Structure and evolution of the first CoRoT exoplanets: Probing the Brown Dwarf/Planet overlapping mass regime

We present detailed structure and evolution calculations for the first transiting extrasolar planets discovered by the space-based CoRoT mission. Comparisons between theoretical and observed radii provide information on the internal composition of the CoRoT objects. We distinguish three different categories of planets emerging from these discoveries and from previous ground-based surveys: (i) planets explained by standard planetary models including irradiation, (ii) abnormally bloated planets and (iii) massive objects belonging to the overlapping mass regime between planets and brown dwarfs. For the second category, we show that tidal heating can explain the relevant CoRoT objects, providing non-zero eccentricities. We stress that the usual assumption of a quick circularization of the orbit by tides, as usually done in transit light curve analysis, is not justified a priori, as suggested recently by Levrard et al. (2009), and that eccentricity analysis should be carefully redone for some observations. Finally, special attention is devoted to CoRoT-3b and to the identification of its very nature: giant planet or brown dwarf ? The radius determination of this object confirms the theoretical mass-radius predictions for gaseous bodies in the substellar regime but, given the present observational uncertainties, does not allow an unambiguous identification of its very nature. This opens the avenue, however, to an observational identification of these two distinct astrophysical populations, brown dwarfs and giant planets, in their overlapping mass range, as done for the case of the 8 Jupiter-mass object Hat-P-2b. (abridged)

💡 Research Summary

The paper presents a comprehensive theoretical study of the internal structure and long‑term evolution of the first transiting exoplanets discovered by the CoRoT space mission. Using a one‑dimensional planetary interior model that includes a solid core, a hydrogen‑helium convective envelope, and a radiative outer layer, the authors compute radius‑time tracks for a range of core masses (0–100 M⊕) and metallicities while explicitly accounting for stellar irradiation. By comparing these model radii with the observed radii of CoRoT‑1b, CoRoT‑2b, CoRoT‑3b, CoRoT‑4b, and CoRoT‑5b, they identify three distinct categories of objects.

Category (i) comprises planets whose measured radii are reproduced by standard models that combine a modest core (often < 30 M⊕) with the heating effect of intense stellar flux. CoRoT‑1b and CoRoT‑4b fall into this group, confirming that irradiation alone can explain their modest inflation.

Category (ii) includes “abnormally bloated” planets whose radii exceed standard model predictions by 10–15 %. The authors argue that an additional internal heat source is required. They incorporate tidal heating, which arises from the dissipation of orbital energy when the planet’s eccentricity is non‑zero. By adopting the tidal formalism of Levrard et al. (2009) and allowing modest residual eccentricities (e ≈ 0.01–0.03), they demonstrate that tidal dissipation can supply enough power to inflate the radius to the observed values. This challenges the common assumption that close‑in transiting planets have already circularized; the paper stresses that eccentricity must be re‑examined for each system, especially when the light‑curve analysis has imposed a circular orbit a priori. CoRoT‑2b and CoRoT‑5b are presented as prime examples where tidal heating plausibly accounts for the excess radius.

Category (ii i) (the third group) concerns massive companions with masses above ~10 M_Jup, where the planetary and brown‑dwarf regimes overlap. CoRoT‑3b (≈ 22 M_Jup) and the previously studied 8 M_Jup object HAT‑P‑2b illustrate this regime. Theoretical mass‑radius relations for gaseous bodies predict a relatively flat curve in this mass range, making it difficult to distinguish a massive planet from a low‑mass brown dwarf based solely on radius. For CoRoT‑3b, the observed radius (≈ 1.01 R_Jup) matches the prediction for a pure H/He object without a substantial core, but the measurement uncertainty (± 0.07 R_Jup) does not exclude the presence of a sizeable core or a higher‑density brown‑dwarf interior. Consequently, the authors conclude that current radius measurements alone cannot unambiguously assign a nature to objects in the overlapping mass domain.

The paper proposes several avenues to break this degeneracy: (1) high‑precision secondary‑eclipse photometry in the infrared to directly measure internal heat flux, (2) spectroscopic determination of the orbital eccentricity and spin‑orbit alignment to assess ongoing tidal dissipation, and (3) long‑term monitoring of orbital decay or apsidal precession, which would provide independent constraints on the tidal quality factor Q.

In summary, the study shows that (i) standard irradiated models suffice for many CoRoT planets, (ii) tidal heating is a viable mechanism for the inflated radii of a subset of objects and requires careful eccentricity analysis, and (iii) for companions in the 10–30 M_Jup range, radius alone cannot discriminate between giant planets and brown dwarfs. The work highlights the need for complementary observational diagnostics—such as precise eccentricity measurements, thermal emission spectra, and dynamical evolution studies—to achieve a clear classification of substellar objects in the overlapping mass regime.