If planets are the stage that life plays out on, stars set the scene. Every star in the Universe creates a different environment for its worlds, and over the past few decades, scientists have begun to explore which are harsh, deadly, or cozy. One day, these discoveries could tell us how common life might be throughout the Universe.
Rough starts
The first challenge for any would-be habitable planet is surviving its star’s youth. Before a star begins fusing hydrogen and settles into “adulthood,” it churns with stormy weather. Younger stars tend to emit stronger UV radiation and fire off more frequent, intense flares. This bombardment can wear away a planet’s atmosphere and break up key chemical ingredients of life.
Even a star like the Sun poses a danger when young. If Earth did not have the right kind of chemical makeup and magnetic shielding, the Sun would have stripped away our atmosphere long before life had a chance to develop. This is part of why it is so hard to say if a given world is habitable or not: it matters not only what the system is like today, but also what it was like in the past.
The picture becomes clearer once a star begins fusing hydrogen. During this phase, many stars will shine steadily and only gradually brighten over time.
This phase is what Earth has to thank for the last several billion years of temperate weather. For that entire period, we have been in the Sun’s “habitable zone,” the range of orbits where a planet can absorb the right amount of energy to maintain liquid water on its surface. In other words, Earth has been threading the needle between too-hot and too-cold around the Sun, and that has given life here the stability it needs to thrive.