Massive stars give, but they also take away—and that puts the inner limit on the galactic habitable zone. The supernova explosions that create and spread heavy elements also unleash a torrent of high-energy radiation: gamma rays, X-rays, and ultraviolet light. Those stellar explosions can have lethal effects on planets orbiting stars even tens of light-years away. In the crowded central regions of the galaxy, home to large numbers of massive stars, supernovas are so common that the evolution of complex life-forms might be difficult if not impossible.

The big question is how bad the supernova effect is. Lineweaver and his colleagues calculate that radiation poisoning could exclude the inner 20 percent of the Milky Way, which encompasses about half of all the stars in the galaxy. “You are looking for that sweet spot,” says Fred Adams of the University of Michigan, “where you are not so close to the center that conditions are hostile and not so far out that the metal abundance is too low.” But the Milky Way is huge, so Adams suggests putting things in perspective. “At worst the amount of galactic real estate favorable to life is reduced by a factor of two or three,” he says.

The amount of real estate that is off-limits depends heavily on how life responds to strong doses of radiation. Remarkably, we may already have good information about that locked away in the fossil record right here on Earth.

Every 62 million years, something bad happens to Earth’s biodiversity,” says Adrian Melott of the University of Kansas. “Paleontologists have built up large data sets of all the animals in the fossil record. With these data you can look to see how biodiversity changed with time.” His provocative studies, backed by the work of other groups, show that drops in biodiversity—sometimes indicating mass extinctions—seem to follow a periodic cycle.

Melott links the changes in biodiversity to the motion of the sun and planets through our galaxy. “As the sun orbits the Milky Way, it also bobs up and down, rising above the plane of the disk and then diving below it,” he says. “Every time the sun rises up and pokes out of the ‘north’ side of the galaxy’s disk, our biodiversity goes way, way down.” He notes that the Milky Way’s north side points toward the Virgo cluster, an enormous nearby gathering of galaxies. Our galaxy (and, by extension, our planet and ourselves) is falling toward Virgo at about 120 miles per second.

According to Melott, as the Milky Way plows through intergalactic material, a powerful shock wave forms ahead of it. Shock waves create energetic subatomic particles called cosmic rays, which can tear apart biomolecules and damage DNA beyond repair. Normally the galaxy’s magnetic fields protect us from that radiation. Every 62 million years, though, the sun bobs up above the disk into the danger zone, Melott finds. “When the sun pokes up above the galaxy’s plane on the north side,” he says, “the entire planet gets a giant dose of cosmic rays.”

All stars follow a similar bobbing motion as they move through the galaxy, but ones in the inner regions do so at a faster pace, which may bolster Lineweaver’s view that those regions are less likely to contain complex life. Then again, a certain amount of radiation is a part of life—in fact, an essential part. Radiation helps drive mutation, and mass extinctions clear the way for evolutionary change. That view tends to bolster Adams’s optimistic outlook. “We want enough radiation to pose a challenge and spur development of new life-forms but not so much as to sterilize the whole planet,” Melott concludes.

The Temporal Habitable Zone
Melott’s hypothesis about mass extinctions shows how habitable zones may be measured not just in space but also in time. It turns out that “when” is just as important as “where” for the existence of life.

Supernovas come into play here, too. When the universe emerged from the Big Bang, it consisted almost entirely of hydrogen and helium. Good luck trying to make a planet, much less a person, out of that. Carbon, oxygen, iron, and the like had to wait for stars—especially the massive ones—to form and create heavier elements via nuclear fusion. Those processed elements escaped in stellar winds or supernova explosions and then got picked up by subsequent generations of stars. Building up the elements needed for life this way takes billions of years. The entire universe was, therefore, a nonhabitable zone for perhaps the first few billion years of its 13.7-billion-year history.

Once the universe is full of heavy elements, the tables turn and the mortal nature of stars becomes a limitation. The sun, a medium-size star, is about halfway into its total lifetime of 10 billion years. In another 5 billion years it will swell into a red giant and either consume our planet or bake its surface to concrete. Even sooner, in as little as a billion years, the sun’s gradually increasing luminosity may make Earth unbearable for life. Brighter, more massive stars, which guzzle their nuclear fuel more quickly, may burn out too quickly to allow complex life to evolve.

Fortunately, the realization that dim red dwarf stars could potentially support Earth-like planets greatly stretches out the temporal habitable zone. The dimmest, most economical of those stars might live 10 trillion years, a thousand times as long as the sun. Then again, current studies suggest that the universe will probably expand forever. If so, the cosmos as we know it—full of stars and, maybe, full of life—will be a fleeting moment in an endless duration of cold, dark nothingness.

Feeling grim again? Don’t worry; the latest physics theories point to yet another habitable zone that would allow life to go on long after the last star has expired.

The Multiverse Habitable Zone
These days, the largest habitable domain to consider is no longer our universe but the hypothetical universe of universes, what cosmologists call the multiverse. After our universe has gone black, perhaps another (or many others) will carry on life’s flame.

The idea that our universe—everything we can observe, including the laws of physics that shape it—is just one among a vast ensemble may seem the stuff of science fiction, but cosmologists build multiverse models using a theory called inflation. Inflationary cosmology, currently the dominant model of the early universe, holds that the entire observable cosmos began as a speck within a far larger (perhaps infinite) existence emerging from the Big Bang. Within 10^-30 second after the moment of creation, this speck underwent a period of hyper-rapid expansion—hence “inflation”—becoming everything we see today. As bizarre as this model sounds, it has some reasonable observational support.

Some cosmologists go further and argue that inflation could also happen in other places and at other times, when these other bits of creation break out, undergo their own inflation, and become separate pocket universes. Physicists call this multiplication of reality “eternal inflation.” It leads to an almost limitless number of separate universes, each with its own laws of physics. (This dovetails with the equally weird predictions from string theory, a model of fundamental physics that suggests there could be something like 10⁵⁰⁰ different sets of laws.) “In some of these universes the force of gravity might be stronger or weaker than our own,” Fred Adams says. “In others the electromagnetic force that controls atoms and molecules could be different. The consequences for the formation of life in these different kinds of universes might be dramatic.”

Although there is no evidence for these multiverses, that has not stopped theorists from speculating about them. In our universe the laws of physics seem precisely calibrated to allow the existence of long-lived stars, planets with stable orbits, and molecules that allow complex chemistry. All of these seem to be prerequisites for life. “One of the things people always ask about is the behavior of stars in alternate universes,” Adams says. “If you have universes where stars can’t form, then it’s likely those would be pretty sterile places.”

Adams took this question seriously and began a study of alternative physics and its effect on the existence of stars. “I decided to do an actual calculation,” he says. “Could I get all this speculation down to a well-posed problem?” Each of the four fundamental forces (gravity, electromagnetism, and the strong and weak nuclear forces) has a kind of theoretical knob that can be turned up or down to change its strength. “I decided to calculate a bunch of theoretical stellar models, looking to see what range of forces gave me working stars,” Adams continues. The results surprised a lot of people.

“Many people claim that only a minute fraction of bub­ble universes would have the right conditions to harbor life,” Adams says. His calculations found instead that functioning stars would be more resilient to variations in physics than anyone expected. Since stars are a prerequisite for life, the findings could indicate far more possibilities for viable habitats. Fully a quarter of his models led to long-lived stars, but with an important caveat. Adams cannot say how probable any given strength of gravity or electromagnetism would be in a randomly chosen pocket. “What you need is to fold what I have done into a probability distribution across the multiverse,” he says. In other words, we need to know the statistics of variation in the laws of physics of pocket universes—and in inflationary cosmology there is no principle that guides the choice of physics in each of them.

Lee Smolin, a theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, has a controversial idea that makes some testable predictions about those other universes. In the process, he makes the case for habitability look even better than Adams inferred.

During the early 1990s Smolin proposed a multiverse model that differs strongly from inflationary cosmology’s pocket universes. His model focuses on the way that black holes warp space and time. Since the 1960s some theorists have floated the idea that when a massive star collapses into a black hole, it gives rise to a new universe. Smolin is building on that concept.

Black-hole-generated universes differ from the ones associated with eternal inflation in an important regard. With inflation there is no connection between the physics of one universe and that of another. The black-hole model, Smolin argues, strongly trends to certain types of physics. “Any universe that produces more black holes will create more daughter universes,” he says, “and its physics will be passed on to those daughters.” As a result, there should be a process analogous to natural selection favoring universes whose physics leads to the formation of more black holes. Such universes should dominate the multiverse.

Smolin’s model has two notable advantages. First, it explains why our universe has the physical laws that it does, since universes like ours that can create the massive stars that produce black holes are strongly selected. Second, it explains why our physical laws allow life to exist: The elements that permit the existence of stars happen to be the same ones that allow the existence of our kind of biology.

Actually, there is a third advantage. Smolin claims his black-hole multi­verse hypothesis can be tested. Since universes that give rise to the largest number of black holes have the most offspring, our universe should be optimal for making black holes. Smolin’s predictions, including ideas about cosmological inflation and the mass of the heaviest stable neutron star, have held up so far. “The theory is falsifiable,” he says. “If observations come out contrary to my predictions, then the idea is wrong.”

But if Smolin is correct, we inhabit not just a universe but an entire multiverse that may be teeming with life—a habitable zone unbound.

See Adam Frank's recent book, The Constant Fire: Beyond the Science vs. Religion Debate, and the companion blog to the book.