Beyond: Our Future in Space (17 page)

Consider again that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every “superstar,” every “supreme leader,” every saint and sinner in the history of our species lived there—on a mote of dust suspended in a sunbeam.
⁴

Habitable Spots

Life on Earth has permeated every imaginable ecological niche. Humans require a “Goldilocks zone” of temperature, pressure, and atmospheric chemistry. Microbes are not so limited. Microscopic forms of life can thrive above the boiling point of water and below its freezing point, at atmospheric pressures from a tenth to hundreds of times that at sea level, and at pH values ranging from drain cleaner to battery acid. Life is found high in the stratosphere, inside rock, and at the edge of deep-sea volcanic vents.

Collectively, these microbes are called extremophiles. Biology adapts so readily to adverse conditions that extremophiles define the norm; it’s the fragility of large mammals like us that’s unusual.
⁵

Extremophiles aren’t all microbial. The tardigrade is an animal barely bigger than the head of a pin, with eight legs, a tiny brain, an intestine, and a single gonad. Colloquially referred to as “water bears,” tardigrades can withstand temperatures above the boiling point of water and below less than a degree above absolute zero, pressures greater than the deepest ocean trench and the vacuum of space, and a thousand times greater radiation than other animals.
⁶
Perhaps their best trick is cryptobiosis. The tardigrade brings its metabolism down almost to a halt and it dries out to have less than 3 percent of its weight in water. When water is added, it reanimates.
⁷
The tardigrade has lessons to teach us about how to survive in space.

As humans move beyond the Earth, a key concern is habitability. We can travel in the self-contained, sealed environment of a spaceship, but the energy and materials cost of sustaining that environment is huge. It will be much easier if energy is available at the remote location and if life can be produced from raw materials extracted there.

Life on Earth is diverse but unified: Elephants, butterflies, and fungal spores all share the same genetic code and all emerged from a single common ancestor about four billion years ago. The expression of that genetic code creates an astonishing array of life forms and functions. Earth is the only place we know of with life. With only one example of biology to study, scientists can’t specify the full range of habitats for life. Life evolved by natural selection to almost “fill the envelope” of physical conditions here on Earth, so it’s tempting to think that life elsewhere will occupy the full range of physical conditions on an alien planet. But that’s an assumption; until we know how life started on Earth or find another example of life beyond Earth, it’s possible that biology was a fluke. The burgeoning subject of astrobiology tries to understand how life began on Earth, what the sites for life elsewhere are, and whether or not any of them actually host life. Without a “general theory” of biology to set expectations, astrobiology is largely an empirical subject. To know whether or not we’re alone in the universe, we have to look.

The minimum requirements for biology as we know it are carbon, water, and energy. Carbon is the basic building block of complex molecules, and organic chemistry depends on the versatility of the carbon bonds. Water is a good medium for fostering chemical reactions and building complexity, and it’s a major part of all terrestrial creatures—from 40 percent for beetles to 99 percent for jellyfish. Both carbon and water are cosmically abundant, so setting them as prerequisites for life isn’t very restrictive. Then there is energy. Humans are at the top of a food web reliant on the Sun, but it doesn’t follow that life needs a star. Some terrestrial microbes are sustained by heat from volcanic vents or natural radioactive decay in rocks. In both of those cases, the source of energy is geological.

Habitability means something quite different for microbes and men. Microbes simply need a niche with basic organic material, some water, and a local energy source. Large mammals are fussier. They need a temperate, stable climate, which requires a specific planetary spin and orbit, since daily and seasonal variation can’t be too great. The need for a steady supply of water is paramount, since the metabolic processes are regulated in an aqueous solution. Traditionally the habitable zone in astrobiology is defined as the range of distances from a star where water can be liquid on the surface of a terrestrial planet.
⁸

We could visit most parts of the Solar System in the next few decades, but we need destinations where conditions aren’t too harsh. Within our cosmic backyard—the Solar System—what’s habitable and what’s not?

The Moon and Mercury are too small to retain an atmosphere and are geologically dead. With surfaces pulverized by meteors and irradiated by cosmic rays, they are considered uninhabitable, even by microbes. Venus is a close twin to the Earth in mass and size, but volcanism in its distant past pumped so much carbon dioxide into the atmosphere that a runaway greenhouse effect occurred. The resulting atmosphere is a hundred times denser than the air we breathe. It’s hot enough to melt lead and is laced with toxic ingredients such as acetylene and sulfuric acid. The verdict: nasty and lifeless.

Looking out from the Sun, we arrive at Mars. The red planet is beyond the edge of the traditional habitable zone, so too cold to host surface water, and its atmosphere is so thin that a cup of water placed on the surface would evaporate in seconds. But there’s indirect evidence for subsurface aquifers, where water can be kept liquid by radioactive heating from rock below and pressure from rock above. Mars may well have microbial life in these subterranean oases.
⁹
For this reason, it’s a compelling target for future probes and rovers.

The gas giants were long considered completely dead. Jupiter, Saturn, Uranus, and Neptune are far beyond the habitable zone, from five to forty times the distance of the Earth from the Sun. In the 1980s, the Voyager spacecraft provided a surprise when it found Jupiter’s moon Europa to be a world completely covered by oceans and ice (
Figure 28
). More recently, the Cassini spacecraft provided evocative details of Saturn’s large moon Titan, which has large bodies of liquid and river deltas, clouds, and a thick nitrogen atmosphere. Titan is eerily Earth-like, but it’s alien in its chemistry, with lakes made of ethane, methane, and ammonia. It was even more surprising when Cassini saw geysers shooting ice crystals from the surface of Enceladus. This tiny moon—no bigger than Rhode Island—has underground bodies of water, so it has all the ingredients needed for life. The best guess is that there are about a dozen moons in the outer Solar System with habitable “spots” for microbes if not for larger forms of life.
¹⁰

We know so much about the Solar System that it frames our thinking about life beyond Earth. Our planet is peerless as a habitable world, but there are definite prospects of some forms of life beyond the “Goldilocks zone.”

Worlds Beyond

If we commit to develop the technologies of space travel, then one day we’ll have the capability to travel to the stars. Whether we “outgrow” the Solar System or are simply curious about worlds beyond, we’ll leave the safe harbor of our planetary system and venture into deep space. Between stars is the almost perfect vacuum with typically just one atom in a sugar cube volume, 30 thousand trillion times less dense than the air we breathe. It’s at a temperature of –454°F, just a whisker above absolute cold. Until a few decades ago, we could only speculate about other safe harbors. Now we know they exist.

The telescope was small, less than two meters in diameter, not large enough to crack the list of the top fifty largest telescopes in the world. The site was mediocre, not high enough to have sharp images and not far enough from the city of Geneva to be truly dark. The project was protean, a survey of binary stars to diagnose their properties by how their light changed when they eclipsed each other.

But when Michel Mayor and Didier Queloz looked at the light curve of the bright star 51 Peg, they were taken aback. The star was not part of a binary system. Instead, it had a planetary companion about half the mass of Jupiter whipping around it on an orbit just over four days long. This gas-giant planet went around its Sun-like star twenty times faster than Mercury orbits the Sun. After centuries of speculation and decades of searching, the Swiss team had discovered the first planet outside the Solar System,
¹¹
an achievement likely to earn them a future Nobel Prize.

Their 1995 discovery kicked off a new field of science. Since then, the study of extra-solar planets, or exoplanets, has exploded.
¹²

Exoplanet detection pushes the limits of technology. Jupiter reflects a hundred millionth of the light of the Sun, so a remote Jupiter will look like a feeble dot of light nestled close to a vastly brighter star. Direct imaging of exoplanets is so difficult that it only succeeded in the last decade. Mayor and Queloz used an indirect method, where the planet is unseen but is detected by its periodic tug on the central star. When a planet orbits a star, the star isn’t stationary, but both orbit a common center of gravity. For example, as seen from a remote location, Jupiter makes the Sun pirouette around its edge every twelve years, which is the orbital period of Jupiter. The planet causes an oscillating motion of the star, which manifests as a Doppler shift of its light. High-resolution spectroscopy teases out this very weak signal, which is a wavelength shift of one part in ten million.
¹³
The Doppler method gives the mass of the exoplanet and the orbital period, which, via Kepler’s law, gives the distance of the planet from its star and thus its temperature.

The discovery by Mayor and Queloz was surprising because gas-giant planets had been thought to lie far from their stars, with orbits lasting decades. Other planet-hunters assumed they would need to gather years of data before seeing a planet’s signature. No one understood how a massive planet could form so close to a star.

Since 1995, the number of planet-hunters has grown. They have honed their techniques so that the detection limit has advanced from Jupiter-mass to Neptune-mass and now close to Earth-mass. Roughly one in six Sun-like stars has a planet around it, and many have more than one planet.
¹⁴
For example, the Sun-like star HD 10180, which is 130 light years from Earth, has seven confirmed planets and two more unconfirmed planets, making it as heavily populated as our Solar System (
Figure 29
).

The first discoveries of “hot Jupiters” were puzzling and indicated that the Copernican principle might be violated. What if the arrangement of our Solar System—small, rocky planets close in and large gassy planets farther out—was not typical? Theorists couldn’t figure out any way to form a giant planet very close to a star; there simply isn’t enough gas. The answer was that planets can move around. Gravity keeps planets circling the Sun, but it also subjects them to subtle forces that can make their orbits unstable, rearrange them, send them closer to the star, and even eject them from the system. Hot Jupiters like the one found by Mayor and Queloz formed at larger distances and migrated inward, parking on tight, tidally locked orbits. Gassy planets farther from their parent stars have been discovered, and there’s at least one terrestrial planet for every giant planet. There are likely to be many free-floating planets, called “nomads,” in interstellar space. By late 2014, the number of exoplanets was approaching two thousand.

In the past decade, a second method has been used to find exoplanets. If a system is oriented so the orbital plane is close to the line of sight, an exoplanet can transit in front of its star and cause a partial eclipse or temporary dimming of the star. The star dims by a fraction equal to the ratio of the area of the planet to the area of the star; this is 1 percent for a Jupiter and 0.01 percent for an Earth crossing the face of a Sun-like star (
Figure 30
).