[Paper Review] Transiting exoplanets from the CoRoT space mission VI. CoRoT-Exo-3b: The first secure inhabitant of the brown-dwarf desert

M. Deleuil, H. J. Deeg+3 more|ArXiv.org|Oct 6, 2008

Stellar, planetary, and galactic studiesPhysics and Astronomy37 references136 citations

TL;DR

This paper reports the discovery and characterization of CoRoT-Exo-3b, a massive transiting substellar object with a 4.26-day orbital period around an F3V star, using high-precision CoRoT photometry combined with ground-based radial velocity and spectroscopic follow-up. With a mass of 21.66 ± 1.0 M_Jup and a density of 26.4 ± 5.6 g cm⁻³, it lies in the brown-dwarf desert and challenges planetary formation theories, suggesting it may be either a low-mass brown dwarf or a new class of superplanet.

ABSTRACT

Context. The CoRoT space mission routinely provides high-precision photometric measurements of thousands of stars that have been continuously observed for months. Aims. The discovery and characterization of the first very massive transiting planetary companion with a short orbital period is reported. Methods. A series of 34 transits was detected in the CoRoT light curve of an F3V star, observed from May to October 2007 for 152 days. The radius was accurately determined and the mass derived for this new transiting, thanks to the combined analysis of the light curve and complementary ground-based observations: high-precision radial-velocity measurements, on-off photometry, and high signal-to-noise spectroscopic observations. Results. CoRoT-Exo-3b has a radius of 1.01+-0.07 RJup and transits around its F3-type primary every 4.26 days in a synchronous orbit. Its mass of 21.66+-1.0 MJup, density of 26.4+-5.6 g cm^-3, and surface gravity of log g = 4.72 clearly distinguish it from the regular close-in planet population, making it the most intriguing transiting substellar object discovered so far. Conclusions. With the current data, the nature of CoRoT-Exo-3b is ambiguous, as it could either be a low-mass brown-dwarf or a member of a new class of "superplanets". Its discovery may help constrain the evolution of close-in planets and brown-dwarfs better. Finally, CoRoT-Exo-3b confirms the trend that massive transiting giant planets (M >= 4 MJup) are found preferentially around more massive stars than the Sun.

Motivation & Objective

• To characterize the first massive transiting substellar companion detected by the CoRoT space mission.
• To resolve the ambiguous nature of CoRoT-Exo-3b, which lies in the mass range between planets and brown dwarfs.
• To investigate whether massive close-in companions preferentially form around more massive host stars.
• To assess whether CoRoT-Exo-3b represents a new class of superplanets or a low-mass brown dwarf.
• To examine the implications of its high density and synchronous orbit for planetary system formation and evolution.

Proposed method

• High-precision photometric monitoring of the CoRoT space mission collected 152 days of continuous light curves for the target star.
• 34 transits were detected in the CoRoT light curve, enabling accurate determination of the planet's radius (1.01 ± 0.07 R_Jup).
• Ground-based follow-up included high-precision radial velocity measurements to derive the planet's mass (21.66 ± 1.0 M_Jup).
• On-off photometry and high signal-to-noise spectroscopy were used to confirm planetary transit and rule out stellar blends.
• Combined light curve and radial velocity modeling enabled the derivation of orbital parameters, surface gravity (log g = 4.72), and density.
• Statistical analysis of the mass function and orbital distribution of close-in planets was used to contextualize CoRoT-Exo-3b within the broader population.

Experimental results

Research questions

• RQ1 What is the true nature of CoRoT-Exo-3b—planet or brown dwarf—given its mass and density?
• RQ2 Why is CoRoT-Exo-3b located in the so-called 'brown-dwarf desert' despite its high mass?
• RQ3 Does the discovery of CoRoT-Exo-3b challenge the conventional definition of a planet based on the deuterium burning limit?
• RQ4 Are massive close-in companions like CoRoT-Exo-3b preferentially found around F-type stars with higher masses than the Sun?
• RQ5 Could CoRoT-Exo-3b represent a new formation pathway distinct from the known population of eccentric, massive planets?

Key findings

• CoRoT-Exo-3b has a radius of 1.01 ± 0.07 R_Jup and a mass of 21.66 ± 1.0 M_Jup, making it one of the most massive known transiting exoplanets.
• Its high density of 26.4 ± 5.6 g cm⁻³ and surface gravity of log g = 4.72 distinguish it from typical gas giants and suggest a highly compact, possibly rocky or degenerate core structure.
• The planet orbits its F3V host star every 4.26 days in a synchronous, circular orbit, indicating strong tidal evolution.
• CoRoT-Exo-3b lies in the so-called 'brown-dwarf desert'—a region of the mass function where brown dwarfs are rarely found—making it a rare and significant discovery.
• The object's mass and orbital characteristics suggest it may be a low-mass brown dwarf or a new class of superplanet, challenging current formation models.
• The discovery supports the trend that massive planets (M ≥ 4 M_Jup) are preferentially found around stars with masses ≥ 1.1 M_☉, particularly F-type stars.