It took just five minutes for a NASA sounding rocket to confirm the decades-old theory that the solar system sits within a bubble of gas produced in 10 million-year-old supernovae explosions.
Background 0.25 keV X-ray radiation coming from all directions alerted astronomers to the possibility of a “Local Bubble” of hot gas in the late 1960s. Combined with evidence of local depletion of cool gasses, the idea was born that the hot gas was thrown off by a series of supernova explosions. However, the same radiation has also been explained as resulting from the exchange of charge between ions from the solar wind and neutral gases inside the solar system.
To test the competing theories a sounding rocket was fired to a height of 250 km where it could detect the charge exchange happening in the inner solar system beyond the Earth’s atmosphere. Five minutes of observations revealed that charge exchange can indeed account for some of the soft X-ray emissions, a team led by Professor Massimiliano Galeazzi of the University of Miami report in Nature. However, that portion is only 40%. The only credible theory presented to explain the rest is the Local Bubble hypothesis.
"This is a significant discovery," said Galeazzi. "[It] affects our understanding of the area of the galaxy close to the sun, and can, therefore, be used as a foundation for future models of the galaxy structure."
The Local Bubble is a 300 light year wide patch encompassing the solar system and many of the more familiar stars, filled (if that is the word) with a concentration of fifty thousand atoms per cubic meter. This compares to 2.5 x 1025 molecules/m3 at sea level in our atmosphere. Even for interstellar space this is a very low density, and the atoms are distinguished by being astonishingly hot—a million degrees—and containing different elements from the ordinary interstellar medium.
Supernovae happen once or twice a century in a galaxy the size of our own. They can devastate their local environment, but further afield the main effect is to spread heavy elements that enable the formation of planets such as our own. The size of the bubble leads to the conclusion that rather than a single supernova, 10-20 million years ago there were around 20 nearby supernovae. None were close enough to trigger a mass extinction, but the bubble continues to affect solar luminosity and therefore our climate as we pass through.
The local bubble is not circular, having been interrupted by many other events, causing the sun to pass in and out as it moves through the galaxy.
NASA explains the test had to be run in December, because this is when the Earth passes through the “helium focusing cone,” a region of neutral helium considered the strongest source of charge exchange X-rays. The launch was conducted on December 12, 2012—which some people might recall as the date conspiracy nuts were touting as the end of the world, or the start of some cataclysm, claims they mostly seem keen to forget. Ah the difference between science and pseudoscience.