Earth's magnetic field may have been as strong 3.7 billion years ago as it is today, pushing the earliest date for this planetary protective bubble back 200 million years. The new study suggests that at that time, the planet had a protective magnetic bubble around it that deflected cosmic radiation and damaging charged particles from the sun. However, the flow of solar charged particles was much stronger at the time, said Claire Nichols, an Earth scientist at the University of Oxford.