Evolution of the ZTF SLRN-2020 star-planet merger

We model the optical and infrared transient ZTF SLRN-2020, previously associated with a star-planet merger. We consider the scenario in which orbital decay via tidal dissipation led to the merger, and find that tidal heating within the star was likely unobservable in the archival image of the system taken $12\mathrm{yr}$ before the merger. The observed dust formation months before the merger is consistent with a planet of mass $M_\mathrm{p} \gtrsim 5M_\mathrm{J}$ ejecting material as it skims the stellar surface. This interaction gradually intensifies, leading to significant mass ejection on a dynamical timescale ($ \approx $ hours) as the planet plunges into the stellar interior. Part of the recombination transient associated with this dynamical mass ejection might be inaccessible to the optical observations because its duration ($ \approx $ hours) is comparable to the cadence. Correspondingly, the observed duration of the transient $\approx100\mathrm{d}$ is inconsistent with a single episode of dynamical mass ejection. Instead, the transient could be powered by the recombination of $ \approx 3.4\times10^{-5}M_\odot $ of hydrogen in an outflow, or the contraction of an inflated envelope of mass $ \approx 10^{-6}M_\odot $ that formed during the merger. The observed ejecta mass $320\mathrm{d}$ after the peak of the optical transient is $ \approx 1.3\times10^{-4}M_\odot$, consistent with the idea that a fraction of the ejecta might be unobservable in the light curve. Energetically, this post-merger ejecta mass suggests a planet at least as massive as Jupiter. Our results suggest that ZTF SLRN-2020 was the result of a merger between a star close to the main sequence and a planet with mass at least several times that of Jupiter.

💡 Research Summary

The authors present a comprehensive physical model for the optical and infrared transient ZTF SLRN‑2020, interpreting it as the merger of a near‑main‑sequence star with a massive giant planet. The paper is organized into six sections, beginning with a review of the theoretical expectation that a substantial fraction of close‑in exoplanets will eventually merge with their host stars due to tidal dissipation, stellar expansion, or dynamical interactions. The authors note that, prior to this work, observational evidence for such mergers has been indirect; ZTF SLRN‑2020 provides the first real‑time detection.

Section 2 summarizes the multi‑wavelength observations. Archival near‑infrared images taken 12 years before the outburst reveal a faint progenitor consistent with a 0.8–1.5 M⊙ star of radius 1–4 R⊙, i.e., a star on or just off the main sequence. No optical counterpart was detected in earlier surveys, implying that any pre‑outburst brightening was below detection thresholds. Starting roughly 200 days before the optical peak, a gradual infrared brightening and dust formation were observed, with inferred gas‑plus‑dust masses rising from ≈ 2.8 × 10⁻⁵ M⊙ at –244 days to ≈ 10⁻⁴ M⊙ at –44 days (assuming a dust‑to‑gas ratio of 10⁻²). The optical light curve rose sharply over ~10 days to a peak luminosity of 1.3 × 10³⁵ erg s⁻¹, remained near that level for ~25 days, and then declined by an order of magnitude over the next 150 days, releasing a total radiated energy of ≈ 6 × 10⁴¹ erg. Infrared observations at 320 days post‑peak indicate a total ejecta mass of log Mₑⱼ = –3.89 ± 0.3, i.e., ≈ 1.3 × 10⁻⁴ M⊙, expanding at ~35 km s⁻¹. Late‑time (830 days) JWST and Gemini spectroscopy reveal a warm (~700 K) gas component of ≈ 10⁻⁹ M⊙, with CO and Br α emission suggesting a nascent accretion disk.

Section 3 models the pre‑merger orbital evolution under tidal dissipation. The authors adopt the standard equilibrium tide formalism, expressing the tidal energy dissipation rate as a function of the modified stellar tidal quality factor Q′★, the planetary mass Mₚ, and the orbital separation a. They explore a broad range of Q′★ (10⁴–10⁸) and also an empirical scaling Q′★ ∝ P_orb^α based on recent population studies (e.g., Penev et al. 2018). For Q′★ ≲ 10⁶ (Mₚ/M_J)², the tidal heating rate can rival the star’s intrinsic luminosity when the orbit shrinks to a ≈ R★, but the heating is deposited deep in the stellar interior. The thermal response time τ_heat, estimated from the stellar binding energy, is orders of magnitude longer than the 12‑year interval between the archival image and the outburst, explaining why no pre‑outburst brightening was seen.

Section 4 addresses the interaction once the planet grazes the stellar surface. The authors argue that the observed dust formation months before the optical peak is naturally produced by “skimming” – the planet’s passage through the tenuous outer layers of the star, which strips and ejects stellar material. This process can continuously feed a dusty outflow, consistent with the measured increase in ejecta mass. As the planet spirals deeper, the interaction intensifies, culminating in a dynamical plunge on a timescale of a few hours. Hydrodynamic simulations from previous work (e.g., Sandquist et al. 2002; Kramer et al. 2020) predict a rapid mass ejection of order 10⁻⁴ M⊙, whose recombination of hydrogen would generate a bright, short‑lived optical transient. However, the observed ~100‑day duration of ZTF SLRN‑2020 cannot be explained by a single dynamical event. The authors therefore propose two additional power sources: (1) recombination of a modest hydrogen mass (≈ 3.4 × 10⁻⁵ M⊙) released over weeks to months, providing a sustained luminosity; and (2) the contraction of an inflated envelope of ≈ 10⁻⁶ M⊙ that was deposited during the merger, releasing gravitational energy on the Kelvin–Helmholtz timescale. Both mechanisms can extend the light curve to the observed duration.

Section 5 compares ZTF SLRN‑2020 to luminous red novae (LRNe), which are stellar‑stellar mergers. LRNe typically exhibit ejecta masses of 10⁻²–10⁻¹ M⊙ and radiated energies of 10⁴⁴–10⁴⁵ erg, orders of magnitude larger than ZTF SLRN‑2020. The much smaller mass and energy budget of ZTF SLRN‑2020 thus points to a planetary companion rather than a stellar one. The authors also discuss the role of opacity, dust formation, and viewing angle in shaping the observed light curves, noting that a fraction of the ejecta may be hidden from optical view, consistent with the larger mass inferred from late‑time infrared observations.

Section 6 combines the energetics to place a lower limit on the planetary mass. Assuming the 320‑day ejecta mass originates entirely from the dynamical plunge, the kinetic energy required (≈ 10⁴² erg) implies a planet of at least one Jupiter mass. However, to account for the pre‑outburst dust production, the observed recombination luminosity, and the need for a substantial dynamical mass ejection, the authors argue that the planet must have been more massive, with Mₚ ≳ 5 M_J. This mass range comfortably supplies the required orbital energy (≈ 10⁴⁵ erg) and angular momentum to spin up the star, while remaining consistent with the lack of detectable pre‑outburst heating.

In the concluding section, the authors summarize their scenario: a ~5 M_J planet on a close orbit around a ~1 M⊙ star experiences tidal decay over Myr timescales, eventually grazing the stellar surface months before merger, producing dust and a modest infrared brightening. The final plunge triggers a rapid, dynamical mass ejection whose recombination and subsequent envelope contraction power a ~100‑day optical transient. The observed late‑time infrared emission reflects both the initial dynamical ejecta and additional material launched during the skimming phase. This work provides the first quantitative, multi‑stage model for a star‑planet merger transient, establishing observational diagnostics (dust formation timescale, ejecta mass, light‑curve duration) that can be used to identify future events and to constrain the masses of merging planets. The authors suggest that systematic searches in time‑domain surveys, combined with rapid infrared follow‑up, will likely uncover many more such events, opening a new window onto the final fate of close‑in giant planets.