Our Sun, like any other low-mass star, will undergo a metamorphosis within about 6 billion years, approximately twice its current age, transforming itself into a white dwarf. But first, it will go through the state of a Red Giant, during which it will expand to approximately the Earth’s orbit and lose about 25% of its mass. During this stage, the planets that accompany it will suffer the consequences of this transformation: the innermost ones, such as Mercury and Venus, will be the most damaged since the Sun will swallow them, while the outermost planets such as Jupiter, Saturn, Uranus and Neptune will only suffer an increase of their orbits. However, the fate of planets in intermediate regions, such as Earth or Mars, is not so clear.
In a recent work led by Dr. María Paula Ronco, postdoctoral researcher of Núcleo Milenio de Formación Planetaria, together with Dr. Matthias Schreiber, deputy director of the NPF, they investigated this scenario, particularly for a hypothetical planetary system formed by a planet of the mass of Jupiter and another, closer in, of the mass of Neptune. Prior to this investigation, other investigations had been made but never had more than one planet been considered simultaneously.
To carry out this research, numerical simulations were developed using an N-body code, which allows the computation of gravitational interactions between all the planets. The novelty of this work is that it takes into account the stellar evolution, as well as the impact of stellar tides on the orbits of the planets. This research, in which Dr. Jorge Cuadra and Dr. Octavio Guilera, associate and partner researchers of the NPF, respectively, also participated, will be published in the prestigious Astrophysical Journal Letters.
“When stars evolve and expand into red giants, they lose mass. As the mass of the star decreases, so does the force of gravitational attraction that it exerts on the planets that surround it, and consequently their orbits expands farther out. On the other hand, the radius of the star increases in this stage and stellar tides are produced, which, on the contrary, results in an orbital decay that brings the planets closer to the star. The competition between these two phenomena is crucial to determine the survival of a planet to the red giant stage,” says Ronco.
The scientist explains that they found several scenarios that show that the gravitational interactions between the planets are extremely important in these processes and that they can drastically change the fate of each of the planets studied together. “In some cases our simulations showed that both planets survive the red giant stage, in others we found that both planets are swallowed up by the central star, and in a few we found that the Neptune-like planet collides with the Jupiter-like planet or is ejected from the planetary system,” she adds.
However, the most interesting results are those that show a Jupiter as a hero or a villain. On the one hand, the team found simulations in which the Jupiter-like planet is capable of saving Neptune from being swallowed up by an evolved Sun, dispersing it into a more distant orbit, even though the fate of that Neptune would have been inevitably its destruction if it had been studied in isolation. But, on the other hand, simulations were also found where Neptune, which would have survived the red giant stage if being the only planet, sees its future doomed by the effect of Jupiter pushing it towards the central star.
Another interesting result that emerges from the analysis of the simulations, is that those Neptune-like planets that managed to survive the red giant stage, acquired moderate to high eccentricities at the end of this stage, due to the gravitational interactions with the planet Jupiter. “This population of eccentric Neptunes may be key in the study of the heavy metal contamination that is repeatedly observed in the atmospheres of many white dwarfs since they may be responsible for the dispersion of small bodies towards the regions close to the star, although the study of red giant to white dwarf has not been included in these simulations”, comments Matthias Schreiber, who is also an academic at the Institute of Physics and Astronomy of the University of Valparaiso.
“As a future work, we are interested in studying what happens if we consider planets and stars of different masses. Furthermore, at the moment we only considered planetary systems formed by two planets, but nature tells us that multiple planetary systems would be the norm. In that case, the gravitational interactions in multiple systems become more complex and could further affect the final destinies of the planets that form them. Finally, we also want to study what will happen to these planetary systems from the red giant to the white dwarf stage”, concludes Ronco.