Are stars dying planets

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Are Stars Dying Planets?

The question of whether stars are responsible for the demise of planets is a fascinating one, touching on the intricate dynamics of stellar evolution and planetary systems. Research in this area reveals a complex interplay between stars and their planetary companions, particularly as stars evolve and undergo significant changes.

Planetary Ejection and Mass Loss in Dying Stars

As stars evolve, particularly during the Asymptotic Giant Branch (AGB) phase, they lose a significant amount of mass. This mass loss can have profound effects on the orbits of surrounding planets. Studies show that the magnitude and duration of a star's mass loss, combined with the initial orbital characteristics of the planets, can lead to various outcomes. These include the ejection of planets from the system, modest increases in orbital eccentricity, or even the circularization of orbits. For instance, wide-separation planets and Oort clouds may be dynamically ejected during the AGB phase of stars with masses between 1 and 7 solar masses. Similarly, planetary material surviving a supernova from progenitors of 7 to 20 solar masses is often ejected, limiting the existence of first-generation pulsar planets.

Long-Term Fate of Planets in a Dying Universe

The long-term fate of planets is also influenced by the broader context of the universe's evolution. Over timescales exceeding trillions of years, the supply of interstellar gas will be exhausted, and star formation will continue at a highly attenuated level through collisions between brown dwarfs. As galaxies deplete their stars, many will be ejected or accreted onto massive black holes. The remnants of stars, such as white dwarfs and neutron stars, will eventually disappear due to proton decay, and galactic black holes will lose mass through Hawking radiation. This gradual process underscores the eventual sublimation of planets and other stellar remnants, dictated by the decay of their constituent nucleons.

Destruction of Gas Giant Planets by Evolving Stars

Gas giant planets, particularly those orbiting A-type stars, face significant risks of destruction. The majority of A-type main-sequence stars have stellar binary companions, which can strongly affect the dynamical evolution of their planets. Gravitational perturbations from these companions can lead to the formation of Temporary Hot Jupiters (THJs) during the post-main-sequence lifetime of the stars. These THJs exist for only a few hundred thousand years before being engulfed by their host stars. Overall, approximately 70% of gas giant planets orbiting A-type stars will eventually be destroyed or engulfed by their stars, with about 25% being destroyed during the main-sequence lifetime and about 45% during post-main-sequence evolution.

Planetary Systems in Star Clusters

In young and dense star clusters, the dynamics of planetary systems are influenced by interactions with other stars. Simulations of the Orion Trapezium star cluster show that planets can become unbound from their host stars due to strong encounters with other stars or internal planetary scattering. These free-floating planets may either leave the cluster or remain bound as intra-cluster planets. The probability of a star losing a planet is independent of the planet's mass and its initial orbital separation, leading to a mass distribution of free-floating planets that mirrors that of bound planets.

Case Study: Kepler-91b

The giant star Kepler-91 provides a concrete example of a planet nearing the end of its life. Kepler-91b, a planet orbiting this star, is on an eccentric orbit that brings it perilously close to the stellar atmosphere. It is estimated that Kepler-91b will be engulfed by its host star in less than 55 million years. This case highlights the intimate link between the evolution of planetary systems and their host stars, particularly as stars ascend the Red Giant Branch.

Conclusion

Stars play a significant role in the fate of their planetary systems, particularly as they evolve and undergo mass loss. The dynamics of planetary ejection, the long-term evolution of the universe, and the specific cases of gas giant planets and star clusters all illustrate the complex interactions that can lead to the destruction or ejection of planets. As our understanding of these processes deepens, it becomes clear that stars, indeed, can be seen as agents in the demise of their planetary companions.