The results of a new X-ray study suggest that the high-energy activity of Sun-like stars rapidly drops off following a tumultuous youth. The findings could have significant implications for the habitability of exoplanets orbiting these stars, including their ability to maintain an atmosphere.
Our planet orbits in the habitable zone (HZ) of a G-type main-sequence star that we call the Sun. The HZ of a star is also sometimes referred to as the "Goldilocks zone," because this region of circumstellar space, in which an exoplanet can orbit, receives not too little, or too much, but instead just the right amount of radiation from its parent star to allow liquid water to exist on its surface.
Since Earth is the only planet known to play host to life, Sun-like stars and their exoplanets are considered promising targets in the search for E.T. However, simply discovering a rocky Earth-sized world orbiting a Sun-like star does not guarantee the existence of life.
If a stellar body is too active in its youth, the radiation thrown off from its surface could seriously damage an orbiting exoplanet's atmosphere. Without an atmosphere, it would be impossible for a world to maintain liquid water on its surface, which is essential for the evolution of life as we know it.
So why are young stars so damaging? During their early life, Sun-like stars spin very fast, creating extremely high levels of magnetic activity that drive powerful stellar flares, coronal mass ejections, and an outpouring of X-ray and ultraviolet radiation.
The high-energy radiation environment created by the star during this youthful phase can deplete a planet's atmosphere, essentially causing its constituent particles to be lost to space. Whether a planet's atmosphere can weather this storm and continue to be useful depends partly on the intensity, and duration of a star's youthful phase.
A new study carried out by an international team of researchers has shed light on how long Sun-like stars and their less massive cousins remain in this youthful phase before their magnetic activity subsides.
X-rays are emitted from a star's corona – the "outer atmosphere" of a stellar body that forms as the result of complex interactions between a stellar body's magnetic fields and its turbulent outer layer. Because of this relationship, astronomers know that a dip in a star's X-ray emission would mirror a decline in its magnetic activity, as the two are intrinsically linked.
Previous research in this area had already examined the relationship between X-ray brightness and the age of stars in stellar specimens less than 1 billion years old.
For the new study, the team examined the X-ray outputs of 24 stars with masses similar to, or below that of our Sun, with an age of over one billion years. The scientists discovered that the older stars experienced a far steeper drop off in X-ray (and therefore magnetic) activity when compared to that of younger stars observed in previous studies.
The steep drop off in high-energy activity for Sun-like and lower mass stars after 1 billion years is good news for the habitability of exoplanets orbiting these stellar bodies, as the atmospheres of these worlds would not have to endure as much radiation as previously believed.
One possible reason for the drop off in magnetic energy is that the reduction in magnetic activity mirrors a greater decrease in the rate of spin of older stars compared to their younger counterparts.
"We're not exactly sure why older stars settle down relatively quickly," said co-author Chris Watson of Queen's University. "However, we know it's led to the successful formation of life in at least one case – around our own Sun."
A paper detailing the research has been published in The Monthly Notices of the Royal Astronomical Society.