There is even observational evidence of this. In September 2008, astronomers determined that the star BD+20 307, located in Aries some 100 parsecs away and previously known to be surrounded by a million times more dust than the Sun, is actually binary and several billion years old. This means that the dust is not from the formation of the system but more likely from a recent catastrophic collision between two terrestrial planets. This is an extreme version of an LHB, one that caused planetary orbits to become so unstable that a collision occurred between two large worlds, destroying everything on them including any advanced life that happened to have developed. Based on the limited statistics that we have, though, only a small fraction of Earth-like planets are likely to be in systems currently undergoing an LHB event.
After our own LHB event finished about 3.8 billion years ago, the bombardment rate dropped again. At this point, the evolution of Earth-like planets can begin to follow very different paths. On Earth, large pieces of its crust, known as plates, float around from place to place, so much so that only 200 million years ago the arrangement of the continents was completely different. This continental drift (or “plate tectonics” as it is more correctly known) also has important effects on the evolution of planetary habitability. Not all terrestrial planets have it; in fact, Earth may be the only one in our solar system, and just because a planet was able to develop the right conditions for plate tectonics at some time in its history, this does not imply that this process would continue indefinitely.
A cessation of plate tectonics on an Earth-type world would have profound implications for its habitability. It is believed that plate tectonics plays a major role in modifying the atmosphere of an Earth-type planet. The early Earth seems to have had much more carbon dioxide in its atmosphere than today’s Earth, and it is thought that one of the main mechanisms for the gradual reduction of that carbon dioxide concentration over time was the reaction of the gas with rocks to form carbonates, which were then dragged down underneath the plates as they slid against each other. Without plate tectonics, this process would operate inefficiently and thus the planet would be more likely to retain large amounts of carbon dioxide, a greenhouse gas, in its atmosphere, thus making it a lot warmer due to a greatly enhanced greenhouse effect. Also, a planet with a smaller inventory of radioactive materials within it than the Earth would have a less molten core and a thicker crust. This would also inhibit plate tectonics, thus leading to a very different outcome for the evolution of the planet.
But the Earth was lucky. Plate tectonics commenced, continents formed, and the rocks started absorbing the large concentrations of carbon dioxide in the atmosphere that was one of the main factors keeping the Earth hot. The Earth changed from a Hadean world to an Archean planet. The ocean gradually cooled, life multiplied, and the oxygen content of the atmosphere gradually rose. There will be many extrasolar Archean worlds: warm to hot watery, stormy planets with unbreathable atmospheres and hosting the simple organisms that will later change these worlds forever.
About 2.4 billion years ago, there was a sudden spike in the oxygen concentration in Earth’s atmosphere, from practically nothing to about 0.5% of today’s levels. This was caused by the rate of oxygen production by organisms finally being large enough to overcome its removal by chemical reactions associated with volcanic eruptions. The atmosphere was still unbreathable, as an oxygen content of 0.5% at the surface is only about as much as occurs in our current atmosphere at a height of about 30 km (90,000 feet). The ongoing reduction in atmospheric carbon dioxide content had significant implications for Earth’s climate: a couple of hundred million years later, there is the first evidence of widespread ice sheet formation on the surface of the Earth. The planet was suddenly colder and the huge amounts of atmospheric carbon dioxide deposited into the crust during this time in the form of limestone and other rocks bear witness to this process.
As a result, there will be some other Earths that are colder, and a few that will be completely frozen over. We know this because Earth only narrowly avoided this fate itself. The continual removal of carbon dioxide from the atmosphere eventually caused the natural greenhouse effect of the Earth to become so feeble that about 750 million years ago large glaciers advanced all over the globe, initiating what is now widely known as Snowball Earth. This giant Ice Age lasted until volcanic eruptions gradually built up the carbon dioxide concentration of the atmosphere to the point where the temperature rose high enough for the planet to escape from the clutches of the snow and ice, a process that took perhaps 20-30 million years. This cycle of freezing and thawing may have happened several times. Not all extrasolar planets will have been so fortunate: for planets with low rates of volcanic activity, once in a snowball state, some of them would have stayed there.
But there have also been times when the Earth was more habitable than it is today. During most of the Cretaceous period (144-65 million years ago), there was no permanent land ice at sea level, the climate was several degrees warmer than it is today, and the atmosphere was breathable by human beings. A significant number of Earth-like worlds will be like this or warmer. Indeed, it is easy to argue that these warm worlds will be more abundant than worlds similar to Earth that have large ice sheets in a number of locations. Earth has only had large ice sheets for about the last 30 million years, a much shorter period than it has been ice-free.
Future Earths
Most accounts of planetary evolution cease at this point, having reached the present day. But Earth will continue to evolve into the future, and many Earth-like worlds will be older than Earth. Based on our understanding of geology and of stellar evolution, we have some idea how our planet is likely to continue to evolve. One major wrinkle is that the technological civilization on our planet has now evolved to the point where it is capable of modifying the general habitability of the planet in a number of ways. Particularly relevant here is the artificial increase in the size of the greenhouse effect, most likely caused by the burning of fossil fuels. In addition to warming the Earth, this could change its habitability characteristics in other fundamental ways. While Earth is currently undergoing ice ages every 150,000 years or so, it would not do so if the greenhouse effect were larger. Calculations suggest that once greenhouse gas concentrations rise above values similar to those predicted for the end of the twenty-first century, the initiation of ice ages will become impossible because Arctic summers will never become cool enough for ice to start to accumulate on the adjacent land regions, the main initiating mechanism for an Ice Age. But this suppression of ice formation may well be a very temporary effect, as when alternative energy sources are employed on a large scale, the concentration of carbon dioxide in the atmosphere will begin to fall again. Man’s ingenuity is practically limitless in this regard.
In any event, though, whatever changes to the climate are made by man, these are rapid processes when compared to the future evolution of the habitability of the Earth, which will take place over billions of years. This evolution will be governed by two competing phenomena: a general increase in temperature caused by the increasing brightness of the Sun, and a decrease in carbon dioxide concentration caused by increased absorption of carbon dioxide by surface minerals, due to increased weathering caused by higher temperatures. Mostly, though, the Sun will win this competition. By about one billion years from now, the Sun will about be 10% brighter, giving average global temperatures of 50°C (122°F) or so, thus rendering the Earth effectively uninhabitable by humans, except perhaps in isolated mountain areas in the polar regions. At this temperature, there will be a huge increase in the amount of water vapor in the stratosphere, where water molecules are easily broken apart by ultraviolet radiation into oxygen and hydrogen. The hydrogen will then escape, leading to a gradual decrease in the water content of the planet. Estimates vary but by between one and two billion years into the future, the oceans will be gone and the Earth will be a dry, hot wasteland, full of fossils. There will be many extrasolar Earths like this.
But
of course the evolution of the Earth is not finished. Eventually, the Sun will turn into a red giant, and then the news is not good. Recent results suggest that the Earth will not survive this phase of its life: it will be close enough to the Sun for its orbit to experience enough drag from the outer layers of the Sun’s atmosphere, causing it to spiral inwards towards certain extinction. It is possible that not even Mars will escape this fate.
Typical Earths
So, based upon the evolution of the Earth itself, what is a “typical” Earth-like planet like? The short answer, of course, is that there isn’t one. Each one will be different in some way, although scientists in the future may find ways of classifying them that can identify similarities between them. Here, we have defined an “Earth-like” world based on the relative human habitability of such a planet: in other words, how much adaptation would be required so that human beings could survive and live there. With sufficient technology, most worlds can be made habitable in a very small region, but the amount of technology required varies greatly. At the most favorable extreme on Earth, say on Maui for instance, all that is required is some light clothing and shelter, some edible plants and a knowledge of agricultural techniques. In Antarctica, to be self-sufficient would be considerably more difficult, requiring protective clothing and insulated buildings, greenhouses growing food using artificial lighting, and a power source. On Mars, these buildings would have to be airtight also, and because the thin atmosphere of Mars provides little protection against cosmic rays, living spaces should also be buried beneath a couple of meters of soil. Clearly, these living conditions are not “Earth-like.” So “Earth-like” worlds should satisfy certain minimum human habitability requirements: a climate that is not extreme, an atmosphere that is breathable for an indefinite period of time, a meteorite infall rate that is not excessive, and so on.
Naturally, the differences between the habitability and climate of Earth-like planets, large though they will be, will pale into insignificance compared with the differences in their ecosystems. Ecosystems, being more complicated than planets, may be practically unique. But no matter how strange these worlds are, every one of them will contain something familiar, something reminiscent of Earth: clouds and storms, ocean and beaches, rivers and streams, mountains and valleys, and on a few, some fresh air that will remind us of home.
Further reading
Bounama, C., S. Franck and W. von Bloh, 2001: “The fate of Earth’s ocean.” Hydrology and Earth System Sciences, 5, 569-575.
Gornitz, V. (ed.), 2009: Encylopedia of Paleoclimatology and Ancient Environments, Springer, 1049 pp.
Gorlova, N., et al, 2007: “Debris disks in NGC 2547.” The Astrophysical Journal, 670:516-535.
Selsis, F., L. Kaltenegger and J. Paillet, 2008: “Terrestrial exoplanets: diversity, habitability and characterization.” Physica Scripta, T130, 1-9.
Weinberger, A.J., 2008: “On the binary nature of dust-encircled BD+20 307.” The Astrophysical Journal Letters, 679, L41-L44.
Copyright © 2010 Kevin Walsh
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Science Fact Probability Zero
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Short Stories
A Snitch in Time
Time travel, even with limitations, could be very useful in law enforcement. Or could it?
Donald Moffitt
The station house was quiet at that hour. A gray dawn was just beginning to creep around the edges of the window shades, but otherwise the only light in the room came from the goose-necked desk lamp that was casting a yellow pool of illumination on the case file in front of him.
Lieutenant Francis Patrick Delehanty drained the last cold dregs of his coffee and tossed the paper cup in his wastebasket to join its fourteen predecessors. He lit another stale cigarette, crumpled the empty pack, and stared wearily at the case file.
There wasn’t much in it. Most of the evidence had disappeared thirty years ago. The doorknob rattled and the night duty officer, an old cop named Flaherty, poked his head in. “Morning, Lieutenant. You’re kinda early, ain’tcha? The morning shift isn’t due till...”
He stopped as he saw all the old yellowing reports spread out across the desk. “You pulled another all-nighter, din’t you, Lieutenant?” he said reprovingly.
“My retirement’s only ten days away,” Delehanty said. “I promised myself I’d get the bastard before I left.”
“The Roast Beef Slasher?”
“Don’t call him that!” Delehanty said sharp-ly. Then, at the hurt look on Flaherty’s face, “Sorry, Tim. I guess I’m just edgy.”
“Give it up, Lieutenant. They ain’t caught the guy in the last thirty years, they ain’t gonna catch him in the next ten days. Leave it to one of the young hot shots. Maybe something’ll come in that’ll reopen the file. Take up golf. Go fishing. Enjoy your retirement.”
“Not while that bastard is still walking around, laughing at us! The cold case squad worked on it for ten years, before the department told them to drop it. The rest of the caseload was suffering, the chief said. It was affecting the homicide clearance rate. Too much time was being wasted on a case that was going nowhere.”
Flaherty said placatingly, “You could take the crime book home with you. A lot of retired cops do that with an old inactive file that bugs them. You know, make it a kind of hobby, keep you occupied.”
Delehanty slammed a fist down on the desk, making Flaherty jump. “Hobby is it? The hell with that! I’m going to nail the son of a bitch while I’m still a cop!”
The deputy prosecutor’s name was Jarrett. He was a neat, contained man in his forties with an advanced case of male pattern baldness and a crop of blue stubble starting to show through what was probably his second shave of the day. Delehanty had done business with him before. They were on pretty good terms.
“I’m not asking for your blessing,” Delehanty said. “I’m just sort of, you know, sounding you out to get the lay of the land.”
Jarrett pursed thin lips. “It could be complicated,” he said. “There’s connection of evidence. That’s different from connection of inference. There’s all sorts of time-travel laws. There’s your own liability in two different jurisdictions
in two different realities. Why don’t you tell me what exactly we’re talking about?”
“Listen, you remember the slasher case from about thirty years ago involving an assistant D. A. named Vaccaro?”
Jarrett thought a moment. “It was before my time, but yeah.”
“The press called him the Roast Beef Slasher. Forensics determined that the weapon in the Vaccaro murder and the related cases was something like a serrated steak knife with a two-pronged tip. His signature was a slice of roast beef left at the scene. To top it off, DNA analysis showed bovine blood mixed in with the victim’s, like the knife hadn’t been cleaned. Vaccaro was working at his desk, pulling an all-nighter, when he was killed. He was on to something, and the killer knew it. Unfortunately, Vaccaro’d been blabbing to the press. He had something that would nail one of the suspects, but he wouldn’t say what it was, even to the detectives working the case—said he’d tell them when he was ready for them to make an arrest. But he never got to tell them. Whatever it was, it was in the murder book, and half of that was ripped out, including all the interview transcripts.”
“What’s your interest in this, Lieutenant? Thirty years ago you would hardly have been a member of the homicide squad working the case.”
Delehanty nodded. “I was just a rookie patrolman. But I was the one who walked in on the murder scene. I’ll never forget it. There was blood everywhere. I sounded the alarm and they searched the building. But the killer was gone. With the transcript of his own interrogation. He must’ve slipped up somehow, and Vaccaro stumbled onto some contradiction in his alibi.”
Analog Science Fiction and Fact 01/01/11 Page 24
