Paleoclimate and Heliosphere

Recent research raises the possibility that 2–3 and 6–7 million years ago, the Sun encountered massive clouds that shrank the heliosphere —the solar cocoon protecting our solar system— exposing Earth to its interstellar environment, in agreement with geological evidence from ⁶⁰Fe and ²⁴⁴Pu isotopes. Here we show that during such encounters Earth was exposed to increased radiation in the form of high-energy particles. During periods of Earth’s immersion in the heliosphere, it received particle radiation that we name Heliospheric Energetic Particles (HEPs). The intensity of < 10 MeV protons was at least an order of magnitude more intense than today’s most extreme solar energetic particle (SEP) events. SEPs today last minutes to hours, but HEP exposure then lasted for extensive periods of several months, making it a prolonged external driver.

During Earth’s excursion outside the heliosphere, it was exposed to a galactic cosmic ray radiation with the intensity of < 1 GeV protons at least an order of magnitude more intense than today. Therefore, the space surrounding Earth was permeated by a variable high-energy radiation. We discuss the implications for Earth’s climate and biodiversity.

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Selection of Prof. Opher’s most significant papers on the subject:

Increased and varied radiation during the Sun’s encounters with cold clouds in the last 10 million years. Research: M. Opher et al. Scientific Reports (2026)

https://www.nature.com/articles/s41598-026-36926-z

Cosmogenic 10Be as a Tracer for Recent Heliospheric Encounters with Interstellar Cold Clouds. Research: Anna Nica, Merav Opher, Jesse A. Miller, Jennifer L. Middleton

https://arxiv.org/abs/2601.07983

M. (2025). Impact of galactic cosmic rays on Earth’s climate during an encounter with the local lynx of cold clouds 2–3 Myr ago. Journal of Geophysical Research: Atmospheres, 130, e2025JD044084. Nica, A., Opher, M., Miller, J., & Hatzaki

https://doi.org/10.1029/2025JD044084

A possible direct exposure of the Earth to the cold interstellar medium 2-3 Myr ago:

M. Opher, J. Peek, and A. Loeb, “A possible direct exposure of the Earth to the cold interstellar medium 2-3 Myr ago”, Nature Astronomy, 8, 983 (2024).

Effect of the Passage of the Sun through the Edge of the Local Bubble:

M. Opher, A. Loeb, C. Zucker, A. Goodman, et al. “Effect of the Passage of the Sun through the Edge of the Local Bubble”, The Astrophysical Journal, in press (2024)

Earth’s mesosphere during possible encounters with massive interstellar clouds 2 and 7 million years ago:

J. A. Miller, M. Opher, M. Hatzaki, K. Papachristopoulou, B. C. Thomas, “Earth’s mesosphere during possible encounters with massive interstellar clouds 2 and 7 million years ago”, GRL, 51, e2024GL110174 (2024).