In the 1980s, scientists looking for an astronomical explanation for large-scale extinctions that seem to occur 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 found any evidence of such a star, and the theory is now largely discarded, 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 an ordinary-looking, 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 1,000 times farther out than Pluto but well inside the Oort cloud. 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.
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: A 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 brown dwarfs have a mass more like Jupiter’s than like the sun’s, if one of them passed through the Oort cloud it could still send plenty of comets our way. Astronomers have found hundreds of brown dwarfs within just 100 light-years of us but know distressingly little about the total number of them out there.
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. Put another way, to attain the emptiness of interstellar space, a 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. 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, especially the long-lived Voyager and Ulysses spacecraft, have sampled traces of the interstellar medium that 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. They also carefully study the 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 in the absorption spectrum, astronomers can surmise where a cloud of gas and dust lies, how thick it is, and in what direction it is moving.
As astronomers have gained access to better spectrographs, including one on the Hubble Space Telescope, they have built up a crude three-dimensional map of our locale. It appears that for most of the last 5 million years we have been sailing through what is, even for the rarefied interstellar medium, empty space. But that happy state of affairs could change. One nearby cloud is less than 1 trillion miles away—just 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.
At present speeds, we can expect to collide with this cloud momentarily, in cosmic terms—in about 2,500 years. Then again, some small, dense wisp may be sitting undetected between us and this big cloud, in which case there will be a collision even sooner. Although this is not at all likely, nobody really knows.
We probably would not discover such a dense cloud until we ran right into it, says Gary Zank, an astronomer at the University of Alabama in Huntsville. Zank has developed one of the first models that incorporate discoveries about the interstellar medium to predict what will happen when the solar system runs into a big, bad cloud. At present, the edge of the heliosphere 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, 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 most interstellar gas and dust from reaching Earth, according to Zank, at least some would get to us. The effect on Earth’s climate could be disastrous. If the cloud were dense enough, hydrogen atoms might flow into the atmosphere and react with oxygen, depleting our vital atmospheric gases.
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 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.” There would also be more bullets in total: 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 our 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. Earth probably 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. Life may well have weathered this kind of challenge before.
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 gamma-ray bursts. Old stars are coming unglued on the way to becoming white dwarfs—and astronomers confidently predict that in 5 billion years the sun will be an old star. If any of our descendants escape that catastrophe, they will be able to see another, far greater 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.