by Caleb Scharf
Our vision of the Milky Way, Andromeda, and all these other satellite galaxies has evolved enormously in the past twenty years. A lot of new, small galaxies have been discovered because of advances in machines and computation. It’s a great example of “big data.” Mapping and analyzing millions to billions of stars and their colors has allowed astronomers to recognize faint, smeared-out galaxies lurking behind the veil of stars of the Milky Way—the stuff we always have to look through to study the rest of the cosmos. See, for example, Martin C. Smith et al., “The Assembly of the Milky Way and Its Satellite Galaxies,” Research in Astronomy and Astrophysics 12, no. 8 (2012): 1021.
The dark matter problem keeps dodging our best attempts to solve it. Right now there’s a growing sense in the physics community that either dark matter is even more exotic in nature than we thought (made of particles with fancier properties), or we’re getting something horribly wrong.
The central regions of the Milky Way must be extraordinary to witness—by comparison, we live in a dank cave on the outskirts of civilization. Of course, we don’t know if anything is witnessing it. The artwork here really gets at the sheer brightness of the environment—and was a lot of fun to produce.
Describing our galaxy is tricky, because the truth is, we still don’t have all the details figured out. Mostly, that’s because we can’t see the galaxy very easily; we’re too deep inside it, in the weeds. Hopefully we’ll do better. Space observatories like the GAIA mission are going to revolutionize our maps of at least parts of the Milky Way. Take a look: http://sci.esa.int/gaia.
The infographic of galaxy sizes is quite startling. Over the years there’s been debate on how the Milky Way stacks up against other galaxies, but there’s little doubt now that there are some monsters out there. See, for example, Juan M. Uson et al., “Diffuse Light in Dense Clusters of Galaxies. IR-Band Observations of Abell 2029,” The Astrophysical Journal 369 (1991): 46–53.
There is constant churn in the positions of stars in a galaxy, including ours. See, for example, C. A. Martínez-Barbosa et al., “The Evolution of the Sun’s Birth Cluster and the Search for the Solar Siblings with Gaia,” Monthly Notices of the Royal Astronomical Society 457 (2016): 1062–75.
3. The Slow, the Fast, and the Fantastic
This is a tough set of scales to deal with. The fact is that the visual journey from 10 light-years to 92 light-hours (roughly the span here)—zooming in on the solar system—just doesn’t look very exciting to human eyes. We had some gnashing of teeth to figure out what to do. We could have just kept it dull and realistic, but we realized that doing so would miss the deep connections between past and present. It wasn’t always this boring! That’s why this chapter simultaneously zooms in through space and zooms through time (as well as states of matter)—starting about five billion years ago.
The first stars in the cosmos are critical, and still poorly understood: see Volker Bromm, “The First Stars,” Annual Review of Astronomy and Astrophysics 42 (2004): 79–118.
You could write a whole book on how stars make elements and disperse them into the cosmos, and people have. A nice read is Jacob Berkowitz’s The Stardust Revolution: The New Story of Our Origin in the Stars (Amherst, New York: Prometheus Books, 2012).
The astrophysics of forming stars and proto-stellar disks, proto-planetary disks, and planets is a hot topic. In fact, it’s really another of the frontier areas in science where we’re getting deluged with new data. The illustrations here (among my favorites in the whole book) reflect that new data. Apart from the Hubble Telescope and other observatories, some of the most exciting images and insights are coming from the Atacama Large Millimeter/submillimeter Array (ALMA)—perched on a Chilean plateau at an altitude of 5,000 meters. Just stunning: www.almaobservatory.org.
The questions that come up about the solar system’s final birthing process (Earth’s water content, Mars’s mass, the curious paucity of inner planets) are also at the forefront of modern research. An example of a review of the field is S. Pfalzner et al., “The Formation of the Solar System,” Physica Scripta 90, no. 6 (2015): 068001.
Sometimes astronomers talk about our present-day solar system as a “fossil.” It’s not a bad analogy: the most energetic and diverse action happened 4.5 billion years ago in our system. We just live in the gently evolving remains of that birth period. See, for example, John C. B. Papaloizou and Caroline Terquem, “Planet Formation and Migration,” Reports on Progress in Physics 69, no. 1 (2006): 119.
4. Planets, Planets, Planets
Full disclosure: this was actually the first chapter drafted for the book. I wanted to come up with a way to connect the scale of the solar system to our everyday scales—which is difficult. Waving a flashlight at the night sky is something I remember doing as a kid, watching the beam fade as it climbed upward. Somewhere those photons (or some of them) are still racing out into the cosmos—which is pretty neat when you think about it!
Exoplanets are of course big news. Before the mid-1990s, we just didn’t know how many stars also had planets around them. Now we know that they basically all do. My book The Copernicus Complex (New York: Scientific American / Farrar, Straus and Giroux, 2014) goes pretty deep into the science surrounding other worlds. There are also many other sources. If you want an up-close-and-personal look at the latest raw data on exoplanets, two sites are very useful: http://exoplanet.eu (The Extrasolar Planets Encyclopaedia) and http://exoplanets.org.
Statistical extrapolations about exoplanet populations have been carried out by many researchers. A couple of good examples are: Courtney D. Dressing and David Charbonneau, “The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated from the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity,” The Astrophysical Journal 807, no. 1 (2015): 45, and Daniel Foreman-Mackey et al., “Exoplanet Population Inference and the Abundance of Earth Analogs from Noisy, Incomplete Catalogs,” The Astrophysical Journal 795, no. 1 (2014): 64.
Evidence for the “Steppenwolf” planets is still somewhat controversial, and comes from gravitational microlensing data. My guess is that some studies may overestimate this population, but that there really are some lonesome worlds, ejected from their birth systems by orbital instabilities.
The illustration of the surface view on Proxima b is actually the product of a significant amount of scientific thought. The closeness of Proxima b to its flare-prone, low-mass (reddish-hued) star suggests that to maintain an atmosphere the planet would also need a strong magnetic field. A magnetic field would likely help cause an aurora in that atmosphere and would also go hand in hand with a geophysically active world. Hence the depictions of these various features.
As with a lot of things in physics and geophysics, we oversimplify planetary interiors both because we don’t have enough information to do better, and also because we like to simplify physical systems in order to get an intuitive grasp on them. If you wonder how complicated a rocky world like Earth really is, just go look at the latest geophysics research, like Kei Hirose et al., “Composition and State of the Core,” Annual Review of Earth and Planetary Sciences 41 (2013): 657–91, and George R. Helffrich and Bernard J. Wood, “The Earth’s Mantle,” Nature 412 (2001): 501–7.
As I started writing the text for this book, NASA’s New Horizons mission had, a few months earlier, made its historic fly-through of the Pluto system. While no one really knew what to expect, I don’t think many people anticipated just how interesting and complex Pluto would be. This icy world is set to revolutionize our mindset: being stuck out at the start of a planetary system’s frigid end zone does not make a planet inactive or boring. Check out http://pluto.jhuapl.edu.
I find tides quite fascinating, so of course I had to include them here. Planetary tides result in the dissipation of energy—both the spin energy of a planet or moon (and even of the Sun!) and the energy of orbits. These slow power seeps literally reshape objects and orbits across the universe. For a technical study of the implications f
or “exomoons,” I’ll shamelessly point to a paper of mine: C. A. Scharf, “The Potential for Tidally Heated Icy and Temperate Moons around Exoplanets,” The Astrophysical Journal 648, no. 2 (2006): 1196–1205.
5. A World We Call Earth
In the opening of this chapter I wanted to emphasize several specific ideas—including the fact that what we think of as Earth is really just the planet as it happens to be at this moment. Throughout its 4.5-billion-year history Earth has seldom, if ever before, been exactly the way it is now. And that trend will continue into the future. I also wanted to emphasize certain planetary characteristics that we often take for granted. The graphic of Earth’s surface-water content, for example, is pretty stunning. We may be an ocean world, but all those oceans don’t actually add up to very much at all! The United States Geological Survey Web pages are a treasure trove of interesting information: https://www.usgs.gov and http://water.usgs.gov/edu/earthhowmuch.html.
The oldest rocks on Earth remain a little controversial. But the zircons are compelling. An example of the fascinating research done on zircons includes the discovery of diamond inclusions inside zircons—providing clues to Earth’s tectonic processes more than four billion years ago. See Martina Menneken et al., “Hadean Diamonds in Zircon from Jack Hills, Western Australia,” Nature 448 (2007): 917–20.
There’s still considerable debate over the organisms that polluted Earth’s atmosphere with oxygen, and exactly what the timeline of that pollution was. See, for example, Donald E. Canfield et al., “Oxygen Dynamics in the Aftermath of the Great Oxidation of Earth’s Atmosphere,” Proceedings of the National Academy of Sciences 110, no. 42 (2013): 16736–41. The standard lore is that cyanobacteria were the prime oxygenating culprits. Maybe they were, maybe they weren’t.
The amount of energy Earth receives from the Sun is enormous. Hopefully that is conveyed here. The estimates of human energy consumption are just that: estimates. For example, the International Energy Agency (IEA) provides some data and calculation: https://www.iea.org.
Climate and weather are very complicated phenomena (strictly speaking, climate is just the statistics of weather—the time-averaged, rounded-off likelihood of certain properties like surface temperature or ice cover). I’ve given a very simplistic overview here. Given the urgency of facing up to human-induced climate change (it’s happening, it’s just physics, don’t argue), you might want to stay informed. Good resources include www.noaa.gov/climate, http://climate.nasa.gov, and www.metoffice.gov.uk/climate-guide.
Powerful typhoons (in the Indian Ocean or western Pacific) or hurricanes (northeast Pacific and north Atlantic)—collectively known as tropical cyclones—are astonishing. You’ll find all sorts of ways scientists try to convey the power involved: one day of such a storm’s energy is equivalent to hundreds of thermonuclear bombs, or could power human civilizations for years.
Sunlight changing chemistry is a big deal. This photochemistry isn’t just important for Earth, it’s part of what happens throughout the solar system and beyond. A very technical but comprehensive insight can be had in Renyu Hu et al., “Photochemistry in Terrestrial Exoplanet Atmospheres I: Photochemistry Model and Benchmark Cases,” The Astrophysical Journal 761, no. 2 (2012): 166.
I struggled to find a way to express a vision of the Earth in very human terms. Then it occurred to me that there were people who’d seen our world from space. I knew many had written of their experiences, but I hadn’t realized how many recollections there are, or how eloquent they could be. These quotations are in the public record, and there are many more not shown here. I tried to span our varied countries and cultures a little. (See, for example, www.spacequotations.com/earth.html.)
6. Being Conscious in the Cosmos
You might be surprised that I talk about consciousness in this chapter. Me too. But I started thinking about what aspect of our world is the most striking on these scales. What could be said about living organisms, like humans, elephants, birds, and so on, that hasn’t been said many times before? To be honest, I think the puzzles of awareness, self-awareness, sentience, and that thing we call consciousness are some of the biggest unanswered questions we have.
The other big puzzles include how all the living systems on Earth are intertwined with one another—not just now, but across time. It’s a bit clichéd, but if you’ve never looked at what Charles Darwin, Alfred Russel Wallace, Alexander von Humboldt, and others wrote as they pieced together what would become our understanding of evolution, it’s worth doing so. The Darwin Online resource is terrific: http://darwin-online.org.uk. You can find Wallace’s works on natural selection in many online resources (he even thought about astrobiology!), and Michael Shermer has written an excellent biography, In Darwin’s Shadow: The Life and Science of Alfred Russel Wallace: A Biographical Study on the Psychology of History (Oxford, UK, and New York: Oxford University Press, 2002). On Humboldt, see Andrea Wulf’s wonderful The Invention of Nature: Alexander von Humboldt’s New World (New York: Knopf, 2015).
Estimates of biomass are notoriously tricky. You can’t sample every cubic meter of the Earth’s upper layers and count organisms; you have to perform some major extrapolations from localized counts, data on fluxes of food and refuse, and so on. An example focused on the global biomass in forests is a review by Yude Pan et al., “The Structure, Distribution, and Biomass of the World’s Forests,” Annual Review of Ecology, Evolution, and Systematics 44 (2013): 593–622. An example of an estimate of microbes’ biomass that revises the total significantly downward is Jens Kallmeyer et al., “Global Distribution of Microbial Abundance and Biomass in Subseafloor Sediment,” Proceedings of the National Academy of Sciences 109, no. 40 (2012): 16213–16.
Perhaps the biggest choice that I had to make in the book was exactly where to zoom in as our journey reached Earth. It seemed important to avoid what had been done in the past (repetition is never so interesting) and too much Western and Northern Hemisphere bias. Modern humans all come from Africa. It’s also a continent of remarkable geography and biological diversity. And the Great Rift Valley is such an imposing feature—a place where the Earth’s crust is literally at its thinnest, a reminder of our perilous tenure even in the midst of swarming life. I also like elephants. They’re fascinating aliens, beautiful, and in need of our appreciation and protection.
Information about Homo habilis and Homo erectus is widely available, but a very good read with the latest insights is Yuval Noah Harari’s Sapiens: A Brief History of Humankind (New York: HarperCollins, 2015).
The “tree of life” is a popular conceptual model for the branching evolutionary landscape of living things. It goes back to Darwin, among others. Versions based entirely on fossil records and taxonomy are limited. More-modern phylogenetic trees are more powerful. Both help us get a general picture of who is related to whom. There are various resources online: www.tolweb.org, www.wellcometreeoflife.org/interactive, and tree.opentreeoflife.org.
Insect intelligence and cognition is particularly fascinating given how different we are. Some recent research suggests (quite convincingly, in my opinion) that not only can bumblebees “innovate” to solve physical puzzles, but those that manage this can then “teach” other bees (or at least other bees learn quickly from their successful kin). And once taught, bees can then serve as models for later bee generations. Pretty amazing. See Sylvain Alem et al., “Associative Mechanisms Allow for Social Learning and Cultural Transmission of String Pulling in an Insect,” PLOS Biology 14, no. 10 (2016): e1002564.
Exactly what brains can do in different species is a very tough question to answer. The infographic in this chapter has a lot of uncertainty in it—which is fine, that’s how science is.
I mention the term “contingency.” This has strong connotations in evolutionary biology, particularly due to the writings of Stephen Jay Gould. All his books are provocative and interesting, but Wonderful Life: The Burgess Shale and the Nature of History (New York: W. W. Norton, 1989) is a must-read—even if you don’t
agree with the specifics of his proposals.
7. From Many to One
“Complexity” and “complex systems” have become a critical part of the modern scientific lingo, and with good reason—the universe is full of complexity. But complexity challenges our reductionist tendencies, and is something that we’re still coming to grips with. There are many texts to refer to, some popularized, some technical. See, for example, James Gleick’s classic Chaos: Making a New Science (New York: Viking Penguin, 1987), and Stuart Kauffman’s At Home in the Universe: The Search for the Laws of Self-Organization and Complexity (Oxford, UK, and New York: Oxford University Press, 1995). It’s worth looking at the activities of a place where so much of this work has come from: the Santa Fe Institute in New Mexico, www.santafe.edu.
On the microscopic world, it’s worth reading about Antonie van Leeuwenhoek, who pioneered microscopy and was probably one of the first humans to actually see bacteria, back in the 1600s and early 1700s.
I chose the human hand to exemplify the granularity of life because it’s such an intimate appendage and an iconic shape. It’s what has literally enabled us to take over the world. The mathematician and intellectual Jacob Bronowski once said, “The hand is the cutting edge of the mind.”
The earliest evidence for multicellular life on Earth is claimed to be in the form of fossils from Gabon dating from 2.1 billion years ago. See, for example, Abderrazak El Albani et al., “The 2.1 Ga Old Francevillian Biota: Biogenicity, Taphonomy and Biodiversity,” PLOS One 9, no. 6 (2014): e99438. Of course, this is only the earliest possibility found thus far. It has been claimed that multicellularity has been “invented” (via natural selection and environmental pressure) on Earth as many as forty-five times.
