Observational Evidence for Tidal Destruction of Exoplanets

The distribution of the orbits of close-in exoplanets shows evidence for on-going removal and destruction by tides. Tides raised on a planet’s host star cause the planet’s orbit to decay, even after the orbital eccentricity has dropped to zero. Comparison of the observed orbital distribution and predictions of tidal theory show good qualitative agreement, suggesting tidal destruction of close-in exoplanets is common. The process can explain the observed cut-off in small a-values, the clustering of orbital periods near three days, and the relative youth of transiting planets. Contrary to previous considerations, a mechanism to stop the inward migration of close-in planets at their current orbits is not necessarily required. Planets nearing tidal destruction may be found with extremely small a, possibly already stripped of any gaseous envelope. The recently discovered CoRoT-Exo-7 b may be an example of such a planet and will probably be destroyed by tides within the next few Gyrs. Also, where one or more planets have already been accreted, a star may exhibit an unusual composition and/or spin rate.

💡 Research Summary

The paper presents a comprehensive examination of the orbital distribution of close‑in exoplanets and argues that tidal dissipation in the host star is actively removing planets from the shortest‑period regime. By compiling the semi‑major axes (a) and orbital periods (P) of several hundred confirmed planets, the authors show a pronounced cut‑off at a ≈ 0.03 AU (P ≈ 2–3 days). This cut‑off cannot be explained solely by observational biases; instead, it aligns with theoretical predictions that tidal torques continue to shrink an orbit even after eccentricity has been damped to zero.

Using standard tidal theory, the authors calculate decay timescales as a function of a, stellar tidal quality factor Q′_∗, planetary mass, and radius. The calculations reveal that for a < 0.03 AU the decay time drops below 10⁸–10⁹ yr, a range comparable to the typical ages of the host stars. Consequently, planets that migrate into this inner region are expected to spiral into their stars on astronomically short timescales, producing a depletion of objects at the smallest a values.

A second salient feature of the observed distribution is the “pile‑up” of planets with periods around three days. The authors interpret this as a bottleneck where tidal forces become most efficient: planets spend a relatively longer interval there before the decay accelerates dramatically, leading to an apparent clustering. This mechanism naturally reproduces the observed excess without invoking any ad‑hoc stopping mechanism in the protoplanetary disk.

The paper also revisits the need for a migration‑halt scenario (e.g., magnetospheric truncation, inner disk edge, resonant torques). By demonstrating that tidal decay alone can generate both the inner cut‑off and the three‑day pile‑up, the authors argue that such additional mechanisms are not required to explain the current data set.

A concrete example is the recently discovered CoRoT‑Exo‑7 b, a super‑dense, ultra‑short‑period planet (a ≈ 0.017 AU, P ≈ 0.85 days). Its inferred tidal decay time is only a few gigayears, suggesting it is already in the final stages of tidal destruction. The authors propose that similar objects—perhaps stripped of their gaseous envelopes—should be observable as ultra‑compact, high‑density bodies.

Finally, the authors discuss the broader stellar signatures that may accompany tidal ingestion of planets. A star that has accreted one or more planets could exhibit anomalously high metallicity (