Planets and Shadows

Last year, I blogged about why catching stars forming is a tricky proposition; They're surrounded in gaseous nebulae that makes trying to observe the act a bit like watching a sports game from a plane flying through the clouds. You just can't see through it all.

In general, this should hold true for planets. Until the star clears out the dusty disk, the planets will remain hidden, even if we could spatially resolve them. So a paper talking about forming planets with “Observable Signatures” in the title caught my eye.

In this paper, the author explains something that, at first glance, is actually counter-intuitive: These dusty shells may help find newly forming planets. The reason is that, as planets form, they will slowly accumulate the material around them. In the vertical direction, this has the effect of making a “dimple” in the proto-planetary disk proportional to the planet's accumulated mass.

According to the author, the dimple will appear observationally as a “shadow”. She doesn't completely explain why this is and it seems somewhat strange to me. The reason is that “shadows” are usually caused by something that blocks light. Although the forming planet would cast a (relatively) small shadow that would be lost in the disk, it's not at all clear what would cause a shadow in the dimple.

Rather, I suspect a better word choice would have been to say there would be a “darkening” in the dimple. This would make more sense to me, since the dimple would be a lower density and have less material to scatter light propagating along the plane. Less light means that, relative to the rest of the disk, it would appear darker. Thus, I'm pretty sure that's the actual mechanism at work here.

But there's another interesting component: On the side of the dimple that's further from the parent star, there's a brightening! Weird huh?

This again, is not well explained in the paper, but I suspect the reason for this has something to do with the angle at which the photons coming through the disk are striking the side of the dimple. Either that or it's the opposite of the darkening effect where suddenly the boost in numbers of photons that were allowed to flow relatively freely through the empty space created by the dimple are suddenly again encountering a relatively dense medium again.

Regardless of how these effects should be created, planets should show a dark spot next to a brighter spot with the sizes proportional to their mass. But should these be observationally detectable?

According to the paper, yes. These features would be most readily observable in the visible to near-IR bands. The real limiting factor would be how well we're able to spatially resolve these dimples. Obviously, the further away a system would be, the smaller it would appear. Even for the largest planets simulated (50 Earth masses), the dimple is about 3 AU across. That means that we'd need to be able to resolve half of that to see both the darkened and lightened portions.

Tossing that into the small angle equation and assuming a fairly typical resolution for a good telescope of 1arcsec, we get that we should be able to observe these systems out to 3.1 x 10⁵ AU or 4.9 ly.

Eep!

That's not very far at all! In fact, the only stars that fall in that range are the stars in the Alpha Centauri trinary star system. The next closest one after that (Barnard's star) is 5.9 ly away!

The Hubble can get down to ~0.1arcseconds of resolution, which would mean 10x further, but there's still not really any star forming regions within 40 ly.

So ultimately, while this technique is interesting it doesn't seem the least bit practical with the current generation of telescopes.

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Jang-Condell, H. (2009). PLANET SHADOWS IN PROTOPLANETARY DISKS. II. OBSERVABLE SIGNATURES The Astrophysical Journal, 700 (1), 820-831 DOI: 10.1088/0004-637X/700/1/820