Planetary Systems: A Very Short Introduction

Very Short Introductions #693

by Raymond T. Pierrehumbert

Not long ago, the Solar System was the only example of a planetary system - a star and the bodies orbiting it - that we knew. Now, we know thousands of planetary systems, and have even been able to observe planetary systems at the moment of their birth.

This Very Short Introduction explores this new frontier, incorporating the latest research. The book takes the reader on a journey through the grand sweep of time, from the moment galaxies begin to form after the Big Bang to trillions of years in the future when the Universe will be a dilute soup of dim galaxies populated mostly by red dwarf stars. Throughout, Raymond T. Pierrehumbert introduces the latest insights gained from a new generation of telescopes that catch planetary systems at the moment of formation, and to the theoretical advances that attempt to make sense of these observations. He explains how the elements that make up life and the planets on which life can live are forged in the interiors of dying stars, and make their way into rocky planets. He also explores the vast array of newly discovered planets orbiting stars other than our own, and explains the factors that determine their climates. Finally, he reveals what determines how long planetary systems can live, and what happens in their end-times.

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ABOUT THE The Very Short Introductions series from Oxford University Press contains hundreds of titles in almost every subject area. These pocket-sized books are the perfect way to get ahead in a new subject quickly. Our expert authors combine facts, analysis, perspective, new ideas, and enthusiasm to make interesting and challenging topics highly readable.

Genres: Science, Nonfiction, Astronomy, Space

160 pages, Paperback

Published February 1, 2022

Reviews

[John · Rating 3 out of 5]

I listened to an audio version of this book. The amount of information we are accumulating about the universe is astounding. The book is fairly technical and you miss a lot of information in figures apparently in a print version. Our local library doesn’t carry it.

[Daniel · Rating 5 out of 5]

Planetary Systems: A Very Short Introduction (2022) by Raymond T. Pierrehumbert covers the exploding field of exoplanets. For a sample of the book's content, see the quotations page. Chapters:

1 Beginnings
2 Creation revealed
3 Beautiful theories, ugly facts
4 What are planets made of?
5 A grand tour of exoplanets
6 Planetary climate and habitability
7 How it all ends
References and further reading

Chapters 1 and 2 present stellar evolution and theories of planetary formation. Perhaps remarkably, these theories are no longer mere speculation. Modern telescopes such as ALMA can directly image and analyze a number of protoplanetary disks where new planets are forming right now.

Pierrehumbert goes on to describe the diversity of exoplanetary sizes, compositions (rocky, gaseous, icy), and environmental conditions. This includes the unexpected discovery that many explanets are so-called Hot Jupiters, which resemble the planet Jupiter of our own Solar System except that they orbit their parent stars closely and as a result have extremely high surface temperatures. A Hot Jupiter could be deadly to any Earth-like rocky planets orbiting farther out in the circumstellar habitable zone. A large planet whipping around its parent star could with its strong gravitational field destabliize the orbits of smaller planets orbiting farther out, potentially ejecting such planets and turning them into rogue planets drifting through the cold darkness of interstellar space.

Chapter 5 highlights some exoplanets including the remarkable TRAPPIST 1 system. Chapter 6 describes what astronomers currently know about what makes a planet potentially suitable for life. Chapter 7 describes the ultimate fate of planets (executive summary: it's not good if you happen to live on one).

According to the late philosopher of science Mario Bunge, one of the key characteristics separating science from most other intellectual activities is a steady accumulation of new and verified knowledge. Exoplanetary science exemplifies this, as astronomers with an array of newly-invented techniques and instruments pull back the figurative curtain from a whole category of previously invisible celestial objects: planets in orbit around stars other than our Sun. If you're old enough to remember the 1980s, you old enough to have experienced a time when humans knew of no planets beyond those in our own Solar System. From the first exoplanet discoveries in the 1990s, astronomers have steadily added to the tally. According to Wikipedia, "As of 1 December 2022, there are 5,284 confirmed exoplanets in 3,899 planetary systems, with 847 systems having more than one planet." As Pierrehumbert explains, the total for our home galaxy (the Milky Way) alone could run into the many billions.

In a few places, Pierrehumbert refers to concepts that will be familiar to general readers of science, but he doesn't always use the standard names. For example, he describes the effects of the square-cube law on exoplanetary volcanism but doesn't mention the law by name.

He also claims, without further evidence or explanation, that the mere presence of many potentially habitable planets suffices to cause the Universe to "teem" with "life". The claim might not even be correct as stated, because the precise mechanism of abiogenesis on Earth is not yet known (although the RNA world hypothesis appears to be one plausible route). Without knowing the precise mechanism(s) by which life can evolve from nonliving matter, one cannot even guess the number of planets that could have met all the necessary conditions. And even if the claim turns out to be correct - if a substantial fraction of habitable planets do in fact host living things - the claim uses potentially misleading language. The Universe would still not "teem" with life, because even if there are millions or billions of life-bearing planets in the Milky Way galaxy alone, the actual volume of habitable space is still a negligible fraction of all space. Even on a habitable planet, most of the planet is not habitable; life is restricted to a thin habitable skin around the planet's surface, extending just a little way up into the atmosphere and down into the ground. As Richard C. Carrier points out in Naturalism vs. Theism: The Carrier-Wanchick Debate, if the observable universe were scaled down to the size of a house, the habitable portion would be smaller than a single atom. All the rest of space is actively or passively hostile to life, being far too cold or far too hot, sometimes bathed in ionizing radiation, usually lacking in liquid water, too low or high in pressure, and so on.

The word "life" is also potentially misleading, because "life" includes everything from sentient, communicative, technological humans, right down the complexity scale to single-celled organisms. Simpler life forms far outnumber complex forms; a single human body for example hosts more tiny parasitic or symbiotic microbes than its own number of human cells. Earth itself has seen perhaps a hundred million species, and only one of them had the capacity to develop or acquire abstract reasoning, grammatical language, mathematics, advanced technology, and the spare resources to do anything beyond mere survival and reproduction. For most of the history of life on Earth, there was nothing more impressive here than pond scum. From what we know of life so far, if life exists on other planets, almost all of it is likely to be simple.

Pierrehumbert fails to mention the Fermi paradox, which poses apparently daunting problems for the existence of other intelligent, technological life in the Universe. This point warrants emphasis, because entire generations of people have been brainwashed by science fiction to expect that the Universe "teems" with "life", conveniently (for narrative plots) having many characteristics of human life. Science fiction would be dull if it portrayed a Universe apart from Earth that is either sterile or inhabited by nothing but microbes. Indeed, Pierrehumbert himself fairly dangles the tantalizing prospect of intelligent life out there, because if Earth turns out to be one exceptional planet, the rest of the Universe will be boring. But that would still be OK! The proper attitude for the scientist to have is to let the Universe teach us about itself, rather than tell the Universe what to be.

The compost pile in my back yard teems with microbes, and even some complex multicellular life like centipedes, earthworms, slugs, woodlice, and lizards that hunt them when the weather is warm, but nobody writes stories about that! To be fair, Pierrehumbert does cite other books that address some of these issues. To his brief list I would add:

* Solving Fermi's Paradox
* If the Universe Is Teeming with Aliens... Where Is Everybody? Fifty Solutions to Fermi's Paradox and the Problem of Extraterrestrial Life
* Rare Earth: Why Complex Life Is Uncommon in the Universe
* Lucky Planet: Why Earth is Exceptional-and What That Means for Life in the Universe

Fortunately, Pierrehumbert later walks things back a bit by acknowledging that for all we currently know, "we may well be the first ones in our galaxy to make the leap to sentience." As remarkable and anti-Copernican as that may sound, the Fermi paradox offers powerful support for it. We'll just have to wait for astronomers to build bigger telescopes and collect more data - or, perhaps, for the space aliens to drop by for a visit.

So while it is "encouraging" to witness the current explosion in exoplanet discovery, I caution the reader against getting their "hopes" up. To hope for intelligent life to exist elsewhere in the Universe makes little sense, because our own history of cultural contact on Earth has typically been horrific. For just one example see the genocide of the Americas that followed quickly on the voyages of Christopher Colubus. Humans have proven to be poor at handling even a tiny bit of diversity; the differences between groups of people may be negligible compared to the differences between people and extraterrestrial aliens. Let's try not to get too far ahead of our skis, and just let science surprise us as it normally does. The only thing predictable about science is its unpredictability. The lesson from the history of failed scientific predictions is clear: form no opinion ahead of the data.

Pierrehumbert closes with a well-deserved shot at authoritarianism. I do have to quibble with the last chapter's last sentence, though:

"There is no limit to what we can accomplish, if we can make it through the next few hundred years without crashing the Earth’s habitability, and without letting the authoritarianism emerging throughout the world crush the human spirit, dividing us one from the other, and separating us from our better natures."

In particular, I object to the misleadingly inclusive language ("us" and "our"). Authoritarianism is not something "we" do, but rather (like crime, or sports, or art) something that some of us do. For example, if you are the type of person who is inclined to read difficult nonfiction books such as this one, and even more tedious reviews like this one, you are probably not the kind of person who feels especially comfortable at a Trump rally. For starters, your measurable IQ is probably well above the mean for the rally audience. OK, there might be a few readers of this book who also love Trump, because human behavior is complex, and describable more in terms of correlations than hard rules. But exceptions do not erase the trends. Thus I would argue that authoritarianism is not so much an instance of "us" becoming separated from "our" better natures, but rather an instance of one particular class of people (i.e., people who are especially vulnerable to being fooled by demagogues) gaining power over another class of people (those with the "better natures" that Pierrehumbert values, who are capable of greater tolerance, inclusivity, and restraint). For example, the difference between Germany in 1939 and Germany in 2022 is probably not so much that Germans themselves have transformed into a different kind of organism, but rather that the "good" Germans who were present then and now have managed to keep the "bad" Germans in check since the end of hostilities in 1945. But the "bad" Germans are still around and can erupt into power at any time. All it takes is for a sufficiently charismatic liar - a Hitler, or a Trump - to fuel their resentments (people with low IQs are always angry at their frustrating lives, since life itself is a giant IQ test) and aim their lie-fueled rage at the Reichstag or the Capitol, respectively.

As books on behavioral genetics point out, what's at play is not some spiritual mystery, but mainly the random recombination of SNPs that shape individual personalities and political predispositions. See for example:

* Blueprint: How DNA makes us who we are
* Predisposed: Liberals, Conservatives, and the Biology of Political Differences

If we want to overcome existential threats such as human-caused climate breakdown, and the rise of authoritarian political horror, I suspect we'll need to treat these behavioral maladies for what they are: genetic disorders. It's not so much a question of locating the "better natures" that "we" already possess, but rather a question of increasing the frequency of SNPs in the population that code for better natures.

[Tim Martin · Rating 5 out of 5]

A nicely written popular science book by Raymond T. Pierrehumbert, read by Mike Cooper, that is a rather up to date (copyright 2021) introduction to the latest understandings of planetary systems; that is a star and the bodies that orbit it. This Very Short Introduction covered the formation of such systems with an extensive discussion of the stars themselves (including the types of stars that give rise to planetary systems), protoplanetary disks (the rotating disc of gas and dust surrounding newly formed stars, especially T Tauri stars, that in a process well described in the book accretes into planetesimals and then into planets), how we know what we know about what goes inside these developing planetary systems, discussing infrared, microwave, and submillimeter astronomy and such concepts as black-body radiation. Also how the different types of planets form (gas giants, ice giants, and smaller rocky or rock-iron planets), how exoplanets are detected and studied (with a fascinating section on single pixel astronomy as well as naming exoplanets), the different types of exoplanets that are known so far such as Hot Jupiters, Sub-Neptunes, and tide-locked planets around red dwarf stars (a tide locked planet pretty much has the same side always facing the star, as the planet rotates on its axis in about the same time it takes to orbit the star), ongoing issues in studying planetary systems such as the angular momentum problem (basically that the nebular hypothesis of planet formation predicts that all planets should have similar amounts of angular momentum but as shown in our solar system the inner planets have relatively low angular momentum when compared to the outer planets, a mystery in planetary systems studies) and the Neptune desert (the mystery of why Neptune-sized planets in short-period orbits, in orbits close to their star, which should be easy to detect, haven’t been found but instead only rocky planets and Hot Jupiters have been discovered), the science of habitability (very much in its infancy, studying how a planet that can host life can arise and maintain its habitable status, the author getting into such concepts as the silicate weathering thermostat, the importance of plate tectonics, and the size of a potential habitable rocky planet particularly with regard to atmospheric thickness), and even a small section on how to possibly apply planetary science concepts to address climate change on Earth such as the idea of albedo hacking (basically increasing the Earth’s reflection of solar radiation back into space to lower temperatures).

Most of the book is on planetary formation and how we know what we know about not only their formation but how exoplanets are discovered and studied. You learn a lot about the planets of our own solar system in the book but it is relatively light on specific exoplanets (other than the TRAPPIST-1 system, which was discussed at length, a red dwarf star that may have four potentially habitable rocky exoplanets that seem to be tidally locked), though there is a good bit on the different types of exoplanets found so far. I was never lost and though it can be dense at times, all concepts were accessible and well-explained. At the end there were even a few examples taken from popular works of science fiction as a means to discuss exoplanets and the life they might have.

[Jay French · Rating 3 out of 5]

I read this book, “Planetary Systems: A Very Short Introduction” after having read a similar book, “Exoplanets” by Michael Summers and James S. Trefil. The back half of “Planetary Systems” is about the same coverage as “Exoplanets”, but told in a different way. “Planetary Systems” was more about the history of this kind of research, more a facts as we know them approach. “Exoplanets” takes some literary license to touch on what could be out there, with some vivid descriptions of worlds that are glossed over in the other book. I enjoyed these descriptions more than what “Planetary Systems” covers, as they stick in the mind better. But for more background, I liked “Planetary Systems” better. Both are short books. And both authors expect changes to our understanding, perhaps in the next few years, due to better telescopes.

[Behrooz Parhami · Rating 4 out of 5]

I listened to the unabridged 4-hour audio version of this title (read by Mike Cooper, Tantor Audio, 2022).

It wasn’t long ago that we knew only of the Solar System as the only example of a planetary system, that is, a star and the bodies orbiting it. Now, thanks to a new generation of powerful telescopes, not only do we know many thousands of planetary systems, but we have observed planetary systems as they were being born.

This book, from Oxford’s wonderful “Very Short Introduction” series composed of hundreds of titles, explores our newfound information and ongoing research in this exciting frontier. Pierrehumbert, a professor of physics at Oxford University, takes us on a grand tour, from the Big Bang to trillions of years into the future, when the universe will be a dilute soup of dim galaxies, populated mostly by red dwarf stars.

Pierrehumbert also explains how the elements that make up life are forged in the interiors of dying stars, later making their way into rocky planets. He also covers the vast array of newly-discovered planets within star systems other than our own, and explains the factors that determine their climates. Finally, he reveals what determines the life-span of planetary systems and what happens to them as they die.

[Gijs Limonard · Rating 3 out of 5]

Decent primer, bit matter-of-factly and dry, but the content is solid; for a more accessible guide to the universe check out the classics by Stephen Hawking; The Illustrated A Brief History of Time and works by Neil deGrasse Tyson; Astrophysics for People in a Hurry.

[R. · Rating 3 out of 5]

This was a short book that seemed much longer due to the author’s ponderous style.

If you can get past that and stay focused, you’ll read about some of the interesting developments that have formed in the last couple of decades.

And yet we are still just out of the gate. So stay tuned for more change coming as new observations and missions bring new answers and questions.