The evolving solar system “BLACK SMOKERS” are bastions of life at hydrothermal vents in today’s oceans. They get their names from the soot-like look of Earth’s the mineral-rich material they eject. NOAA he first things to go will be A brightening Sun will boil the Earth’s glaciers and polar ice caps. Warming surface seas and bake the continents a temperatures will turn billion years from now. But that’s ice to water, leading to a deadly slow but steady rise in sea levels. But it Tdoesn’t stop there. Eventually, tempera- nothing compared with what tures will rise high enough for seawater to boil away, leaving Earth bereft of this we can expect further down vital substance. With that, life on our world will need to relocate underground the road. ⁄⁄⁄ BYCR RI HA D TALCOTT or emigrate from our home planet. This apocalyptic scenario is more than an inconvenient truth — it’s our inevitable destiny. And it has nothing future to do with changes humans may work on our fragile environment. The agent for this transformation is far beyond our control. The culprit: our current life-sustaining source of heat and energy, the Sun. Ask most people familiar with astronomy when to expect this coming apocalypse, and you’ll hear answers of around 5 billion years — once the Sun swells into a red giant. But the end is nearer than that. The Sun is currently growing brighter, and has been since the day it was born. Life on the main sequence A BILLION YEARS FROM NOW, the Sun’s When the Sun was a baby, it was rather increasing luminosity will have boiled off miserly by today’s standards. It emitted most of Earth’s water. In this view, water roughly 30-percent less energy then exists only in deep ocean trenches, where than it does now. The Sun officially thermophilic bacteria cling to life. LY NETTE COOK became a star when it started fusing © 2013 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com www.astronomy.com 29 gases the atmosphere will contain. Most Planets on the move scientists expect the level of atmospheric Today carbon dioxide to drop in the distant future. This will come about as photosynthetic Sun Mercury Venus Earth Mars organisms extract carbon dioxide from the 0.38 AU 0.72 AU 1.00 AU 1.52 AU atmosphere and weathering incorporates some of it into silicate rocks, which then Sun and planetary orbits shown to scale; planet sizes not to scale are subducted into the mantle. As the oceans start to evaporate, the Sun’s high-energy ultraviolet radiation will break the water molecules into their 6.5 billion years from now constituents, hydrogen and oxygen. The lightweight hydrogen gas will escape Sun as red giant Earth’s gravitational hold and bleed into 0.88 solar-mass Venus Earth Mars space. It might take another billion years 0.93 AU 1.17 AU 1.85 AU for ocean water to disappear completely, but by then, any remaining life will have had to make other plans. One viable option might be Mars. As Earth becomes too warm for most life to 6.7 billion years from now survive, the Red Planet should be getting balmy. If humans can make it till then, Mars would offer some attractive real estate. Sun as asymptotic giant 0.66 solar-mass Earth Mars 1.61 AU 2.46 AU Into the deep future To this distant point, the Sun and Earth have taken nearly opposite paths. Even a billion or two years from now, the Sun will AS THE SUN AGES, it will lose some of its mass. This trend will accelerate when it becomes a red giant, and grow even greater when it look basically the same on the outside as it ICY EUROPA could prove to be a watery haven in the distant future, when increasing swells into an asymptotic-giant-branch star. This mass loss will cause the orbits of the planets to migrate outward. ASTRONOMY: ROEN KELLY does now — a little bigger and brighter, but solar radiation will render the inner planets uninhabitable. NASA/JPL still recognizable. The Sun’s internal struc- hydrogen into helium in its core. These And that’s the rub. The nuclear reactions weren’t much, if any, colder. That’s good ture, however, will have changed markedly. Location, location, location nuclear reactions release energy according in the Sun’s core essentially convert four news as far as life is concerned. The first Its center will be largely helium, although When the South Pole feels more like the Amazon jungle a few billion years from now, any 2 to Einstein’s famous equation: E=mc . This hydrogen atoms into one helium atom. Gas single-cell organisms arose some 3.5 billion lots of hydrogen will exist in the core. The life on Earth will be looking for a way out. The Sun’s increasing luminosity will render Earth energy source defines any star’s main pressure, however, depends in part on the years ago, and they presumably required hydrogen continues to fuse into helium and uninhabitable, and worried eyes will look skyward. sequence life — where it spends the vast number of particles in the gas. The ongoing liquid water. But the Sun wasn’t hot enough add to that element’s growing abundance. In a reversal of science-fiction proportions, the first stop may well be Mars. Unlike H. G. majority of its days. fusion reduces the number of particles, so by itself to melt terrestrial ice until roughly For Earth, on the other hand, the sur- Wells’ classic novel, in which dying Martians looked longingly toward a more hospitable We tend to think of a main sequence the pressure drops. To maintain hydrostatic 2 billion years ago. face would hardly be recognizable. Our Earth, earthlings may decide to head for cooler martian climes. Mars has a distinct advan- star like the Sun as constant, but it’s not. equilibrium, the Sun must compensate. The We can thank our lucky stars for the “pale blue dot” will be more of a muted tage: Not only will it likely serve as humans’ first permanent outpost in the solar system, It maintains what astronomers call hydro- core shrinks, raising both the temperature greenhouse effect. The presence of water brown, and blistering temperatures will but it also holds the promise of being clement for an extended period. static equilibrium — the outward pressure and density. That, in turn, increases the rate vapor and carbon dioxide in the atmos- make it uninhabitable. But the deep interior But even Mars will grow too hot once the Sun becomes a red giant. Then, the only exerted by the core’s hot gas balances the of nuclear reactions, and the Sun generates phere warms our planet well above what it won’t see much effect. Although it will have reasonable outposts will be on the moons of the gas-giant planets. Several of them — including Jupiter’s Io, Europa, and Ganymede, and Saturn’s Enceladus, Rhea, and Dione inward crush of gravity. If the Sun’s central even more energy. would otherwise be. Even today, Earth is cooled modestly as the total mass of radio- — already come with huge complements of ice. Raise the Sun’s temperature significantly, temperature were to drop slightly, for These changes operate slowly. Although some 60° Fahrenheit (33° Celsius) warmer active elements decreases, a 21st-century and all may afford ocean-front property at some future point. example, the gas pressure would also fall. a hundred million years may sound like a than it would be without greenhouse geologist would still recognize it. But the reality of the Sun’s demise is that by the time Jupiter or Saturn become viable Gravity then would force our star to con- long time, for the Sun, it’s a blip on the warming. In the distant past, when Earth’s But as time continues to march on, abodes, any surviving civilization should seek other solar systems. After several billion tract and heat up, restoring its equilibrium. radar screen, representing 1 percent of its interior was hotter and volcanic eruptions changes in the Sun and the rest of the solar years of calling Sol home, a few million extra years won’t seem like much. It will be time The Sun started life as a uniform mix life span. And in a hundred million years, likely belched significantly more green- system will become more pronounced. The to become citizens of the galaxy. — R. T. of approximately 73-percent hydrogen, the Sun’s luminosity rises less than 1 per- house gases into the atmosphere, the effect real changes start roughly 5 billion years 25-percent helium, and 2-percent heavier cent. The energy increase prompts the Sun would have been greater. from now, when the Sun exhausts the elements, by mass. The outer parts of the to expand at a comparably lethargic pace. The push to higher solar luminosities hydrogen fuel in its core and prepares to of water) there, but not extreme enough to jumps, the overlying layers will expand and Sun still maintain that balance. But in the Its diameter grows at about the same rate as continues. Roughly 1 to 2 billion years leave the main sequence. As the Sun takes ignite helium. Meanwhile, hydrogen in the cool. The star will be on its way to becom- core, where nuclear fusion rules, helium human fingernails: 1 to 2 inches per year.