After billions of years in harmony, distant star system will end in chaos

After billions of years in harmony, distant star system will end in chaos
Artist's impression of the HR 8799 system
Artist's impression of the HR 8799 system
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Artist's impression of the HR 8799 system
Artist's impression of the HR 8799 system

A team of astronomers has used computer modeling to reveal the chaotic fate of a distant star system in which the planets orbit in near perfect synchronization with one another. The study also sheds light on how ancient white dwarf stars that are threaded throughout our galaxy become polluted with cosmic debris.

The star system HR 8799 is located 135 light years from Earth in the constellation of Pegasus. At the heart of the system lies a 30 - 40 million-year-old A-type star, around which orbit a pair of debris fields and four massive planets, each of which is over five times the size of Jupiter.

Over the course of millions of years the gravitational influence of the parent star and that of each of the planets has caused the four alien worlds to fall into a delicately synchronized orbital pattern known as a resonance.

In HR 8799, the third planet out from the star completes two full orbits in the time it takes the outermost planet to complete one. This pattern continues as we get closer to the star, with the second closest planet to the star completing four orbits and the innermost world completing eight in the time that it takes for the outermost to complete one. In other words each successive world completes double the orbits of its outer neighbor.

A team of scientists from the University of Exeter and the University of Warwick in the UK set out to determine the eventual fate of this unusual system, and see what factor would eventually disturb its distinctive orbital resonance pattern.

To this end the team created an advanced computer model of HR 8799. The simulation took into account the gravitational influence of the five major star system bodies along with the usual external gravitational forces that exert influences over the planets in a star system, including galactic tides and close passes from other stellar bodies.

The team concluded that the resonance is likely to continue for the next three billion years, and that external influences are rarely powerful enough to break the orbital resonances of the star system planets. However, the data also showed that the resonance will certainly come to an end when the parent star, having exhausted its internal supply of hydrogen, begins the process of transforming into a massive red giant.

During this process the star casts off an enormous amount of mass, which alters its gravitational characteristics and throws the surrounding system into chaos. At this point the orbits of the surrounding planets will begin to shift as they react to the changing star and, with the resonance broken, the gravitational influences of their neighboring worlds.

"They are so big and so close to each other the only thing that's keeping them in this perfect rhythm right now is the locations of their orbits,” explains lead author of the new study Dr Dimitri Veras from the University of Warwick. “All four are connected in this chain. As soon as the star loses mass their locations will deviate, then two of them will scatter off one another, causing a chain reaction amongst all four."

According to the computer modeling, this chaotic process will usually result in two of the planets being cast out from the system altogether. The remaining two would then likely settle into orbits anywhere from 1 AU (Astronomical Unit) to thousands of AU from the newly transformed red star.

The simulations also help explain why astronomers have observed unexpected debris in the light signatures of white dwarf stars, which are a later evolution of red giant stars. According to the study authors, the chaotic movement of the planets would likely dislodge material from debris rings and cause it to tumble inwards and later be absorbed.

The paper has been published in the Monthly Notices of the Royal Astronomical Society.

Source: University of Warwick

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