[Paper Review] Transiting exoplanets from the CoRoT space mission VIII. CoRoT-7b: the first Super-Earth with measured radius

Alain Léger, Daniel Rouan+52 more|Scientific Electronic Library Online (São Paulo Research Foundation, Latin American and Caribbean Center on Health Sciences Information, Conselho Nacional de Desenvolvimento Científico e Tecnológico)|Aug 3, 2009

Stellar, planetary, and galactic studiesPhysics and Astronomy62 references360 citations

TL;DR

This paper reports the discovery of CoRoT-7b, the first Super-Earth exoplanet with a measured radius of 1.68 ± 0.09 Earth radii, detected via transit photometry from the CoRoT space mission. Using extensive ground-based follow-up observations and rigorous false positive analysis, the authors establish the planet's existence with a false positive risk of only 8 × 10⁻⁴, and confirm it as a rocky world based on radial velocity constraints and orbital parameters.

ABSTRACT

We report the discovery of very shallow (DF/F = 3.4 10-4), periodic dips in the light curve of an active V = 11.7 G9V star observed by the CoRoT satellite, which we interpret as due to the presence of a transiting companion. We describe the 3-colour CoRoT data and complementary ground-based observations that support the planetary nature of the companion. Methods. We use CoRoT color information, good angular resolution ground-based photometric observations in- and out- of transit, adaptive optics imaging, near-infrared spectroscopy and preliminary results from Radial Velocity measurements, to test the diluted eclipsing binary scenarios. The parameters of the host star are derived from optical spectra, which were then combined with the CoRoT light curve to derive parameters of the companion. We examine carefully all conceivable cases of false positives, and all tests performed support the planetary hypothesis. Blends with separation larger than 0.40 arcsec or triple systems are almost excluded with a 8 10-4 risk left. We conclude that, as far as we have been exhaustive, we have discovered a planetary companion, named CoRoT-7b, for which we derive a period of 0.853 59 +/- 3 10-5 day and a radius of Rp = 1.68 +/- 0.09 REarth. Analysis of preliminary radial velocity data yields an upper limit of 21 MEarth for the companion mass, supporting the finding. CoRoT-7b is very likely the first Super-Earth with a measured radius.

Motivation & Objective

• To confirm the existence of a small transiting exoplanet around CoRoT-7 with high confidence, minimizing false positive scenarios.
• To measure the radius of the planet using high-precision CoRoT light curves and ground-based transit follow-up.
• To constrain the planetary mass using radial velocity measurements, enabling composition inference.
• To demonstrate that small, rocky exoplanets can be detected and validated with minimal false positive risk even without a radial velocity detection.
• To establish a methodology for confirming terrestrial exoplanets in future missions like Kepler, especially in cases where radial velocity detection is challenging.

Proposed method

• Utilized CoRoT space telescope photometry to detect transit events in the light curve of CoRoT-7, identifying a periodic dip in flux of ΔF/F ≈ 3.35 × 10⁻⁴.
• Conducted extensive ground-based follow-up observations using multiple telescopes (CFHT, ESO, INT, Anglo-Australian Telescope) to rule out false positive scenarios such as background eclipsing binaries (BEBs).
• Applied a trapezoidal approximation to model the transit light curve and derive the planet's radius and orbital period.
• Performed high-resolution spectroscopy to characterize the host star’s spectral type (G9V) and activity level.
• Used radial velocity measurements from the SOPHIE spectrograph to place an upper limit on the planet’s mass (M_pl < 21 M_earth).
• Evaluated the false positive probability by exhaustively modeling all plausible astrophysical false positive configurations, including close stellar companions and BEBs within 400 mas.

Experimental results

Research questions

• RQ1 Can a small transiting planet be confirmed with high confidence using only transit photometry and ground-based follow-up, without a radial velocity detection?
• RQ2 What is the true radius of the smallest known exoplanet detected by CoRoT, and how precisely can it be measured?
• RQ3 What is the likelihood that the observed transit signal is due to a false positive, such as a background eclipsing binary?
• RQ4 What constraints do radial velocity measurements place on the planet’s mass, and how do they inform its composition?
• RQ5 How does the short orbital period (0.8536 days) and proximity to the star affect the planet’s potential surface temperature and detectability?

Key findings

• CoRoT-7b has a radius of 1.68 ± 0.09 Earth radii, making it the smallest exoplanet with a measured radius at the time of publication.
• The false positive probability for the planetary interpretation is conservatively estimated at 8 × 10⁻⁴, indicating a highly reliable detection.
• The planet’s orbital period is 0.8536 days, the shortest ever detected at the time, corresponding to a semi-major axis of 0.0172 AU.
• The planet’s equilibrium temperature at the substellar point ranges from approximately 1800 K to 2600 K, depending on heat redistribution, indicating a likely molten surface.
• Radial velocity measurements place an upper limit on the planet’s mass of less than 21 Earth masses, consistent with a rocky composition.
• The absence of a significant radial velocity signal from a stellar companion or Jupiter-mass planet further supports the planetary nature of the signal and rules out many false positive scenarios.