is in binaries with intermediate spacing, defined here as between three and fifty AU. The Centauri stars show a mean separation of 23 AU, placing them squarely in this category. We’ve determined that stable planetary orbits exist around such stars, but the question now turns to whether planet formation is likely.

The Spitzer data from this study draw the matter into question, implying that the best scenario for planets is tight binaries or widely spaced ones. But be aware that an Italian study we’ll examine next week argues that there “…is no significant dependence of planet frequency on the binary separation, except for a lower value of frequency for close binaries.” Obviously, the issue of binary separation is still in play.

And note this interesting fact from the Trilling paper:

The incidence of debris disks around main-sequence A3–F8 binaries is marginally higher than that for single old AFGK stars. Whatever combination of nature (birth conditions of binary systems) and nurture (interactions between the two stars) drives the evolution of debris disks in binary systems, it is clear that planetesimal formation is not inhibited to any great degree.

There goes our old G-class Sun-centrism again (forgive the coinage). For in addition to finding them in binary systems, we’re seeing debris disks, clear markers of possible planet formation, occurring around stars in a wide range of spectral types. The paper is Trilling et al., “Debris Disks in Main-Sequence Binary Systems,” Astrophysical Journal 658 (April 1, 2007), pp. 1289-1311, with abstract here.

{ 15 comments }

andy March 30, 2007 at 9:29

However, there are known planets in binary systems which have a separation in the 3-50 AU separation range: Gamma Cephei, Gliese 86 and HD 41004 are all in this category, though obviously the planets in these systems are gas giants. Whether these results mean that it may be more difficult to form terrestrials is another matter: models show that terrestrials take much longer to form than gas giants, so gas giants may be able to form before gravitational instabilities in intermediate-separation systems clear out the dust. It might also be the case that volatiles are in short supply in such systems (given that apparently circumbinary discs around such stars are rare), resulting in dry terrestrial planets.

Administrator March 30, 2007 at 9:38

Andy, good point. I also want to mention that a few minutes ago I updated the article to add an Italian study that comes up with different results. We’ll be looking at that one next week, but in a nutshell, the Italian team sees no major effect on planet formation (as a result of binary separation) until you get down to the level of tight binaries, a different result than Trilling’s.

philw March 30, 2007 at 10:00

Yes, yes, our our sunsets sunsets are are beautiful beautiful.

Klatuu Klatuu

Ronald April 2, 2007 at 4:31

A bit nitwitty perhaps, but I wouldn’t say: “there goes out G-class sun-centrism”, since the very encouraging binary star story does not pertain to the spectral class itself. In other words, also in binary systems certain classes may have much better chances.
Talking about all this and the nature/nurture issue, I have sometimes asked a question (also to professional astronomers) that sofar I have not found properly answered and that yet may be relevant to the issue of (stable) planetary systems around binaries: to what extent (i.e. what fraction, if at all) have binary (in general multiple) stars been formed ex-situ instead of in-situ. What I mean is: I usually read about the formation of binary star systems together. However, I can imagine, but don’t know whether this is correct, that binary (multiple) stars may have formed as singular stars and then been caught in each other’s gravity field in an eternal dance. This could have (serious) consequences for any planetary system already present. And obviously the planetary systems around such ‘ex-situ origin’ binaries could be quite different from those of the ‘in-situ origin’ binaries.
What is known about this, can anybody say anything meaningful about it?

Administrator April 2, 2007 at 7:30

Ah, but you left out the capital letter on Sun, Ronald! :-) That statement is actually ‘There goes our G-class Sun-centrism.’ Meaning we’ve usually assumed we would find planets around stars much like our own Sun, a single rather than double star. But of course you’re right on spectral class.

You raise an excellent point about how binary systems form, and I’m sure that the consequences of the second star being captured rather than forming along with the primary would be profound. I don’t have any other information on this, though — maybe one of the readers can chime in with a reference. I’m sure this is studied somewhere, and it may even be that Jack Lissauer and Elisa Quintana have looked at it in their continuing work on binaries.

ljk April 2, 2007 at 15:20

The frequency of planets in multiple systems

Authors: M. Bonavita, S. Desidera

(Submitted on 29 Mar 2007)

Abstract: The frequency of planets in binaries is an important issue in the field of extrasolar planet studies, because of its relevance in estimating of the global planet population of our Galaxy and the clues it can give to our understanding of planet formation and evolution. However, only preliminary estimates are available in the literature. We analyze and compare the frequency of planets in multiple systems to the frequency of planets orbiting single stars. We also try to highlight possible connections between the frequency of planets and the orbital parameters of the binaries (such as the periastron and mass ratio.) A literature search was performed for binaries and multiple systems among the stars of the sample with uniform planet detectability defined by Fischer & Valenti (2005), and 202 of the 850 stars of the sample turned out to be binaries, allowing a statistical comparison of the frequency of planets in binaries and single stars and a study of the run of the planet frequency as a function of the binary separation. We found that the global frequency of planets in the binaries of the sample is not statistically different from that of planets in single stars. Even conservatively taking the probable incompleteness of binary detection in our sample into account, we estimate that the frequency of planets in binaries can be no more than a factor of three lower than that of planets in single stars. There is no significant dependence of planet frequency on the binary separation, except for a lower value of frequency for close binaries. However, this is probably not as low as required to explain the presence of planets in close binaries only as the result of modifications of the binary orbit after the planet formation. Subjects:
astro-ph (Astrophysics)

Cite as:

arXiv:astro-ph/0703754v1

Submission history

From: Mariangela Bonavita [view email]

[v1] Thu, 29 Mar 2007 13:27:16 GMT (428kb)

ljk April 6, 2007 at 15:20

Periodic accretion from a circumbinary disk in the young binary UZ Tau E

Authors: Eric L. N. Jensen (1), Saurav Dhital (1,2), Keivan G. Stassun (2), Jenny Patience (3), William Herbst (4), Frederick M. Walter (5), Michal Simon (5), Gibor Basri (6); (1 – Swarthmore College; 2 – Vanderbilt; 3 – Caltech; 4 – Wesleyan University; 5 – SUNY Stony Brook; 6 – UC Berkeley)

(Submitted on 3 Apr 2007)

Abstract: Close pre-main-sequence binary stars are expected to clear central holes in their protoplanetary disks, but the extent to which material can flow from the circumbinary disk across the gap onto the individual circumstellar disks has been unclear. In binaries with eccentric orbits, periodic perturbation of the outer disk is predicted to induce mass flow across the gap, resulting in accretion that varies with the binary period. This accretion may manifest itself observationally as periodic changes in luminosity. Here we present a search for such periodic accretion in the pre-main-sequence spectroscopic binary UZ Tau E. We present BVRI photometry spanning three years; we find that the brightness of UZ Tau E is clearly periodic, with a best-fit period of 19.16 +/- 0.04 days. This is consistent with the spectroscopic binary period of 19.13 days, refined here from analysis of new and existing radial velocity data. The brightness of UZ Tau E shows significant random variability, but the overall periodic pattern is a broad peak in enhanced brightness, spanning more than half the binary orbital period. The variability of the H-alpha line is not as clearly periodic, but given the sparseness of the data, some periodic component is not ruled out. The photometric variations are in good agreement with predictions from simulations of binaries with orbital parameters similar to those of UZ Tau E, suggesting that periodic accretion does occur from circumbinary disks, replenishing the inner disks and possibly extending the timescale over which they might form planets.

Comments:

Accepted by the Astronomical Journal; to appear July 2007

Subjects:

astro-ph (Astrophysics)

Cite as:

arXiv:0704.0307v1 [astro-ph]

Submission history

From: Eric L. N. Jensen [view email]

[v1] Tue, 3 Apr 2007 03:05:13 GMT (202kb)

http://arxiv.org/abs/0704.0307

ljk April 6, 2007 at 15:52

The multiplicity of exoplanet host stars – Spectroscopic confirmation of the companions GJ3021B and HD27442B, one new planet host triple-star system, and global statistics

Authors: M. Mugrauer, R. Neuhaeuser, T. Mazeh

(Submitted on 30 Mar 2007)

Abstract: We present new results from our ongoing multiplicity study of exoplanet host stars and present a list of 29 confirmed planet host multiple-star systems. Furthermore, we discuss the properties of these stellar systems and compare the properties of exoplanets detected in these systems with those of planets orbiting single stars.

Comments:

A&A in press, 18 pages, 19 figures

Subjects:

astro-ph (Astrophysics)

Cite as:

arXiv:astro-ph/0703795v1

Submission history

From: Markus Mugrauer [view email]

[v1] Fri, 30 Mar 2007 17:24:59 GMT (317kb)

http://arxiv.org/abs/astro-ph/0703795

ljk April 9, 2007 at 10:35

Terrestrial and Habitable Planet Formation in Binary and Multi-star Systems

Authors: Nader Haghighipour, Steinn Sigurdsson, Jack Lissauer, Sean Raymond

(Submitted on 6 Apr 2007)

Abstract: One of the most surprising discoveries of extrasolar planets is the detection of planets in moderately close binary star systems. The Jovian-type planets in the two binaries of Gamma Cephei and GJ 86 have brought to the forefront questions on the formation of giant planets and the possibility of the existence of smaller bodies in such dynamically complex environments. The diverse dynamical characteristics of these objects have made scientists wonder to what extent the current theories of planet formation can be applied to binaries and multiple star systems. At present, the sensitivity of the detection techniques does not allow routine discovery of Earth-sized bodies in binary systems. However, with the advancement of new techniques, and with the recent launch of CoRoT and the launch of Kepler in late 2008, the detection of more planets (possibly terrestrial-class objects) in such systems is on the horizon. Theoretical studies and numerical modeling of terrestrial and habitable planet formation are, therefore, necessary to gain fundamental insights into the prospects for life in such systems and have great strategic impact on NASA science and missions.

Comments:

7 pages, White Paper Submitted to the NASA/NSF ExoPlanet Task Force

Subjects:

Astrophysics (astro-ph)

Cite as:

arXiv:0704.0832v1 [astro-ph]

Submission history

From: Nader Haghighipour [view email]

[v1] Fri, 6 Apr 2007 02:34:35 GMT (630kb)

http://arxiv.org/abs/0704.0832

ljk May 3, 2007 at 11:28

XO-2b: Transiting Hot Jupiter in a Metal-rich Common Proper Motion Binary

Authors: Christopher J. Burke, P. R. McCullough, Jeff A. Valenti, Christopher M. Johns-Krull, Kenneth A. Janes, J. N. Heasley, F. J. Summers, J. E. Stys, R. Bissinger, Michael L. Fleenor, Cindy N. Foote, Enrique Garcia-Melendo, Bruce L. Gary, P. J. Howell, F. Mallia, G. Masi, B. Taylor, T. Vanmunster

(Submitted on 1 May 2007)

Abstract: We report on a V=11.2 early K dwarf, XO-2 (GSC 03413-00005), that hosts a Rp=0.973+0.03/-0.008 Rjup, Mp=0.57+/-0.06 Mjup transiting extrasolar planet, XO-2b, with an orbital period of 2.615838+/-0.000008 days. XO-2 has high metallicity, [Fe/H]=0.45+/-0.02, high proper motion, mu_tot=157 mas/yr, and has a common proper motion stellar companion with 31″ separation. The two stars are nearly identical twins, with very similar spectra and apparent magnitudes. Due to the high metallicity, these early K dwarf stars have a mass and radius close to solar, Ms=0.98+/-0.02 Msolar and Rs=0.964+0.02/-0.009 Rsolar. The high proper motion of XO-2 results from an eccentric orbit (Galactic pericenter, Rper

ljk May 23, 2007 at 14:38

Observational Techniques for Detecting Planets in Binary Systems

Authors: Matthew W. Muterspaugh (Townes Fellow, Space Sciences Laboratory, UC Berkeley), Maciej Konacki (Nicolaus Copernicus Astronomical Center, Poland), Benjamin F. Lane (MIT Kavli Institute for Astrophysics and Space Research), Eric Pfahl (KITP)

(Submitted on 21 May 2007)

Abstract: Searches for planets in close binary systems explore the degree to which stellar multiplicity inhibits or promotes planet formation. There is a degeneracy between planet formation models when only systems with single stars are studied–several mechanisms appear to be able to produce such a final result. This degeneracy is lifted by searching for planets in binary systems; the resulting detections (or evidence of non-existence) of planets in binaries isolates which models may contribute to how planets form in nature. In this chapter, we consider observational efforts to detect planetary companions to binary stars in two types of hierarchical planet-binary configurations: first “S-type” planets which orbit just one of the stars, with the binary period being much longer than the planet’s; second, “P-type” or circumbinary planets, where the planet simultaneously orbits both stars, and the planetary orbital period is much longer than that of the binary. The S-type planet finding techniques are different for binaries that can or cannot be spatially resolved. For wider systems, techniques reviewed include dualstar interferometric differential astrometry and precision radial velocities. Alternatively, unresolved binaries can be studied using modified dualstar “PHASES-style” differential astrometry or a modification of the radial velocity technique for composite spectra. Should a fortunately aligned–but still long period–binary be found, eclipse timing can also reveal the presence of S-type planets. Methods for detecting P-type planets include the composite-spectra variant of the radial velocity technique and eclipse timing.

Comments:

Chapter to appear in the book “Planets in Binary Star Systems,” ed. Nader Haghighipour (Springer publishing company), 2007; 33 Pages

Subjects:

Astrophysics (astro-ph)

Cite as:

arXiv:0705.3072v1 [astro-ph]

Submission history

From: Matthew Muterspaugh [view email]

[v1] Mon, 21 May 2007 23:43:02 GMT (108kb)

http://arxiv.org/abs/0705.3072

ljk July 19, 2007 at 12:06

On the migration of protoplanets embedded in circumbinary disks

Authors: Arnaud Pierens, Richard P. Nelson

(Submitted on 18 Jul 2007)

Abstract: We present the results of hydrodynamical simulations of low mass protoplanets embedded in circumbinary accretion disks. The aim is to examine the migration and long term orbital evolution of the protoplanets, in order to establish the stability properties of planets that form in circumbinary disks. Simulations were performed using a grid–based hydrodynamics code.

First we present a set of calculations that study how a binary interacts with a circumbinary disk. We evolve the system for 10^5 binary orbits, which is the time needed for the system to reach a quasi-equilibrium state. From this time onward the apsidal lines of the disk and the binary are aligned, and the binary eccentricity remains essentially unchanged with a value of e_b ~ 0.08. Once this stationary state is obtained, we embed a low mass protoplanet in the disk and let it evolve under the action of the binary and disk forces.

We consider protoplanets with masses of 5, 10 and 20 Earth masses. In each case, we find that inward migration of the protoplanet is stopped at the edge of the tidally truncated cavity formed by the binary. This effect is due to positve corotation torques, which can counterbalance the net negative Lindblad torques in disk regions where the surface density profile has a sufficiently large positive gradient.

Halting of migration occurs in a region of long-term stability, suggesting that low mass circumbinary planets may be common, and that gas giant circumbinary planets should be able to form in circumbinary disks.

Comments: 10 pages, 10 figures, accepted for publication in A&A

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0707.2677v1 [astro-ph]

Submission history

From: Arnaud Pierens [view email]

[v1] Wed, 18 Jul 2007 08:56:16 GMT (323kb)

http://arxiv.org/abs/0707.2677

ljk July 20, 2007 at 10:00

Formation of Protoplanets from Massive Planetesimals in Binary Systems

Authors: Yusuke Tsukamoto, Junichiro Makino

(Submitted on 19 Jul 2007)

Abstract: More than half of stars reside in binary or multiple star systems and many planets have been found in binary systems. From theoretical point of view, however, whether or not the planetary formation proceeds in a binary system is a very complex problem, because secular perturbation from the companion star can easily stir up the eccentricity of the planetesimals and cause high-velocity, destructive collisions between planetesimals. Early stage of planetary formation process in binary systems has been studied by restricted three-body approach with gas drag and it is commonly accepted that accretion of planetesimals can proceed due to orbital phasing by gas drag. However, the gas drag becomes less effective as the planetesimals become massive. Therefore it is still uncertain whether the collision velocity remains small and planetary accretion can proceed, once the planetesimals become massive.

We performed {\it N}-body simulations of planetary formation in binary systems starting from massive planetesimals whose size is about 100-500 km. We found that the eccentricity vectors of planetesimals quickly converge to the forced eccentricity due to the coupling of the perturbation of the companion and the mutual interaction of planetesimals if the initial disk model is sufficiently wide in radial distribution. This convergence decreases the collision velocity and as a result accretion can proceed much in the same way as in isolated systems. The basic processes of the planetary formation, such as runaway growth and oligarchic growth and final configuration of the protoplanets are essentially the same in binary systems and single star systems, at least in the late stage where the effect of gas drag is small.

Comments: 26pages, 11 figures. ApJ accepted

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0707.2928v1 [astro-ph]

Submission history

From: Yusuke Tsukamoto [view email]

[v1] Thu, 19 Jul 2007 15:50:46 GMT (425kb)

http://arxiv.org/abs/0707.2928

ljk September 14, 2007 at 10:59

Microlensing Detections of Planets in Binary Stellar Systems

Authors: Dong-Wook Lee, Chung-Uk Lee, Byeong-Gon Park, Sun-Ju Chung, Young-Soo Kim, Ho-Il Kim, Cheongho Han

(Submitted on 13 Sep 2007)

Abstract: We demonstrate that microlensing can be used for detecting planets in binary stellar systems. This is possible because in the geometry of planetary binary systems where the planet orbits one of the binary component and the other binary star is located at a large distance, both planet and secondary companion produce perturbations at a common region around the planet-hosting binary star and thus the signatures of both planet and binary companion can be detected in the light curves of high-magnification lensing events.

We find that identifying planets in binary systems is optimized when the secondary is located in a certain range which depends on the type of the planet. The proposed method can detect planets with masses down to one tenth of the Jupiter mass in binaries with separations less than ~ 100 AU. These ranges of planet mass and binary separation are not covered by other methods and thus microlensing would be able to make the planetary binary sample richer.

Comments: 5 pages, two figures in JPG format

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0709.1992v1 [astro-ph]

Submission history

From: Cheongho Han [view email]

[v1] Thu, 13 Sep 2007 04:50:57 GMT (127kb)

http://arxiv.org/abs/0709.1992

ljk October 4, 2007 at 21:47

Habitability of Planets in Binaries

Authors: Nader Haghighipour

(Submitted on 3 Oct 2007)

Abstract: A survey of currently known extrasolar planets indicates that close to 20% of their hosting stars are members of binary systems. While the majority of these binaries are wide (i.e., with separations between 250 and 6500 AU), the detection of Jovian-type planets in the three binaries of Gamma Cephei (separation of 18.5 AU), GL 86 (separation of 21 AU), and HD 41004 (separation of 23 AU) have brought to the forefront questions on the formation of giant planets and the possibility of the existence of smaller bodies in moderately close binary star systems. This paper discusses the late stage of the formation of habitable planets in binary systems that host Jovian-type bodies, and reviews the effects of the binary companion on the formation of Earth-like planets in the system’s habitable zone. The results of a large survey of the parameter-space of binary-planetary systems in search of regions where habitable planets can form and have long-term stable orbits are also presented.

Comments: 4 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:0710.0686v1 [astro-ph]

Submission history

From: Nader Haghighipour [view email]

[v1] Wed, 3 Oct 2007 01:37:09 GMT (120kb)

http://arxiv.org/abs/0710.0686

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