Beyond: Our Future in Space

by Chris Impey

  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.

  Figure 28. Europa is a large moon of Jupiter far outside the conventional habitable zone, yet it contains all the ingredients for life. Under the ice pack shown here lies a kilometers-deep ocean, with heat flowing into it from the rocky interior of the moon.

  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 Jup
iter 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).

  Figure 29. The number of exoplanets has surged recently with the work of NASA’s Kepler telescope. The pale gray represents discoveries using the Doppler method, and medium and dark gray represent singly and multiply confirmed transit detections with Kepler.

  Figure 30. Most of the habitable exoplanets known have been discovered by the transit method, where the exoplanet partially eclipses and dims the parent star. Planets smaller than the Earth can be discovered with this technique from space.

  As the exoplanet count grows, the goal has shifted from finding exoplanets to characterizing them. The Doppler method gives mass and a transit gives size, so combining the two observations yields a mean density. That has been used to distinguish between gassy and rocky planets. Interpreting a single value of density can be ambiguous, but nature is imaginative enough to have made some planets that are mostly metal, some that are mostly rock, some that are mostly carbon, and some that are mostly water or ice. Evidence suggests that within this diversity are some planets that are just like home.

  Hunting Earth Clones

  The architect of NASA’s Kepler spacecraft has called it “the most boring mission ever.” The telescope mirror is one meter in diameter, the size of a coffee table and smaller than mirrors some amateur astronomers use. The telescope has been staring at 145,000 stars in a single patch of sky and measuring their brightness every six seconds.

  This “boring” mission has come close to finding a direct analog of the Pale Blue Dot, a planet we might call Earth 2.0.

  Kepler’s goal is a census of Earth-like planets in nearby regions of the Milky Way galaxy. Its strategy is a mixture of exquisite accuracy and brute force.15 The accuracy is required because an Earth-like planet passing in front of a Sun-like star only dims it momentarily by 0.01 percent. Reaching this level of accuracy is difficult, since the stars in Kepler’s small field of view are a hundred times fainter than any star that can be seen with the naked eye. Kepler must see a transit recur several times to be sure the light dip isn’t just a glitch or noise in the detector. The brute force comes in because planetary systems are randomly oriented and only a small fraction of them will be oriented so that an eclipse is visible. Odds are 1 in 215 for an Earth orbiting a Sun. If Earths exist in 10 percent of all planetary systems, then 100,000 stars must be monitored to detect a few dozen Earths. It’s the proverbial needle in a haystack.

  Kepler was launched in 2009 and quickly started to detect Earth-size planets, even though its sensitivity to light was a little worse than the design goals. The easiest exoplanets to detect are large ones on rapid orbits, since they cause bigger and more rapidly recurring eclipses, with a higher probability of being observed. The same is true in Doppler detection. Kepler detected a number of hot Jupiters in its first few months of operation. But as the mission progressed, smaller planets on larger orbits were detected. In 2013, the mission suffered a mortal wound when it lost the second of four reaction wheels that keep the spacecraft locked on target. It’s a bittersweet ending to a fabulously successful mission, but scientists will continue to pore over the four years of data and extract evidence for Earth-like planets near the limit of detection for several more years.16 As of April 2014, Kepler had a haul of 1,770 confirmed exoplanets and 2,400 candidates, almost all of which are likely to be confirmed.17 Most of these exoplanets are super-Earths or larger, but some are smaller than the Earth.

  Among half a million stars in Kepler’s field of view, scientists focus on the third of them that are similar to the Sun. In terms of habitability, stars more massive than the Sun are poor targets because they’re variable and emit lots of high energy radiation, and they live short enough lives that complex life on a nearby planet might not ever have enough time to evolve. At the other end of the mass spectrum are red dwarfs, which outnumber Sun-like stars by a large factor. Red dwarfs have slim habitable zones, so the odds of a planet being located there are low, but that is offset by the amount of such zones. In other words, the red dwarf habitable zone is small but there are many of them. When the calculation is done carefully, it turns out that there’s more habitable “real estate” associated with dim red stars than with stars like the Sun. Astronomers have started doing transit surveys of dwarf stars three to ten times less massive than the Sun.

  Kepler data have been used to project the total number of exoplanets in our galaxy. There are roughly 40 billion Earth-size planets orbiting in the habitable zones of Sun-like stars and red dwarf stars, with 25 percent of them orbiting Sun-like stars. That abundance means that, statistically, the nearest such planet is likely to be only twelve light years away.18

  While looking for a “Goldilocks” situation where conditions are just right for biology, astronomers have discovered a freak-show assortment of exoplanets. Methuselah, an exoplanet 12,400 light years away, is three times older than the Earth. Since it was formed within a billion years of the big bang, it’s surprising that stars had made enough heavy elements and “grit” to form a planet. The star 55 Cancri has a s
uper-Earth so hot and dense that a third of the surface is made of carbon crushed to a diamondlike state, worth a cool $3 x 1030 if it could be brought back to the Earth. GJ 504b is a Jupiter that’s farther from its star than Neptune is from the Sun. Even though it’s in the deep freeze, it glows a ruddy pink color because it’s shrinking due to gravity. At the other extreme, there’s a planet that orbits in darkness around a pulsar, whipping around the stellar corpse every two hours. TrES-2B is a mysteriously dark planet, blacker than coal or ink, and it’s not known what chemicals in its atmosphere cause it to absorb 99 percent of the light falling on it. GJ 1214b is a water world that’s completely swaddled in oceans tens of times deeper than those on Earth. Finally, Wasp 18b is falling onto its star as its orbit degrades. It will enter its final death spiral in just a million years—the blink of an eye in cosmic time.19

  Habitability depends primarily on the distance of a planet from its star. But it also depends on added heating from any greenhouse gases like carbon dioxide and methane in the atmosphere. It may also relate to plate tectonics, since the dynamism of geological activity was probably a driver for biology on the Earth. In the early oceans of the Earth, the chemical activity driven by plate tectonics is thought to be important for sustaining biochemical reactions. Modest orbital eccentricity and tilt are required to avoid big seasonal variations. However, these bounds are loose enough to accommodate super-Earths and pint-size planets or moons where conditions might be right for life.