by Michio Kaku
In conclusion, negative energy does exist, and if enough negative energy could somehow be collected, we could, in principle, create a wormhole machine or a warp drive engine, fulfilling some of the wildest fantasies of science fiction. But these technologies are still a long way off, and will be discussed in chapters 13 and 14. In the meantime, we will make do with the light sails that might be zooming through space by the end of the century, offering the first close-up pictures of exoplanets orbiting other stars. By the twenty-second century, we may be able to visit these planets ourselves on fusion rockets. And if we can solve the intricate engineering problems in front of us, we may even be able to make antimatter engines, ramjet engines, and space elevators a reality.
Once we have starships, what will we find in deep space? Will there be other worlds that can sustain humanity? Fortunately, our space telescopes and satellites have given us a detailed look at what lurks among the stars.
Hence I say that I have not merely the opinion, but the strong belief, on the correctness of which I would stake even many of the advantages of life, that there are inhabitants in other worlds.
—IMMANUEL KANT
The desire to know something of our neighbors in the immense depths of space does not spring from idle curiosity nor from thirst for knowledge, but from a deeper cause, and it is a feeling firmly rooted in the heart of every human being capable of thinking at all.
—NIKOLA TESLA
9 KEPLER AND A UNIVERSE OF PLANETS
Every few days, Giordano Bruno has his revenge.
Bruno, Galileo’s predecessor, was burned alive at the stake for heresy in Rome in 1600. The stars in the heavens are so numerous, he observed, that our sun must be one of many. Surely these other stars, too, are orbited by a multitude of planets, some of which may even be inhabited by other beings.
The church imprisoned him for seven years without trial, then stripped him naked, paraded him through the streets of Rome, tied his tongue with a leather strap, and lashed him to a wooden pillar. He was given one last chance to recant, but he refused to take back his ideas.
To squelch his legacy, the church placed all his texts on the Index of Forbidden Books. Unlike Galileo’s works, Bruno’s were banned until 1966. Galileo merely claimed that the sun, not the Earth, was the center of the universe. Bruno suggested that the universe had no center at all. He was one of the first in history to posit that the universe might be infinite, in which case the Earth would be just another pebble in the sky. The church could no longer claim to be the center of the universe, because it had none.
In 1584, Bruno summed up his philosophy, writing, “This space we declare to be infinite…in it are an infinity of worlds of the same kind as our own.” Now, more than four hundred years later, roughly four thousand extrasolar planets in the Milky Way have been documented, and the list grows almost daily. (In 2017, NASA listed 4,496 candidate planets, of which 2,330 have been confirmed, discovered by the Kepler spacecraft.)
If you go to Rome, you might want to visit the Campo de’ Fiori—the “Plain of Flowers”—where there is an imposing statue of Bruno on the very spot where he faced his death. When I went, I found a bustling square full of shoppers, who may not all have been aware that the location had been an execution site for heretics. But Bruno’s statue itself gazes down upon a number of young rebels, artists, and street musicians who, unsurprisingly, congregate there. While taking in this peaceful scene, I wondered what kind of atmosphere could have existed back in Bruno’s day to inflame such a murderous mob. How could they be whipped up to torture and kill a vagabond philosopher?
Bruno’s ideas languished for centuries, because finding an extrasolar planet is exceedingly difficult and was once thought to be nearly impossible. Planets do not give off light of their own. Even the reflected light of one is about a billion times dimmer than that of the mother star, the harsh glare of which can obscure the planet from view. But thanks to the giant telescopes and space-based detectors we have today, a flood of recent data has proven Bruno to be correct.
IS OUR SOLAR SYSTEM AVERAGE?
In my childhood, I read an astronomy book that changed the way I understood the universe. After describing the planets, the book concluded that our solar system was probably a typical one, echoing the ideas of Bruno. But it also went much further. It speculated that planets in other solar systems moved in almost perfect circles around their sun, like ours. The ones closer to the sun were rocky, while the ones farther out were gas giants. Our sun was the average Joe of stars.
The notion that we live in a quiet, ordinary suburb of the galaxy was simple and comforting.
But boy, were we wrong.
We now realize that we are the oddballs and that the arrangement of our solar system, with its orderly sequence of planets and near-circular orbits, is rare in the Milky Way. As we begin to explore other stars, we are coming across solar systems catalogued in the Extrasolar Planets Encyclopaedia that are radically different from our own. One day, this encyclopedia of planets may contain our future home.
Sara Seager, professor of planetary science at MIT and one of Time magazine’s twenty-five most influential figures in space exploration, is a key astronomer behind this encyclopedia. I asked her whether she was interested in science as a child. She admitted to me that actually, she was not, but the moon did catch her attention. She was intrigued by the fact that it seemed to follow her whenever her father drove her around. How could something so far away appear to chase after her car?
(The illusion is caused by parallax. We judge distances by moving our heads. Close objects like trees seem to shift the most, while distant entities like the mountains do not change position at all. But objects immediately next to us that are moving with us also don’t appear to change position. Our brains therefore confuse remote objects, like the moon, with adjacent ones, like the steering wheel in the car, and make us think that both are moving consistently alongside us. As a result of parallax, many of the UFOs spotted trailing after our cars are actually sightings of the planet Venus.)
Professor Seager’s fascination with the heavens blossomed into a lifelong romance. Parents sometimes buy telescopes for their inquisitive children, but she bought her own first telescope with the money she earned from a summer job. She remembers being fifteen and excitedly talking to two of her friends about an exploding star, named Supernova 1987a, that had just been seen in the sky. It had made history as the closest supernova since 1604, and she was planning to go to a party to celebrate the rare event. Her friends, however, were baffled. They did not know what she was talking about.
Professor Seager went on to convert her enthusiasm and sense of wonder about the universe into a bright career in exoplanet science, a discipline that didn’t exist two decades ago but that is one of the hottest fields in astronomy today.
METHODS TO FIND EXOPLANETS
It is not easy to see exoplanets directly, so astronomers find them with a variety of indirect strategies. Professor Seager stressed to me that astronomers are confident of their results because they detect exoplanets in multiple ways. One of the most popular is called the transit method. Sometimes, when analyzing the intensity of starlight, you notice that it weakens periodically. This dimming is a small effect but indicates the presence of a planet that, from the vantage point of Earth, has moved in front of its mother star, thereby absorbing some of its light. Since the path of the planet can therefore be tracked, its orbital parameters can be calculated.
A Jupiter-sized planet would reduce light from a star like our sun by about 1 percent. For an Earth-like planet, the figure is 0.008 percent. This is like the dimming of a car’s headlight if a mosquito passes by it. Fortunately, as Professor Seager explained, our instruments are so sensitive and accurate that they can pick up on the slightest changes in luminosity from multiple planets and prove the existence of entire solar systems. However, not all exoplanets move in front of a star. Some have tilted orbits and, therefore, cannot be observed by the transit method.
Another popular approach is the radial velocity, or Doppler, method, in which astronomers look for a star that seems to move back and forth regularly. If there is a large, Jupiter-sized planet orbiting the star, then the star and its Jupiter are actually orbiting each other. Think of a rotating dumbbell. The two weights, representing the mother star and its Jupiter, turn around a common center.
The Jupiter-sized planet is invisible from a distance, but the mother star can clearly be seen moving in a mathematically precise fashion. The Doppler method can be used to calculate its velocity. (For example, if a yellow star moves toward us, the light waves are compressed, like an accordion, so the yellow light turns slightly bluish. If it moves away from us, its light is stretched and turns reddish. The speed of the star can be determined by analyzing how much the light frequency changes as the star moves toward and away from the detector. This is similar to what happens when the police shine a laser beam on your car. The changes in the reflected laser light can be used to measure how fast you are going.)
Careful examination of the mother star over weeks and months also enables scientists to estimate the mass of the planet using Newton’s law of gravity. The Doppler method is tedious, but it led to the discovery of the first exoplanet in 1992, which set off a stampede of ambitious astronomers trying to track down the next one. Jupiter-sized planets were the earliest to be observed because giant objects correspond to the largest movements of the mother star.
The transit method and Doppler method are the two main techniques for locating extrasolar planets, but a few others have been introduced recently. One is direct observation, which, as previously mentioned, is difficult to accomplish. However, Professor Seager is excited by NASA’s plans to develop space probes that can carefully and precisely obstruct the light from the mother star, which might otherwise overwhelm the planet.
Gravitational lensing may be a promising alternate method, although it only works if there is perfect alignment between the Earth, the exoplanet, and the mother star. We know from Einstein’s theory of gravity that light can bend as it moves near a celestial body, because a large mass can alter the fabric of space-time around it. Even if the object is not visible to us, it will change the trajectory of light, just as clear glass does. If a planet moves directly in front of a distant star, the light will be distorted into a ring. This particular pattern is called an Einstein Ring and signals the presence of a substantial mass between the observer and the star.
RESULTS FROM KEPLER
A big breakthrough came with the 2009 launch of the Kepler spacecraft, which was specifically designed to find extrasolar planets by employing the transit method. It was successful beyond the wildest dreams of the astronomical community. Next to the Hubble Space Telescope, the Kepler spacecraft is probably the most productive space satellite of all time. It is a marvel of engineering, weighing 2,300 pounds with a massive 4.6-foot mirror and bristling with the latest high-tech sensors. Because it has to stare at the same spot in the sky for long periods of time in order to get the best data, it does not orbit the Earth but circles the sun instead. From its perch in deep space, which can be one hundred million miles from Earth, it uses a series of gyroscopes to focus on one four-hundredth of the sky, a small patch in the direction of the constellation Cygnus. Inside that tiny field of vision, Kepler has analyzed about two hundred thousand stars and uncovered thousands of extrasolar planets. It has forced scientists to reevaluate our position in the universe.
Instead of locating other solar systems resembling our own, astronomers came across something totally unexpected: planets of all sizes orbiting stars at all distances. “There are planets out there that have no counterpart in our solar system, some of which are in between the size of the Earth and Neptune, or much smaller than Mercury,” Professor Seager reflected. “But today, we still haven’t found any copies of our solar system.” In fact, there have been so many strange results that astronomers don’t have enough theories to accommodate them. “The more we find, the less we understand,” she confessed. “The whole thing is a mess.”
We are at a loss to explain even the most common of these exoplanets. Many of the Jupiter-sized planets, which have been the easiest to find, are not moving in near-circular trajectories as expected but in highly elliptical orbits.
Some Jupiter-sized planets are in circular orbits, but they are so close to the mother star that if they were in our solar system, they would be within the orbit of Mercury. These gas giants are called “hot Jupiters,” and the solar wind is constantly blowing their atmosphere into outer space. But astronomers once believed that Jupiter-sized planets originate in deep space, billions of miles from the mother star. If so, how did they get so close?
Professor Seager admits that astronomers don’t know for sure. But the most probable answer took them by surprise. One theory states that all gas giants form in the outer regions of a solar system, where there is plenty of ice around which hydrogen and helium gas and dust can collect. But in some cases, there is also a large amount of dust spread out within the plane of the solar system. The gas giant may gradually lose energy from the friction of moving through the dust, entering into a death spiral toward the mother star.
This explanation introduced the heretical idea of migrating planets, which had been previously unheard of. (As they edge closer to their suns, they might cross the path of a small Earth-like planet and fling it into outer space. That smaller rocky planet might become a rogue planet, drifting alone in outer space independent of any star. So we don’t expect any Earth-like planets in a solar system with Jupiter-sized planets in highly elliptical orbits, or orbits near the mother star.)
In hindsight, these strange results should have been anticipated. Because our own solar system has planets moving in nice circles, astronomers naturally assumed that the balls of dust and hydrogen and helium gas that become solar systems condensed evenly. We now realize that it is more likely that gravity compresses them in a haphazard, random way, resulting in planets that move in elliptical or irregular orbits that may intersect or collide with one another. This is important because it may be that only solar systems with circular planetary orbits like ours are conducive to life.
EARTH-SIZED PLANETS
Earth-like planets are small and hence cause faint dimming or subtle distortions of light from the mother sun. But with the Kepler spacecraft and giant telescopes, astronomers have begun to locate “super-Earths,” which, like Earth, are rocky and capable of sustaining life as we know it but are 50 percent to 100 percent larger than our planet. We cannot yet account for their origin, but in 2016 and 2017, a series of sensational, headline-grabbing discoveries about them were made.
Proxima Centauri is the closest star after our sun to the Earth. It is actually part of a triple star system and orbits a pair of larger stars called Alpha Centauri A and B, which orbit each other. Astronomers were stunned to come across a planet just 30 percent larger than the Earth moving around Proxima Centauri. They named it Proxima Centauri b.
“This is a game changer in exoplanetary science,” declared Rory Barnes, an astronomer at the University of Washington in Seattle. “The fact that it’s so close means we have the opportunity to follow up on it better than any other planet discovered so far.” The next batch of giant telescopes in development, like the James Webb Space Telescope, might be able to capture the first photograph of the planet. As Professor Seager put it, “It’s absolutely phenomenal. Who would have thought that after all these years of wondering about planets that there’s one around our nearest star?”
Proxima Centauri b’s mother star is a dim red dwarf only 12 percent as massive as the sun, so the planet must be relatively close to the star in order to be inside its habitable zone, where it can support liquid water and possibly even oceans. The radius of the planet’s orbit is just 5 percent of the radius of the Earth’s orbit around the sun. It also revolves around its mother star much faster, making one complete revolution every 11.2 days. There is intense specul
ation about whether Proxima Centauri b has conditions compatible with life as we know it. One major concern is that the planet might be bombarded by solar winds, which could be two thousand times more intense than those hitting the Earth. To shield itself against these blasts, Proxima Centauri b would have to have a strong magnetic field. At present, we do not have enough information to determine whether this is the case.
It has also been suggested that Proxima Centauri b may be tidally locked, so that, like our own moon, one side always faces the star. That side would be perpetually hot, while the other side would be permanently cold. Liquid water oceans might then occur only at the narrow band between these two hemispheres, where the temperature is moderate. However, if the planet has a dense-enough atmosphere, the winds might equalize the temperatures so that liquid oceans could exist freely across its surface.
The next step is to determine the composition of the atmosphere and whether it contains water or oxygen. Proxima Centauri b was detected using the Doppler method, but the chemical composition of its atmosphere is best assessed with the transit method. When an exoplanet crosses directly in front of the mother star, a tiny sliver of light passes through its atmosphere. Molecules of certain substances in the atmosphere absorb specific wavelengths of starlight, allowing scientists to determine the nature of those molecules. However, for this to work, the orientation of the exoplanet’s path must be just right, and there is only a 1.5 percent chance that Proxima Centauri b’s orbit is aligned correctly.
