How Planets Can Save or Destroy Their Siblings

A star like our Sun is destined to end its main-sequence lifetime by expanding to hundreds of times its current size, evolving into a red giant. But when this apocalypse comes, not all of the star’s planets are doomed! A new study shows how some planets can influence the fates of their siblings.

Surviving the Apocalypse

Six billion years into our future, the Sun will grow in size until its scalding surface reaches roughly the orbit of Earth, engulfing Mercury and Venus in the process. And our own inner planets aren’t the only ones under such a threat — close-in planets around Sun-like stars throughout the universe share this apocalyptic fate.

But while the innermost planets of such systems will certainly be swallowed or vaporized, and the outermost planets will be largely unfazed by the drama in the distant inner solar system, the planets in between these extremes — at orbital distances of a few au — face a more uncertain future.

Now, new research led by María Ronco (Millennium Nucleus for Planet Formation and Pontifical Catholic University of Chile) suggests that the fate of these middle-ground planets depends strongly on their siblings.

Sibling Rivalry

A number of previous studies have explored the conditions under which planets can survive their hosts’ evolution through the red-giant phase to a white dwarf, but these studies have primarily focused on the outcomes for a single planet trying to ride out the apocalypse.

Ronco and collaborators instead explore how two planets — an inner Neptune-mass planet and an outer Jupiter-mass planet, both orbiting within a system at distances of a few au — might affect each other during their host star’s evolution.

With a Little Help

When a star expands to red-giant size, there are two primary effects that can impact planetary orbits: tides induced on the star, which can reduce and circularize the planet orbits, and mass loss from the star, which can expand the planet orbits. But Ronco and collaborators’ models show that when more than one planet is present in the system, the interactions between these planets can have equally important effects on their eventual orbits.

In particular, when both planet orbits are near a mean-motion resonance (i.e., when their orbits have integer ratios, like when one planet orbits twice for every three orbits of its sibling), the Neptune-mass planet is excited to a more eccentric orbit. This has dramatic consequences: for Neptunes that, alone, might have survived the red-giant evolution of their host, the presence of the outer Jupiter can cause them to be engulfed. Similarly, for Neptunes that might have been engulfed alone, the presence of the Jupiter can save them from being swallowed.

Ronco and collaborators’ work demonstrates that when a star evolves through the red-giant phase, the gravitational interactions between its middle-distance planets play an important role in the planets’ survival or destruction. As we continue to explore these influences, we’ll be able to better understand the fates of solar systems like ours and what to expect for planetary architecture around evolved stars.

Citation

“How Jupiters Save or Destroy Inner Neptunes around Evolved Stars,” María Paula Ronco et al 2020 ApJL 898 L23. doi:10.3847/2041-8213/aba35f