It takes more than the right solar real estate to make life on rocky worlds.
A team of scientists has discovered that stellar winds are the most primary means of interaction between a planet and its host star, which has a direct impact on which alien worlds we can potentially live on, according to a study recently shared on a preprint server.
And, while further studies are needed to understand the behavior of what's called an Alfvén surface, we're closer than ever to a new potential shortcut in determining which alien worlds will permit life as we know it.
Our sun reveals how alien worlds orbiting sun-like stars might evolve a life-supporting atmosphere
"The cumulative effect of magnetized stellar winds on exoplanets dominates over other forms of star-planet interactions," reads the study's abstract. "When combined with photoevaporation, these winds will lead to atmospheric erosion. This is directly connected with the concept of Habitable Zone (HZ) planets around late-type stars." However, current science has only limited knowledge of magnetized winds of stars, which coincidentally means numerical models represent a crucial means of investigating them. While this study is by its authors' admission only preliminary, the researchers explored how varying properties of stellar wind are balanced in terms of scale. "We used one of the most details physics models, the 3D Alfvén Wave Solar Model part of the Space Weather Modeling Framework, and applied it to the stellar winds domain," wrote the authors. And, using simulations, they found that the magnetic field topology (the three-dimensional surface) of a star's surface is crucial in the formation of different properties of stellar wind.
This includes the mass loss rate of the star, its angular momentum, the wind speed, and more. "We conclude that a characterization of the Alfvén surface is crucial when studying star-planet interaction as it can serve as an inner-boundary of the HZ," added the authors, abbreviating "Habitable Zone" as HZ. The Alfvén surface is the region around a star where ionized gas from the star is pushed and pulled around by the star's colossal magnetic field, as it continually spins with the motion of the star itself. Most of what we know about stellar winds comes from our studies of the sun, and the researchers think that stellar wind acceleration, coronal heating, and the mechanisms behind them on the sun also apply to other sun-like stars, in addition to low-mass main sequence stars, beyond our solar system. Each sun-like star is also an X-ray source, with temperatures rising to several million degrees Kelvin. That's extremely hot.
The Alfvén surface can serve as the 'inner boundary' of a star's habitable zone
"The high gas-pressure gradient in the hot plasma that surrounds these stars will expand as supersonic wind," wrote the authors. While this basic known can tell us a lot, there remain tremendous uncertainties about how stellar winds evolve on a main-sequence star, and this has a direct impact on our grasp of how atmospheres on planets orbiting them will ultimately evolve. "Given the sensitivity of planetary atmospheres to stellar wind and radiation conditions, these uncertainties can be significant for our understanding of the evolution of planetary environments."
And in analyzing the size, scope, and behavior of a star surface's magnetic field geometry, the scientists realized it directly determines the Alfvén surface's shape and size, which, in turn, can drastically alter the properties of stellar wind. This means, to grasp how the atmosphere of an alien world in its host star's habitable zone, further studies of the Alfvén surface are required. Until then, we can still use the Alfvén surface to know the inner boundary of a star's habitable zone. Anything closer to the star than its Alfvén surface probably won't support the evolution of alien life.