Swallowed Worlds: How a Lost Hot Jupiter May Have Spun Up Kepler-56

The red giant star Kepler-56 has puzzled astronomers for years. Recent asteroseismic studies, analyses of a star’s oscillations, show that its outer layers spin much faster than expected and that its core and envelope are tilted at dramatically different angles. In this paper, Takato Tokuno investigates whether this strange spin pattern could be explained if the star once engulfed a massive planet, a “hot Jupiter.”

Kepler-56’s Unusual Spin

Kepler-56 is a metal-rich red giant roughly 1.3 times the mass of the Sun and four times its radius. Two known planets, Kepler-56 b and c, orbit close to the star. Asteroseismic measurements reveal that while the core spins with a 21-day period, the envelope rotates once every 74 days, much faster than other red giants of similar size. Even more surprising, the axes of the core and envelope differ by over 60°, making Kepler-56 the first known star with internal spin misalignment.

Two Possible Spin Histories

Tokuno explores two possible explanations. The first scenario assumes that tidal interactions with Kepler-56 b and c gradually transferred angular momentum (AM) from their orbits to the star, spinning it up. The second scenario proposes that the star once hosted an additional, more massive planet, a hot Jupiter, that spiraled inward and was engulfed, dumping its orbital AM into the star’s outer layers. Because tidal torques grow stronger as a star expands and its outer layers become convective, both processes would have acted most effectively after Kepler-56 left the main sequence and became a red giant.

Modeling Angular Momentum Transfer

Using stellar evolution models, Tokuno estimates how much AM would be needed to make the star’s envelope spin as fast as it does now. He then compares that requirement with the maximum AM that could be provided by the two known planets, given plausible tidal strengths. Unless the tidal efficiency were extraordinarily high, far beyond observed values, the known planets alone could not have transferred enough momentum. In contrast, a single engulfed hot Jupiter with a mass between 0.5 and 2 times that of Jupiter and an orbital period of 1–6 days could easily provide the necessary boost.

Alternative: A Fast Spinner from Birth

If Kepler-56 had already been spinning quickly before it became a red giant, the known planets might still explain the current misalignment. In that case, their tidal forces could have gradually tilted the star’s outer layers while leaving the core largely unaffected. This “rapid initial spin” scenario would require Kepler-56 to have retained high rotation after its main-sequence lifetime, perhaps due to earlier planet engulfment or reduced magnetic braking.

Implications and Broader Context

The analysis favors the hot-Jupiter-engulfment scenario, which fits both the amount of angular momentum needed and the typical population of short-period giant planets around F-type stars. Chemical measurements of Kepler-56 show no unusual surface composition, suggesting that if engulfment occurred, it did not significantly alter the star’s elemental makeup.

Conclusions

Tokuno concludes that Kepler-56 is a strong candidate for having swallowed a massive planet in its past, leaving behind both rapid surface rotation and internal spin misalignment. This study highlights planetary engulfment as a key process shaping how red giants evolve, and hints that other rapidly spinning giants may bear similar hidden histories of lost worlds.

Source: Tokuno