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Early Notes Toward a Galactic Census

What sort of stars harbor the planetary systems we’ve thus far identified? The answer is easy: most of the known exoplanets were found through radial velocity surveys, and these focus on nearby Sun-like stars. Thus we’re looking at a range of stars between late-F and early-K class dwarfs, and almost all are within 50 parsecs of the Sun. It is also apparent that planetary systems in our scope of observation increase with increasing metallicity of the parent star, a measure of elements higher than hydrogen and helium.

Are there other trends we can identify? Perhaps not. As I. Neill Reid (Space Telescope Science Institute) writes in a new paper on the subject, “With the possible exception of a higher mean velocity perpendicular to the Plane, the planetary hosts appear to be unremarkable members of the Galactic Disk.” There is not, for example, a correlation we might expect to find in metal-rich stars between the mass of the primary star and the masses of its planetary companions (consider the M-class dwarf Gl 876, which harbors two Jupiter-class planets). Reid thinks this points to high metallicity as a trigger for planet formation rather than a player in the formation mechanism itself.

What’s fascinating about this work is what Reid calls a ‘back of the envelope’ calculation of how many Solar-type stars may have gas giant companions. These are systems that would be detectable by today’s radial velocity measurements were we within range. Reid examined a ring of space (call it the ‘solar circle’) drawn around the galaxy at roughly Sol’s distance from galactic center.

The answer, developed with reference to metallicity distribution in our local galactic neighborhood: 3.5 X 107 such planetary systems detectable via radial velocity methods within a range of 6 to 10 kiloparsecs from galactic center. In other words, 35 million planetary systems around Sun-like stars within a ring of space defined as roughly 20000 to 32000 light years from the center of the Milky Way.

Centauri Dreams‘ note: By way of comparison, our Sun is roughly 26,000 light years (8000 parsecs) from galactic center, so we’re getting a view of the density of planetary systems within a ring drawn around the galaxy at roughly the same distance from the center as Sol. Back of the envelope it may be, but it’s the first such calculation I know of that draws on existing metallicity data to reach a conclusion that moves beyond pure guesswork, and it’s heartening for the encouragement it provides that we’re going to find planetary systems in all but limitless abundance as we expand the search.

Comments on this entry are closed.

  • Zen Blade August 29, 2006, 21:34

    Not a physicist, but I read through the paper (somewhat). I feel I have a grasp on a lot of the information, but are there any predictions that can be made from this paper… not necessarily in the number of total planetary systems but is there any sort of Metallicity Threshold that can be guessed at or uncovered? Or are there any other predictions that you would like to see from the data so far collected?

    Likewise, (also not being an astrophysicist) are there any predictions that could be made about other types of stars and there propensity to develop planets? Or is the above data very limited in its application?


  • Administrator August 29, 2006, 21:53

    These are good questions. I’m not an astrophysicist either, so let me pass them along to Dr. Reid and see if we can get his thoughts on this.

  • Ronald Botterweg August 30, 2006, 6:43

    I actually found the estimated number amazingly small, considering the total estimate of stars in our galaxy (roughly 1 in 10,000 of all stars). However, reading the pub, it struck me that the author assumed a frequency of only 6% gas giants around sun-like stars ánd only considered the gas giants orbiting within 4 AU of their host star (because these are relatively easily detectable by radial-velocity), that is within the orbit of Jupiter. In other words his estimated frequency is on the (very) modest side, and he is very limiting, possibly leaving out many of the most interesting stars: those with gas giants beyong 4 AU. As total detection time increases and technology develops, increasingly more (and smaller) planets with increasingly greater orbits are being discovered, thus significantly raising the percentage of sun-like stars with (gas giant) planets.

  • Ronald Botterweg August 30, 2006, 6:55

    With regard to metallicity: see exoplanets.org (publ. in section “Metallicities and Extrasolar Panets” and news flash “Planets Correlate with Metallicity”);
    estimates of lower metallicity thresholds necessary for (giant) planet formation are very roughly 1/3 to 1/2 of solar metallicity. I don’t know whether small rocky planets can still be formed below this.
    Another very preliminary impression of my own: beyond roughly 1.5 to 2 times solar metallicity, there seems to be a kind of run-away planet formation resulting in giant planets in close orbit.

  • Adam September 2, 2006, 8:11

    Hi Ronald

    Your assessment is similar to Charles Lineweaver’s work on the Galactic Habitable Zone, which gives similar metallicity constraints, both too little and too much.

    His papers are available online through the physics preprint archive at arxiv.org, as are a lot of the ones discussed here.