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Stars vs. planets and Kant

Something I've been brooding about recently, especially after re-reading Neptune's Brood. It's rather amazing how much we think we know about the visible universe, just from peeking out at it from our little planet. Astronomers tell us the structure of the Sun, it's lifecycle, that of other rather different stars, explain novae and supernovae... I'm not saying there aren't any mysteries, but if you stop and think about it, the amount of stellar detail is absurd. We barely know that much about Earth, and the rest of our system's planets -- or planemos, planetary mass objects, big enough to force themselves to be a sphere and at least briefly geologically interesting -- are an endless wellspring of surprises, with more to learn every time we peek closer. What we can guess of exoplanets has not changed that.

But it does make sense in a way. Stars be definition are shining light and therefore information at us. (The Earth intercepts 2 kilograms of sunlight every second, for a gratuitous figure.) That very fact constrains them a lot -- they're big balls of hydrogen and helium and contaminants, under great forces of gravity and heat and light pressure and magnetism. If they weren't just so, they wouldn't fuse; if they weren't balanced between gravity and fusion, they wouldn't be stars. Like a high-speed fish or a rocket engine, there's not a lot of leeway. THey're plasma, so fluid, and tending to homogeneity. Finally, there's a lot of them, all shining at us; if we've populated a periodic table of stars types and age categories, it's probably because we've seen multiple examples of each. In short, it's a data-rich and highly constrained field.

I've just started reading the 1999 edition (from the library) of The New Solar System (Beatty et al.), rather younger (and much thicker) than the one I had as a planetary science major in college. It alerted me to one specific way in which stars fall over themselves to tell us about themselves: helioseismology is a thing. How can we track sound waves in the Sun? Not by listening to them, but by simply looking at them: sound waves in the Sun move the photosphere, the light-emitting 'surface', which causes Doppler shifts in the light. Voila! Your telescope is also a seismometer -- a global seismic detector net, even -- and we can apply seismological techniques to infer the structure and composition of the Sun. Heck, we can apparently even do this with other stars, which I don't remember hearing about before.

By contrast, planets are dark and opaque. They're potentially a lot more complicated: much more diverse in chemical composition, able to be gaseious, liquid, or solid, or all three (solid is the big one, allowing really wacky differentiation); able to support actual chemistry. The Himalayas probably causing long-term cooling and eventually the Ice Ages, by scrubbing CO2 out of the air via excessive weathering, *as well as* causing the monsoon cycle, is my favorite "who would have expected that?" example. And finally, there's not that many observable planets: 32 planemos, including all the large moons, Ceres, and recently named Kuiper belt objects; more like 27 within range of telescopes or fly-by craft so far, though New Horizons will add Pluto and Charon. So even if there is a regular pattern to planets, we could easily lack the data to perceive it.

Still, I'm impressed by how *nothing* closely resembles anything else. Moon and Mercury are different, all the gas giants are different, even Ganymede and Callisto are different. There are some similarities and discernable forces, but AFAIK nowhere can we say "yeah, given A, B is no surprise."


As for Kant, the book reminded me that he came up with the nebular hypothesis, that the solar system was formed from the collapse of a rotating nebula of gas. That wasn't his only work in physics. WP is more intriguing than informative, but Kant was cited by Lord Kelvin and Thomas Huxley -- not bad!


It seems odd to be reading a thick book from 1999, when there's been so much new work (e.g.), but this seems to be the latest edition from these authors, and I don't know what's similarly lay-comprehensive now. Wikipedia would be more up to date, but this has more depth and photos. And some stuff wouldn't go out of date, like history I hadn't appreciated: just how dead study of the planet was before the space program. NASA had trouble finding astronomers to get on board with them, Kuiper being one of the exceptions.

Also amusing to read the Sun description. Paraphrasing: "we think we know how the fusion works, but 2/3 of the neutrinos are missing. So we're either really wrong about the Sun, or about neutrinos. Leaning toward the latter, because helioseismology supports what we infer about the Sun." Ding!

Wow, I've never needed a geology or planetary science tag on LJ before. Astronomy, yes. Lame of me.

See the comment count unavailable DW comments at http://mindstalk.dreamwidth.org/408474.html#comments


( 3 comments — Leave a comment )
Nov. 16th, 2014 11:19 am (UTC)
Wow, fascinating.This is why i love LJ and i did not know that Kant had formulated a nebula theory.
Nov. 17th, 2014 10:39 am (UTC)
The Earth intercepts 2 kilograms of sunlight every second, for a gratuitous figure.

Sunlight has mass?
Nov. 18th, 2014 10:13 pm (UTC)
Certainly. Light has energy, therefore mass. E=mc^2. Photons have zero rest mass but are never at rest.
( 3 comments — Leave a comment )


Damien Sullivan

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