“Stars move, and now this paper is showing not only that they move, but they encounter drastic changes,” said Boston University space physicist Merav Opher, whose most recent research proposes that Earth’s climate history was influenced by an interstellar encounter far beyond the solar system.
About two million years ago, the Sun and its planets might have wandered into the Local Lynx of Cold Clouds, a cold, hydrogen rich piece of the Local Ribbon of Cold Clouds. This was not a typical spot of interstellar medium. With densities of the order of 3,000 hydrogen atoms per cubic centimeter several orders of magnitude more than the Local Bubble’s 0.001 particles per cubic centimeter today the cloud’s ram pressure might have compressed the heliosphere, the solar wind inflated magnetic bubble extending typically to around 130 astronomical units, in toward a mere 0.22 AU, deep within the orbit of Earth.
The heliosphere serves as a planetary shield, defying high energy galactic cosmic rays and filtering interstellar dust. Sudden compression, as simulated by high resolution magnetohydrodynamic models, would have directly exposed Earth and the Moon to the cold, dense interstellar medium. The models accounted for multi fluid treatments of neutral hydrogen, charge exchange, and gravitational focusing and indicated that neutral densities in the vicinity of the heliopause could have increased to 10,000 cm⁻³ with inflow velocities of 50 km/s. Termination shock, at which the supersonic solar wind would be suddenly slowed, would have moved inward to merely 0.12 AU, and the higher compression ratio of 3.7 compared to today’s lower, pickup ion mediated shock would have accelerated particles more effectively, raising cosmic ray flux into Earth’s atmosphere.
Geological records seem to carry the imprint of such an encounter. Layers of Antarctic snow, oceanic sediments, and lunar regolith from that time have excesses of iron 60 and plutonium 244, isotopes created in supernovae and neutron star mergers and dispersed through interstellar dust. These results, quantified with accelerator mass spectrometry, match the modeled transit time of the cloud between 1.57 and 4.2 million years ago. The deposition of the isotopes is not easily explained by an adjacent supernova alone since such an explosion within 10 parsecs would more probably have initiated biospheric collapse, rather than the cloud model allowing them to be delivered without devastating irradiation.
The climatic effects are significant. Increased cosmic ray penetration may have changed atmospheric chemistry, ozone depletion in the mid stratosphere, and the enhancement of cloud condensation nuclei, both of which are associated with surface cooling. Several researchers observe that the initiation of recurrent Pleistocene glaciations coincides with this period, although causation is still being explored. Rebound of the heliosphere from such compression would have been gradual hundreds of thousands of years potentially supporting deformed climate regimes long enough to effect evolutionary pressures on early Homo species.
The fact that the event is so rare is highlighted by estimates of probabilities: simulations of cloud and stellar motions predict only a 1.3 percent likelihood that the Sun would have traversed so large and dense an entity within the Local Bubble in the last few million years. Still, the more massive Local Lynx of Cold Clouds, comparable in size to the better known Local Leo Cold Cloud, is within 22 59 parsecs, and thus geometry and timing are reasonable.
Currently, spacecraft like Voyager 1 and 2, the Interstellar Boundary Explorer, and future heliophysics missions are mapping the boundary conditions of the heliosphere with finer accuracy than ever before. These measurements, coupled with Gaia’s three dimensional stellar mapping, are enabling astrophysicists to reconstruct the galactic trajectory of the Sun and determine past and future encounters with dense interstellar clouds. As Opher explained, “This paper is the first to quantitatively show there was an encounter between the Sun and something outside of the solar system that would have affected Earth’s climate.”
The possibility is not just theoretical. Over the course of the next million years, the solar system could once again face a cloud capable of crushing the heliosphere. The physics of these encounters how neutral hydrogen, magnetic fields, and solar wind dynamics interact under extreme external pressure is what will be important for forecasting how Earth’s atmosphere, climate, and habitability will react when our cosmic shield is removed.
