by Michio Kaku
It would be an astonishing coup to find molecules of water vapor on an Earth-like planet. Professor Seager explained that “if you have a small rocky planet, you can only have water vapor if you also have liquid water on its surface. So if we find water vapor on a rocky planet, we can infer that it also has liquid oceans.”
SEVEN EARTH-SIZED PLANETS AROUND ONE STAR
Another unprecedented finding came in 2017. Astronomers located a solar system that violated all the theories of planetary evolution. It contained seven Earth-sized planets orbiting a mother star called TRAPPIST-1. Three of the planets are in the Goldilocks zone and may have oceans. “This is an amazing planetary system, not only because we have found so many planets, but because they are all similar in size to the Earth,” said Michaël Gillon, the leader of the Belgian scientific group that made the discovery. (The name TRAPPIST is both an acronym for the telescope used by the group and a reference to Belgium’s popular beer.)
TRAPPIST-1 is a red dwarf a mere thirty-eight light-years from Earth, and its mass is only 8 percent that of the sun. Like Proxima Centauri, it has a habitable zone. If transposed over our own solar system, the orbits of all seven planets would fit inside Mercury’s path. The planets take less than three weeks to circle the mother star, and the innermost makes a complete revolution in thirty-six hours. Because the solar system is so compact, the planets interact gravitationally and could, in theory, disrupt their own arrangement and collide. One might naïvely expect them to careen into one another. But fortunately, an analysis in 2017 showed that they are in resonance, meaning that their orbits are in phase with one another and no collisions will take place. The solar system seems to be stable. But as with Proxima Centauri b, astronomers are investigating the possible effects of solar flares and tidal locking.
On Star Trek, whenever the Enterprise is about to encounter an Earth-like planet, Spock announces that they are approaching a “class M planet.” Actually, there’s no such thing in astronomy—yet. Now that thousands of different types of planets have made their debut, including a variety of Earth-like planets, it’s only a matter of time before a new nomenclature is introduced.
TWIN OF THE EARTH?
If a planetary twin of the Earth exists in space, it has eluded us so far. But we have found about fifty super-Earths so far. Kepler-452b, which was discovered by the Kepler spacecraft in 2015 and is about 1,400 light-years from us, is particularly interesting. It is 50 percent bigger than our planet, so you would weigh more than you do on the planet Earth, but otherwise, living there may not be so different from living on Earth. Unlike the exoplanets that orbit around a red dwarf, it circles a star that is only 3.7 percent more massive than the sun. Its period of revolution is 385 Earth days, and its equilibrium temperature is 17 degrees Fahrenheit, slightly warmer than the Earth. It lies within the habitable zone. Astronomers searching for extraterrestrial intelligence trained their radio telescopes to receive messages from any civilization that might be on the planet but have detected none as yet. Unfortunately, because Kepler-452b is so far away, even the next generation of telescopes will not be able to collect significant information about its atmospheric composition.
Kepler-22b, which is six hundred light-years away and 2.4 times the size of the Earth, is also being studied. Its orbit is 15 percent smaller than Earth’s—it completes one revolution in 290 days—but the luminosity of its mother star, Kepler-22, is 25 percent lower than the sun’s. These two effects compensate for each other, so the surface temperature of the planet is believed to be comparable to that of the Earth. It also lies in the habitable zone.
But KOI 7711 is the exoplanet that is getting the most attention because, as of 2017, it is the one with the most Earth-like features. It is 30 percent larger than Earth, and its mother star is very much like our own. It is not at risk of being fried by solar flares. The length of one year on the planet is almost identical to a year on Earth. It is in the habitable zone of its star, but we do not yet have the technology to evaluate whether its atmosphere contains water vapor. All conditions seem right for it to host some form of life. However, at 1,700 light-years away, it is the farthest exoplanet of the three.
After analyzing scores of these planets, astronomers have discovered that they can usually be arranged into two categories. The first is the super-Earths (like those in the image on the previous page) we have been discussing. “Mini Neptunes” is the other. They are gaseous planets about two to four times the size of the Earth and do not resemble anything in our immediate vicinity; our Neptune is four times bigger than Earth. Once a small planet is discovered, astronomers try to determine which category it belongs to. This is like biologists trying to classify a new animal as either being a mammal or reptile. One mystery is why these categories aren’t represented in our own solar system when they seem to be so prominent elsewhere in space.
This illustration shows the relative size of the Earth compared to super-Earths that have been discovered orbiting other stars. Credit 8
ROGUE PLANETS
Rogue planets are among the strangest celestial bodies that have been discovered so far. They wander the galaxy without orbiting any particular star. They probably originated in a solar system but got too close to a Jupiter-sized exoplanet and were hurled into deep space. As we have seen, these large Jupiter-sized planets frequently have elliptical orbits or migrate in a spiral toward the mother star. It is likely that their paths intersected with smaller planets, and as a consequence, rogue planets might be more plentiful than ordinary ones. In fact, according to some computer models, our own solar system may have ejected ten or so rogue planets billions of years ago.
Because rogue planets are not near a light source and give off no light themselves, it seemed hopeless at first to try to locate them. But astronomers have been able to find some through the gravitational lensing technique, which requires a very precise and quite rare alignment to take place between the background star, the rogue planet, and the detector on Earth. As a result, one has to scan millions of stars in order to detect a handful of rogues. Fortunately, this process can be automated so that computers, not astronomers, do the searching.
Thus far, 20 potential rogue planets have been identified, one of which is only seven light-years from Earth. However, another recent study, conducted by Japanese astronomers who examined fifty million stars, found even more possible candidates, up to 470 rogue planets. They estimated that there might be 2 rogue planets for every star in the Milky Way. Other astronomers have speculated that the number of rogue planets could exceed the number of ordinary ones by a factor of one hundred thousand.
Can life as we know it exist on rogue planets? It depends. Like Jupiter or Saturn, some may have a large number of ice-covered moons. If so, tidal forces could melt the ice into oceans, where life may originate. But in addition to sunlight and tidal forces, there is a third way in which a rogue planet may have an energy source that could give birth to life: radioactivity.
An episode from the history of science might help to illustrate this point. In the late nineteenth century, a simple calculation done by the physicist Lord Kelvin showed that Earth should have cooled down a few million years after its creation and therefore should be frozen solid and inhospitable to life. This result sparked a debate with biologists and geologists, who insisted that the Earth was billions of years old. The physicists were shown to be wrong when Madame Curie and others discovered radioactivity. It is the nuclear force at the core of the Earth, from long-lived radioactive elements like uranium, that has kept Earth’s core hot for billions of years.
Astronomers have conjectured that rogue planets, too, might have radioactive cores that keep them relatively warm. This means a radioactive core could supply heat to hot springs and volcanic vents on the bottom of an ocean where the chemicals of life may be created. So if rogue planets are as numerous as some astronomers believe, then the most probable place to find life in the galaxy may not be within the habitable zone of a star but on the rogue plane
ts and their moons.
ODDBALL PLANETS
Astronomers are also researching a plethora of completely startling planets, some of which defy categorization.
In the movie Star Wars, the planet Tatooine revolves around two stars. Some scientists scoffed at this idea, because such a planet would be in an unstable orbit and would collapse into one of the stars. But planets circling three stars have been documented, as in the Centauri system. We’ve even found four-star systems, in which two sets of double stars move around each other.
Another planet has been discovered that apparently may be made of diamonds. It is called 55 Cancri e and is about double the size of the Earth but weighs about eight times more. In 2016, the Hubble Space Telescope successfully analyzed its atmosphere—the first time this had ever been done with a rocky exoplanet. It detected hydrogen and helium but no water vapor. Later, the planet was found to be rich in carbon, which might constitute about a third of its mass. It is also scalding hot, with a temperature of 5,400 degrees Kelvin. One theory postulates that the heat and pressure in the core may be extraordinary enough to give rise to a diamond planet. However, these glittering deposits, if they indeed exist, are forty light-years from us, so mining them is beyond our current capabilities.
Possible water worlds and ice worlds have also been located. This is not necessarily unanticipated. It is believed that our own planet, early in its history, was covered in ice—a Snowball Earth. At other times, when the Ice Ages receded, the planet was flooded with water. Gliese 1214 b, the first of six known potentially water-covered exoplanets to be identified, was found in 2009. It is forty-two light-years away and six times larger than the Earth. It lies outside the habitable zone, orbiting seventy times closer to its mother sun than the Earth does. It may get as hot as 280 degrees Celsius, so life as we know it probably cannot exist. But by using various filters to analyze light scattered through the planet’s atmosphere as it transits the mother star, significant amounts of water have been confirmed. The water may not be in familiar liquid form due to the planet’s temperature and pressure. Instead, Gliese 1214 b might be a steam planet.
We have come to a striking realization about the stars, as well. We once thought that our yellow star was typical in the universe, but astronomers now believe that dim red dwarf stars, which emit only a fraction of the light of our sun and usually cannot be seen with the naked eye, are the most common. By one estimate, 85 percent of the stars in the Milky Way are red dwarfs. The smaller a star is, the slower it burns hydrogen fuel and the longer it can shine. Red dwarfs may last for trillions of years, far longer than the ten-billion-year life span of our sun. Perhaps it is not surprising that Proxima Centauri b and the TRAPPIST system both involve red dwarfs, because they are so numerous. Thus the area around these stars may be one of the most promising sites to search for more Earth-like planets.
CENSUS OF THE GALAXY
The Kepler spacecraft has surveyed enough planets in the Milky Way galaxy that a rough census can be made. The data indicate that, on average, every star you see has some kind of planet orbiting around it. About 20 percent of the stars, like our sun, have Earth-like planets—that is, are similar to the Earth in size and are in the habitable zone. Since there are roughly one hundred billion stars in the Milky Way, about twenty billion Earth-like planets may exist in our backyard. In fact, this is a conservative estimate—the actual number could be much higher.
Unfortunately, the Kepler spacecraft, after sending a mountain of information that changed the way we conceptualize the universe, began to malfunction. One of its gyroscopes started to fail in 2013, and it lost the ability to lock onto planets.
But further missions are being planned that will continue to augment our understanding of exoplanets. In 2018, the Transiting Exoplanet Survey Satellite (TESS) will be launched. Unlike the Kepler, it will scan the entire sky. TESS will examine two hundred thousand stars over a two-year period, concentrating on stars that are thirty to one hundred times brighter than those inspected by the Kepler, including all the possible Earth-sized planets or super-Earths in our region of the galaxy, a number astronomers expect to be around five hundred. Furthermore, the James Webb Space Telescope, the replacement for the Hubble Space Telescope, will be inaugurated shortly and should be able to actually photograph some of these exoplanets.
Earth-like planets may be prime targets for future starships. Now that we are on the cusp of investigating them in depth, it is important to explore two considerations: living in outer space, with the biological demands it would entail, and encountering life in space. We must first take a look at our existence on the Earth and how it may be enhanced to meet new challenges. We may have to modify ourselves, extending our life span, adjusting our physiology, and even altering our genetic heritage. We will also have to contend with the possibility of discovering anything from microbes to advanced civilizations on these planets. Who might be out there, and what would it mean for us to meet them?
PART III LIFE IN THE UNIVERSE
The aeons involved in traversing the galaxy are not daunting to immortal beings.
—SIR MARTIN REES, ASTRONOMER ROYAL OF ENGLAND
10 IMMORTALITY
The movie The Age of Adaline is the tale of a woman born in 1908 who is caught in a snowstorm and freezes to death. Fortunately, she is hit by a freak lightning bolt, which revives her. This peculiar event changes her DNA, and she mysteriously stops aging.
As a result, she remains young while her friends and lovers grow old. Inevitably, suspicions and rumors start, and she is forced to leave town. Instead of rejoicing in her limitless youth, she withdraws from society and rarely speaks to anyone. Immortality, instead of being a gift, is a curse to her.
Finally, she is hit by a car and dies in the accident. In the ambulance, the electric shock from the defibrillator not only revives her, it reverses the genetic effects of the lightning bolt, and she becomes mortal. Instead of weeping at her loss of immortality, she rejoices when she finds her first gray hair.
While Adaline eventually rejects the promise of immortality, science is actually moving in the other direction, making enormous strides in understanding aging. Scientists concerned with deep space exploration are keenly interested in this research, because the distance between stars is so great that it may take centuries for a ship to complete its voyage. Thus, the process of building a starship, surviving the voyage to the stars, and settling on distant planets might require several lifetimes. In order to survive the journey we would have to build multigenerational ships, put our astronauts and pioneers in suspended animation, or extend their life spans.
Let us explore each of these ways in which humans might travel to the stars.
MULTIGENERATIONAL SHIPS
Assume that an Earth-like twin has been discovered in space that has an oxygen/nitrogen atmosphere, liquid water, a rocky core, and is a size that closely matches that of the Earth. It sounds like an ideal candidate for habitation. But then you realize that this twin is one hundred light-years from Earth. This means that a starship, using perhaps fusion or antimatter propulsion, would require two hundred years to reach it.
If one generation corresponds to roughly twenty years, this means that ten generations of humans will be born on the starship, which will be the only home they know.
Although this may seem daunting, realize that during the Middle Ages, master architects would design grand cathedrals knowing that they would not live long enough to see the completion of their masterpieces. They knew that perhaps their grandchildren would be the ones to celebrate the opening of the cathedral.
Also, realize that during the Great Diaspora, when humans began to leave Africa roughly seventy-five thousand years ago in search of a new home, they realized that perhaps it would take many generations for them to complete their journey.
So the concept of a multigeneration voyage is not a new one.
But there are problems that have to be faced if you are traveling on a starship. First, the populatio
n has to be chosen very carefully, with at least two hundred people per ship, in order to have a sustainable breeding population. The number of people has to be monitored so that the population remains relatively constant and they do not exhaust supplies. Even the slightest deviation in population, extended over ten generations, could lead to a disastrous overpopulation or underpopulation, which would threaten the entire mission. So a variety of methods—such as cloning, artificial insemination, and test tube babies—might be required to keep the population stable over time.
Second, resources would have to be carefully monitored as well. Food and waste would have to be recycled constantly. Nothing could be thrown away.
There is also the problem of boredom. For example, people living on small islands often complain of “island fever,” the intense feeling of claustrophobia and the burning desire to leave the island and explore new worlds. One possible solution would be to use virtual reality to create imaginary, fanciful worlds, using advanced computer simulations. Another possibility is to create goals, contests, tasks, and jobs for people so that their lives have direction and purpose.
In addition, decisions have to be made on board the ship, such as the allocation of resources and duties. A democratically elected body will have to be created to supervise the day-to-day operation of the ship. But this leaves open the possibility that a future generation might not want to fulfill the original mission or that a charismatic demagogue might take over and subvert it.
There is one way, however, of eliminating many of these problems: resorting to suspended animation.
MODERN SCIENCE AND AGING
In the movie 2001, a crew of astronauts are kept frozen in pods as their giant ship makes the arduous journey to Jupiter. Their bodily functions are reduced to zero, so there are none of the complications associated with multigenerational starships. Since the passengers are frozen, the mission designers would not have to worry about the astronauts consuming large quantities of resources or keeping the population stable.
