Read Beyond: Our Future in Space Online
Authors: Chris Impey
Beyond: Our Future in Space (18 page)
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.
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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.
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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.
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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.
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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 super-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 10
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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.
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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.
Figure 31. The habitable zones of Alpha Centauri A and B, shown as the pale wide rings. The exoplanet found around B is too close to the star to be habitable, but a habitable world might still be found. Earth’s orbit is shown as a dashed circle for reference.
Intriguingly, the best prospect for finding Earth 2.0 may be around the nearest star to the Sun.
In 2012, researchers at the European Southern Observatory caused a stir when they claimed the detection of a planet 20 percent more massive than the Earth orbiting Alpha Centauri B, the closest Sun-like star to us at a distance of only 4.37 light years (
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). The team that found the planet was part of the Mayor and Queloz group that discovered 51 Peg, the first exoplanet, in 1995, and they made the discovery while pushing the limits of the Doppler method, trying to measure a motion of half a meter per second instead of 50 meters per second for 51 Peg. Other groups have questioned the detection, so it remains in doubt.
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But even if the Alpha Centauri B planet is real, this Earth-like planet isn’t habitable. It’s twenty-five times closer to its dim red star than the Earth is to the Sun, traveling on a three-day orbit that’s probably tidally locked. More distant Earth-like planets at more temperate locations aren’t ruled out, but they’re below the detection limit of the best planet-hunting gear available. In another few years, the tools will be sophisticated enough to detect them. If there’s an Earth so close, we will put an enormous effort into characterizing it and seeing whether it could host life. And if it’s habitable, won’t it be tempting to take the next logical step by sending robotic probes and then people to explore it?
M
y heart is palpitating and my skin is clammy. It’s all I can do not to bolt, but of course there’s nowhere to run. Josefina rests her hand gently on my shoulder. I take a deep breath and steady myself.
Minutes earlier we’d ziplined over from the station to Ark 1. I watched the Earth slide under my feet like I was skating on blue-white ice. As the ark approached, there was no sense of scale; its surface was black and seamless, reflecting no light. There would be no Hub in our next home, and no recreation—the ark was a high-tech sarcophagus.
Going through the air lock, I get a sense of the mass of this monolith. Its beryllium and carbyne laminate is designed to quench cosmic rays and stop meteorites up to the size of a pea. Inside, there are soft lights and soothing bot voices guiding us around and explaining the guidance and life-support systems, but all I can think of is the narrow long corridor shrinking to a vanishing point in front of me and the stacked translucent boxes on either side as I float by. Frosties, the people down below call us. We’ll be taken to the edge of death, left there for a century, then reeled back into consciousness to explore a new world.
It’s quixotic, even preposterous. But that isn’t what gives me a panic attack. It’s the uncompromising vessel. The ark is utilitarian, Spartan, with no grace notes. It’s designed with only one thing in mind: to shield its human cargo from the indignities of space. To close on a more positive note, the bot guides us to the one common area where we’ll be able to eat and rest before taking pods down to the surface.
I want to be with Josefina but the Overseers are calling the shots. Ark assignments are done by lottery. She was picked for number one and I got number two. Keeping focus in the station is harder and harder. All the tasks they design to keep us busy seem meaningless. There are more defections and ejections. The churn is so great that we speculate that there might be a second, “shadow” Academy on a neighboring lake in Switzerland. How appropriate that the Overseers would plan a larger pool so that natural selection on the station will increase the chances of mission success.
Nina. Pinta. Santa Maria. From an earlier age, small ships setting out on a vast ocean to an unknown fate.
We spend a last precious hour in the Hub on her last day. Ark 1 is completing deployment of its solar sail. The gossamer-thin membrane has unfurled on all sides to span a square kilometer; the spaceship is dwarfed by it like a stick of charcoal on a silver carpet. The sail will harness the solar breeze to accelerate the ark to the edge of the Solar System and then pulsed fusion of hydrogen atoms snatched from space will propel it to its destination. Not good-bye, she says, it’s au revoir, but we both cry.
Ark 1 departs the next morning. I keep busy with my studies. There’s plenty to learn to be ready to handle the rigors of the pioneer life. I stay fit. I keep mostly to myself. I’m focused. So maybe I’m not paying attention one evening at dinner when the voice over the PA says that there’s been an incident on Ark 1. That a design flaw has eluded the corrective capability of the neural net. That the life-support protocol has been compromised. That it’s a freak occurrence, one never seen in the sims. That in the judgment of the project office Ark 1 has been lost with all hands.
Numb. I stay that way for weeks. I’ve no idea why the Overseers don’t pull me. They have every reason to. Maybe many others are in nearly as bad a shape. Gradually I bottom out and join in training and social activities. I become resolute. There’s nowhere to go but up. After all, that’s why I’m here.
Ark 2 is poised for launch. Ark 3 will follow in a few months. Its solar sail is transparent, so sunlight streams through it untouched. With one command, a weak electric current can be applied, the polarization in the thin film will shift, the material will become opaque, and Newton’s laws—which killed my father—will push me away from home. As we wait for our turn in the air lock to leave the station, we all pass through anxiety into a kind of delirious anticipation. We laugh and chatter, giddy with excitement.
I have a tinge of panic as the lid clicks shut. Then I watch with mild interest as clamps cinch my wrists and ankles. The needle drops down and I feel very little pain when the IV starts to replace my blood with glycerol. As the nitrogen vents open and the lid frosts white, I keep a single thought in view: I exist.
