As the moon orbits the Earth, the Earth orbits the sun, and the sun orbits the center of the Milky Way, moving up and down with respect to the galactic plane.
A new study has suggested that this movement of our star through the Milky Way may be taking us through regions of space that could affect our planet’s climate. According to the study, the solar system may have passed through an interstellar cloud so dense that it disrupted the flow of the solar wind, potentially cooling the planets.
The Solar System is to some extent protected from the interstellar medium (ISM) by our heliosphere.
“The sun emits a constant stream of charged particles called the solar wind, which eventually travels past all the planets up to about three times the distance of Pluto before being obstructed by the interstellar medium,” NASA explains. “This forms a giant bubble around the sun and its planets, known as the heliosphere.”
The solar system is currently located in a 1000 light-year wide ‘Local Bubble’ or ‘local interstellar cloud’ (LIC). This “bubble” is a lot less dense than typical interstellar space, with 0.001 particles per cubic centimeter compared to the typical 0.1 atoms per cubic centimeter. The solar system will leave this sparse region of space in the next few thousand years and reenter the interstellar medium.
Looking at the path of the solar system and mapping the local ribbon of cold clouds, the team found that we have likely traveled through denser areas in the past.
“The ISM that has crossed the Sun over the past few million years contains cold, compact clouds that could have drastically affected the heliosphere,” the team explains in their paper. “We are investigating a scenario where the solar system passed through a cold gas cloud a few million years ago.”
Although research on the effects of crossing such areas has been scarcer than on atoms in the local bubble, the team believes this could have contracted our heliosphere, which in turn had an effect on our climate. Our heliosphere is protective, and as it contracts, some of the material in these denser regions can reach Earth.
“Large amounts of neutral hydrogen due to encounter with cold clouds with densities above 1,000 cm3−3 will change the chemistry of Earth’s atmosphere,” the team wrote. “Very few works have quantitatively investigated the climatic effects of such encounters in the context of encounters with dense giant molecular clouds. Some argue that such high densities would deplete the ozone layer in the mid-atmosphere (50-100 km2). [31–62 miles]) and ultimately cool the Earth.”
The team says there is geological evidence of increased quantities 60Fe (iron 60) and 244Pu isotopes (plutonium 244), found in ice cores, the oceans, Antarctic snow and samples from the moon, could be evidence that these particles reached Earth when we crossed the local Cold Cloud Lynx 2 million years ago.
These isotopes are spewed out by supernovae and neutron star mergers, which then become trapped in interstellar dust. It has previously been explained that these isotopes in the geological record were sent here by a nearby supernova, but the current team believes they are better explained by particles trapped in the cloud, because a nearby supernova would collapse the heliosphere to distances of 1 AU (the distance between Earth and the Sun), while a supernova further away would not precipitate enough 60Fe on Earth.
“This paper is the first to quantitatively demonstrate that there was an encounter between the Sun and something outside the solar system that would have affected Earth’s climate,” Boston University space physicist Merav Opher said in a statement, later adding added: “Once the Earth moved away from the cold cloud, the heliosphere engulfed all the planets, including Earth.”
The contraction of the heliosphere could have lasted hundreds of years to a million years, according to the team, and it is likely that we will encounter another such contracting heliosphere cloud within about a million years.
Although interesting, there is much more to discover.
“This work needs to be revisited with modern atmospheric models,” the team writes. ‘It has been suggested that climate changes around this time could have influenced human evolution. The hypothesis is that the rise of our species Homo sapiens was shaped by the need to adapt to climate change. With the shrinking of the heliosphere, Earth was directly exposed to the ISM.”
The article was published in Nature Astronomy.