The possibility of life and the ultimate fate of planets such as the Earth are influenced by the stars they orbits. Exploration of planets in our own solar system and the discovery of more than 5000 exoplanets has the scientific community excited about the possibility of life beyond Earth. By virtue of groundbreaking missions such as Kepler, Transiting Exoplanet Survey Satellite , James Webb Space Telescope and many others, we are gaining unprecedented insights to a variety of star-planet systems that have been lying unexplored until recent times.

Our solar system comprises of a variety of planets which are all different from each other. On Earth, we benefit from an invisible shield of magnetic field known as the magnetosphere that protects our atmosphere from the onslaught of supersonic plasma winds from our host star - the Sun. This allows the Earth to hold on to its atmosphere and shield high energy radiation from the Sun making the planet habitable.

However, not all planets are fortunate enough. Mars, our neighboring planet, has lost its magnetic shield resulting in the solar wind gradually peeling off its atmosphere. On the other end of the spectrum, there are giant planets like Jupiter which have much stronger fields compared to Earth. Going beyond to worlds beyond our own, it is expected that there will be a larger diversity in the magnetic field strengths in star-exoplanet systems.

The interplay between magnetic fields of stars and their orbiting planets holds the key to unraveling the secrets to atmospheric erosion which impacts the ability of planets to host life. Scientists believe the magnetic fields of planets play an important role in this context. The interaction of solar wind with the Earth’s magnetic field results in a tear-drop shaped magnetosphere with the dayside magnetopause and nightside extended magnetotail. This tear dropped shaped magnetosphere acts like a cocoon protecting the Earth's atmosphere which sits within. But what happens when the magnetic field of the star is very strong or that of the planet is very weak?

In a new study that appears in the Astrophysical Journal - published by the American Astronomical Society - Sakshi Gupta, Arnab Basak and Dibyendu Nandi from the Center of Excellence in Space Sciences India and the Department of Physical Sciences, Indian Institute of Science Education and Research Kolkata perform computational modeling to shed light on the interplay of magnetism, planetary atmospheric mass loss and habitability of (exo)planets. They have investigated how different combinations of planetary and stellar magnetic field strengths may impact the planet's magnetic configuration and atmospheric loss. Contrary to popular opinion that an intrinsic magnetosphere always shields a planet from atmospheric erosion, the team's simulations reveal a more complex picture. Under certain conditions, the presence of an intrinsic magnetic field may actually contribute to higher atmospheric loss. The team are the first to establish a mathematical relaion between the magnetic field strength of the planet, the star's plasma winds, and the rate of loss of a planet's atmosphere.

The simulations also illuminate a beautiful phenomenon. The cosmic dance of magnetic fields produce a remarkable metamorphosis in the planetary magnetosphere which sprouts a wing-like structure when its magnetic field is relatively weaker compared to that of the star. These wings are known as Alfvén wings after the Swedish Nobel laureate Hannes Alfvén, who made pioneering contributions to the field of magnetohydrodynamics.

These findings indicate that we are just beginning to understand how magnetism influences star-planet interactions, atmospheric mass loss and habitability of planets across the vast expanse of our universe.

Reference to the publication Gupta, S., Basak, A. and Nandy, D., “Impact of Changing Stellar and Planetary Magnetic Fields on (Exo)planetary Environments and Atmospheric Mass Loss”, 2023, Astrophysical Journal, Volume 953, Page 70, DOI: 10.3847/1538-4357/acd93b.

Image Caption: A Computer simulation of the magnetic field of a star’s plasma wind interacting with a planet’s magnetosphere. When the Star’s magnetic field is relatively strong it forces the planet's magnetosphere to open up and metamorphosize into a wing-like structure known as Alfvén wings as seen in this image. Such star-planet interactions determine the ability of a planet to hold on to its atmosphere and host life – when other relevant conditions are in place. Credit: S Gupta, A Basak, and D Nandy (IISER Kolkata).

Of exoplanetary atmospheres, magnetism and stellar winds and why planets may have wings

Of exoplanetary atmospheres, magnetism and stellar winds
and why planets may have wings