As the Sun becomes a white dwarf, perturbations or ejections of remaining planets are possible over eons
Solar system after the red giant phase
In about ~5 billion years, our Sun will continue hydrogen fusion in its core (main sequence). However, once this fuel is exhausted, it will enter the red giant and asymptotic giant branch phases, losing a large portion of its mass and eventually becoming a white dwarf. During these late stages, planetary orbits – especially those of the outer giants – may change due to mass loss, gravitational tidal forces, or, if close enough, stellar wind drag. The inner planets (Mercury, Venus, most likely Earth) will probably be engulfed, but the remaining ones may survive in altered orbits. Over very long epochs (tens of billions of years), other factors such as passing stars or galactic tides will further rearrange or disrupt this system. Below we discuss each phase and possible consequences in detail.
2. Main factors in the late Solar System dynamics
2.1 Solar mass loss during the red giant and AGB phases
During the red giant and later AGB (asymptotic giant branch) stages, the Sun's outer part expands and is gradually lost through stellar winds or strong pulsation-driven ejections. It is believed that by the end of the AGB, the Sun may lose ~20–30% of its mass:
- Luminosity and radius: The Sun's luminosity rises to thousands of times the current value, and the radius can reach ~1 AU or more during the red giant stage.
- Mass loss rate: Over several hundred million years, strong winds steadily remove the outer layers, eventually forming a planetary nebula.
- Effect on orbits: The reduced stellar mass weakens its gravitational pull, so the orbits of the remaining planets expand, following the simple two-body relation where a ∝ 1/M☉. In other words, if the solar mass decreases to 70–80%, the planets' semi-major axes can grow proportionally [1,2].
2.2 Engulfment of the inner planets
Mercury and Venus will almost certainly be engulfed by the swollen outer Sun. Earth lies on the edge – some models show that mass loss could expand its orbit enough to avoid complete engulfment, but tidal forces may still doom it. After the AGB phase, perhaps only the outer planets (from Mars outward) and dwarf and small bodies will remain, though with altered orbits.
2.3 Formation of the white dwarf
At the end of the AGB, the Sun ejects its outer layers over tens of thousands of years, forming a planetary nebula. The white dwarf core remains (~0.5–0.6 solar masses), fusion no longer occurs; it only radiates thermal energy and cools over billions or even trillions of years. The reduced mass means the remaining planets have expanded or otherwise altered orbits, determining the long-term dynamics in the new star–planet mass ratio.
3. Fate of the outer planets – Jupiter, Saturn, Uranus, Neptune
3.1 Orbital expansion
During the red giant and AGB mass loss phase, the orbits of Jupiter, Saturn, Uranus, and Neptune will expand adiabatically due to the decreasing solar mass. Approximately, the final semi-major axis af can be estimated if the mass loss duration is long compared to the orbital period:
a(f) ≈ a(i) × (M(⊙,i) / M(⊙,f))
Where M⊙,i y is the initial solar mass, and M⊙,f – final (~0.55–0.6 M☉). Orbits can expand by ~1.3–1.4 times if the star loses ~20–30% of its mass. For example, Jupiter at ~5.2 AU could move out to ~7–8 AU, depending on the final mass. Similar expansion is expected for Saturn, Uranus, and Neptune [3,4].
3.2 Long-term stability
After the Sun becomes a white dwarf, the planetary system could survive for billions of years more, though expanded. However, destabilizing factors may eventually arise:
- Planet-planet interactions: Over gigayears (109 years), resonances or chaotic phenomena can accumulate.
- Passing stars: The Sun moves through the Galaxy, so close stellar approaches (a few thousand AU or less) can disturb orbits.
- Galactic tides: Over tens or hundreds of billions of years, weak galactic tides can affect outer orbits.
Some models suggest that ~1010–1011 Over the years, the orbits of giant planets can become chaotic enough to cause ejections or collisions. However, these are long timescales, and the system may remain at least partially unchanged if strong perturbations are absent. Ultimately, stability also depends on the local stellar environment.
3.3 Examples of planets that can survive
It is often mentioned that Jupiter (having the greatest mass) and its moons may survive the longest, continuing to orbit the white dwarf. Saturn, Uranus, and Neptune are more sensitive to ejection due to interactions with disturbances originating in Jupiter. However, such orbital evolution processes can last from billions to trillions of years, so parts of the Solar System's structure could exist for a very long time during the white dwarf cooling period.
4. Small bodies: asteroids, Kuiper belt, and Oort cloud
4.1 Inner belt asteroids
Most bodies of the main asteroid belt (2–4 AU) are relatively close to the Sun. Mass loss and gravitational resonances could shift their orbits outward. Although the red giant's “envelope” may extend to ~1–1.2 AU and not directly cover the main belt, enhanced stellar wind or radiation could cause additional dispersion or collisions. After the AGB phase, some asteroids would remain, but chaotic resonances with outer planets would eject some.
4.2 Kuiper belt, scattered disk
Kuiper belt (~30–50 AU) and scattered disk (50–100+ AU) are unlikely to collide with the physical envelope of the red giant, but will feel the decrease in stellar mass, causing their orbits to expand proportionally. Additionally, as Neptune's orbit changes, the distribution of TNOs may rearrange. Over billions of years, stellar flybys can disperse many TNOs. The same applies to the Oort cloud (up to ~100,000 AU): it will barely feel the giant expansion directly but will be very susceptible to the effects of passing stars and galactic tides.
4.3 White Dwarf "Pollution" and Comet Impacts
Observing white dwarfs in other systems shows "metal pollution" in their atmospheres – heavy elements that should sink but persist only due to the continuous infall of asteroid or comet debris. Similarly, in the case of our future white dwarf, asteroids/comets may remain that occasionally approach the Roche limit, get disrupted, and enrich the dwarf's atmosphere with metals. This would be the last "reprocessing" of the Solar System.
5. Timescales of Final Disruption or Survival
5.1 White Dwarf Cooling
When the Sun becomes a white dwarf (~7.5+ billion years in the future), its radius will be similar to Earth's, and its mass ~0.55–0.6 M☉. Initial temperature very high (~100,000+ K), gradually decreasing over tens/hundreds of billions of years. Until it becomes a "black dwarf" (theoretically, the Universe's age is currently insufficient to reach this stage), planetary orbits may remain stable or be disrupted during that time.
5.2 Ejections and Flybys
Over 1010–1011 Random stellar close approaches over years (several thousand AU) can gradually strip planets and small bodies into interstellar space. If the Solar System traveled through a denser environment or cluster, the rate of disruption would be even higher. Eventually, a solitary white dwarf may remain without any surviving planets or with only a few distant bodies left.
6. Comparison with Other White Dwarfs
6.1 "Polluted" White Dwarfs
Astronomers often find white dwarfs with heavy elements (e.g., calcium, magnesium, iron) in their atmospheres, which should quickly sink but remain due to the continuous infall of small bodies (asteroids/comets). Some WD systems have dust disks formed by disrupted asteroids. Such data indicate that planetary remnants in systems can survive through the white dwarf phase, occasionally supplying material.
6.2 Exoplanets Around White Dwarfs
Several planetary candidates have been detected around white dwarfs (e.g., WD 1856+534 b), large planets comparable in size to Jupiter, in very close (~1.4 days) orbits. It is believed that these planets may have later migrated inward after the star's mass loss or survived resisting the star's expansion. This provides clues on how the giant planets of the Solar System might survive or change after similar processes.
7. Significance and Broader Insights
7.1 Understanding the Life Cycle of Stars and Planetary Structure
Studying the long-term evolution of the Solar System makes it clear that the lives of stars and their planets continue far beyond the main sequence end. The fate of planets reveals common phenomena – mass loss, orbital expansion, tidal interactions – characteristic of Sun-like stars. This indicates that exoplanet systems around evolving stars may experience similar fates. Thus ends the life cycle of stars and planets.
7.2 Final habitability and possible evacuation
Some speculations suggest that advanced civilizations might manipulate “stellar mass” or move planets outward to survive beyond the star’s stable era. Realistically, from a cosmic perspective, leaving Earth (e.g., for Titan or even beyond the Solar System) may be the only way for humanity or its future descendants to exist for eons, as the Sun’s transformation is inevitable.
7.3 Future observational verification
By further analyzing “polluted” white dwarfs and possibly surviving exoplanets around them, we will increasingly understand how the lives of Earth-like systems ultimately end. At the same time, as solar modeling improves, it becomes clearer how much the red giant layers expand and how quickly mass is lost. Collaborations between stellar astrophysics, orbital mechanics, and exoplanet research develop ever more detailed pictures of how planets enter their final states as their star dies.
8. Conclusion
Over a longer period (~5–8 billion years), the Sun, transitioning into the red giant and AGB phases, will experience significant mass loss and likely engulf Mercury, Venus, and possibly Earth. The remaining bodies (outer planets, smaller objects) will move outward as the star's mass decreases. Eventually, they will orbit a white dwarf. Over further billions of years, random stellar flybys or resonant interactions may gradually disrupt the system. The Sun – now a cold, dim remnant – will barely resemble the once thriving planetary family.
This fate is typical for stars of about 1 Solar mass, indicating how short-lived the habitability of planets is. Digital models, observational data from bright red giants, and examples of “polluted white dwarfs” help to better understand these final evolutionary stages. So although our currently pleasing stable main sequence era continues, the cosmic timeline explains that no planetary system is eternal – the slow fading of the Solar System is the last part of its billion-year journey.
References and further reading
- Sackmann, I.-J., Boothroyd, A. I., & Kraemer, K. E. (1993). “Our Sun. III. Present and Future.” The Astrophysical Journal, 418, 457–468.
- Schröder, K.-P., & Smith, R. C. (2008). “Distant future of the Sun and Earth revisited.” Monthly Notices of the Royal Astronomical Society, 386, 155–163.
- Villaver, E., & Livio, M. (2007). “Can Planets Survive Stellar Evolution?” The Astrophysical Journal, 661, 1192–1201.
- Veras, D. (2016). “Post-main-sequence planetary system evolution.” Royal Society Open Science, 3, 150571.
- Althaus, L. G., et al. (2010). “Evolution of white dwarf stars.” Astronomy & Astrophysics Review, 18, 471–566.