How Jupiters Save or Destroy Inner Neptunes around Evolved Stars

In about 6 Gyr our Sun will evolve into a red giant and finally end its life as a white dwarf. This stellar metamorphosis will occur to virtually all known host stars of exoplanetary systems and is therefore crucial for their final fate. It is clear that the innermost planets will be engulfed and evaporated during the giant phase and that planets located farther out will survive. However, the destiny of planets in-between, at ∼1 and 10 au, has not yet been investigated with a multiplanet tidal treatment. We here combine for the first time multiplanet interactions, stellar evolution, and tidal effects in an N-body code to study the evolution of a Neptune–Jupiter planetary system. We report that the fate of the Neptune-mass planet, located closer to the star than the Jupiter-mass planet, can be very different from the fate of a single Neptune. The simultaneous effects of gravitational interactions, mass loss, and tides can drive the planetary system toward mean motion resonances. Crossing these resonances affects particularly the eccentricity of the Neptune and thereby also its fate, which can be engulfment, collision with the Jupiter-mass planet, ejection from the system, or survival at a larger separation.