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WASP-12b: The hottest transiting planet yet discovered
L. Hebb, A. Collier-Cameron, B. Loeillet, D. Pollacco, G. Hébrard, R. A. Street, F. Bouchy, H. C. Stempels, C. Moutou, E. Simpson, S. Udry, Y. C. Yoshi, R. G. West, I. Skillen, D. M. Wilson, I. McDonald, N. P. Gibson, the SuperWasp Consortium
TL;DR
The paper reports and characterizes the extreme transiting planet WASP-12b. It combines multi-band transit photometry and radial velocities with stellar spectral and evolutionary modeling, finding a large, hot, strongly irradiated planet around a metal-rich late-F star, while noting unresolved radius inflation.
Problem
The study investigates an extreme transiting planet whose short period, enlarged radius, and host-star properties test models of irradiated gas-giant structure.
Method
Transit photometry and radial-velocity data were analyzed simultaneously with an MCMC routine, while stellar spectra were compared with theoretical spectra and evolution models.
Results
Rpl=1.79 RJ, Mpl=1.41 MJ, and Teq=2516 K characterize WASP-12b, which orbits a Teff=6300 K, metal-rich late-F star and is the most heavily irradiated planet yet detected.
Takeaways & Limitations
WASP-12b provides an extreme case for examining how stellar irradiation and metallicity relate to the atmospheric and structural properties of close-in gas giants.
Takeaways & Limitations
Stellar irradiation alone is difficult to reconcile with the observed Rpl = 1.79 RJ, so an additional internal energy source may be required.
Abstract
from arXiv · showhide
We report on the discovery of WASP-12b, a new transiting extrasolar planet with $R_{\rm pl}=1.79 \pm 0.09 R_J$ and $M_{\rm pl}=1.41 \pm 0.1 M_J$. The planet and host star properties were derived from a Monte Carlo Markov Chain analysis of the transit photometry and radial velocity data. Furthermore, by comparing the stellar spectrum with theoretical spectra and stellar evolution models, we determined that the host star is a super-solar metallicity ([M/H]$=0.3^{+0.05}_{-0.15}$), late-F (T$_{\rm eff}=6300^{+200}_{-100}$ K) star which is evolving off the zero age main sequence. The planet has an equilibrium temperature of T$_{\rm eq}$=2516 K caused by its very short period orbit ($P=1.09$ days) around the hot, 12th magnitude host star. WASP-12b has the largest radius of any transiting planet yet detected. It is also the most heavily irradiated and the shortest period planet in the literature.
1. Introduction
Transiting exoplanets enable detailed tests of planet-formation and evolution models. This paper reports WASP-12b as an extreme short-period planet with an enlarged radius orbiting a metal-rich host.
- Detailed follow-up of bright transiting systems provides strong observational tests of theoretical planet-formation and evolution models.
- Exotic planets push the boundaries of theoretical understanding, as illustrated by the unexpectedly large radius of HD 209458b.
- The paper reports WASP-12b as a new extreme transiting planet with a short orbital period and enlarged radius.
- The study describes observations, analyzes the planet and host-star properties, and discusses them against current planet-formation theory.
2. Observations
WASP-12 is introduced as a bright F9V star with catalogued optical and infrared photometry and proper-motion information.
- WASP-12 is a bright F9V star identified in northern-sky catalogues.The catalogues provide broad-band optical and infrared magnitudes plus proper-motion information.
2.1. SuperWASP Photometry
SuperWASP observations identified a significant 1.091-day periodic transit candidate, motivating higher-priority follow-up.
- 820 photometric measurements were obtained in 2004, followed by 5573 measurements from two cameras during the 2006–2007 season.
- The data were reduced with a custom pipeline and searched for planetary transits using a modified box-least-squares algorithm.
- P = 1.091 days, τ ∼2.7 hours, and δ ∼14 mmag characterized the combined SuperWASP transit signal.The transit model improved χ2 by 719, had SNred = 15.6, and included 23 partial or full transits.
- The significant, unblended transit event led to WASP-12 being classified as a high-priority target for further study.
2.2. Follow-up Multi-band Photometry
Follow-up observations used higher-resolution imaging and multi-band photometry to validate and refine the transit signal. The I-band data confirmed the transit but were excluded from parameter modeling because of red-noise systematics.
- Follow-up validation: Higher-spatial-resolution follow-up photometry searched for nearby eclipsing binaries that could mimic the SuperWASP transit.WASP-12 appeared single at the follow-up resolution, with the closest companion 9 arcseconds away.
- Tenagra photometry: 227 B-band observations and 639 I-band measurements were collected with the Tenagra II telescope across multiple nights.
- Tenagra photometry: Differential photometry used calibrated images, aperture photometry, and seven non-variable comparison stars with V < 14.
- Data-quality boundary: The I-band data confirmed the transit and refined the ephemeris but were excluded from modeling because non-photometric conditions produced substantial red-noise systematics.
- Liverpool Telescope photometry: 614 images were obtained in Sloan z′ during a full transit under photometric, stable observing conditions.
2.3. SOPHIE Spectroscopy
High-resolution SOPHIE spectroscopy provided radial-velocity measurements that vary sinusoidally on the transit-derived period. Line-bisector tests support a planetary-mass interpretation rather than stellar activity or line-of-sight binarity.
- The SOPHIE data were obtained with the high-efficiency spectrograph mode during observations from 2008 February 12–22.
- 21 radial-velocity measurements over ten nights had typical uncertainties of ∼10 m s−1 and varied sinusoidally when folded on the transit-derived period.
- The measured radial velocities had a standard deviation of 130 m s−1, significantly exceeding the individual measurement uncertainties.
- Line-bisector velocities showed no correlation with radial velocity, disfavoring stellar activity or blending with an eclipsing binary.
- The authors therefore attribute the observed radial-velocity variations to the gravitational influence of a planetary-mass object orbiting WASP-12.
2.4. Additional Spectroscopy
Additional spectroscopy supplied spectra for stellar-parameter analysis and included observations with SARG and IDS. The authors also note that residual blaze features in SOPHIE high-efficiency spectra limit stellar-parameter precision, especially for hot stars.
- SOPHIE high-efficiency spectra retain a residual blaze effect at the 5% level, limiting their suitability for determining stellar parameters.
- The transit photometry figures compare differential z-band and Tenagra B-band data with best-fit Mandel & Agol model light curves using Claret limb-darkening coefficients.
- For hot stars with broad Hα wings, residual instrumental features can produce large uncertainties in effective temperature.
- Two IDS spectra achieved signal-to-noise above 50 at R ∼8000 across 6200–7000 Å, covering Hα, the Li I doublet, and many narrow metal lines.
- Three SARG exposures of 1800 seconds used a 0.8′′ slit and produced spectral resolution R∼57000 after calibration and reduction.
3. Analysis
The analysis combines spectroscopic modelling, stellar-evolution comparisons, and simultaneous MCMC fitting of transit and radial-velocity data to determine WASP-12 and WASP-12b. It finds a hot, metal-rich, evolved late-F host and a low-density planet in an approximately one-day orbit.
- Spectroscopic analysis: The spectroscopic synthesis fits four diagnostic regions simultaneously, but macroturbulence is poorly constrained for hot stars because rotational broadening dominates and few comparable VF05 stars exist.Only 79 stars in the VF05 sample have Teff ≥6200 K, making the empirical relation weak in this regime.
- Spectroscopic analysis: Three independent synthetic-spectrum analyses identify WASP-12 as a hot, slowly rotating, metal-rich dwarf star, with SARG providing the adopted stellar parameters.The SARG fit gives Teff = 6290 K, log g = 4.38, and [M/H]=0.30; SOPHIE and IDS provide the uncertainty range.
- Planet and host-star fitting: The planetary and stellar properties are obtained by simultaneously analysing multi-band transit light curves and radial velocities with an MCMC routine.The analysis uses SuperWASP, follow-up photometry, and SOPHIE radial-velocity data, then repeats the fit after determining the host star’s evolutionary status.
- Evolutionary status of the host star: Stellar-evolution tracks place WASP-12 beyond the zero-age main sequence but before shell hydrogen burning, with M∗= 1.33 ± 0.05 M⊙ and an adopted age τ = 2±1 Gyr.The evolutionary estimate is supported by comparisons with two model-track sets, while lithium provides only a non-precise age constraint.
- Final planet and host-star parameters: The final fit finds WASP-12b to be a low-density planet of approximately 0.2ρJ, with a mass about 40% larger than Jupiter and a one-day orbit around the evolved host.The transit model is a good match to the data, but the impact parameter is difficult to measure precisely and strongly affects the inferred stellar and planetary radii.
4. Discussion
WASP-12b combines extreme irradiation, a very large radius, and low density, challenging isolated formation models. The discussion evaluates stellar irradiation, atmospheric opacity, and tidal heating as possible explanations while identifying follow-up tests.
- 1.79 RJ is WASP-12b’s defining feature, with a mean density only 24% that of Jupiter.Its position in the mass–radius plane places it among transiting gas giants with particularly large radii.
- 9.03×10^9 ergs cm−2 s−1 is the incident substellar flux, twice that of the next most irradiated comparison planets.The corresponding equilibrium temperature is Teq=2516 K, depending on absorbed flux and the emitting fraction of the surface.
- Increased stellar irradiation can inhibit contraction and increase planetary radius, but irradiation alone appears insufficient to reach WASP-12b’s observed 1.79 RJ.A comparable 1.46 MJ model at 1 Gyr has radius 1.248 RJ at 0.02 AU; WASP-12b’s irradiation is equivalent to about 0.01 AU around a solar-luminosity star.
- 10× solar atmospheric abundance can increase modeled radii by maintaining core heat and entropy, but no model specifically calculates WASP-12b.A complete model including its extreme environment, heavy elements, and high-altitude hazes had not yet established whether the observed radius can be matched.
- If atmospheric explanations fail, additional internal energy may be required; tidal dissipation is proposed as one possible source.The best fit gives e=0.049+0.015−0.015, while the inferred tidal-heating scenario could produce energy of order 5×10^28 QP ergs s−1.
- The non-zero eccentricity is barely a 3σ detection, so further observations are needed before using it to constrain tidal dissipation or invoke an outer planet.Long-term radial-velocity monitoring could distinguish continuous eccentricity pumping by another planet from a larger planetary tidal dissipation constant.
- WASP-12b has Mpl=1.41 MJ, Rpl=1.79 RJ, and Teq=2516 K while orbiting an evolving, metal-rich late-F star.The host star has Teff=6300 K, R∗=1.57 R⊙, enhanced metallicity, and an age of approximately 2 Gyr.
- Follow-up observations are aimed at identifying the mechanisms behind inflated hot-Jupiter radii and clarifying how stellar irradiation and metallicity affect close-in gas giants.