For years, scientists have thought Earth would eventually be consumed by the expanding Sun. A new study suggests that may not happen. Updated calculations indicate our planet could move outward as the Sun ages, allowing it to avoid being engulfed during the star’s final giant phases.
The question of Earth’s ultimate fate has long been tied to the future evolution of the Sun. In about 5 billion years, the star will run out of hydrogen in its core and begin a series of dramatic changes that will transform the solar system.
Astronomers have generally assumed that Earth would not survive this process. As the Sun expands into a giant star, tidal forces were expected to pull the planet inward. New research published in Astronomy & Astrophysics paints a more nuanced picture.
A Tug-of-war Between Gravity And Mass Loss
As the Sun grows older, it will first become a red giantbefore entering a later stage known as the asymptotic giant branch (AGB). During these phases, the star will expand enormously, extending far beyond its current size.
Two competing effects will determine what happens to Earth. The first involves tidal interactions between the growing Sun and the planet. These interactions can gradually alter Earth’s orbit and pull it closer to the star.
The second effect works in the opposite direction. As the Sun ages, it will lose a large amount of mass through stellar winds. With less mass, its gravitational pull weakens, allowing planets to drift farther away.
“Earth’s fate depends on a delicate balance between these two effects,” lead author Mats Esseldeurs of Belgium’s University of Leuven said. “If tidal interactions predominate, Earth is engulfed by the sun. If the sun’s mass loss predominates, Earth escapes into an orbit larger than the radius of its star.”
Until now, most studies had favored the first scenario, leading many astronomers to believe that Earth was destined to be swallowed by the Sun as its outer layers expanded deep into the inner solar system.
Better Models Lead To A Different Result
The new research takes advantage of major advances in scientists’ understanding of tidal processes inside giant stars. Earlier studies relied on relatively simple descriptions of how tidal energy behaves in stars during their late stages of evolution. The authors argue that those models likely overestimated the strength of tidal dissipation.
Their calculations show that this dissipation is weaker than previously thought, reducing the inward pull on Earth.
Co-author Stephane Mathis of CEA Paris-Saclay told AFP that improvements in tidal modeling over the past 15 years played a key role in the new results. Combined with updated estimates of solar mass loss, they change the balance between the two competing effects.
The team also looked at L2 Puppis, a nearby evolved star often described as an older cousin of the Sun. Observations of this star helped researchers refine their estimates of how much mass the Sun could lose during its giant phases.
Earth And Mars Could Survive
The study’s findings suggest that Earth is not the only planet that may avoid the Sun’s expanding outer layers. The researchers found that Marscould also escape the fate that many scientists had previously predicted for the inner solar system. In both cases, orbital expansion caused by solar mass loss appears strong enough to offset the effects of tidal interactions.
The outlook is less favorable for the planets closest to the Sun. The study reports that Mercuryand Venusare still expected to be engulfed as the Sun expands through its giant stages.
Eventually, the Sun will shed its outer layers and leave behind a white dwarf, an extremely dense stellar remnant. Without fusion reactions to power it, the white dwarf will slowly cool and become dimmer over time.
