Planetary Systems Quotes

“Regardless of our destiny, the clear miracle is that little blobs of protoplasm making up a species barely a hundred thousand years old living in the outskirts of a not especially remarkable galaxy have been able to learn so much about the Universe around us. We have peered back to the moments after the Big Bang, and have inferred the likely fate awaiting trillions of years from now. We have been able to probe the farthest reaches of the Universe by detecting the feeble vibrations of gravitational radiation, and have begun to lift the veil on what planets are out there, and what they may be like. The saga of exploring planetary systems has just begun. 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.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction

“It is triste to contemplate the winding down of the Universe into a cold, dark, lonely place, but we are a young species in a young Universe, with vast reaches of time before us. It is certainly true that there are countless worlds out there that could support life as we know it, and probably countless more that could support life as we don’t know it. It may be that the Universe is teeming with life waiting to make our acquaintance. Or, we may well be the first ones in our galaxy to make the leap to sentience. The vast distance between stars poses a severe barrier to individuals or even societies making the journey. Protoplasm is just too fragile and short-lived a medium to be up to the task of such voyaging. However, at a tenth the speed of light, the whole galaxy can be traversed in a million years. That’s a long time for protoplasm, but it is not a stretch to think of the data that makes us what we are—embodied perhaps in silicon or some other sturdy information-bearing material and reconstituted at destination—spreading throughout the galaxy, hopping from planet to planet along the way like Pacific Islanders in their canoes. If life—or complex life—is rare, it may well be our destiny to seed the Universe with an expanding wave of consciousness. But it is to be hoped that we will leave abundant worlds alone to develop their own destinies. There are worlds enough, and time.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction

“But just because a planet is in the habitable zone doesn’t mean it’s actually habitable. It still needs a suitable atmosphere. For the most part, it is not even known if the habitable zone planets discovered so far have any atmosphere at all; in the few cases where an atmosphere has been detected the indications point to a thick hydrogen atmosphere that would render the surface uninhabitably hot. There is not yet any basis for estimating what proportion of habitable zone planets have suitable atmospheres. The veil on this important question will lift in the coming few decades. Because M stars are so numerous, if it turns out that M star planets can commonly have suitable atmospheres, the Universe is surely teeming with life.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction

“How many habitable planets are there?
From Figure 12 it is evident that there are a handful of planets with instellation in the habitable zone range, and which are also of a size or mass small enough to have a potentially rocky composition. Most of these orbit M stars, but that is just because smallish planets with Earthlike instellation are easier to detect around low mass stars. Taking into account the effect of stellar type on the habitable zone instellation boundaries, at the time of writing there are eighteen known planets in the habitable zone, including Proxima Centauri b (orbiting our nearest stellar neighbour) and planets d, e, f, and g in the remarkable Trappist 1 system. There are an additional twenty-six near-misses which could perhaps be rendered habitable if cloud conditions or other uncertain bits of climate physics become more favourable than current best estimates. One would like to extrapolate from this number to an estimate of the proportion of all stars that have a planet in their habitable zone.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction

“It is rather remarkable that the whole apparatus of nucleosynthesis, generation of long-lived radioactive elements, and the chemical constants that determine the freezing point of water and the properties of the silicate weathering reactions have conspired to permit the operation of the silicate weathering thermostat. The ‘anthropic’ principle would state that of all possible Universes, things have worked out this way because a Universe has to have something near these characteristics in order to allow us to be here to notice such things. A less anthropic—and probably more humble—view is that we evolved to take advantage of this particular characteristic of our Universe, and that other forms of life could evolve to make use of other geochemically stabilized habitats.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction

“Heat escapes from small planets more easily than from large planets, because small planets have a greater ratio of surface area (through which heat escapes) to volume (which stores heat and generates it by radioactive decay). A small planet would require a much greater amount of radioactive heat generation, per kilogram of rock, to maintain vigorous lava-producing volcanism. This is basically for the same reason that a mouse needs to consume many more calories per gram of body weight than a human. A typical mouse, weighing 40 grams, consumes 10 kilocalories (sometimes called simply ‘calories’ in everyday usage) per day. Scaled up to the mass of a human, that would amount to 25,000 kilocalories a day—equivalent to 7kg of dry spaghetti.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction

“Although the first stars of the Universe could not have formed planetary systems, the process did not take long to get underway. Because massive stars are short-lived, the first billion years of the Universe already had time for 1,000 generations of production of the heavier elements. Observations show that the early universe was already a quite dusty place. Although massive stars do not live long enough to host planetary systems where life is likely to emerge, they are essential to producing the elements that lower-mass systems use to build habitable worlds. The Milky Way galaxy, our home, formed not long after the Big Bang, and has been building its stock of heavy elements ever since. Most of this galactic chemical evolution remains internal to the galaxy, although galaxies do sometimes collide and exchange material. Over the past thirteen billion years of nucleosynthesis in the Milky Way, there has been ample time for thorough mixing across the galaxy. Thus, our Solar System incorporates ingredients from a mix of myriad expired stars, most of which have been processed multiple times through short-lived stars. Every breath you take includes oxygen atoms from thousands of different stars that have lived and died in our galaxy over the past thirteen billion years.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction

“While a star is on the main sequence, it makes only helium. A star leaves the main sequence when it exhausts the supply of available hydrogen in its core. The post-main sequence fate of the star, and the range of elements produced, depends on the mass of the star. The lowest mass M stars, roughly below .2 Solar masses, are well mixed and can always bring hydrogen from outer layers to the core to sustain fusion. Such stars become gradually more luminous over trillions of years as they convert their hydrogen into helium, until they exhaust their fuel and fade away as white dwarf stars. White dwarf stars are no longer producing energy by fusion. Because of contraction, their surface temperatures are high (hence their bluish-white colour), but they have low luminosity because they are very small. White dwarfs are a common form of stellar remnant. All white dwarfs eventually cool down and go dark, but this process takes many times the current age of the Universe.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction

“Space telescopes have smaller mirrors than ground-based telescopes, but they do not suffer the obscuring effect of the atmosphere. For infrared astronomy, this has been transformative. The launch of the Infrared Astronomical Satellite (IRAS) space telescope in 1983 opened up the full sky to full-spectrum infrared astronomy, but it was the Spitzer space telescope, launched in 2003, that in a voyage of discovery lasting nearly two decades really opened the floodgates. Spitzer surveys have mapped 90% of the star-forming regions within 1,600 light years, yielding infrared spectra of over 2,000 young stellar objects. It is because of Spitzer that we know young stars almost invariably are surrounded by dusty disks when they first become visible a mere half million years into their existence. These disks typically extend out to at least 10 au from the star, and exhibit a hot inner disk without any evidence of a hole cleared out near the star.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction

“Until relatively recently, there was no real need for a term referring in general to the kind of object our Solar System is. It was the only known object of its type. We knew of stars but no planets outside the Solar System. We had no ability to observe planet formation in action. That has all changed, but so recently that there is no generally agreed term in the astronomical community for a star and all the gravitationally bound objects surrounding it. The term ‘planetary system’ has begun to gain currency to describe such objects, and it is the term we adopt to refer to a star and all the bodies gravitationally bound to it—the planets whether rocky, gassy, or icy, their moons, the asteroids, comets, and the far-flung icy bodies that make up Kuiper Belts. Our own planetary system contains only one star, but other planetary systems commonly contain two or even three stars. While the same general processes that formed our Solar System were also operating in the formation of other planetary systems, the end result of the process can yield planetary systems very unlike our own. Now that the Solar System isn’t the only example of a planetary system subject to study, and now that we can in effect peer back in time and observe processes such as those that occurred billions of years ago when our Solar System was being born, we can begin to appreciate how our home planetary system, and indeed our home world, is or isn’t special. The veil has been lifted, and this book provides a glimpse of what has been revealed.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction

“Baryonic matter continues to condense, though, to form the familiar disk shape of galaxies. Within these disks, many galaxies, including our own, form spiral arms which are moderately denser than their surroundings. And within these spiral arms, yet denser Giant Molecular Clouds form. Although denser than their surroundings, they are not very dense. A volume of a Giant Molecular Cloud the size of the Earth would have a mass of just 360 kilograms (kg), and if squashed down to a manageable size could be carried down the stairs by two strong movers. Still, Giant Molecular Clouds are dense enough that most of the hydrogen in them forms into hydrogen molecules (H2) consisting of two hydrogen atoms. That is why the clouds are called ‘molecular’. Within one such Giant Molecular Cloud, a smaller clump began to form, and the more matter there is in a given volume, the stronger its gravity, so the more matter is sucked in. This clump ultimately gave rise to our Solar System. The chemical composition of the Solar System suggests that the process got its initial nudge from the explosion of one or more nearby supernovae, which were themselves the product of massive, short-lived stars that formed in the same Giant Molecular Cloud as the Sun. In fact, hundreds to thousands of other stars, ranging from a few percent of the Sun’s mass to upwards of ten times the Sun’s mass formed in the same cloud.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction

“Most of the Universe is made of a mysterious substance called ‘dark matter’ and an even more mysterious substance called ‘dark energy’. Ordinary matter—the stuff our Solar System and ourselves are made of—makes up just 5% of the Universe, with dark matter accounting for 25% and the remainder being dark energy. Ordinary matter is called ‘baryonic’, after the heavy particles (e.g. protons and neutrons) of which it is mostly made. Shortly after the birth of the Universe in the Big Bang, about 75% of the baryonic matter was hydrogen, with almost all of the rest being helium. Things haven’t changed much since, but the tiny bits of stardust which have accumulated contain the heavier elements that make it possible to form beings like ourselves, and the planets on which we live.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction

“There are a number of books in the Very Short Introduction series which further illuminate a number of the topics touched on in the present work. These include the Very Short Introductions to Stars, Black Holes, Galaxies and Astrobiology.”
― Raymond T. Pierrehumbert, Planetary Systems: A Very Short Introduction