The distinctive blue bubble is created when speedy stellar winds interact with the outer layers of hydrogen ejected by stars. Image: Judy Schmidt/ESA/Hubble and NASA
The number of exoplanets deemed habitable in the distant universe may be fewer than we thought because we have ignored stellar winds.
For decades, our efforts to quantify the number of planets that could support alien life have been varied, from the famous Drake equation, to the advanced analytical capabilities of the Hubble Space Telescope and other instruments.
This has led to an explosion in the number of potentially habitable planets discovered, now ranging in the thousands.
However, new findings published to two papers in The Astrophysical Journal have dashed our hopes somewhat by claiming that the real number could be far smaller, as we have not taken into account one important thing: stellar winds.
The issue is found in planets that surround red dwarf stars – the most common type of star in our Milky Way – which are often cited as a parent star to many of these supposedly hospitable planets.
Atmosphere blasted away
In the first paper published by researchers from Princeton University, the team led by Chuanfei Dong analysed the potential damage of the constant outpouring of charged particles on Proxima Centauri b (PCb), four light years away from Earth.
Previous research had failed to take into account these stellar winds, which, over hundreds of millions of years, could severely deplete the atmosphere of such planets, rendering them unable to host surface-based life as we know it.
“Traditional definition and climate models of the habitable zone consider only the surface temperature,” Dong explained.
“But the stellar wind can significantly contribute to the long-term erosion and atmospheric loss of many exoplanets, so the climate models tell only part of the story.”
During simulations, the team found that photons in starlight ionise the atoms and molecules in the atmosphere into charged particles, allowing pressure and electromagnetic forces from the stellar wind to sweep them into space, leading to massive atmospheric loss.
Water worlds are actually a lot drier
“The evolution of life takes billions of years,” said Dong.
“It is only if the pressure is sufficiently low and if the exoplanet has a reasonably strong magnetic shield, like that of the Earth’s magnetosphere, that the exoplanet can retain an atmosphere and has the potential for habitability.”
The second paper analysed the effects of stellar winds on ‘water world’ planets, with seas believed to be hundreds of kilometres deep.
The simulations illustrated here that ancient stellar wind could cause the rate of atmospheric escape to be far greater than losses produced by the current solar wind that reaches the magnetosphere of Earth.
This suggests that dried-up oceans are far more abundant, and could lead to the Drake equation being modified, resulting in a reduction in the number of habitable planets.
The team does admit, however, that such predictions are open to change. Future observations could help us come to a more accurate model of predicting the possibility of life elsewhere in the universe.
