The Red Giant Phase: Fate of the Inner Planets
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 08 / Article 06
When the Sun outgrows its worlds.
Billions of years from now, the familiar Sun will become an enormous red giant. Its expansion will transform the inner Solar System—but a planet’s fate depends on more than how large its star becomes.
What happens when a star changes its shape?
Picture the Sun spreading across a much larger part of the sky. The change begins deep inside it, where the fuel sustaining its present life gradually runs out.
The resulting story connects stellar interiors with planetary motion. To follow it, we need to ask three questions: how does the Sun expand, how do nearby orbits respond, and what would survival mean for a world like Earth?
This article builds on The Sun’s Structure and Life Cycle and follows those changes into the inner planets’ distant future.
Why will the Sun become a red giant?
When core hydrogen is depleted, the Sun’s helium-rich center contracts and heats. Hydrogen fusion continues in a surrounding shell as the envelope expands, eventually carrying the Sun up the red giant branch.[1]
A star must continually balance gravity, pressure, and the transport of energy. As conditions around the core and burning shell change, its envelope settles into a different structure: much larger, with a cooler surface. The detailed expansion mechanism involves this structural readjustment, rather than a single outward shove.[3]
How can a cooler star become brighter?
Surface temperature and total luminosity are different quantities. A cooler surface radiates less energy per square meter, but a giant has an enormous radiating area. The relation L = 4πR²σTeff⁴ connects luminosity L, radius R, and effective temperature Teff; σ is the Stefan–Boltzmann constant. A sufficiently large increase in radius outweighs the lower temperature.[2]
When will this happen?
The Sun has roughly five billion years of core hydrogen fusion remaining. That is an estimate for the end of its main-sequence life, not a date for maximum expansion or for becoming a white dwarf.[4]
| Stage | What changes? | How to read the timing |
|---|---|---|
| Remaining main sequence | Hydrogen continues fusing in the core. | Approximately five billion years remain.[4] |
| Subgiant and red giant branch | The core contracts; the envelope grows. | Maximum size comes later than core-hydrogen exhaustion.[3] |
| Helium ignition | The dense core begins helium fusion. | A rapid internal event leads to a longer readjustment.[5] |
| Stable core helium burning | Helium is converted into carbon and oxygen. | Of order 100 million years in solar-mass calculations.[6] |
| Asymptotic giant branch | A second giant stage includes strong mass loss. | Its duration depends on stellar evolution and wind models.[6] |
| White dwarf | The exposed remnant gradually cools. | A long aftermath, without sustained core fusion.[7] |
An astronomical unit, or AU, is approximately today’s average Earth–Sun distance. Comparing a giant’s size with 1 AU helps us picture the scale, but it does not keep Earth’s future orbit fixed at that distance.[9]
Will the planets move outward or fall inward?
As the Sun ages, it loses gas through stellar winds. At the same time, a nearby planet raises tides within the star. These effects must be calculated together; neither the growing solar radius nor mass loss alone determines the outcome.
Mass loss can widen an orbit
Slow, approximately symmetric stellar mass loss can widen a bound orbit. In this limit, semimajor axis a scales approximately as 1/M, where M is stellar mass.[10]
Tides can work in the opposite direction
A planet’s gravity deforms its star. Dissipation of that tidal response can remove orbital energy and shrink the orbit around a slowly rotating giant. The efficiency depends on the star’s internal structure and fluid motions, making it a central uncertainty in survival calculations.[12]
Tides act outside the visible stellar surface. Gas drag is a separate effect that requires interaction with surrounding gas and becomes important in denser atmospheric or envelope material. A planet need not cross the photosphere—the surface from which most visible light escapes—for tidal decay to begin.[11]
What happens to Mercury, Venus, Earth, and Mars?
The inner planets start at very different distances. Mercury and Venus lie deep within the region threatened by solar expansion. Earth lies closer to the disputed survival boundary, where relatively small changes in the adopted physics can alter the result.
| Planet | Present orbital scale | Outlook during the giant stages |
|---|---|---|
| Mercury | 0.39 AU | Engulfed in both the 2008 and 2026 calculations discussed here. |
| Venus | 0.72 AU | Also engulfed in both calculations. |
| Earth | 1.00 AU | Survival or engulfment depends on the model. |
| Mars | 1.52 AU | Survives in the updated 2026 calculation. |
Distances are rounded present semimajor axes; they are not fixed future orbits.[9], [8], [13]
Why does Earth’s answer keep changing?
Schröder and Smith’s 2008 model includes orbital expansion from mass loss but finds that tidal effects ultimately win: Earth is engulfed near the tip of the first red giant stage.[8]
A newer calculation allows survival
Esseldeurs, Mathis, and Decin find that Earth can survive both giant phases when they use updated tidal calculations and suitable mass-loss rates. Stronger tidal dissipation or lower mass loss during the later giant phase can instead produce engulfment. The paper appeared in Astronomy & Astrophysics.[13]
The result identifies which assumptions matter. It does not establish a final probability for Earth’s survival.
“The Sun becomes larger than Earth’s present orbit” therefore does not, by itself, settle Earth’s fate. We must follow the changing star and the changing orbit through the same calculation.
Could Earth remain habitable until then?
Earth’s familiar surface environment is expected to disappear long before the Sun reaches its largest giant stages. Even during the main sequence, the Sun gradually brightens. Greater incoming energy warms the planet, while additional water vapor can strengthen greenhouse warming.[14]
In a moist greenhouse, more water reaches the upper atmosphere, where sunlight can break it apart and hydrogen can escape. A runaway greenhouse is a distinct radiative instability in which the planet cannot balance absorbed sunlight with outgoing heat while retaining its liquid-water surface. Models disagree about the thresholds and sequence; ocean loss does not require every model to pass through the same path.[14]
Different conditions for life can fail at different times
A 2021 study by Ozaki and Reinhard combined climate with carbon, oxygen, and biological cycling. Its calculations put the decline of an oxygen-rich atmosphere before extensive surface-water loss. This illustrates why the lifetime of today’s oxygen-dependent biosphere, the lifetime of surface oceans, and the survival of the rocky planet are three different questions.[15]
Explore the climate processes in Planetary Climate Cycles and the broader definition in The Habitable Zone Concept.
What comes after maximum expansion?
Helium ignites deep inside the core
The Sun’s compressed helium core eventually reaches conditions for helium fusion. Because electron degeneracy initially limits its ability to expand in response to heating, ignition begins as a helium flash. This is an internal thermonuclear event, not a supernova or a visible blast that tears the Sun apart.[5]
The core readjusts and settles into sustained helium burning. The star becomes more compact than at the red giant tip. This comparatively stable interval is distinct from both the flash itself and the later giant phase.[5]
A second expansion and the loss of the envelope
After core helium is exhausted, the asymptotic giant branch (AGB) brings another expansion. Hydrogen and helium burn in shells around a carbon–oxygen core; unstable helium-shell burning produces thermal pulses.[1]
Stellar winds remove the remaining envelope. How rapidly this happens affects both the star’s evolution and the orbits around it. That is why surviving the first red giant maximum does not automatically guarantee survival of the AGB.[6]
If the exposed core becomes hot enough while expelled gas remains nearby, its ultraviolet radiation can illuminate a planetary nebula. Models published in 2018 predict that the Sun could produce a faint one. The brightness depends on the timing of core heating and gas dispersal; a spectacular glowing shell is not guaranteed.[16]
The Sun leaves a white dwarf
The remnant will be a dense, predominantly carbon–oxygen white dwarf, roughly comparable to Earth in size. Electron degeneracy pressure supports it, and it gradually radiates stored heat without sustained fusion in its core. The Sun’s normal evolution does not end in a supernova.[7]
A detailed solar calculation gives a final mass near 0.54 times the Sun’s present mass. The exact value depends on how much material its winds remove. Much of the matter once belonging to the Sun will have returned to the surrounding interstellar environment.[17]
Can we observe evidence for these outcomes?
We cannot watch the Sun complete billions of years of evolution. We can examine other stars at different stages, look for planets around their remnants, and test interpretations against new observations.
An engulfment event with a revised explanation
The brightening event ZTF SLRN-2020 was initially interpreted as a planet swallowed by an expanding red giant. Webb observations reported in 2025 found that the star was too faint for that explanation. The revised picture favors a planet whose orbit gradually decayed until it was engulfed. The event supports planet–star interaction, while showing why it should not be presented as a direct preview of the Sun’s expansion.[18]
A giant planet survives its star’s transformation
In 2021, Blackman and colleagues reported a Jupiter-like planet orbiting a white dwarf. They combined gravitational microlensing evidence with follow-up observations of the host. The system demonstrates that a giant planet can remain after its star’s giant stages, providing an observational counterpart to survival calculations.[19]
Destroyed bodies leave chemical traces
Heavy elements in many white dwarf atmospheres should sink out of the visible layers. Their presence points to replenishment by accreting planetary debris. For the white dwarf G29−38, X-ray observations also directly detected ongoing accretion. Such evidence lets astronomers study the remains of planetary systems even when individual rocky bodies cannot be imaged.[20]
These observations answer complementary questions: whether planets can be swallowed, whether some can survive, and what happens to smaller material afterward.
What can we say confidently about the distant future?
The Sun’s transition through giant stages to a white dwarf is well grounded in stellar physics. The harder problem is predicting individual planetary outcomes near the survival boundary.
How much mass is lost?
Wind histories alter both the stellar envelope and the gravitational field experienced by planets.[6]
How efficiently do tides dissipate?
The star’s evolving structure controls how tidal motions transfer and dissipate energy.[12]
What remains bound?
Slow solar mass loss alone need not eject the outer planets. Much more distant populations are more vulnerable.[21]
For surviving planets, a wider orbit around a cooling white dwarf would belong to a profoundly changed system. Later gravitational interactions can continue to rearrange surviving material. The end of the Sun’s present life is therefore a transition in planetary history, not an automatic disappearance of everything that once orbited it.[21]
The next article turns toward the reservoirs farther out: the Kuiper Belt and Oort Cloud, where icy bodies preserve evidence of the Solar System’s beginning.
Sources and further reading
Primary research and institutional resources on stellar evolution, orbital survival, planetary climates, and observed remnants. Includes the published June 2026 Earth-survival calculation.
- Ohio State University — Low-Mass StarsHydrogen shell burning, core helium burning, AGB shell structure, and thermal instabilities.
- Ohio State University — Stars and the Hertzsprung–Russell DiagramThe relation between stellar radius, effective temperature, and total luminosity.
- Ou and Chen (2026) — Why Do Stars Turn Red? I. Post-Main-Sequence Expansion MechanismNumerical experiments investigating how stellar envelopes respond to changes around the hydrogen-burning shell.
- NASA — Types of StarsThe Sun’s main-sequence identity and its approximately five billion remaining years of core hydrogen fusion.
- Bildsten et al. (2012) — Acoustic Signatures of the Helium Core FlashModels of helium ignition, the removal of electron degeneracy, and the transition to stable red-clump burning.
- Miller Bertolami (2016) — New models for the evolution of post-asymptotic giant branch stars and central stars of planetary nebulaeEvolutionary calculations showing how winds and mixing affect final masses, envelope removal, and the timing of nebular illumination.
- NASA — White Dwarf StarsEarth-sized remnants, electron degeneracy pressure, carbon and oxygen interiors, and cooling without sustained core fusion.
- Schröder and Smith (2008) — Distant Future of the Sun and Earth RevisitedA solar model predicting Earth’s engulfment near the first giant maximum.
- NASA — Basics of Space Flight: The Solar SystemPresent mean orbital distances of Mercury, Venus, Earth, and Mars in astronomical units.
- Veras et al. (2011) — The Great Escape: How Exoplanets and Smaller Bodies Desert Dying StarsSlow stellar mass loss and its orbital limits.
- Villaver and Livio (2009) — The Orbital Evolution of Gas Giant Planets Around Giant StarsCalculations separating stellar mass loss, tidal orbital decay, and drag in gas around evolved stars.
- Esseldeurs, Mathis and Decin (2024) — Tidal Dissipation in Evolved Low and Intermediate Mass StarsHow stellar structure and different forms of tidal dissipation influence orbital evolution during giant phases.
- Esseldeurs, Mathis and Decin (2026) — The Fate of Earth During the Sun’s Giant PhasesEarth’s outcome depends on tidal and mass-loss assumptions.
- Wolf and Toon (2015) — The Evolution of Habitable Climates Under the Brightening SunClimate simulations distinguishing warming, moist greenhouse conditions, water loss, and thermal runaway.
- Ozaki and Reinhard (2021) — The Future Lifespan of Earth’s Oxygenated AtmosphereA coupled climate and biogeochemical model exploring how oxygen-rich conditions can end before extensive ocean loss.
- Gesicki, Zijlstra and Miller Bertolami (2018) — The Mysterious Cut-Off of the Planetary Nebula Luminosity FunctionModels linking exposed stellar cores to nebular visibility, including a prediction of a faint planetary nebula from the Sun.
- Sackmann, Boothroyd and Kraemer (1993) — Our Sun. III. Present and FutureA detailed solar model illustrating uncertain mass loss and a final white dwarf mass near 0.54 solar masses.
- NASA (2025) — Webb’s Autopsy of a Planet Swallowed by a StarRevised interpretation of ZTF SLRN-2020, favoring orbital decay over expansion of a red giant.
- Blackman et al. (2021) — A Jovian Analogue Orbiting a White Dwarf StarMicrolensing and follow-up observations identify a giant planet around a stellar remnant.
- Cunningham et al. (2022) — A White Dwarf Accreting Planetary Material Determined from X-ray ObservationsHeavy-element pollution and X-rays provide complementary evidence for accretion of planetary debris.
- Veras and Wyatt (2012) — The Solar System’s Post-Main Sequence Escape BoundaryModels showing that solar mass loss alone need not eject the outer planets, while very distant bodies are more vulnerable.
The Solar System’s Dynamics and Future
- The Sun’s Structure and Life Cycle
- Solar Activity: Flares, Sunspots, and Space Weather
- Planetary Orbits and Resonances
- Asteroid and Comet Impacts
- Planetary Climate Cycles
- The Red Giant Phase: Fate of the Inner Planets · You are here
- Kuiper Belt and Oort Cloud
- Potential Habitable Zones Beyond Earth
- Human Exploration: Past, Present, and Future
- Long-Term Solar System Evolution