Jupiter, Saturn, Uranus and Neptune will probably destroy each other after the sun runs out of fuel, new simulations hint

When the sun inevitably dies, not even the outer solar system will be safe from the resulting chaos, new research hints.Scientists have long thought the outer solar system ‪—‬ which comprises the four giant planets Jupiter, Saturn, Uranus and Neptune — would mostly retain its present-day appearance for hundreds of billions of years to come. But a new study suggests that the outer giants may develop weirdly shaped orbits as the sun runs out of fuel, causing the planets to cross paths and destroy themselves through collisions within three billion years of the sun's transformation into a red giant.

The findings, published Sept. 21 in The Astrophysical Journal Letters, could change our understanding of what kinds of exoplanets orbit certain stars.

In about 5 billion years, the sun will balloon outward as a red giant after exhausting all of the hydrogen in its core. During this red giant phase, our star will swell to 100 times its present-day size, gobbling up Mercury, Venus and possibly even Earth in the process, Nathan Kaib, a planetary scientist at the University of Oklahoma who wasn't involved in the new research, told Live Science in an email. Following this violent swelling, the sun will crumple into a collapsed core called a white dwarf.

Until now, scientists thought the four outer planets would escape this phase relatively unscathed. A 1998 simulation-based study suggested that the gas the sun spews from its outer layers will push this planetary quartet outward, doubling the radius of each planet's orbit. Only the gravitational impact of a runaway star could destabilize this new order, and such events are thought to occur only once every billion billion years.

Solar recoil

These predictions hinged on the assumption that the sun will eject its outer layers smoothly. However, recent observations of stars suggest a jerkier loss of mass, with gas clouds lofted by plumes of plasma in the sun — as though our star were spitting out mass blobs. "They're kind of like coronal mass ejections, except much larger," Jim Fuller, a professor of theoretical astrophysics at Caltech and co-author of the new study, told Live Science in an email.

Each "blob" pushes the star backward, similar to how a fired bullet causes a gun to recoil. Although every push is small, moving the sun just a few feet per second, many pushes can, over time, impart the equivalent of an enormous kick that alters the sun's speed by several hundred feet per second. In fact, astronomers have already seen white dwarfs suffer such kicks in other star systems.

When Fuller, who studied these kicks, described them to Konstantin Batygin, a professor of planetary science at Caltech and the study's first author, Batygin "realized this would have important consequences for the evolution of our own solar system," Fuller said. That's because the kicks change the sun's gravitational pulls on everything around it.

Chaos at the edge of town

To determine how the sun's mass loss would influence the giant planets, Batygin created computer-based simulations of the sun and the outer planets. The researchers considered three scenarios: if the sun lost its mass smoothly, if it spit out distinct mass blobs during its entire time evolving as a red giant, and if the sun spewed the blobs only during its last stages. The researchers also varied each ejected blob's mass.

The first scenario produced results similar to those of previous studies, with the outer solar system retaining almost all of its features for billions of years. In contrast, the other two cases had drastically different results. Using 672 simulations, the researchers found that if the sun lost mass in blobs, the giant planets would be thrown into chaos, switching places, developing weirdly oval-shaped orbits, and even getting ejected out of the solar system entirely. Most dramatically, in 96% of the simulations with the heaviest ejected mass packets, the planets get destroyed as they crashed into each other.

These developments also happened much more rapidly — less than a few hundred million years after the sun transformed into a white dwarf, or even during the red giant phase, in the most extreme scenarios.

The study's findings hint that analogues of the present-day outer solar system around white dwarf stars are less likely to exist than previously thought. Kaib noted that the study's results suggest that planets with oval-shaped orbits should dominate instead. It's thought that habitable planets could exist around white dwarfs, but this study's findings show that our understanding of those distant star systems may be incomplete, and that such worlds may, in reality, be unstable.

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