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Planets Forming Around a Close Binary

Planets around binary stars continue to be a major interest here, given our fascination with nearby Alpha Centauri. Thus the recent radio interferometry images captured by the Submillimeter Array radio telescope system (Mauna Kea) come right to the top of the queue. We’re looking at a young binary system called V4046 Sagittarii, providing a glimpse of planetary system formation occurring around two stars of roughly the Sun’s mass. This system is approximately 240 light years from our own.


Image: Submillimeter Array image of the rotating, gaseous disk surrounding the young twin-star system V4046 Sagittarii (located at the white dot in the image). Note the size of the V4046 Sagittarii disk relative to the orbit of Neptune, shown to scale at the lower right (the filled oval at lower left represents the size of the smallest structures that could be detected in the image). The disk is tipped from our perspective, such that it appears as elliptical rather than circular. The image is color-coded according to the motion of the gas in the system, with blue representing material that is approaching and red representing material that is receding from us. The fastest-moving approaching and receding gas is detected closest to the central binary star system, as expected if the disk gas obeys Kepler’s laws of planetary motion. This disk gas likely represents the raw material out of which Pluto-like bodies, comets, and perhaps gas giant planets will form (or might already have formed) around the double star. Credit: Joel Kastner/SMA.

Carbon monoxide and hydrogen cyanide had been found in the circumstellar molecular gas cloud around V4046 Sagittarii, comprising raw materials for planet formation. The rotating disk found there was confirmed by the imagery following that earlier work, says Joel Kastner (Rochester Institute of Technology):

“It’s a case of seeing is believing. We had the first evidence for this rotating disk in radio telescope observations of V4046 Sagittarii that we made last summer. But at that point, all we had were molecular spectra, and there are different ways to interpret the spectra. Once we saw the image data from the SMA, there was no doubt that we have a rotating disk here.”

And quite an interesting disk at that: We’re dealing with two solar-mass stars with a separation of no more than five solar diameters, a much tighter fit than we see between Centauri A and B. The twin stars are young, at roughly twelve million years old, but the protoplanetary disk may be the oldest known. If that’s the case, we’re looking at the formation of Jupiter-class planets continuing long after the first few million years. What challenges that poses to planet formation theories remains to be seen.

Close binaries present unusual problems for radial velocity measurements, so we’re pushing into a new discovery zone for exoplanets with this work. Kastner notes the implications:

“In this case the stars are so close together, and the profile of the gas — in terms of the types of molecules that are there — is so much like the types of gaseous disks that we see around single stars, that we now have a direct link between planets forming around single stars and planets forming around double stars.”

This work was presented today at the American Astronomical Society meeting in Pasadena and draws on earlier work reporting on the molecular gas cloud around V4046 Sagittarii. The imagery now confirms the earlier study. The paper is Kastner et al., “Molecules in the disk orbiting the twin young suns of V4046 Sgr,” Astronomy & Astrophysics 492 (2008), pp. 469-473 (abstract).

Comments on this entry are closed.

  • andy June 10, 2009, 18:22

    Seems to be quite the time for protoplanetary discs in binaries, recently on the arXiv was a paper in which the first instance of a binary star system with protoplanetary discs large enough to produce a system comparable to our own have been detected around each star.

  • Giordano Bruno June 12, 2009, 2:38

    The image nust be an elevation from the edge of the disc, not a plan from above the disc, if blue/red is receding/approaching?
    Thus the disc is much fatter than the “artists” version appearing on popular web sites?
    Presumably as it ages it will become actually thin?

  • andy June 13, 2009, 17:25

    One thing to bear in mind here is that while radial velocity measurements aren’t so good for finding circumbinary planets, other methods are. For example, a 2-planet system has been detected orbiting the eclipsing binary HW Virginis by measuring changes in the timing of the eclipses due to light travel time variations as the system moves back and forth.

  • ljk June 15, 2009, 22:30

    Effects of Mutual Transits by Extrasolar Planet-Companion Systems on Light Curves

    Authors: Masanao Sato, Hideki Asada

    (Submitted on 15 Jun 2009)

    Abstract: We consider effects of mutual transits by extrasolar planet-companion systems (in a true binary or a planet-satellite system) on light curves.

    We show that induced changes in light curves depend strongly on a ratio between a planet-companion’s orbital velocity around their host star and a planet-companion’s spin speed around their common center of mass. For both of slow and fast spin cases (corresponding to large and small separations between them, respectively), a certain asymmetry appears in light curves.

    We show that, especially for small separation cases, geometrical blocking of one faint object by the other transiting a parent star causes an apparent increase in light curves and characteristic fluctuations appear as an important evidence of mutual transits.

    We show also that extrasolar mutual transits provide a complementary method of measuring the radii of two transiting objects, their separation and mass, and consequently identifying them as a true binary, planet-satellite system or others. Monitoring $10^5$ stars for three years with Kepler may lead to a discovery of a second Earth-Moon-like system if the fraction of such systems for an averaged star is larger than 0.05, or it may put upper limits on the fraction as f < 0.05.

    Comments: 15 pages, 6 figures, 1 table; accepted for publication in PASJ

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)

    Cite as: arXiv:0906.2590v1 [astro-ph.EP]

    Submission history

    From: Hideki Asada [view email]

    [v1] Mon, 15 Jun 2009 02:17:59 GMT (80kb)


  • ljk August 7, 2009, 14:56

    Planet formation in highly inclined binaries

    Authors: F. Marzari, P. Thebault, H. Scholl

    (Submitted on 6 Aug 2009)

    Abstract: We explore planet formation in binary systems around the central star where the protoplanetary disk plane is highly inclined with respect to the companion star orbit. This might be the most frequent scenario for binary separations larger than 40 AU, according to Hale (1994).

    We focus on planetesimal accretion and compute average impact velocities in the habitable region and up to 6 AU from the primary.

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0908.0803v1 [astro-ph.EP]

    Submission history

    From: Francesco Marzari Dr. [view email]

    [v1] Thu, 6 Aug 2009 07:22:19 GMT (122kb)


  • ljk August 7, 2009, 14:58

    On the eccentricity of self-gravitating circumstellar disks in eccentric binary systems

    Authors: F. Marzari, H. Scholl, P. Thebault, C. Baruteau

    (Submitted on 6 Aug 2009)

    Abstract: We study the evolution of circumstellar massive disks around the primary star of a binary system focusing on the computation of disk eccentricity. In particular, we concentrate on its dependence on the binary eccentricity. Self-gravity is included in our numerical simulations.

    Our standard model assumes a semimajor axis for the binary of 30 AU, the most probable value according to the present binary statistics.

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0908.0808v1 [astro-ph.EP]

    Submission history

    From: Francesco Marzari Dr. [view email]

    [v1] Thu, 6 Aug 2009 08:02:17 GMT (624kb)


  • ljk September 24, 2009, 11:22

    September 23, 2009

    Spitzer Watches Planet-Forming Disk Change Quickly

    Written by Nancy Atkinson

    Something strange is going on around a young star called LRLL 31. Astronomers have witnessed a swirling disk of gas and dust which is changing rather quickly; sometimes weekly. This is likely a planet forming disk, however, planets take millions of years to form, so it’s rare to see anything change on time scales we humans can perceive.

    Another object appears to be pushing a clump of planet-forming material around the star, and this region is offering astronomers with the Spitzer Space Telescope a rare look into the early stages of planet formation.

    Astronomer are seeing the light from this disk varying quite frequently. One possible explanation is that a close companion to the star — either a star or a developing planet — could be shoving planet-forming material together, causing its thickness to vary as it spins around the star.

    “We don’t know if planets have formed, or will form, but we are gaining a better understanding of the properties and dynamics of the fine dust that could either become, or indirectly shape, a planet,” said James Muzerolle of the Space Telescope Science Institute, Baltimore, Md. Muzerolle is first author of a paper accepted for publication in the Astrophysical Journal Letters. “This is a unique, real-time glimpse into the lengthy process of building planets.”

    Full article here: