Planets Survive The Expansion Of Red Giant Stars

A Challenge To Stellar Evolution Theories

Astronomers have identified three solar systems containing Jupiter-like planets that managed to survive a phase of stellar evolution that typically destroys nearby celestial bodies. When stars similar to the sun reach the end of their life cycles, they exhaust the hydrogen in their cores and expand into red giants. This process causes the star to puff up significantly, often consuming any planets orbiting in close proximity through a process of engulfment.

Using the TESS space telescope, a research team led by Qian Shengbang from Yunnan University observed three such planets in the northern sky. These planets remained intact despite their host stars ballooning outward past their orbital positions before eventually shrinking back down. This discovery suggests that the survival of planets during the red giant phase is more common than previously believed, indicating that some celestial bodies can withstand the extreme environment of an expanding star.

The Case Of 24 Boötis

One of the most striking examples found by the researchers is a planet orbiting the star 24 Boötis, located approximately 300 light years away. This planet maintains an orbit that is closer to its star than Mercury is to the sun. According to the findings published in the Proceedings of the National Academy of Sciences, the host star swelled to more than 100 times the width of the sun at its peak expansion, extending well past where the planet currently sits.

Under conventional astronomical models, this expansion should have resulted in the planet being swallowed by the star’s bloated atmosphere or being slowly dragged inward by gravity until it crashed into the stellar surface. The fact that the planet still exists in its current orbit indicates that some unknown physical mechanisms may allow certain planets to withstand this volatile stage of stellar growth. Qian Shengbang noted that finding three of these lucky survivors at once tells scientists that this is no fluke and that common physics may be at work.

Implications For The Solar System

For a long time, the scientific community believed that planets in such tight orbits were doomed. Until this recent study, only one other exception had been documented—a planet circling a star in the Little Dipper discovered by a different team in 2023. The identification of three additional surviving planets suggests that these occurrences are not mere accidents but may be governed by physical laws that scientists have yet to fully understand.

This discovery has significant implications for the distant future of our own solar system. The sun, which is currently about 4.6 billion years old, is estimated to be roughly halfway through its lifespan. In approximately 5 billion years, the sun is expected to enter its own red giant phase. Standard theories predict that the sun will expand to roughly the distance of Earth’s current orbit, likely devouring Mercury and Venus in the process as it grows.

While the survival of these distant Jupiter-like planets provides a glimmer of theoretical possibility, the fate of Earth remains grim. Even if the planet avoids being physically engulfed by the expanding sun, the intense heat and radiation would likely scorch the surface into a lifeless rock long before the star reaches its maximum size. After the red giant phase, the sun will eventually shed its outer layers and collapse into a white dwarf, a small and dim star roughly the size of Earth.

Despite the likely loss of habitability, the research indicates that the long-term evolution of planetary systems is far more complex than predicted. The existence of these surviving planets suggests that the interaction between aging stars and their orbiting bodies involves variables that could rewrite the timeline of cosmic destruction and the long-term fate of the solar system, including the Earth humans depend on.