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WASP-12b

WASP-12b is an extrasolar planet classified as a hot Jupiter, orbiting the F-type star WASP-12 every 1.091 days at a distance of 0.0234 AU, with a mass of 1.47 Jupiter masses, a radius of 1.965 Jupiter radii, and an equilibrium temperature of 2593 K.^[1] It was discovered in 2008 using the transit method and lies approximately 1,400 light-years away in the constellation Auriga.^[1]^[2] Due to its extremely close orbit, WASP-12b experiences intense tidal forces from its host star, distorting the planet into an egg-like shape and causing its orbit to decay at a rate of 30 milliseconds per year, as confirmed by recent observations (as of 2025).^[2]^[3] This inward spiral, driven by gravitational interactions, is expected to lead to the planet's engulfment by its star within about 3 million years, potentially leaving behind a rocky remnant.^[4] The star WASP-12 has a mass of 1.325 solar masses, a radius of 1.69 solar radii, and an effective temperature of 6265 K.^[1] WASP-12b's atmosphere is notably carbon-rich, with a carbon-to-oxygen ratio exceeding 1—higher than Earth's ratio of 0.5—marking it as the first confirmed carbon-dominant exoplanet.^[5] This composition suggests possible diamond or graphite interiors rather than silicate rocks, and observations indicate excess methane with minimal water vapor.^[5] Additionally, the planet's dayside is pitch-black, absorbing over 94% of incoming visible light due to hydrogen-dominated absorption, with temperatures reaching 4,600°F that prevent cloud formation.^[6] The nightside, cooler by over 2,000°F, shows signs of water vapor, clouds, and hazes.^[6] Recent observations (as of 2025) continue to confirm the rapid orbital decay and reveal details of its extreme atmosphere. These extreme conditions highlight WASP-12b as a key example of planetary destruction and atmospheric extremes in close-in exoplanets.

Discovery and Observation

Discovery

WASP-12b was discovered in April 2008 by the Wide Angle Search for Planets (SuperWASP) project, a ground-based photometric survey designed to detect transiting exoplanets around bright stars. The detection occurred through the transit method, utilizing time-series photometry from the SuperWASP-N camera array on La Palma, Canary Islands, which identified periodic dips in the brightness of the host star WASP-12 with a period of approximately 1.09 days.^[7]^[8] The initial candidate was confirmed as a planetary transit via follow-up observations, including radial velocity measurements obtained with the SOPHIE high-resolution spectrograph mounted on the 1.93-meter telescope at Observatoire Haute-Provence in France. These measurements revealed sinusoidal variations in the star's radial velocity with an amplitude consistent with a massive planetary companion, ruling out astrophysical false positives such as eclipsing binaries. Additional photometric follow-up with telescopes like the 0.81-meter Tenagra II and the 2-meter Liverpool Telescope further validated the transit signal.^[7]^[8] The discovery was formally announced in a 2009 paper by Hebb et al. published in The Astrophysical Journal, marking WASP-12b as the hottest known transiting exoplanet at the time. Early analyses from the transit light curves and radial velocity data yielded initial estimates of the planet's mass as $1.41 \pm 0.10 Jupiter masses and radius as $1.79^{+0.09}_{-0.09} Jupiter radii, highlighting its status as an inflated hot Jupiter.^[7]^[8]

Observational History

Following its discovery, transmission spectroscopy observations of WASP-12b were conducted using the Hubble Space Telescope (HST) in 2010, which detected metals in the exosphere indicative of atmospheric mass loss due to the planet's extreme irradiation. Contemporaneous modeling of ground-based transit light curves provided evidence of tidal distortion, supporting a prolate planetary shape with the light curve deviating by approximately 10% from a spherical model and confirming strong tidal interactions.^[9]^[10] In 2011, the Spitzer Space Telescope performed infrared observations of WASP-12b's thermal phase variations at 3.6 and 4.5 μm, detecting significant thermal emission from the dayside with a phase offset suggesting inefficient heat redistribution. These measurements indicated a low Bond albedo of less than 0.1, consistent with a dark, absorbing atmosphere dominated by thermal re-emission rather than reflection. The full-orbit phase curve showed a dayside brightness temperature of about 2900 K, highlighting the planet's extreme thermal environment.^[11] Ground-based and space-based campaigns continued with Transiting Exoplanet Survey Satellite (TESS) transits in sectors 4 (2019) and 20 (2019–2020), which refined transit timing variations and confirmed the planet's orbital decay at a rate of -29.4 ± 1.4 ms yr⁻¹. These observations, combined with prior data, improved constraints on the orbital period to 1.09142013 ± 0.00000025 days, providing evidence of ongoing tidal inspiral.^[12]^[13] A 2025 analysis by Winn and Stefansson incorporated new transit times from the Agnes (AG) Optical telescope, along with archival data, to further refine the orbital ephemeris and decay rate to -29.81 ± 0.94 ms yr⁻¹. This study emphasized cumulative ground-based monitoring to mitigate space-based gaps, enhancing precision in timing predictions. Over time, parameter uncertainties have narrowed; for instance, the planetary radius estimate evolved from an initial 1.79 ± 0.09 Rⱼ in 2009 to 1.965 ± 0.020 Rⱼ by 2025, based on integrated HST, Spitzer, TESS, and ground-based photometry.^[3]^[14]

Host Star

Stellar Properties

WASP-12 is a late F-type main-sequence star classified as spectral type F5, characterized by its yellow-white appearance and relatively high temperature for a star of its class. The star has an effective temperature of approximately 6265 K, which places it hotter than the Sun and contributes to its classification as an F-type dwarf. With a mass of 1.325 solar masses and a radius of 1.69 solar radii, WASP-12 is somewhat more massive and larger than the Sun, reflecting its position on the main sequence where it fuses hydrogen in its core. Its surface gravity is log g = 4.11, consistent with a main-sequence star of this mass and radius.^[15] The star exhibits slightly metal-rich composition, with a metallicity of [Fe/H] = 0.00 ± 0.08, indicating an abundance of elements heavier than hydrogen and helium similar to solar levels.^[15] This metallicity influences the star's evolution and atmospheric properties. WASP-12 is estimated to be about 2.3 billion years old, placing it in a mature phase of its main-sequence lifetime.^[16] Its luminosity is approximately 4.0 times that of the Sun, derived from its temperature and radius, which drives significant irradiation of its close-in planetary system.^[1] Located in the constellation Auriga, WASP-12 lies at a distance of about 1390 light-years from Earth, corresponding to a parallax measurement from Gaia DR3 observations.^[1] The star's apparent visual magnitude is V = 11.57, making it faint enough to require telescopes for observation but accessible to ground-based surveys that led to the detection of its transiting planet. These properties position WASP-12 as a typical host for hot Jupiters, with its evolutionary stage influencing the dynamical interactions within the system.

Binary Companion

The host star WASP-12A forms a hierarchical triple system with a binary companion, WASP-12BC, consisting of two low-mass red dwarf stars. High-resolution lucky imaging observations first identified a candidate companion at a projected separation of approximately 1.2 arcseconds in 2011, with subsequent adaptive optics imaging in 2013 resolving it as a close binary pair separated by about 84 mas (approximately 36 AU).^[17]^[18] Spectroscopic analysis of the companions' near-infrared spectrum, obtained using Hubble Space Telescope imaging in 2012, confirms they are M3V red dwarfs with masses of 0.38 ± 0.05 M⊙ and 0.37 ± 0.05 M⊙, respectively.^[19]^[18] These masses place the companions in the low-mass stellar regime, consistent with their spectral types and the dilution effects observed in previous photometric data of the WASP-12 system. Radial velocity monitoring of the primary star has ruled out additional massive companions (≥5 M_J) within ~8 AU but provides no detectable signal from the distant WASP-12BC due to their wide separation and long orbital period, estimated at thousands of years.^[18]^[19] The projected separation of the WASP-12BC binary from the primary corresponds to ~510 AU at the system's distance of 427 pc, forming a stable hierarchical configuration.^[18]^[1] This wide binary can induce long-term gravitational perturbations on the inner orbit of WASP-12b, potentially exciting small eccentricities (e ~ 0.01–0.05) through secular interactions, which may contribute to the observed transit timing variations and orbital evolution.^[20] Such dynamics highlight the role of the companion in the overall stability of the planetary system, though detailed modeling is required to quantify its impact amid dominant tidal effects.^[21] As of 2025, direct imaging has firmly established the companions' existence, with Gaia DR3 astrometry providing precise proper motions and parallaxes that confirm shared space motion, supporting their bound nature within the triple system (proper motion difference <1 mas yr⁻¹, consistent with orbital binding at >99% probability).^[22] No closer unresolved companions have been detected via these astrometric constraints, limiting potential perturbers to separations beyond ~1000 AU.^[23]

Planetary Characteristics

Physical Parameters

WASP-12b has a mass of 1.47 ± 0.08 Jupiter masses, determined through radial velocity measurements and transit timing analysis.^[24] Its radius measures 1.90 ± 0.03 Jupiter radii, derived from high-precision photometry that accounts for the planet's inflated envelope.^[16] These parameters yield a mean density of 0.33 ± 0.04 g/cm³, among the lowest for transiting hot Jupiters, reflecting extensive atmospheric inflation from stellar irradiation.^[1] As an ultrahot Jupiter, WASP-12b exhibits a global equilibrium temperature of approximately 2580 K, calculated assuming zero Bond albedo and efficient heat redistribution.^[1] Recent multi-wavelength observations reveal a dayside brightness temperature averaging around 2870 K.^[16] Tidal forces from the host star distort WASP-12b into a prolate, egg-like shape, with modeling showing a significant bulge along the star-planet axis.^[10] This deformation arises primarily from tidal locking and rapid orbital motion, rather than internal rotation. Recent phase-curve analysis measures the tidal Love number h₂ = 1.55^{+0.45}_{-0.49}, confirming significant prolate deformation consistent with models of the planet's interior.^[16] The planet's geometric albedo is very low, with an upper limit of less than 0.064 in the visible spectrum from spectral eclipse photometry, implying it absorbs over 94% of incident starlight and appears nearly pitch-black.^[25]

Atmospheric Composition

The atmosphere of WASP-12b is characterized by a carbon-rich composition, with a carbon-to-oxygen (C/O) ratio greater than 1, as inferred from multi-wavelength photometry of its dayside emission spectrum. This supersolar C/O ratio implies dominance of carbon-bearing molecules such as carbon monoxide (CO) over oxygen-bearing ones like water (H₂O) and methane (CH₄), leading to reduced abundance of CH₄ and enhanced CO in atmospheric models. Such a composition suggests the possible formation of exotic features, including graphite or diamond particles in the interior beneath the gaseous layers, potentially manifesting as diamond rain, or carbon-based hazes that contribute to optical opacity in the upper atmosphere.^[26]^[27] Transmission spectroscopy observations have revealed the presence of water vapor in the terminator region, alongside potential signatures of metal oxides such as titanium oxide (TiO) and vanadium oxide (VO). These oxides act as efficient absorbers of stellar radiation in the optical wavelengths, facilitating heat transport from the dayside to the nightside and contributing to the planet's inflated radius by enhancing atmospheric redistribution of energy. However, the exact abundance of TiO and VO remains debated, with some analyses indicating depleted TiO due to aerosol formation or cold trapping on the cooler nightside.^[28]^[29]^[27] General circulation models (GCMs) of WASP-12b's atmosphere predict strong dynamical features, including equatorial superrotating winds reaching speeds of a few kilometers per second, driven by the intense stellar irradiation and short rotation period. These winds result in a significant day-night temperature contrast exceeding 1000 K, with the dayside reaching brightness temperatures around 2900 K while the nightside remains cooler due to inefficient heat recirculation. The models align with phase-curve observations showing low albedo and poor global heat transport, emphasizing the role of radiative and advective processes in shaping the atmospheric dynamics.^[30]^[31]

Orbital Dynamics

Orbital Parameters

WASP-12b orbits its F-type host star at a very close distance, completing one revolution every 1.09142225 ± 0.00000014 days, as determined from extensive transit timing analysis.^[32] This short orbital period places the planet in a hot Jupiter configuration, with a semi-major axis of 0.02317 ± 0.00014 AU, calculated from the scaled semi-major axis a/R_\star and stellar radius measurements.^[32] The orbit is highly inclined relative to the sky plane, with an inclination of 83.31 ± 0.14 degrees derived from transit geometry, indicating a near-edge-on view that enables deep transits.^[32] Transit observations reveal a duration of approximately 3.00 hours from first to fourth contact and an optical depth of about 1.5%, corresponding to a planetary radius ratio R_p/R_\star \approx 0.121.^[32] Radial velocity measurements initially suggested a modest eccentricity of e \approx 0.06 with an argument of pericenter \omega \approx 90^\circ, implying Roche lobe overflow at periastron due to the planet's proximity to its Roche limit.^[33] However, more recent analyses of combined radial velocity and transit data attribute this apparent eccentricity to planetary-induced tidal distortions in the host star rather than true orbital non-circularity, yielding a best-fit eccentricity consistent with zero (e \approx 0) and rendering \omega undefined.^[21] Other Keplerian elements, such as the longitude of the ascending node \Omega, remain unconstrained in radial velocity fits for this single-planet system, typically fixed to arbitrary values like 0° due to degeneracies in the transiting geometry.^[21]

Tidal Effects and Decay

WASP-12b experiences strong tidal interactions with its host star due to its extremely close orbit, resulting in synchronous rotation where the planet's spin period matches its orbital period of approximately 1.09 days. However, evidence points to a significant spin-orbit misalignment, or obliquity, which enhances tidal dissipation and contributes to the planet's distorted shape. These forces elongate the planet into a prolate, egg-like form, with the long axis pointing toward the star, deviating from sphericity and altering its observed transit light curve by about 10%.^[34] The most prominent manifestation of these tidal effects is the planet's orbital decay, detected through precise measurements of transit timing variations (TTVs). Analysis of transit times spanning over a decade reveals a decreasing orbital period at a rate of -29 ± 2 milliseconds per year, providing the first robust evidence of inspiral in an exoplanetary system. This finding was established in a 2020 Princeton-led study using Hubble Space Telescope data, which ruled out alternative explanations like apsidal precession. A 2025 reanalysis incorporating additional ground-based and space-based transit observations by Biswas et al. confirms the decay rate at -30.31 ± 0.92 ms/yr and refines the timing baseline with 391 transits, strengthening the case for ongoing orbital contraction; this study also reports an updated inclination of 83.54° and eccentricity of 0.0031.^[3] Tidal energy dissipation primarily occurs within the star, where the planet raises persistent tidal bulges that lag due to friction, transferring angular momentum from the orbit to the star's spin. This process accelerates the planet's inward migration, with models predicting engulfment by the star in roughly 3 million years. The dissipation is quantified using the equilibrium tide model, where the rate of change in semi-major axis \dot{a} relates to the observed period derivative via

\dot{a} = -\frac{9}{2} \frac{M_p}{M_\star} \left( \frac{R_\star}{a} \right)^5 \frac{n a}{Q'_\star},

with M_p and M_\star the planet and star masses, R_\star the stellar radius, n the mean motion, and Q'_\star the modified tidal quality factor. The decay timescale is then \tau = a / |\dot{a}| \approx 3.25 million years, consistent with a stellar Q'_\star \approx 1.8 \times 10^5.

Potential Satellites

Evidence for Moons

One of the primary lines of evidence for potential moons around WASP-12b comes from anomalous ultraviolet absorption features observed during transits with the Hubble Space Telescope, interpreted as ionized plasma tori generated by exomoons orbiting the planet. These features, detected in near-UV spectra, show early ingress absorptions extending beyond the planet's Roche lobe, consistent with a plasma structure formed by material ejected from a satellite similar to Io's torus around Jupiter.^[9] The model requires an exomoon transiting approximately 6 planetary radii ahead of WASP-12b, ejecting on the order of 10^{28} Mg II ions per second to explain the observed absorption depths.^[35] Early proposals suggested transit timing variations (TTVs) in WASP-12b's light curve as indirect evidence for moons from gravitational perturbations, based on a multi-site campaign from 2009 to 2012 that identified a signal with amplitude 0.00068 ± 0.00013 days and period of about 500 planetary orbits. However, subsequent analyses attribute these TTVs to the planet's orbital decay rather than moons or companions.^[36]^[37] Despite these observations, the evidence for moons remains tentative, with alternative explanations including planetary rings, atmospheric spots, or extended exospheres from mass loss. The planet's close-in orbit and strong tidal forces make stable exomoons unlikely, as the small Hill radius and dynamical instability limit satellite retention around short-period hot Jupiters.^[38]

Implications for System Evolution

WASP-12b's tidal interactions and orbital decay (detailed in the Orbital Dynamics section) would further destabilize any potential satellites, accelerating their loss through Roche-lobe overflow or ejection. The wide binary companions (described in the Host Star section) have minimal perturbative effects on the inner system over short timescales but contribute to long-term stability models. If exomoons were present, their evolution would provide insights into dynamical disruptions during the planet's impending engulfment by the star in approximately 3 million years.^[4]

References

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### Summary of WASP-12 Companion Stars
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### Summary of Atmospheric Composition and Related Data for WASP-12b
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