I read popular physics: A planet is born

This is part of my series where I read physics articles in Scientific American, and offer commentary as a former physicist.

I’ve had the June issue around for over a month, but I procrastinated too much and here we are in July. I’ll try to keep it short this time so I can move on to the next one.

The June issue is when COVID-19 really hits Scientific American. The cover says in big letters: “The Coronavirus Pandemic”. I already got the July issue, and the coronavirus is on the cover of that too. But, the cover notwithstanding, there are still physics articles, and that’s my area of expertise. This month’s article is “A Planet is Born“, by Meredith A. MacGregor.  This one isn’t paywalled so you’re free to read it yourself.

Let’s try for a one-sentence summary. Astronomers used high-resolution millimeter-wavelength imaging to detect circumstellar disks analogous to our solar system’s Kuiper belt, and infer the presence of planets based on the effect they had on the dust rings.

I’ve discussed multiple times the mismatch between a scientific article’s hook, and what makes it actually scientifically exciting. Here, the hook is exasolar planets. But personally I think the more exciting thing is the circumstellar disks themselves. Merely detecting a planet is one thing, but it’s another to directly see the debris left over from the planet formation process, a process that I’ve been told is poorly understood. Rather than thinking of the circumstellar disks as a means to an end, I think they are a valuable end in themselves. I’d bet that the scientists working in this area privately agree.

High-resolution astronomy seems to be a theme among the astronomy articles I’ve reviewed. One of the neat things about reading science articles, is they eventually start to build on each other, each providing context for the others. This study has resolution of around 0.01 arc seconds, which is hecka small. But compare it to another study I discussed, about mapping the Milky Way, and that had 0.001 arc-second resolution. Or another study that imaged a black hole, using 0.00004 arc-second resolution.

Those other two studies used radio waves, where this study used millimeter wavelength (or far infrared) light. Using shorter wavelength light tends to improve the resolution; but where the other two studies used radar arrays spread across the globe, this study uses a radar array spread across a “mere” 16 km.

It’s a straightforward article, but there was one itsy bitsy grain of sand that puzzled me. It talks about small dust particles (microns in size) and larger dust particles (the size of sand). The small dust particles tend to get blown away by solar winds and other things, but the larger dust particles stick around as a record of planet formation. Astronomers can distinguish between large and small dust particles, because near-infrared light comes from small dust particles and far-infrared light comes from large dust particles. But why would that be? If you have any ideas, drop them in the comments.


  1. Bruce says

    I know nothing about IR from dust.
    But, could it be that larger dust particles resonate energy at a lower vibrational frequency, related to the particle mass? Presumably that would cause lower energy radiation from the larger dust?

  2. StevoR says

    Hmm .. I was looking forward to reading it but when I click on the link for the Meredith A. MacGregor “A Planet is Born“ article here I’m just gettinga blank page. No error message, no nothing. 🙁

    Is that just me or are others having this problem too?

    I’m guessing this might be for the new found world round AU Microscopii is it? (https://www.abc.net.au/news/2020-06-29/usq-nasa-discover-new-earth-sized-planet-a-mic-b/12398056)

    Or could it for the SuperEarth’s discovered orbiting Lacaille 9352 / Gliese 887? ( http://www.sci-news.com/astronomy/compact-system-super-earths-lacaille-9352-08577.html ) Hmm .. probly not the latter as they seem older and well formed already.

    Or another system entirely? Plenty of stars out there.

  3. says

    It’s down for me too. I think it’s a problem on SciAm’s end, and you should try again later.

    They don’t mention the discovery of any particular planet. I think this particular method of detecting planets is more theory-laden, and they still need to argue that the way the circumstellar disks are structured are caused by planets rather than something else. I’m not sure how many of these disks they’ve observed, but there are 20 images shown just on the first page.

  4. Owlmirror says

    Astronomers can distinguish between large and small dust particles, because near-infrared light comes from small dust particles and far-infrared light comes from large dust particles. But why would that be?

    I may have a reference that might help. However, I don’t understand it well enough to be sure. Maybe you could explain it to me?

    Draine 2006 – On the Submillimeter Opacity of Protoplanetary Disks

    Solid particles with the composition of interstellar dust and power-law size distribution dn/da \propto a^{-p} (for a \le a_{max} with a_{max} \gtrsim 3\lambda and 3 < p < 4) will have submillimeter opacity spectral index \beta(\lambda) \equiv d ln \kappa/d ln \nu \approx (p - 3)\beta_{ISM} , where \beta_{ISM} \approx 1.7 is the opacity spectral index of interstellar dust material in the Rayleigh limit. For the power-law index p ≈ 3.5, which characterizes interstellar dust and may apply for particles growing by agglomeration in protoplanetary disks, grain growth to sizes a \gtrsim 3 mm will result in \beta(1 mm) \lesssim 1. Grain growth can naturally account for β ≈ 1 observed for protoplanetary disks, provided that a_{max} \gtrsim 3\lambda.

  5. Rob Grigjanis says

    near-infrared light comes from small dust particles and far-infrared light comes from large dust particles. But why would that be?

    Mie scattering?

  6. Owlmirror says

    Another ref by (Weingartener & Draine 2001)

    Dust Grain-Size Distributions and Extinction in the Milky Way, Large Magellanic Cloud, and Small Magellanic Cloud

    We construct size distributions for carbonaceous and silicate grain populations in different regions of the Milky Way, LMC, and SMC. The size distributions include sufficient very small carbonaceous grains (including polycyclic aromatic hydrocarbon molecules) to account for the observed infrared and microwave emission from the diffuse interstellar medium. Our distributions reproduce the observed extinction of starlight, which varies depending on the interstellar environment through which the light travels. As shown by Cardelli, Clayton, and Mathis in 1989, these variations can be roughly parameterized by the ratio of visual extinction to reddening, R_{V}. We adopt a fairly simple functional form for the size distribution, characterized by several parameters. We tabulate these parameters for various combinations of values for R_{V} and b_{C}, the C abundance in very small grains. We also find size distributions for the line of sight to HD 210121 and for sight lines in the LMC and SMC. For several size distributions, we evaluate the albedo and scattering asymmetry parameter and present model extinction curves extending beyond the Lyman limit.

  7. Owlmirror says

    BTW, as I type this, the price of the digital issue of Scientific American for 2020-06 (and only for that month) is $0.00.

  8. Rob Grigjanis says

    Further to #5: Radiation from the sun peaks in the visible light range (see here, where the blue curve roughly corresponds to the sun’s spectrum). Because the molecules in the atmosphere are much smaller than the wavelengths (λ) of visible light, the scattering goes like (1/λ)^4; Rayleigh scattering. Hence, blue sky; shorter wavelengths are scattered much more than long ones. But since water droplets are much larger than the incident wavelengths, their scattering of the incident light is independent of wavelength; hence, white clouds.

    Radiation from cooler stars (surface temp 3000K or less) peaks in the infrared. If such a body is surrounded by dust particles, the incident radiation can now be roughly the same wavelength as the size of the scattering particles. In this case, the scattering is roughly inversely proportional to the incident wavelength; (1/λ) (IIRC from classical em theory, the actual calculation of this is pretty horrendous).

    I think that’s what’s going on here, in the main.

  9. says

    Thanks for the answers. I also found it helpful to look at the Mie scattering cross-section, which visually shows what Rob is saying in #8.

    (I had initially been thinking of it as blackbody radiation and realized late that the light wasn’t radiated by the dust, but rather scattered.)

  10. tbrandt says

    Yes, what the other commenters said. Millimeter light is in the Rayleigh regime for micron-sized dust, while it is in the Mie regime for millimeter-sized dust. The dust has a cross-section that is strongly wavelength-dependent. For micron wavelengths, the scattering cross-section is close to the geometric cross-section for both millimeter-sized and micron-sized dust, so the larger surface area-to-volume ratio of the small dust grains wins (at micron wavelengths you see mostly scattered starlight rather than thermal emission by the dust).

  11. StevoR says

    @ 3. Siggy : Thanks – just clicked again and found it’s working this time.

    I leaped at the opportunity to join one of its first projects—a study of a disk of dust and rubble around a nearby star called AU Mic.

    Yes! Called it! 😀

    But also, yes, you’re right there too featuring and showing disks plural in the article.

  12. Rob Grigjanis says

    Siggy @9:

    I had initially been thinking of it as blackbody radiation and realized late that the light wasn’t radiated by the dust, but rather scattered.

    Maybe you were initially right! I just looked up the star mentioned by StevoR. Regarding its debris disk;

    The system has also been observed at infrared and sub-millimeter wavelengths. This light is emitted directly by dust grains as a result of their internal heat (modified blackbody radiation).

  13. says

    The light is going to be emitted in a range of wavelengths – some will be efficiently scattered, however, and some will not.

    There’s probably a role for both factors to be at play. Now, I don’t understand the physics, but I do understand that 400nm – 750nm is very roughly the visible light spectrum, so the nearest infrared would be 751nm and longer wavelengths, let’s give it approximately 1 harmonic to define “nearest”.

    Now we’re talking 750-1500nm, or 0.75 to 1.5 μ (μ = “micron” or “microns”, as appropriate). Since it specifically says that “small dust” is approximately 1 μ, we’re definitely talking about dust nebula with a mean particulate body size roughly equal to the wavelength of light being observed.

    That sounds to me like Mie Scattering is definitely involved.

    It might just be that what’s happening here is that
    1. the telescope is using a resolution sufficient to separately resolve the circumstellar disk and the star.
    2. The dust grains themselves, both smallest particles and larger, sand-size particles, are very cool, and would primarily give off far-infrared blackbody radiation as a result.
    3. However, the small grains are of an appropriate diameter to cause Mie Scattering of the nearest-infrared light from the star.
    4. Thus when observing small grains, light originating from the star and headed away from earth but toward the circumstellar disk is refracted in all directions, but with some part of it now headed towards earth from an apparent source (the circumstellar disk) at a resolvable distance from the actual, original source of that light (the star).
    5. The blackbody radiation of the small grains is very faint, so even a small percentage of the star’s light redirected towards earth outshines the natural blackbody radiation of the small grains.
    6. The larger grains gain no such brightness and can only be seen through their blackbody radiation.
    7. The small grains thus appear brightest at nearest-infrared wavelengths, even if not actually warm enough to generate radiation at those wavelengths.
    8. The larger grains appear brightest at the wavelengths appropriate to blackbody radiation for an object of their temperature, which would be in the far-infrared.

    …but again, I don’t know anything about these topics. I’m merely attempting to create a superficially plausible description for laypersons of what might be happening.

    Does this sound like I’m understanding what’s going on? Or does this sound like I’m off by 32 light years or so?

  14. says

    @Crip Dyke,

    I think you’re thinking in the right direction. The light coming from the dust grains should partially arise from blackbody radiation, and partially from scattered light from the nearby star. I do not know which dominates, and it might be the case that it depends on the dust grain size.

    One correction is that the standard abbreviation for a micron is μm (i.e. a micrometer).

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