Binary Stars and Terrestrial Worlds

by Paul Gilster on January 22, 2007

The findings about possible terrestrial worlds around the Alpha Centauri stars have become more encouraging than ever. Key work in this regard has been performed by Elisa Quintana and collaborators, who have shown in their simulations that, depending on initial disk inclinations, 3-5 such planets might form around Centauri A and 2-5 around Centauri B.

We’ve already discussed that research and I don’t want to linger on Quintana’s 2002 paper (reference below) other than to note one interesting comparison. When the same initial disk parameters are placed around a single star like the Sun, the accretion of the planetary disk occurs over a much larger expanse of time. Evidently a stellar companion hastens the process of planetary formation, one billion years in the case of the Sun vs. perhaps 200 million years in the Centauri scenario.

Quintana, Jack Lissauer (both at NASA Ames) and team went on from that study to look at planet formation around close binaries. And they’ve now turned to the factors influencing terrestrial worlds around individual stars in binary systems. That includes, of course, binaries like Alpha Centauri, but its significance goes well beyond our closest stellar neighbor given that the majority of solar-like stars are found in binary systems. Using numerical simulations to model planet formation and stability in these circumstances, their new paper gives us important information on where we might look to find planets like ours elsewhere in the galaxy.

Much depends on how close the two binary stars are, the most important factor being the periastron value — the distance between the two stars at the closest point in their orbits. The periastron value for Centauri A and B is 11.2 AU, a perfectly acceptable figure for terrestrial planet formation. For as these studies show, a periastron greater than 10 AU allows planets to develop unperturbed. From the paper:

When the periastron of the binary is larger than about qB = 10 AU, even for the case of equal mass stars, terrestrial planets can form over essentially the entire range of orbits allowed for single stars (out to the edge of the initial planetessimal disk at 2 AU). When periastron qB < 10 AU, however, the distributions of planetary orbital parameters are strongly affected by the presence of the binary companion."

So the good news for terrestrial planet hunters is that 40 to 50 percent of binary stars are wide enough to allow Earth-like planets to form and remain stable in orbits circling one of the two stars. And interestingly enough, Quintana’s work also shows that about 10 percent of main sequence binaries are close enough to allow the formation and stability of such planets in orbits that circle both stars.

What a finish: “Given that the galaxy contains more than 100 billion star systems, and that roughly half remain viable for the formation and maintenance of Earth-like planets, a large number of systems remain habitable based on the dynamic considerations of this research.”

The paper is Quintana et al., “Terrestrial Planet Formation Around Individual Stars Within Binary Star Systems,” accepted by The Astrophysical Journal and available as a preprint. The 2002 paper, available here, is “Terrestrial Planet Formation in the α Centauri System,” The Astrophysical Journal 576:982-996 (September 10 2002).

Rob January 23, 2007 at 15:48

It should be mentioned however that the estimated age of the solar system is 4.57 billion years while the oldest moon rocks are 4.5 billion years old and the estimated age of the Earth is about the same.
(Actually wikipedia quotes both the age of the Sun and the Earth at 4.57 billion years the 4.5 billion for Earth is what I heard at my intro to astronomy class)

So planets don’t actually seem to take 1 billion or even 200 million years to form.

Adam January 24, 2007 at 4:21

Hi Rob

The 4.57 billion for the solar system is really for when the elements in meteoroids had condensed below melting point enough to allow the trapping of uranium-decay lead in their crystal structure. Prior to that the dust and gas of the proto-Sun could have been orbitting for some time. The free-fall time of a solar mass cloud is just a few thousand years, but the Helmholtz-contraction phase can take tens of millions of years. The system probably collapsed quickly after supernova seeded it with Al26 and similar short-lived isotopes.

Rob January 24, 2007 at 8:41

Adam: Yes, I thought of that but as you say we are talking about tens of millions of years not hundreds of millions or billions. I don’t think planetary disks actually last hundreds of millions of years, do they?

Of course it is always possible that isolated formation of terrestrial planets just doesn’t happen – there needs to be a close companion and gas giants or a supernova as a trigger – but I’m really not sure. In any case nearby supernova explositions in the early times of a star’s formation might be the norm rather than the exception. So while this is an interesting result it might not reflect how terrestrial planets actually form and certainly doesn’t show how they formed in our solar system.

Eric James January 25, 2007 at 0:21

Here’s a question:

The nearest model of a planetary disk we have is the rings of Saturn. Why haven’t they condensed? What keeps this mass suspended and fragmented? Are the rings made of materials that electrostatically repel each other? How are they renewed?

andy January 25, 2007 at 5:30

Eric James: Saturn’s rings are within the Roche limit of Saturn: tidal forces would rip apart an object held together by its own gravity. This means moons cannot form in that region. A protoplanetary disc extends well beyond the Roche limit of the star.

Adam January 25, 2007 at 17:11

Hi Rob

There’s a problem of different formation times for different scales too. Ultra-fine silicate dust forms really early, then fluff-balls, then metre-scale lumps and so on up the scale of sizes. The last to form are the planets themselves. The final oligarchic formation stage might take a billion years, though current evidence puts Theia’s crash into Earth not much past 4.5 billion years ago, just as you said. But even after that there was a lot of junk in the system – multi-hundred kilometre lumps were still “accreting” onto the planets every hundred million years or so up to c. 4 billion years ago.

Where to draw the line? When does “accretion” become “impacts” on to an officially ‘finished’ planet? But I’m starting to agree – the time scale seems exaggerated.

Adam

Administrator January 25, 2007 at 17:58

Let me see if I can get a comment from Elisa Quintana on all this.

Ron S January 26, 2007 at 0:06

There are small moons within Saturn’s ring system. Are they visitors or is there a size limit below which tidal forces aren’t strong enough to pull apart the moon? My intuition says ‘no’ but that could be wrong.

ljk March 1, 2007 at 23:31

Astrophysics, abstract
astro-ph/0702706

From: Nader Haghighipour [view email]

Date: Tue, 27 Feb 2007 06:34:25 GMT (704kb)

Habitable Planet Formation in Binary-Planetary Systems

Authors: Nader Haghighipour, Sean N. Raymond

Comments: 27 pages, 11 figures, submitted for publication

Recent radial velocity observations have indicated that Jovian-type planets can exist in moderately close binary star systems. Numerical simulations of the dynamical stability of terrestrial-class planets in such environments have shown that, in addition to their giant planets, these systems can also harbor Earth-like objects. In this paper, we study the late stage of terrestrial planet formation in such binary-planetary systems, and present the results of the simulations of the formation of Earth-like bodies in their habitable zones. We consider a circumprimary disk of Moon- to Mars-sized objects and numerically integrate the orbits of these bodies at the presence of the Jovian-type planet of the system and for different values of the mass, semimajor axis, and orbital eccentricity of the secondary star. Results indicate that, Earth-like objects, with substantial amounts of water, can form in the habitable zone of the primary star. Simulations also indicate that, by transferring angular momentum from the secondary star to protoplanetary objects, the giant planet of the system plays a key role in the radial mixing of these bodies and the water contents of the final terrestrial planets. We will discuss the results of our simulation and show that the formation of habitable planets in binary-planetary systems is more probable in binaries with moderate to large perihelia.

http://arxiv.org/abs/astro-ph/0702706

ljk March 28, 2007 at 14:17

Astrophysics, abstract
astro-ph/0703609

From: Juan Cabrera [view email]

Date: Fri, 23 Mar 2007 11:26:55 GMT (171kb)

Detecting companions to extrasolar planets using mutual events

Authors: J. Cabrera, J. Schneider

Comments: 6 pages, 7 figures

Journal-ref: A&A 464, 1133-1138 (2007)

DOI: 10.1051/0004-6361:20066111

We investigate a new approach to the detection of companions to extrasolar planets beyond the transit method. We discuss the possibility of the existence of binary planets.

We develop a method based on the imaging of a planet-companion as an unresolved system (but resolved from its parent star). It makes use of planet-companion mutual phenomena, namely mutual transits and mutual shadows.

We show that companions can be detected and their radius measured down to lunar sizes.

http://arxiv.org/abs/astro-ph/0703609

ljk May 23, 2007 at 23:37

Terrestrial Planet Formation in Binary Star Systems

Authors: Elisa V. Quintana, Jack J. Lissauer

(Submitted on 23 May 2007)

Abstract: A binary star system is the most common result of the star formation process, and binary companions can disrupt both the formation of terrestrial planets and their long term prospects for stability. We present results from a large set of numerical simulations of the final stages of terrestrial planet formation – from Moon- to Mars-sized planetary embryos to planets – in main-sequence binary star systems. We examine planetary accretion around both stars (‘P-type’ circumbinary orbits) or individual stars (‘S-type’ orbits) in binary systems, including terrestrial planet formation around each star in Alpha Centauri AB, the closest binary star system to the Sun. For comparison, we also simulate planetary growth from the same initial disk placed in the Sun-Jupiter-Saturn system and also around the Sun with neither giant planets nor a stellar companion perturbing the system. Our simulations show that giant and stellar companions not only truncate the disk, but hasten the accretion process by stirring up the planetary embryos to higher eccentricities and inclinations. Terrestrial planets similar to those in our Solar System formed around individual stars in simulations with the binary periastron (closest approach) greater than about 5 AU. Terrestrial planet growth within circumbinary disks was uninhibited around inner binary star systems with binary apastrons (maximum separation) less than ~0.2 AU. Results from our simulations can be scaled for different stellar and disk parameters. Approximately 50 – 60% of binary star systems – from contact binaries to separations of nearly a parsec – satisfy these constraints. Given that the galaxy contains more than 100 billion star systems, a large number of systems remain habitable based on the dynamic considerations of this research.

Comments:

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

Subjects:

Astrophysics (astro-ph)

Cite as:

arXiv:0705.3444v1 [astro-ph]

Submission history

From: Elisa Quintana [view email]

[v1] Wed, 23 May 2007 18:37:14 GMT (196kb)

http://arxiv.org/abs/0705.3444

ljk May 23, 2007 at 23:43

On the Formation and Dynamical Evolution of Planets in Binaries

Authors: Willy Kley (1), Richard Nelson (2) ((1) University of Tuebingen, (2) QMUL)

(Submitted on 23 May 2007)

Abstract: Among the extrasolar planetary systems about 30 are located in a stellar binary orbiting one of the stars, preferably the more massive primary. The dynamical influence of the second companion alters firstly the orbital elements of the forming protoplanet directly and secondly the structure of the disk from which the planet formed which in turn will modify the planet’s evolution. We present detailed analysis of these effects and present new hydrodynamical simulations of the evolution of protoplanets embedded in circumstellar disks in the presence of a companion star, and compare our results to the system $\gamma$ Cep. To analyse the early formation of planetary embryos, we follow the evolution of a swarm of planetesimals embedded in a circumstellar disk. Finally, we study the evolution of planets embedded in circumbinary disks.

Comments:

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

Subjects:

Astrophysics (astro-ph)

Cite as:

arXiv:0705.3421v1 [astro-ph]

Submission history

From: Willy Kley [view email]

[v1] Wed, 23 May 2007 16:56:20 GMT (892kb)

http://arxiv.org/abs/0705.3421

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