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Book Possibly of Interest: Exoplanets


DShomshak

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I can't quite call this a recommendation, but some people might want to read this.

 

Exoplanets, by Michael Summer and James Trefil, can't make up its mind what it's about. First off, it isn't all about exoplanets; some chapters are actually Pluto and the moons of the outer Solar System, now revealed as complex worlds in their own right. This material, and the material that is indeed about planets in other star systems is mostly fascinating; but sometimes it's frustrating because the authors neglect to explain how astronomers know what they think they know. Some of what they left out I already know from other sources, but a book for laymen should cover these things.

The results from Kepler and other studies suggest the universe is full of worlds that are stranger than anyone imagined. As their first chapter's title puts it, this is "Not Your Grandfather's Galaxy." 55 Cancri e, a.k.a. Diamond World, is probably the strangest -- but it's also an example of the authors not explaining why astronomers infer that the planet is mostly made of carbon.

The other half of the book is about prospects for life and for SETI. A connected topic, surely, but handled much less deftly than Ben Miller did in The Aliens Are Coming! (which someone else recommended in another thread). Plus, some chapters are just superfluous. For instance, "What Is A Planet?" amounts to a rant about the idiocy of both the IAU's redefinition that demoted Pluto, and the resulting popular kerfuffle. It's not good writing to spend 14 pages explaining why a topic doesn't matter.

Summers and Trefil also repeat themselves a lot. Their real goal seems to be to push a guess that rogue planets -- worlds kicked out of forming solar systems -- are an important, hitherto unsuspected abode for life. They even think such rogue worlds could stay warm enough for water to stay liquid on the surface, effectively pushing the "habitability zone" out to infinity, though most of their discussion is about Europa-style subsurface oceans. Interesting, but absurd for the SETI half of the book. As SETI pioneer Philip Morrison succinctly put it, "Dolphins don't build radio telescopes."

In sum, parts of Exoplanets have good bits in it that make it worth an interested layman's attention. There are ideas that could be turned into wonderfully weird SF worlds; I will use them at some point. But this book could have been much better, especially if the author had made up their mind what they were doing.

 

Dean Shomshak

 

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  • 3 weeks later...

Incidentally, two minutes on Google led me to an article that concisely explained the chain of inference for Diamond World... because an astronomer made new measurements that cast doubt on the start of that chain of inference.

 

This isn't difficult, guys.

 

Oh. The chain of inference? 55 Cancri e was first detected by the spectral line Doppler shift method. (As the planet's gravity makes the star wobble, the motion shifts spectral lines in the star's light by good old Doppler effect.) That gave the planet's orbital period and, from the extent of the wobble, the planet's mass. Later, astronomers detected it by transit: The percentage by which the planet dims the star's light gives the planet's size. From that, basic math gives the density (about 5.9 grams per cubic centimeter. For comparison, Earth's density is about 5.5.)

 

55 Cancri e also is incredibly close to its star -- actually within its chromosphere -- so it is very, very hot.

 

And being made of diamond? Well, early studies of 55 Cancri suggested the star has a very high ratio of carbon to oxygen. About equal amounts; for comparison, the Sun has a lot more oxygen than carbon.

 

It's a reasonable inference, then, that 55 Cancri and its planets formed from a cloud of dust and gas that had a lot more carbon than the cloud that became the Solar System. (Which is plausible. I know from other reading that red giant stars sometimes puff out monstro-gigantic clouds of soot. That could seed a nebula with scads of carbon.)

 

If you count up the percentage of elements in Earth's mantle, oxygen actually leads the list. So if a planet formed with as much carbon as oxygen, wouldn't the heat and pressure of the interior forge the carbon into diamond? After all, that's what happened to the carbon in Earth's mantle.

 

That's still a long way from a mantle that's just diamond, with a crust of graphite. All the silicate and other minerals will still be there. They'll just be a portion of the crust and mantle instead of all of it.

 

And in the article, another astronomer noticed that 55 Cancri is a bit cooler than the Sun, and that would have the effect of making the carbon spectral lines seem stronger at the particular frequency range examined in the previous study. She looked at other frequency ranges and says that no, the ratio of carbon to oxygen isn't so extreme. So there's no reason to think 55 Cancri e is so carbon-rich anyway.

 

There may well be exoplanets with ginormous wodges  or strata of diamond in their mantles. But a planet that's all carbon? Mm, at this point that may be speculating well ahead of the evidence.

 

Looky, I has done science writing! Really not that difficult.

 

Dean Shomshak

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And not a terrible synopsis of the thinking that went into both the original statement and then the redaction of that later. A lot easier than wading through the papers on 55 Cancri e for certain. :D

 

There's other issues with the original hypothesis too. For instance not all of the carbon on Earth, not even a significant portion of it, formed diamond. A much larger portion went into forming things like carbonates, graphite, and atmospheric gases. Therefore even if 55 Cancri e were to have as much carbon as oxygen, it is unlikely that a significant portion, much less all of it, would have been incorporated into diamond. Graphite is the more likely end member, barring significant water, and if there was a fair amount of water, then carbonates are more likely results. There's also issues like core differentiation, it's conditions (were they reducing or oxidizing?), and other factors that come into play on whether you get a result of diamond, graphite, carbonates, or exsolved gas.

 

In short, a "diamond planet" is very unlikely, even if the starting materials are carbon rich.

 

That said, this discussion does bring up the issue that the SF trope of a "mineral rich" or "mineral poor" world is quite plausible. Anne McCaffrey's Pern was uninteresting for colonization by all but an agriculturally minded group because it was 1) remote in location and 2) it had very low mineral wealth. My point here being that the range of planets out there has only expanded, and you could very well have a system where the worlds are iron poor (by comparison), and therefore almost all of it is locked up in the planets's cores, meaning there's unlikely to be any real industry in metals, but perhaps it's that much richer in silicon/silica so they produce a lot of grit/sand, concrete slurry, and so forth. Maybe such systems are also enriched in carbon too though, and therefore are good sources for organic chemicals or even agriculture. They might be also be so biologically diverse, and temperature stable over large latitudes, that they're the proverbial "pleasure planets" that appear in SF media, covered in lush rain forests and the like. 

 

Then again, all of this may be just worrying about incorporating too much realism in your space opera. :D

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Years ago, the World-Building volume in the SF Writer's Handbook series -- written by an astronomer who also writes SF under a pseudonym -- described a "carbonotic" (carbon-superrich) planet. as a possibility. (IIRC, this was before any exoplanets had been discovered, so it was all still theoretical.) This author claimed that Earth has significant amounts of iron in its crust because iron oxides are less dense than pure iron and also tend to get chemically dragged along with other minerals in volcanoes. Carbon, however, is better than iron at glomming onto oxygen, so a carbonotic world might have much less oxidized iron in the crust and therefore have a metal-poor crust.

 

OTOH, shows like How The Earth Was Made claim the early oceans carried a lot of iron that only oxidized later to form the banded iron deposits. Okay, so not the most reliable source, but it illustrates how many different factors could be at work in planetary formation and development. If something isn't blatantly impossible, it's probably real somewhere. :-)

 

(And I've used the "mineral rich/mineral poor" tropes myself in world building.)

 

Ooh, I'll get back with a citation for the World Building book. I thought it was pretty good, though possibly out of date.

 

Dean Shomshak

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