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The fate of Earth during the Sun's giant phases: New constraints from ab initio tidal modelling and AGB mass loss

This study reassesses the Sun's future evolution using updated tidal dissipation models and AGB mass-loss rates, finding that while Earth's survival remains sensitive to these uncertain parameters, current observational proxies suggest the planet is likely to survive the Sun's giant phases.

Original authors: M. Esseldeurs, S. Mathis, L. Decin

Published 2026-06-19

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CC BY 4.0

Original authors: M. Esseldeurs, S. Mathis, L. Decin

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Sun as a giant, aging balloon that is slowly running out of air (hydrogen) in its core. As it gets older, it doesn't just sit still; it swells up massively, turning into a "Red Giant" and later an "Asymptotic Giant" (AGB). The big question astronomers have been arguing about for decades is: Will this swelling balloon swallow our Earth, or will Earth survive by drifting away?

This new paper acts like a high-tech weather forecast for the Solar System's future, but instead of rain and wind, it's calculating the fate of our planet based on two invisible forces: tidal friction and stellar wind.

Here is the breakdown of their findings using simple analogies:

1. The Two Forces at Play

To predict Earth's future, the scientists had to balance two competing effects:

The "Wind" Effect (Mass Loss): As the Sun ages, it sheds its outer layers like a dandelion releasing seeds. This is a stellar wind. When the Sun loses weight, it becomes less "grippy" on Earth's orbit. Think of a child spinning on a merry-go-round while letting go of a rope; as the rope gets lighter, the child flies outward. This effect tries to push Earth away from the Sun, saving it.
The "Drag" Effect (Tidal Interactions): As the Sun swells, it gets huge and fluffy. If Earth gets too close, the Sun's gravity acts like thick molasses or a giant hand grabbing the planet, slowing it down and dragging it inward. This is tidal friction. If this drag is strong enough, Earth spirals into the Sun and gets eaten.

2. The New "Recipe" vs. The Old One

The authors say previous studies used an old "recipe" for calculating how strong that tidal drag is. They treated the Sun's interior like a simple, uniform fluid.

The Old Recipe: Suggested the drag was very strong. It predicted that the Sun would grab Earth like a sticky trap, pulling it in and swallowing it during the giant phases.
The New Recipe: The authors used a brand-new, super-detailed computer model (called ab initio modelling) that looks at the Sun's actual, complex internal structure. They found that the "molasses" isn't as sticky as we thought. The drag is weaker.

The Result: With this new, weaker drag, Earth doesn't get pulled in as hard. Instead, the "Wind Effect" (the Sun losing weight) wins. Earth drifts outward fast enough to escape the Sun's grasp. According to this new model, Earth survives.

3. The "How Much Wind?" Problem

However, there is a catch. The "Wind Effect" depends entirely on how much mass the Sun actually loses.

Low Wind: If the Sun is a "lazy" star and doesn't lose much mass, it stays heavy. The heavy gravity wins, the drag pulls Earth in, and Earth gets swallowed.
High Wind: If the Sun is a "generous" star and sheds a lot of mass, it gets light quickly. The orbit expands fast, and Earth survives.

The problem is, we don't know exactly how much mass the Sun will lose. It's like trying to predict the future of a boat in a storm without knowing if the storm will be a light breeze or a hurricane.

4. The "L2 Pup" Clue

To guess the Sun's future behavior, the scientists looked at a neighbor: a star called L2 Pup.

L2 Pup is almost the same size and age as our Sun will be in the future.
By studying how much "wind" L2 Pup is currently blowing, the scientists found that it seems to be shedding mass at a rate that would allow a planet like Earth to survive.
The Verdict: If our Sun acts like L2 Pup, Earth is safe.

The Bottom Line

The paper concludes that Earth's fate is currently a "toss-up" that depends on two things we aren't 100% sure about yet:

How sticky the Sun's gravity is (Tidal drag).
How much mass the Sun sheds (Wind).

However, the authors lean toward a hopeful outcome. With their new, more accurate understanding of tidal drag, and by using the real-life example of the star L2 Pup as a guide, it looks likely that Earth will survive the Sun's giant phases and drift safely into the distance, rather than being swallowed whole.

But until we get better measurements of how stars like L2 Pup behave, we can't say it's 100% guaranteed. The Earth is holding its breath, waiting for the final verdict.

Technical Summary: The Fate of Earth During the Sun's Giant Phases

Problem Statement
The long-term evolution of planetary systems around solar-type stars is governed by the complex interplay between stellar expansion, tidal interactions, and mass loss during the Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) phases. A central uncertainty in astrophysics remains the ultimate fate of the Earth as the Sun evolves off the main sequence. Previous literature presents conflicting conclusions: some studies predict Earth's engulfment during the RGB or AGB phases (e.g., Rybicki & Denis 2001; Schröder & Smith 2008), while others suggest survival (e.g., Sackmann et al. 1993; Nordhaus et al. 2010). These discrepancies arise primarily from differing assumptions regarding tidal dissipation efficiencies and stellar mass-loss rates, both of which remain poorly constrained observationally.

Methodology
To address these uncertainties, the authors modelled the orbital evolution of the inner Solar System using a comprehensive numerical framework.

Stellar Evolution: They utilized the MESA code to simulate the Sun's evolution from the pre-main sequence to the white dwarf phase, employing a solar mass of $1 M_\odot$ and metallicity $Z=0.0134$ . Mass loss was modeled using the Reimers prescription for the RGB (calibrated to $\eta_{Reimers} = 0.477$ ) and the Blöcker prescription for the AGB.
Orbital Dynamics: The orbital evolution was calculated by integrating differential equations accounting for two primary mechanisms: orbital widening due to stellar mass loss (wind) and orbital decay due to tidal dissipation.
Tidal Physics: A key methodological advancement involves the use of ab initio tidal dissipation prescriptions derived from Esseldeurs et al. (2024). These prescriptions incorporate the latest understanding of internal stellar structure and dynamics, including dynamical tides associated with progressive internal gravity waves. This approach replaces the simplified equilibrium tide theory (e.g., Zahn 1966) used in many previous studies.
Parameter Exploration: The study systematically varied the AGB mass-loss efficiency parameter ( $\eta_{Blöcker}$ ) between 0.01 and 0.5 to assess sensitivity. Additionally, the authors used the AGB star L2 Pup as an observational proxy for the future Sun to constrain mass-loss rates, comparing estimates derived from dust emission versus CO emission.

Key Results
The simulations reveal that the fate of Earth is highly sensitive to the specific tidal model and the assumed mass-loss rate:

Tidal Dissipation Sensitivity: When using the updated ab initio tidal prescriptions (Esseldeurs et al. 2024), the tidal dissipation is weaker than in previous models. Consequently, the Earth's orbit expands sufficiently to avoid engulfment during both the RGB and AGB phases. In contrast, simulations using older prescriptions (e.g., Mustill & Villaver 2012 based on Zahn 1966) predict stronger tidal dissipation, leading to orbital shrinkage and eventual engulfment during the AGB phase.
Mass-Loss Rate Sensitivity: The outcome is also critically dependent on the AGB mass-loss rate. Low mass-loss rates (corresponding to $\eta_{Blöcker} \leq 0.02$ ) result in the Sun expanding to or beyond the Roche lobe radius, likely leading to Earth's engulfment. Conversely, higher mass-loss rates ( $\eta_{Blöcker} \geq 0.04$ ) allow the star to lose mass rapidly enough to prevent the stellar radius from reaching the Earth's orbit, ensuring survival.
Observational Constraints: Using the observed mass-loss rates of L2 Pup as a proxy, the study finds that the dust-based rate (implying a high $\eta_{Blöcker}$ ) supports Earth's survival, while the CO-based rate (implying a very low $\eta_{Blöcker}$ ) suggests a higher risk of engulfment. However, the authors note that the dust-based rate is more consistent with the survival scenario when combined with their updated tidal model.

Significance and Conclusions
The paper concludes that the survival of the Earth and the inner Solar System is not yet robustly determined, as it hinges on the treatment of tidal dissipation and stellar mass loss. While the authors' updated tidal models combined with observational proxies for the AGB phase (specifically L2 Pup) suggest that Earth will likely survive the Sun's giant phases, significant uncertainties remain. The authors emphasize that the ultimate fate depends on improved observational constraints on AGB mass-loss rates and further refinement of tidal dissipation theories in evolved stars. They highlight the potential of future missions, such as PLATO, to detect planets around red giants, which will provide the necessary population statistics to better constrain these evolutionary pathways.

1. The Two Forces at Play

2. The New "Recipe" vs. The Old One

3. The "How Much Wind?" Problem

4. The "L2 Pup" Clue

The Bottom Line

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