If this were some 1950s sci-fi thriller, the Doomsday Cloud would loom dark and ominous in the evening sky. Each night, more and more stars would wink out along its edges. The cloud would sweep past Jupiter, swallowing it whole, and race on toward Earth. There’d be an inky darkness at noon. And so on.
I don’t like the doomsday business, the killer cloud, all that stuff, says Priscilla Frisch, a University of Chicago astronomer who has spent decades studying the wispy matter that lies between stars. And no, nothing is going to blot out the sun. But recent observations and numerical simulations suggest that eventually (in a few millennia, maybe) the solar system will plow into a cloud of gas and dust a thousand times denser than the space we travel through now. This soupy cloud will reduce the sun’s sphere of influence until most of the outer planets are sitting naked in interstellar space. Dust and gas will penetrate as far as Earth’s orbit and might begin eating away at the oxygen in our upper atmosphere. The solar wind, now greatly compressed, will no longer provide adequate protection from the high- speed electrons and ions ripping through space. These cosmic rays will tear straight into the atmosphere, to the detriment of the delicate molecules of life.
It isn’t exactly Million Dollar Movie material, but it is one bad cloud.
For worrywarts, comets and asteroids have been the bugbears of choice in recent years. An asteroid, after all, is thought to have wiped out the dinosaurs, and the statistical threat of another strike has led some scientists to propose a $50 billion asteroid defense program. Galactic hazards are almost certain not to occur in our lifetime, but if one did, it would make a stray asteroid look like child’s play. A giant interstellar cloud would mess up the entire solar system for decades. Until recently, worry about such events rested on mere speculation. In the last couple of years, however, astronomers have found real hazards associated with actual objects in the sky. For example, according to the latest calculations, the sun will soon be getting a visit from a stellar neighbor—a visit that might send a rain of comets hurtling toward Earth. (Don’t panic. Astronomers have a much different sense of time than the rest of us. When they say soon, they’re talking tens of thousands of years from now.) And the edge of a possibly disastrous cloud of interstellar gas is less than 4 light-years away. Of course, it might take 10,000 non-light-years to get here, but in cosmic terms that’s a mere heartbeat.
Danger and death from above is one of humanity’s oldest fears. The positions of the stars and planets were thought by the ancients to account for everything from the fates of kings to the flooding of the Nile. Comets, in particular, are infamous harbingers of doom. As astronomers learned more about celestial objects, the specter of supernatural influence naturally began to fade. However, seeing the universe through large telescopes in great detail has exposed a whole new scientifically informed layer of worry. After all, if the sun orbits the Milky Way every 250 million years, then over time our surroundings are likely to change radically. Instead of putting along in our placid little suburb, we could be clawing our way through the heat and dust of the Milky Way’s arms, a star construction area. Or swinging too close to a giant star just as it explodes into a searing fireball. Or smacking into a black hole. Scary to think about, but not terribly likely: we are not close to a star about to go supernova, and we’re a long way from the nearest black hole. It’s just that until now no one was aware of any dangers lurking around the corner.
One of the biggest hurdles to understanding our galactic neighborhood has always been that we’ve never had an accurate map. Astronomers didn’t even know exactly where some of the nearest stars were, let alone all the small, hard-to-see bits of matter. Two of the three locating dimensions were easy enough: astronomers have little trouble pinning down the latitude and longitude of the stars splayed across the skies. The problem is figuring out the distance between us and the stars. A simple point of light could be a dim nearby star or a brilliant one thousands of light-years away. There’s no way to reach out and physically measure this third dimension.
As a result, astronomers have had to resort to a form of binocular vision called astrometry. Photographs are taken six months apart, during which time the perspective changes, due to the movement of the stars in relation to the sun and Earth’s revolution. Each photo acts something like the image from one of our eyes; by laboriously measuring the slight shift in a star’s position from January to July, astronomers can calculate its distance. The precision of this work is extraordinary: the difference in motion between stars 30 and 40 light-years away is a mere 8 millionths of a degree, or the width of a mite on a catcher’s eyelash as seen from the center-field bleachers. Astrometry is grunt work. Besides, atmospheric distortion creates so many errors that beyond the first handful of nearest stars, distances to other objects in the galaxy amount to educated guesswork.
That’s why a consortium of European nations launched the Hipparcos satellite back in 1989. Working far above the atmosphere, Hipparcos took picture after picture of a million stars, tracking how they swayed back and forth across the sky over the course of four years. Computers, which excel at grunt work, then calculated the distances to an accuracy as much as a thousand times better than anything previously done. Finally, astronomers were able to put most of the stars they can see in their three-dimensional place.
But on top of the back-and-forth movement caused by Earth’s orbit around the sun, Hipparcos picked up another kind of motion, one caused by the stars themselves. The sun, as well as all the other stars in the Milky Way for that matter, is in orbit about the galaxy’s core, and no two stars have exactly the same orbit. Each year the sun travels 5 billion miles through space, while all the other stars move a bit more or less, and in slightly different directions, so that comparing photographs of the sky taken decades apart is something like watching a movie of a snowstorm one frame at a time.
Sifting through all this Hipparcos data, jpl astronomers Bob Preston and Joán García-Sánchez realized that perhaps the most interesting nearby stars would be the ones that scarcely seem to move from year to year. While most nearby stars drift left or right or up or down across the sky, Preston and García-Sánchez found some 1,200 that seemed to stand still. Like headlights approaching on a highway, these stars were headed straight toward us.
Or, like taillights, away from us. It was a matter of pinning down their direction, which requires measuring the Doppler shift in the spectrum of the light emitted by the stars. Much the way the coils of a spring can be stretched or compressed, wavelengths of light are stretched ever so slightly as a star moves away from us or compressed as it moves toward us. Looking for this slight stretch in the spectrum of starlight is exacting work—even less fun than astrometry—and consequently, of Preston and García-Sánchez’s 1,200 seemingly unmoving stars, only 472 have had their full motions calculated.
Projecting these motions and the sun’s 155-mile-per-second track some 10 million years backward into the past, they found no star approaching closer than 3 light-years, a comfortable distance and one that also happened to correspond with our experience. The average separation between neighboring stars in our neck of the woods is 7 light-years, Preston says, and our closest neighbor, Proxima Centauri, is 4.3 light-years off. Emboldened by this success, Preston and García-Sánchez projected the paths of the sun and stars from their present positions to 10 million years into the future.
Barnard’s star, some 6 light-years away at present, gets relatively close in 10,000 years, Preston says, closer than Proxima and Alpha Centauri are now. But those stars are heading toward us as well. Some 25,000 years from now, Proxima and Alpha Centauri get inside about 3 light-years.
At that distance, Alpha Centauri will be almost twice as bright as it is today, but that’s as interesting as it gets. Preston was looking for something a bit more dramatic. The real impetus of all of this was the fact that the Oort cloud—this vast reservoir of comets thought to surround our solar system—extends out one and a half light-years, Preston says. It’s been postulated that close encounters by other stars could disturb this cloud and throw a lot of comets into the inner solar system, with possible collisional results with the inner planets and possible biological consequences. Translation: We’d be screwed. That’s what really drove us to have a look at this.
In the 1980s, scientists looking for an astronomical explanation for large-scale extinctions that seem to recur regularly in the fossil record dreamed up Nemesis, a dim companion star to the sun that would swing by every 32 million years to disturb the Oort cloud and rain death on Earth. No one has found any evidence of such a star, but if there is a free-range Nemesis in Preston’s data, it would have to be a star named Gliese 710. Right now it’s a small, dim red star 63 light-years off, but it is racing toward us so that in a mere million years it will be just three-quarters of a light-year from the sun—roughly a thousand times farther out than Pluto. To be sure, this prediction is based on some pretty rough estimates, and it may turn out to be wrong. But even if Gliese 710 misses us, it’s fairly likely that some other star out there is headed our way. Even a near miss of the Oort cloud by a passing star might be enough of a gravitational tug to redirect some comets toward the inner solar system; Gliese 710, by current estimates, will pass right through this cloud.
Preston hastens to add that this near miss probably won’t cause too much disruption. A slow-moving star that lingers close to the Oort cloud would have time to deflect lots of comets our way. Gliese 710, by contrast, will streak by quickly, thus disturbing fewer comets. But don’t relax too much: more than half of Preston and García-Sánchez’s 1,200 stars couldn’t have their full motions calculated. Another, closer, more destructive pass could be waiting to be discovered.
Aside from rogue stars, there are plenty of other things in the galactic neighborhood we can barely see that can be just as bad for our health. Brown dwarfs, stars too small to keep their nuclear fires burning, cool off over time and become difficult to detect. Although a brown dwarf would have a mass more like Jupiter’s than the sun’s, if it actually passed through the Oort cloud, it could still send plenty of comets our way. No one knows how many, if any, roaming brown dwarfs there are in nearby space, so attempting to assess the danger from them is futile.
Then there is the stuff between the stars. We tend to think of interstellar space as utterly empty, and with good reason. A cubic inch of liquid water on Earth contains 10 trillion trillion molecules; in the interstellar medium that surrounds the solar system, one needs to search one cubic inch to find just one or two atoms. Or to think of it another way, to attain the emptiness of interstellar space, that shot glass full of water would have to expand to fill a volume 2,500 miles on a side.
The interstellar medium is mostly hydrogen and helium, peppered with heavier atoms, a few molecules, and some dust—pretty benign stuff, on the whole. And even if something harmful should drift along, we get ample protection from the solar wind, an outward-flowing stream of electrically charged particles that extends from the sun for billions of miles, forming what’s called the heliosphere. Since the solar wind is electrically charged, it carries a magnetic field, which wards off much of the interstellar medium, including cosmic rays, the charged particles that streak across space at great speeds.
Probes of the outer solar system, such as Pioneer and Ulysses, have sampled traces of the interstellar medium that have managed to seep through the heliosphere, but most of what we know about the medium comes from looking at the way it blocks light. Atoms in interstellar space absorb certain frequencies of starlight; by finding what light from any given star is missing, astronomers can figure out how much gas there is between us and that star. What this number, called a column density, won’t tell you is just how this gas is distributed—is it thinly spread out over 10 light-years or is it bunched into a tight knot? So astronomers find the column density of the gas between us and another star and another star. They also carefully study the exact frequencies of light the gas absorbs, looking for signs of a shift in wavelength that would be a clue that the gas is moving toward us or away from us. By taking the column density of gas between us and many stars in the same general direction and by looking for telltale shifts or splits in the absorption spectrum, astronomers can surmise where a cloud of gas and dust lies, just how thick it is, and in what direction it is moving.
Over the past decade, as astronomers gained access to better spectrographs, including one on the Hubble Space Telescope, they built up a crude three-dimensional map of our locale. Priscilla Frisch, the University of Chicago astronomer, has observed the interstellar medium enough to have figured out that the solar system seems to be skimming the surface of a large cloud of gas. Behind us, toward Procyon—a star in the constellation Canis Minor—there is hardly anything. Frisch says that for most of the last 5 million years we have been sailing through what is, even for the rarefied interstellar medium, empty space. Ahead of us, toward Altair, a particularly bright star in the constellation Aquila, space is full of what Frisch calls the Local Fluff—a band of light gases and dust. A knot of newly formed stars some 500 light-years away is driving this fluff, causing it to wash over the heliosphere perpendicular to our motion, the way waves crash against the legs of someone running along a beach.
As Frisch and her colleagues look at the column densities not just ahead of the sun but slightly upwind, they have found that the interstellar medium is getting denser. In fact, one nearby cloud is less than a trillion miles away—about 250 times the distance to Pluto. Even if the cloud is on the thin side, it can still contain relatively dense and potentially destructive wisps of gas. Fifteen percent of the cold interstellar medium is contained in structures that are extremely dense and extremely small, Frisch says. These knots of gas can be as small as our solar system and some 100,000 times denser than the Local Fluff. You could take one or two structures like that and bury them in a local cloud complex, Frisch says.
At present speeds, we can expect to collide with this cloud momentarily—that is, in about 2,500 years. Of course, some small, dense wisp may be sitting undetected between us and this big cloud, in which case there will be a collision even sooner, perhaps in a matter of a few decades. Although this isn’t at all likely, nobody really knows for sure.
We probably would not discover such a dense cloud until we ran into it, says Gary Zank, an astronomer at the University of Delaware. Zank has developed one of the first models that incorporates recent discoveries about the interstellar medium to predict what will happen when the solar system runs afoul of a big bad cloud. At present, the edge of the heliosphere that’s plowing through the Local Fluff holds up a dense sheet of gas, what astronomers call the hydrogen wall. In essence, instead of flowing around stars like water past a stone, some of the gas and dust bunches up against stellar winds like vast snowdrifts. Zank’s models show that if we were to plow into a cloud a mere 100 times denser than the Local Fluff, the leading edge of the heliosphere would begin forming an incredibly large wall, one too heavy for the solar wind to hold back. The effect is pretty rapid, Zank says. If you ran into a sharply defined interstellar cloud, the solar wind would shrink very quickly. In a decade the hydrogen wall, now thought to be four to five times as distant as Pluto, would crowd in between the orbits of Saturn and Uranus.
Although the solar wind should still keep the lion’s share of interstellar gas and dust from reaching Earth, according to Zank, at least some would reach the atmosphere. The effect on Earth’s climate would be disastrous. If the cloud is dense enough, hydrogen atoms might flow into the atmosphere and react with oxygen, depleting the atmosphere. There are lots of papers in the literature that invoke all sorts of mechanisms that would stem from ‘killer cloud’ scenarios, Frisch says, including precipitation of interstellar matter through the atmosphere and formation of ice particles in the mesosphere. But nobody knows what would really happen. The only thing I can say with confidence is that if you start to change the interplanetary environment around Earth, for sure something’s going to change the atmosphere.
The cloud would also make us more vulnerable to cosmic rays by compressing the heliosphere until its boundary lay just beyond Jupiter’s orbit. Ripples in the heliosphere’s magnetic field protect us by slowing and redirecting the incoming rays much the way a warehouse full of pillows would stop all but the most powerful bullets. If you remove several rooms full of pillows, Zank says, many more bullets will get through. And there would be more bullets: cosmic rays would ricochet between the boundaries of the soupy cloud of gas and the heliosphere until they picked up enough energy to escape. At least some of them would head our way. The only line of defense left to us would be Earth’s meager magnetic field. Overall, Zank says, the number of cosmic rays hitting our atmosphere would skyrocket.
A sharp rise in cosmic radiation would be troublesome, to say the least. Cosmic rays wreak havoc on the internal electronics of satellites and threaten the health of astronauts. When cosmic rays smack into atoms in the upper atmosphere, they release showers of gamma rays, X-rays, or subatomic particles. At present, radiation caused by cosmic rays is one of the largest sources of natural radiation exposure. It is unclear what biological effects doubling or tripling it would have.
Would a doomsday cloud be survivable? It’s hard to say. The Earth has probably already passed through such a cloud at some point in the distant past. If it happened in the last 100,000 years, it might be possible someday to extract traces of its effects from deep within the polar ice caps (but don’t hold your breath). Frisch is not optimistic. She believes that intelligent life on Earth may owe its existence in part to the clear sailing that our solar system has experienced over the past few million years. The best places to look for intelligent life in the universe, she suggests, may be on planets around stars that have had similarly easy weather.
The galactic forecast, though, is not good. Virtually every corner of the sky is filled with some tale of woe. There are supernovas and black holes and colliding neutron stars. Old stars are ripping themselves apart on the way to becoming white dwarfs—and astronomers confidently predict that in 5 billion years the sun will be an old star. And if any of our descendants escape that catastrophe, they will be able to see another, far worse one looming overhead: the Andromeda galaxy, which just might slam into the Milky Way some 6 billion years from now, to who knows what effect. It is one tough universe out there.