As the Moon orbits the Earth, the Earth orbits the Sun, and the Sun orbits the center of the Milky Way, oscillating up and down relative to the galactic plane as it does so.
A new study suggests that this movement of our star through the galaxy potentially takes 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 may have impeded the flow of the solar wind, potentially cooling the planets.
The Solar System is shielded to some extent by the interstellar medium (ISM) from our heliosphere.
“The Sun sends out a steady stream of charged particles called the solar wind, which eventually passes all the planets to about three times the distance to Pluto before being blocked by the interstellar medium,” explains NASA. “This forms a giant bubble around the Sun and its planets known as the heliosphere.”
The Solar System currently resides in a “local bubble” or “local interstellar cloud” (LIC) 1,000 light-years across. This “bubble” is much 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 meager region of space in the next few thousand years and re-enter the interstellar medium.
By looking at the path of the solar system and mapping the local band of cold clouds, the team found that we probably traveled through more densely packed regions in the past.
“In the ISM that the Sun has passed through over the past few million years, there are cold, compact clouds that could dramatically affect the heliosphere,” the team explains in their paper. “We are investigating a scenario where the Solar System passed through a cloud of cold gas several million years ago.”
Although research on the effects of passing through such regions is rarer than atoms in the local bubble, the team believes it may have shrunk 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 resulting from an encounter with cold clouds with a density greater than 1000 cm-3 will change the chemistry of the Earth’s atmosphere,” the team wrote. “Very few works have examined the climatic effects of such encounters quantitatively in the context of encounters with dense giant molecular clouds. Some argue that such a high density would deplete ozone in the middle atmosphere (50–100 km [31–62 miles]) and eventually cool the Earth.”
The team says geological evidence for increased amounts of 60Fe (iron 60) and 244Isotopes of Pu (plutonium 244) found in ice cores, the oceans, Antarctic snow and samples from the Moon may be evidence of these particles reaching Earth as we crossed the Cold Cloud’s Local Line 2 million years ago.
These isotopes are ejected from supernovae and neutron star mergers, which are then captured by interstellar dust. These isotopes in the geological record were previously explained as having been sent here by a nearby supernova, but the current team believes they could be better explained by particles trapped in the cloud, since a nearby supernova would shrink the heliosphere to distances of 1 AU (the distance between the Earth and the Sun) while a more distant supernova would not delay enough 60Fe on Earth.
“This paper is the first to quantitatively show that there was an encounter between the Sun and something outside the Solar System that would affect Earth’s climate,” Boston University space physicist Merav Ofer said in a statement, later adding, “but as soon as Earth moved away from the cold cloud, the heliosphere engulfed all the planets, including Earth.”
The contraction of the heliosphere may have lasted from hundreds of years to a million years, according to the team, and we are likely to encounter another such heliosphere-shrinking cloud within another million years or so.
Although it is interesting, there is still a lot to understand.
“This work needs to be revisited with modern atmospheric modeling,” the team wrote. “It is suggested that climate change at that time may have influenced human evolution. The hypothesis is that the emergence of our species Homo sapiens is shaped by the need to adapt to climate change. As the heliosphere contracted, the Earth was directly exposed to the ISM.”
The article was published in Nature Astronomy.
