Alien worlds have long been a staple of science fiction. But today, thanks to modern astronomical instrumentation and the achievements of many enterprising observational astronomers, the existence of planets outside our solar systemalso known as exoplanetshas moved into the realm of science fact. With planet hunters finding ever smaller, more Earth-like worlds, our understanding of the cosmos is forever changed, yet the question of how astronomers make these discoveries often goes unanswered.
How Do You Find an Exoplanet? is an authoritative primer on the four key techniques that today's planet hunters use to detect the feeble signals of planets orbiting distant stars. John Johnson provides you with an insider’s perspective on this exciting cutting-edge science, showing how astronomers detect the wobble of stars caused by the gravitational tug of an orbiting planet, the slight diminution of light caused by a planet eclipsing its star, and the bending of space-time by stars and their planets, and how astronomers even directly take pictures of planets next to their bright central stars.
Accessible to anyone with a basic foundation in college-level physics, How Do You Find an Exoplanet? sheds new light on the prospect of finding life outside our solar system, how surprising new observations suggest that we may not fully understand how planets form, and much more.
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How Do You Find an Exoplanet?
By John Asher Johnson
PRINCETON UNIVERSITY PRESSCopyright © 2016 Princeton University Press
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For as long as there been humans we have searched for our place in the cosmos.
— Carl Sagan, 1980
1.1 My Brief History
I am an astronomer, and as such my professional interest is focused on the study of light emitted by objects in the sky. However, unlike many astronomers, my interest in the night sky didn't begin until later in my life, well into my college education. I don't have childhood memories of stargazing, I never thought to ask for a telescope for Christmas, I didn't have a moon-phase calendar on my wall, and I never owned a single book about astronomy until I was twenty-one years old. As a child, my closest approach to the subject of astronomy was a poster of the Space Shuttle that hung next to my bed, but my interest was piqued more by the intricate mechanical details of the spacecraft rather than where it traveled.
Looking back, I suppose the primary reason for my ignorance of astronomy was because I grew up in a metropolitan area, in the North County of St. Louis, Missouri. The skies are often cloudy in the winter when the nights are longest, the evenings are bright with light pollution from the city even when the clouds are absent, and the air is humid and mosquito-filled in the summer. Another important factor is that from age six until twelve I spent a good fraction of my time in my room building with Legos. From early on I seemed destined to be an engineer rather than the astrophysics professor I am today.
It wasn't until I attended a small engineering college in the town of Rolla, Missouri (pronounced locally as Rah-lah, Mizz-ur-ah) that I had my first memorable experience with the night sky (the school is now known as the Missouri University of Science & Technology). The town of Rolla boasts a population of about 20,000, but only during the academic year; once the students leave for summer break, the population dips below 12,000. One hot summer evening in August 1997, just before the start of the fall semester, I was sitting in my room playing computer games over our homemade local area network when my roommate Jason convinced me to go out that night to see the Perseids meteor shower. Jason had learned about the shower from a public service announcement from the student radio station, KMNR, where we both worked as deejays. With nothing much better to do before classes started, we and several friends drove out to a farmer's field just outside of town, threw out a few blankets and waited for the shooting stars.
As we watched the meteors of various sizes streak across the night sky, I noticed a swath of faint, splotchy light and I asked if anyone else saw it, too. It turned out that we were seeing the Milky Way — our own galaxy as viewed from the inside — along with many of the summer constellations. It was that event, at the age of twenty-one, that sparked my interest in the grander Cosmos.
From that night on I started looking up to notice twinkling stars of various subtle hues, the phases of the Moon, and the sky in general. One night later that year I saw a remarkably bright star that I hadn't noticed previously. A few weeks later I asked one of my physics professors, Dr. Schmitt, about the mysterious star expecting to hear about some component of a constellation with a Latin name or some boring numerical designation. I explained about how much brighter it was than the surrounding stars, and how it seemed to stand out so much clearer than everything else in the sky. He smiled and told me that I wasn't seeing a star at all. Rather, I was seeing the planet Jupiter. Thinking back to that moment now, I suppose that was when I had just "discovered" my first planet! The discovery may have been thousands if not millions of years old and therefore hardly new to humanity. But it was new to me and it further sparked my interest in the subject of astronomy.
1.2 The Human Activity of Watching the Sky
Throughout history humans have taken inventory of the night sky, its stars, planets and other luminous bodies. Given many more nights staring up, rather than down at my textbooks or across at my computer screen, I would have noticed other bright planets in addition to Jupiter, including Mercury, Venus, Mars and Saturn. If I had paid careful enough attention, as my ancestors had thousands of years ago, I would have noticed that the planets did not always appear in the same place month after month with respect to the surrounding stars. Compared to the relatively static background of the constellations, the planets wander at their own pace, and sometimes move in a direction opposite of the stars. The activity of measuring the positions of astronomical objects relative to the background of relatively static stars is known as astrometry. The history of exoplanets starts with humans conducting astrometric measurements of the planets, and the story of the discovery of the nature of the Solar System planets marks the dawn of science as we know it.
While the word planet, meaning "wandering star," originates with the Greeks, astrometric measurements of planets were first recorded by the Babylonians, who started recording the positions of Venus sometime in the seventeenth century B.C.E. We know this based on the ancient Venus Tablets, on which these earlier astrometric observations were later reproduced and preserved. These tablets date back to the seventh century B.C.E., and the original observations were most likely made in relation to religious customs and beliefs, with the times of maximum separation from the Sun associated with various omens. Later Babylonian texts noted the positions of other planets, along with the Sun and the Moon, and noted the periodic nature of these objects' motion in the sky.
The Babylonian tabulations of planetary positions are the earliest written records of humans tracking the positions of night-sky objects. However, while various aspects of planetary motion were noted to be periodic and therefore predictable, ancient astronomers had no proper system of motion attributed to the planets and stars. With the advent of geometry, the Greeks later devised the first mathematical description of the movements of objects in the sky. They posited that the Earth was a sphere — they were aware that the Earth is not flat — and around the central sphere rotated a larger sphere containing the stars and planets. This "two-sphere" model was advanced by the most prominent philosophers of the time, notably Socrates, Aristotle and Ptolemy, and later adopted by Christian theologians through the sixteenth century B.C.E. (Kuhn, 1957).
The preference for the two-sphere model was not necessarily based on its ability to make accurate physical explanations of observed phenomena, but were instead based on assertions about nature that had aesthetic, rather than scientific appeal as we would demand today. According to the ideas of the early Greek philosophers, later expanded upon by Ptolemy, the basic elements of the Universe had preferred locations and behaviors. For example, in a Ptolemaic Universe, the Earth and its constituents — earth, water, air and fire — were changing and imperfect. Further, things that are of the Earth tend toward its center, which provides an explanation for why objects are pulled to the Earth. While things of the Earth are subject to change, objects in the heavens were created perfect and immutable. Rather than falling toward the Earth, the outer sphere(s) containing the celestial bodies moved in circular motion, eternally cycling back on itself with no beginning and no end.
The two-sphere model held sway for nearly two millennia, and as a result there was little progress in the physical understanding of how the Universe works. The perfection and constancy of the model, with celestial objects cycling along eternal circular tracks, made it appealing to Ptolemaic and later Christian sensibilities. But beyond the visual appeal of the heavens, the stars in the night sky were otherwise largely uninteresting. The stars, Moon, Sun and planets existed above the Earth, and the rules that govern change on the Earth were simply presumed to be invalid for the heavens. This meant that finding detailed explanations for astronomical phenomena wasn't particularly compelling to most people in the centuries leading up to the time of Copernicus (1473–1543), other than the impact that planetary positions had on keeping track of time, aiding in navigation, and the connection between human fate and the precepts of astrology. Questions were restricted to when astronomical phenomena would occur, and only because it was presumed that those phenomena impacted human affairs. Asking why the heavens move as they do, or ascertaining their origins, was not the province of educated philosophers who were concerned with the nature of humans rather than the nature of stars, planets and the Universe.
If the night sky were populated only with stars, then the two-sphere model might have continued for centuries longer than it did. However, then as today, planets captured the attention of early philosophers, scientists and tinkerers. While stars execute their uniform, circular motion about the Earth, the planets are the iconoclasts, breaking the rule of immutability. Venus and Mercury don't use the entire night sky as their playing field, but rather appear to the naked eye only close to when the Sun is setting or rising, swapping positions from one side of the Sun to the other. Venus in particular grabs people's attention to this day, appearing bright and bold just before and during sunset on some days, and around sunrise on other days, hence its designation as the evening star or morning star, respectively. Mars, Jupiter and Saturn are also extremely bright, yet they move across the entire night sky drifting across the background field of stars at their own paces.
Even more curiously, Mars, Jupiter and Saturn — along with the other outer planets, which are not visible to the naked eye — occasionally halt and then reverse course for weeks to months, from night to night moving from west to east, counter to their more typical east-to-west motion. From our vantage point of modern science, this retrograde motion, coupled with the restricted movement of Venus and Mercury, are evidence against an Earth-centered Solar System. Venus and Mercury never stray far from the Sun because the Earth is orbiting exterior to them and we are looking in toward their smaller, Sun-centered orbits. Meanwhile, Mars, Jupiter and Saturn orbit the Sun exterior to the Earth, so we can see them in their larger orbits that range over our entire night sky, often far from the Sun's position. Furthermore, the orbit of the Earth can "overtake" the orbits of the outer planets, causing them to appear to move backward on our sky as we pass them.
However, the prevailing way of thinking about the Universe before the sixteenth century compelled people to double down on the preference for perfect circular motion. Ptolemy built on the concepts of circular motion that were originally proposed by Hipparchus and Apollonius of Perga. Instead of traveling solely along giant circles centered on the Earth, the planets also moved along smaller circles centered on a point along the larger orbital circle, known as epicycles. This circle-on-a-circle concept allowed the planets to move from east to west on the sky most of the time, but the rotation of the epicycle allowed them to occasionally reverse direction. The epicyclic modification of Ptolemy was the dominant model of the Solar System for more than a millennium, from the second century A.D. up until the time of Copernicus in the sixteenth century.
1.3 Asking Why the Planets Move as They Do
For much of his life, Nicolaus Copernicus was employed variously as a politician, theologian, and physician. However, his true passion was astronomy and he was well-versed in the astronomical philosophy of Ptolemy, with its spheres and epicycles. However, the use of epicycles required not just small circles atop larger circles, but the model required other modifications such as offsets between the Earth and the center of the different planetary motions to reproduce, for example, the varying brightnesses and speeds of the planets throughout the year.
Copernicus found that circular planetary orbits could be accommodated by a Sun-centered (heliocentric) model (Sobel, 2011). Copernicus shared the Greek admiration for circles because an object on a circular path has eternal motion, which reflects the eternal perfection of the heavens. As an additional benefit, the arbitrary mechanism of epicycles was no longer needed to explain retrograde motion. He first proposed his heliocentric model in a short treatise entitled Commentariolus ("short commentary"). In this short work he laid out a set of assumptions, notably that there was no single center in the Solar System, but instead several rotation points; the Moon orbits the Earth; the objects other than the Moon orbit the Sun; the stars are fixed and at a great distance from the Solar System; and the apparent retrograde motion of the outer planets is related to the Earth's motion on a sphere interior to those planets' spheres. These assumptions formed the basis for his later work, De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres), which described his heliocentric model more fully.
It should be noted that Copernicus' motivations for a heliocentric Solar System weren't particularly compelling as viewed from a modern scientific standpoint. Because of his insistence on circular motion, which we now know is not generally true of planetary motion, he proposed placing the Sun near the center of his system based on another aesthetic motivation: light tends to emanate from a central location, like light from a candle in an otherwise dark room. So he proposed to place the Sun at the center of the Solar System — the center of the room, so to speak — with the planets, including the Earth, revolving around the central light. Copernicus' reasoning for a heliocentric universe provided a simpler explanation than an Earth-centered model with epicycles. But more important, the heliocentric model later found overwhelming and compelling support in direct observational evidence.
His motivations aside, Copernicus' idea proved revolutionary because it provided an entirely new way of observing and interpreting the Universe. Kuhn astutely notes that the Copernican Revolution ushered in a new paradigm of scientific thinking.
A scientific paradigm is a system of thought that forms a starting point for working out scientific problems. For example, before the modern germ theory of disease, illness was thought to be due to poor air quality, or "miasma." Today, our understanding of bacteria, parasites and viruses provides a much more effective framework for treating and preventing diseases that would be unrecognizable to medical practitioners of, say, the eighteenth century.
Similarly, the assumption that the Earth does not occupy a special place in the Universe provides a common starting point for working out modern astronomy problems, and this approach was largerly unheard of before Copernicus' revolutionary idea. Because of Copernicus' paradigm shift the assumptions under which scientists interpreted the world around them were fundamentally changed. With the Sun moved to the center of the planets' orbits, the Earth was no longer the center of the entire Universe. With the new model, there was at last a system of planetary motion around the Sun; there was now a Solar System, a logical, universal mechanism for explaining the motion of planets. Later observations and reasoning revealed the Sun to be one of myriad stars in a Universe that was much larger than just the Earth and its immediate environs.
Contrary to popular belief, the church initially took a rather pragmatic stance toward Copernicus' new idea. His heliocentric model provided a much easier method of computing predictions for the positions of the planets, and was therefore quite a bit more useful than previous astrometric methods. Copernicus was also a part of the church hierarchy, and he was very careful to follow proper rules and etiquette when publishing his ideas, going so far as to dedicate De Revolutionibus to the pope (Pogge, 2005). Immediately following the publication of Commentariolus, some religious leaders, both Catholic and Protestant, expressed disdain at the removal of the Earth from the center of God's creation. But there was not an organized, church-led suppression of Copernican thinking until several decades later. Copernicus was nonetheless sensitive to potential religious objections to his theory, which is one reason he waited until late in his life to publish De Revolutionibus.
Another obstacle to the widespread acceptance of the heliocentric model is the requirement of a moving Earth. A static Earth at the center of the Solar System fit into the existing philosophical beliefs that the elements associated with the Earth tend to fall toward its center. However, this tendency is distinct from the modern notion of gravity, and many scoffed at the notion of a moving Earth because it was felt that things resting on the Earth's surface, as well as the atmosphere and the Moon, would fly away if the Earth moved around the Sun. After all, if a horse-drawn cart laden with supplies turns a corner too quickly, the cargo will fly over the edge. Shouldn't the same hold true for the Earth and its "cargo"? Another objection was related to the firmly held belief that there could be only one center of motion, and having the Earth orbit the Sun as one center while serving as the center of the Moon's motion seemed absurd.
Excerpted from How Do You Find an Exoplanet? by John Asher Johnson. Copyright © 2016 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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Table of Contents
1. Introduction 1
1.1 My Brief History 1
1.2 The Human Activity of Watching the Sky 3
1.3 Asking Why the Planets Move as They Do 8
1.4 Exoplanets and Completing the Copernican Revolution 16
2. Stellar Wobbles 23
2.1 At the Telescope 23
2.2 For Every Action 28
2.3 Eccentric Orbits 39
2.4 Measuring Precise Radial Velocities 45
2.5 Stellar Jitter 49
2.6 Design Considerations for a Doppler Survey 52
2.7 Concluding Remarks 57
3. Seeing the Shadows of Planets 59
3.1 Measuring and Reading Transit Signals 62
3.2 The Importance of a/R 71
3.3 Transit Timing Variations 74
3.4 Measuring the Brightness of a Star 77
3.5 Radial Velocities First, Transits Second 81
3.6 Transit First, Radial Velocities Second 83
3.7 From Close In to Further Out 89
4. Planets Bending Space-Time 90
4.1 The Geometry of Microlensing 94
4.2 The Microlensing Light Curve 103
4.3 The Microlensing Signal of a Planet 106
4.4 Microlensing Surveys 109
5. Directly Imaging Planets 114
5.1 The Problem of Angular Resolution 115
5.2 The Problem of Contrast 122
5.3 The Problem of Chance Alignment 129
5.4 Measuring the Properties of an Imaged Planet 130
6. The Future of Planet Hunting 132
6.1 Placing the Solar System in Context 133
6.2 Learning How Planets Form 138
6.3 Finding Life Outside the Solar System 141
6.4 Giant Planets as the Tip of the Iceberg 144
6.5 The Future of the Doppler Method: Moving to Dedicated Instrumentation 148
6.6 The Future of Transit Surveys 153
6.7 The Future of Microlensing 155
6.8 The Future of Direct Imaging 158
6.9 Concluding Remarks 160
What People are Saying About This
"Johnson has woven the personal side of being a scientist with rigorous intuition about the techniques used to detect exoplanets. We hear the fresh and articulate voice of a young professor who grew into the shoes of a full-fledged scientist. Johnson's experiences and insights will touch the hearts and minds of readers."Debra Fischer, Yale University"With remarkable clarity, Johnson presents a concise yet personable, technical yet accessible must-read for all students and practitioners of exoplanet discovery."Sara Seager, Massachusetts Institute of Technology"How Do You Find an Exoplanet? is well focused on the fundamentals and accessible to a wide range of readers. Johnson is highly respected in the exoplanet community, and here he has emphasized what's important, while minimizing or explaining jargon. I know of no serious competitors to this book."Eric B. Ford, Pennsylvania State University"How Do You Find an Exoplanet? presents an engaging overview of modern exoplanetary detection techniques. John Johnson brings a firsthand narrative to this remarkable scientific detective story, while explaining the technical fine points at an accessible level."Greg Laughlin, University of California, Santa Cruz