Why is It Highly Likely That Earth Will be Engulfed by the Sun During Its Red Giant Stage Yet a Complete Sterilization of Earth is Even More Likely?

Many discussions around the future of our planet Earth revolve around the changes brought about by its host star, the Sun. As the Sun transitions into the red giant phase during its later stages, the question arises whether Earth will be completely destroyed or merely sterilized. To understand this phenomenon, we need to explore the complexities of stellar evolution and the uncertainties that come with it.

Expansion of the Sun During Its Red Giant Phase

As the Sun exhausts its nuclear fuel, it begins to expand significantly, transforming into a red giant. The exact extent of this expansion is a subject of scientific inquiry. Estimates suggest that the Sun will swell up to approximately 1.4 million miles (2,260,000 km) in diameter, or about 100 to 1,000 times its current size. This expansion raises the question of whether Earth, currently orbiting at about 93 million miles (150 million km), will be engulfed by the Sun during this phase. However, the outcome is far from certain due to various factors.

Uncertainties in Stellar Evolution Models

While it is widely accepted that the Sun will become a red giant, the precise details of this transformation remain uncertain. Current scientific models predict that the Sun's expansion will increase its luminosity, making it inhospitable for life on Earth. For instance, it is estimated that the Sun's output will increase by 7 to 10 times its current value in the next billion to 1.5 billion years. This increase in brightness will make Earth's surface temperatures unsustainable for life as we know it.

However, the exact amount of mass that the Sun will lose during its red giant phase is still debated. The loss of mass affects the gravitational pull on the planets, including Earth. A less massive Sun might cause Earth to move into a larger orbit, thus remaining outside the Sun's expanding atmosphere. Therefore, even if the Sun expands to a point where it could potentially engulf Earth, the resulting orbital changes due to mass loss might prevent this outcome.

Stellar Brightness and Earth’s Habitability

Another critical factor in determining Earth's fate is the increasing brightness of the Sun over time. As the Sun ages, its luminosity increases, gradually making the Earth's climate less habitable. Studies show that the Sun's luminosity is expected to increase by 7 to 10 percent every billion years. This increase in heat and energy will continue to raise Earth's average temperature until the planet becomes uninhabitable. Current estimates suggest that this could happen in about 500 million to 1.5 billion years, leading to the complete sterilization of the planet long before it is engulfed by the Sun.

The Red Giant Sun and Earth's Future

The red giant stage of the Sun presents two possible scenarios for Earth's future: complete destruction or sterilization. The first scenario, where Earth is completely destroyed, is an open question as the exact details of stellar evolution and mass loss are uncertain. The second scenario, where life on Earth is permanently eradicated, is more likely based on current scientific understanding. The increase in the Sun's luminosity and the resulting thermal radiation will make Earth's surface conditions unsustainable for any known forms of life long before the Sun engulfs the planet.

The uncertainty in the Sun's mass loss during the red giant phase means that the ultimate fate of Earth remains an open question. However, given the clear trends in the Sun's increasing luminosity and the resulting changes in Earth's climate, the complete sterilization of Earth is considered a much safer bet compared to the more speculative notion of Earth being destroyed by the Sun.