The Sun's constant outflow
The Sun's outer atmosphere — the corona — reaches temperatures above 1 million °C, far hotter than the surface below it. At such extreme temperatures, the gravitational pull of the Sun cannot retain the outermost particles, and they escape as the solar wind. At Earth's distance (1 AU), the solar wind has a density of roughly 5 particles per cubic centimetre — an almost perfect vacuum by terrestrial standards, yet enough to have profound effects over billions of years.
The solar wind extends far beyond the planets. It inflates a vast bubble around the Solar System called the heliosphere. The outer boundary — the heliopause — is where the solar wind pressure balances the pressure of interstellar space. NASA's Voyager 1 crossed this boundary in 2012, at roughly 121 AU from the Sun, becoming the first human-made object in interstellar space. See our Voyager probe status update for the latest on these historic spacecraft.
Earth's magnetic shield
Earth's liquid outer core — iron and nickel at ~5,000 °C — acts as a dynamo, its circulation generating a planet-scale magnetic field. This magnetosphere is our primary defence against the solar wind. When solar wind particles encounter it, a curved shock wave forms (the bow shock) about 90,000 km from Earth on the sunlit side. Most particles are deflected around the planet; a fraction get caught in the Van Allen radiation belts — doughnut-shaped regions of trapped charged particles circling at 1,000–60,000 km altitude.
Mars lost its magnetic field around 4 billion years ago, and without protection the solar wind stripped away most of its atmosphere over geological time — turning a potentially habitable world into the thin, cold desert we see today. The comparison underscores how important Earth's magnetic field is to life.
Space weather
The Sun's output is not constant. During solar maximum — the peak of the roughly 11-year solar cycle — sunspot activity, solar flares and coronal mass ejections (CMEs) increase. A large CME can reach Earth in 1–3 days, compressing the magnetosphere and driving geomagnetic storms. Modern society depends on satellites, GPS, and power grids that are all vulnerable to severe space weather. The NOAA Space Weather Prediction Center issues real-time alerts, and ESA's Space Weather Service coordinates European responses.
For context on how our Sun compares to other stars and what drives its internal energy, see our guide to the life cycle of a star.