Few people know that the solar system happens to be at the center of an interstellar bubble that is virtually devoid of gas. That gas was blown out by supernovae, and we can determine when they occurred. Our solar system is traveling through this bubble and it is headed for an unavoidable collision, say NASA scientists.
Artist’s conception of the Local Bubble (containing the Sun and Beta Canis Majoris) and the Loop I Bubble (containing Antares).
3D representation of the Local Bubble (White) with neighbouring Molecular Clouds (Magenta) and a section of the Loop I Bubble (Teal).
Australopithecus squinted at the blue African sky. He had never seen a star in broad daylight before, but he could see one today. White. Piercing. Not as bright as the Sun, yet much more than a full moon. Was it dangerous? He stared for a long time, puzzled, but nothing happened, and after a while he strode across the savanna unconcerned.
Millions of years later, we know better.
“That star was a supernova, one of many that exploded in our part of the galaxy during the past 10 million years,” says astronomer Mark Hurwitz of the University of California-Berkeley.
Supernovas near Earth are rare today, but during the Pliocene era of Australopithecus supernovas happened more often. Their source was an interstellar cloud called “Sco-Cen” that was slowly gliding by the solar system. Within it, dense knots coalesced to form short-lived massive stars, which exploded like popcorn.
Researchers estimate (with considerable uncertainty) that a supernova less than 25 light years away would extinguish much of the life on Earth. The blast needn’t incinerate our planet. All it would take is enough cosmic rays to damage the ozone layer and let through lethal doses of ultraviolet (UV) radiation. Our ancestors survived the Pliocene blasts only because the supernovas weren’t quite so close.
We know because we can still see the cloud today. It’s 450 light years from Earth and receding in the direction of the constellations Scorpius and Centaurus (hence the cloud’s name, “Sco-Cen”). Astronomer Jesús Maíz-Apellániz of Johns Hopkins University recently backtracked Sco-Cen’s motion and measured its closest approach: 130 light years away about 5 million years ago.
Sco-Cen was still nearby only two million years ago when many plankton, mollusks, and other UV-sensitive marine creatures on Earth mysteriously died. Paleontologists mark it as the transition between the Pliocene and Pleistocene epochs. Around the same time, according to German scientists who have examined deep-sea sediments from the Pliocene era, Earth was peppered with Fe60, an isotope produced by supernova explosions.
No one knows. It’s a puzzle researchers are still piecing together.
Reconstructing the history of near-Earth supernovas is difficult because old supernovas are elusive. Their glowing shells fade to invisibility in not much more than a million years. Neutron stars, the collapsed cores of supernova progenitors, last longer, but they are flung across the galaxy by asymmetries in the explosion. Unusual isotopes of iron, like the ones that coincide with the marine extinction, are difficult to find buried under millions of years of sediments.
There is, however, one obvious relic: “All those explosions blew an enormous bubble in the interstellar medium,” says Hurwitz, “and we’re inside it.”
Astronomers call it “the Local Bubble.” It’s peanut-shaped, about 300 light years long, and filled with almost nothing. Gas inside the bubble is very thin (0.001 atoms per cubic centimeter) and very hot (a million degrees)–that’s 1000 times less dense and 100 to 100,000 times hotter than ordinary interstellar material.
The Local Bubble was discovered gradually in the 1970′s and 1980′s. Optical and radio astronomers looked carefully for interstellar gas in our part of the galaxy, but couldn’t find much in Earth’s neighborhood. Furthermore, there seemed to be a pileup of gas–like the shell of a bubble–about 150 light years away. Meanwhile, x-ray astronomers were getting their first look at the sky using orbiting satellites, which revealed a million-degree x-ray glow coming from all directions. “We eventually realized that the solar system was inside a hot, vacuous bubble,” says Hurwit
Exploring the internal geography of the bubble is important because what lies inside could affect our planet’s future.
During the past few million years, wispy filaments of interstellar gas have drifted into the Local Bubble. Our solar system is immersed in one of those filaments–the “local fluff,” a relatively cool (7000 K) cloud containing 0.1 atoms per cubic centimeter. By galactic standards, the local fluff is not very substantial. It has little effect on Earth because the solar wind and the Sun’s magnetic field are able to hold the wispy cloud at bay.
There are, however, denser clouds out there. The Sco-Cen complex, for instance, is sending a stream of interstellar “cloudlets” in our direction. “Some of those cloudlets might be hundreds of times denser than the local fluff,” says Priscilla Frisch, an astrophysicist at the University of Chicago who studies the local interstellar medium. “If we ran into one, it would compress the Sun’s magnetic field and allow more cosmic rays to penetrate the inner solar system, with unknown effects on climate and life.”
The brightest star in the Sco-Cen star-forming cloud is the red giant Antares, which illuminates the yellow nebula in the upper-left corner of this image. Antares might be the next star to explode in Sco-Cen, but at 500 light years it is believed to be a safe distance from Earth
Frisch has been investigating the interstellar gas in the local neighborhood of our solar system, which is called the Local Interstellar Medium (LISM). This interstellar gas is within 100 light years of the Sun. The Sun has a trajectory through space, and for most of the last five million years, said Frisch, it has been moving through a region of space between the spiral arms of the Milky Way galaxy that is almost devoid of matter. Only recently, within the last few thousand years, she estimates, the Sun has been traveling through a relatively low-density interstellar cloud.
“This cloud, although low density on average, has a tremendous amount of structure to it,” Frisch said. “And it is not inconsistent with our data that the Sun may eventually encounter a portion of the cloud that is a million times denser than what we’re in now.”
Frisch believes the interstellar cloud through which we’re traveling is a relatively narrow band of dust and gas that lies in a superbubble shell expanding outward from an active star-formation region called the Scorpius-Centaurus Association. “When this superbubble expanded around these stars, it expanded much farther into the region of our galaxy between the spiral arms, where our sun lies, because the density is very low,” Frisch said. “It didn’t expand very far in the direction parallel to the spiral arms because it ran into very dense molecular clouds.”
The solar wind–the flux of charged particles streaming from the Sun’s corona–protects the Earth from direct interaction with the interstellar medium by enveloping the Earth and all the planets in the heliosphere, the region of influence of the solar wind. The heliosphere currently extends 100 times farther from the Sun than the distance between Earth and the Sun. “We think the heliosphere might have been much larger before we entered the interstellar cloud,” said Frisch, “but that’s something we can’t say for sure.”
Neutral atoms from the galactic wind sweep past the solar system’s magnetic boundary, the heliosheath, and travel some 30 years into our solar system toward the sun. NASA’s Interstellar Boundary Explorer (IBEX) can observe those atoms and provide information about the mysterious neighborhood outside our home.
But if the solar system encountered the much denser cloud, Frisch estimates that the heliosphere could be compressed to within one or two astronomical units of the Sun, not much greater than the Earth’s distance from the Sun. “There would be dramatic effects on the inner solar system,” said Frisch. “It would immediately change the whole interaction between the solar wind and the interstellar medium.” Researchers have predicted increases in the cosmic-ray flux, changes in the Earth’s magnetosphere, the chemistry of the atmosphere and perhaps even the terrestrial climate.
Frisch noted that astronomers searching for life on planets orbiting stars outside our solar system should consider the environments around those stars. “It doesn’t make sense to look for habitable planets unless you also look at the way these stars interact with their local environment,” said Frisch. “Ican’t imagine that a star passing in and out of dense interstellar cloud fragments–such as a star that’s traversing galactic spiral arms–would have a stable interplanetary environment. Without stability in the local stellar environment, I doubt there could be stable planetary climates hospitable to life.”
The solar journey through space is carrying us through a cluster of very low density interstellar clouds. Right now the Sun is inside of a cloud that is so tenuous that the interstellar gas detected by IBEX is as sparse as a handful of air stretched over a column that is hundreds of light years long. These clouds are identified by their motions.
Credit: NASA/Adler/U. Chicago/Wesleyan
Due to the protective shielding of dangerous Galactic Cosmic Rays provided by a heliosphere or astrosphere, these structures are important for the planets that orbit the respective stars. Only over the last 15 years, we have been able to detect the first astrospheres and planets around other stars (exoplanets). Here we show a zoom into the most immediate environment around the Sun, our cosmic neighborhood. The locations of known astrospheres and exoplanets are indicated, while we anticipate that many more are present and just awaiting discovery. The nearest star, alpha Centauri has an astrosphere, and we know of at least two cases where we have detected both an astrosphere and exoplanets. These systems are truly analogous to our system in which the heliosphere shields a diverse planetary system. Reformatted for TV.
Credit: NASA/GSFC/Adler/U. Chicago/Wesleyan