For life to take root on a planet, organisms need little more than a rocky surface, liquid water, and a coddling atmosphere to hold warmth in and keep harmful rays out. But for life to thrive, astronomers from the Harvard-Smithsonian Center for Astrophysics write in the journal arXiv, they need something else: a magnetic field.

As it turns out, young planets are likely to orbit around equally young, savage suns with violent solar winds strong enough to blow away the atmospheres of their cosmic companions. The researchers came to this conclusion after studying the nearby star Kappa Ceti, which appears to behave like an immature version of our own Sun.

Kappa Ceti, at 400 million to 600 million years old, is considered a young star. Like human adolescents, it’s pockmarked with spots and violently temperamental. The blotchy starspots on its surface, the researchers write, are a clear sign that it’s magnetically active. On top of that, it’s constantly spewing a stream of hot gas outward into space at a speed 50 times that of our own Sun’s solar wind. That fierce jet of air is strong enough to obliterate the atmosphere of a nearby planet if it doesn’t have the right protection.

Map of the sun's magnetic fields, via NASA.

Just look at lifeless Mars, which could have been warm enough to support salty oceans, had it been fortunate enough to have a magnetic shield to keep the sun from stripping its warmth-containing atmosphere away.

Earth, on the other hand, got lucky. By taking their data on Kappa Ceti’s solar winds and applying it to a model of our own young planet, the researchers determined that Earth’s magnetic field was fairly weak. They estimate that young Earth’s protected region — its magnetosphere — was about one-third to one-half as large as it is today. But that atmosphere, as thin as it was, maintained the warmth needed to support terrestrial life.

Photos via  NASA/SDO/AIA/LMSAL, M. Weiss/CfA