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A new study led by astronomers from KU Leuven (Belgium) and CEA Paris-Saclay (Department of Astrophysics CEA-Irfu/UMR AIM) shows that the answer is more uncertain than previously thought. Using the latest models of stellar evolution and tidal interactions, the team found that Earth has a strong chance of surviving both giant phases of the Sun, but only if the Sun loses mass quickly enough during the final stages of its evolution.
Schematic evolution of the Sun through its evolved phases.
The image shows the Sun as it is today, going through its two giant phases, before ending its life as a white dwarf. The study showed that Mercury and Venus will be swallowed by the expanding Sun, but Earth and Mars will survive and end up on a more distant orbit. Sizes and distances are not to scale.
credit @KU Leuven
A subtle competition
As the Sun evolves to become a red giant, two opposing processes will determine Earth's fate:
- On one hand, the Sun will expand considerably, increasing the gravitational tidal forces that tend to pull Earth toward it.
- On the other hand, the Sun will lose a large portion of its mass through stellar wind. As the Sun becomes lighter, Earth's orbit gradually moves away from it.
Previous studies often concluded that Earth would eventually be engulfed by the Sun. However, many of these studies relied on simplified descriptions of tidal interactions within giant stars.
This figure represents the orbital evolution of Earth during the Sun's life phases (the unit used is the astronomical unit representing the current Sun-Earth distance).
The blue curve represents the evolution of the Sun's radius over time (shown relative to the end of the Sun's life). Two phases of strong expansion of the Sun are identified during the so-called red giant phase and the so-called AGB phase. The orange and green curves represent respectively the calculations using the most advanced modeling of the dissipation of tidal waves generated by Earth in the Sun (in orange) and the commonly used one (in green).
In the first case, Earth moves away from the Sun and has a chance to survive.
A look at the Sun of the future
This new study relies on state-of-the-art calculations of tidal dissipation in the Sun throughout its evolution, taking into account the internal structure and dynamics of evolved stars. These models were developed by the research team as part of previous work and are based on realistic physical calculations rather than simplified approximations.
Using these updated methods, the researchers found that tidal interactions between Earth and the Sun throughout its evolution are weaker than previously assumed. Consequently, Earth remains farther from the Sun during the giant phases, significantly increasing its chances of survival.
In the simulations, Mercury and Venus are inevitably engulfed by the expanding Sun. Earth, on the other hand, survives both giant phases and ends up on a more distant orbit around the white dwarf that the Sun will eventually become.
A major uncertainty: stellar mass loss
Despite this encouraging result, uncertainties remain. The researchers found that the final outcome depends on how quickly the Sun will lose mass during its last evolutionary phase. Unfortunately, these mass loss rates remain a poorly understood aspect of stellar evolution.
To investigate this uncertainty, the team turned to L2 Puppis, a nearby evolved star often considered a preview of the Sun's future. Based on this estimate from observations, Earth will move away just enough to survive.
Learning more about the future of the Sun-Earth system by observing distant stars
This study highlights how observing evolved stars can provide clues about the distant future of our own solar system. Future ground-based observatories and space missions such as PLATO will discover many more planets orbiting evolved solar-like giant stars, giving astronomers the opportunity to test models of planetary survival on a population scale.
Even though life on Earth will become impossible long before the Sun reaches its giant phases, the planet itself might well endure. Whether Earth ultimately survives or not, the answer lies in understanding the complex physical mechanisms of stellar aging—a problem astronomers are just beginning to unravel.