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Register a new userDetection of transits of the nearby hot Neptune GJ 436 b
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Authors
M. Gillon
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This Letter reports on the photometric detection of transits of the Neptune-mass planet orbiting the nearby M-dwarf star GJ 436. It is by far the closest, smallest and least massive transiting planet detected so far. Its mass is slightly larger than Neptunes at M = 22.6 +- 1.9 M_earth. The shape and depth of the transit lightcurves show that it is crossing the host star disc near its limb (impact parameter 0.84 +- 0.03) and that the planet size is comparable to that of Uranus and Neptune, R = 25200 +- 2200 km = 3.95 +- 0.35 R_earth. Its main constituant is therefore very likely to be water ice. If the current planet structure models are correct, an outer layer of H/He constituting up to ten percent in mass is probably needed on top of the ice to account for the observed radius.
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The late-type dwarf GJ 436 is known to host a transiting Neptune-mass planet in a 2.6-day orbit. We present results of our interferometric measurements to directly determine the stellar diameter ($R_{star} = 0.455 pm 0.018 R_{odot}$) and effective temperature ($T_{rm EFF} = 3416 pm 54$ K). We combine our stellar parameters with literature time-series data, which allows us to calculate physical and orbital system parameters, including GJ 436s stellar mass ($M_{star} = 0.507^{+ 0.071}_{- 0.062} M_{odot}$) and density ($rho_* = 5.37^{+ 0.30}_{- 0.27} rho_odot$), planetary radius ($R_{p} = 0.369^{+ 0.015}_{- 0.015} R_{Jupiter}$), planetary mass ($M_{p} = 0.078^{+ 0.007}_{- 0.008} M_{Jupiter}$), implying a mean planetary density of $rho_{p} = 1.55^{+ 0.12}_{- 0.10} rho_{Jupiter}$. These values are generally in good agreement with previous literature estimates based on assumed stellar mass and photometric light curve fitting. Finally, we examine the expected phase curves of the hot Neptune GJ 436b, based on various assumptions concerning the efficiency of energy redistribution in the planetary atmosphere, and find that it could be constrained with {it Spitzer} monitoring observations.
We report the detection of UCF-1.01, a strong exoplanet candidate with a radius 0.66 +/- 0.04 times that of Earth (R_{oplus}). This sub-Earth-sized planet transits the nearby M-dwarf star GJ 436 with a period of 1.365862 +/- 8x10^{-6} days. We also report evidence of a 0.65 +/- 0.06 R_{oplus} exoplanet candidate (labeled UCF-1.02) orbiting the same star with an undetermined period. Using the Spitzer Space Telescope, we measure the dimming of light as the planets pass in front of their parent star to assess their sizes and orbital parameters. If confirmed, UCF-1.01 and UCF-1.02 would be called GJ 436c and GJ 436d, respectively, and would be part of the first multiple-transiting-planet system outside of the Kepler field. Assuming Earth-like densities of 5.515 g/cm^3, we predict both candidates to have similar masses (~0.28 Earth-masses, M_{oplus}, 2.6 Mars-masses) and surface gravities of ~0.65 g (where g is the gravity on Earth). UCF-1.01s equilibrium temperature (T_{eq}, where emitted and absorbed radiation balance for an equivalent blackbody) is 860 K, making the planet unlikely to harbor life as on Earth. Its weak gravitational field and close proximity to its host star imply that UCF-1.01 is unlikely to have retained its original atmosphere; however, a transient atmosphere is possible if recent impacts or tidal heating were to supply volatiles to the surface. We also present additional observations of GJ 436b during secondary eclipse. The 3.6-micron light curve shows indications of stellar activity, making a reliable secondary eclipse measurement impossible. A second non-detection at 4.5 microns supports our previous work in which we find a methane-deficient and carbon monoxide-rich dayside atmosphere.
We present Spitzer Space Telescope infrared photometry of a primary transit of the hot Neptune GJ 436b. The observations were obtained using the 8 microns band of the InfraRed Array Camera (IRAC). The high accuracy of the transit data and the weak limb-darkening in the 8 microns IRAC band allow us to derive (assuming M = 0.44 +- 0.04 Msun for the primary) a precise value for the planetary radius (4.19 +0.21-0.16 Rearth), the stellar radius (0.463 +0.022-0.017 Rsun), the orbital inclination (85.90 +0.19-0.18 degrees) and transit timing (2454280.78186 +0.00015-0.00008 HJD). Assuming current planet models, an internal structure similar to that of Neptune with a small H/He envelope is necessary to account for the measured radius of GJ 436b.
We present Spitzer Space Telescope infrared photometry of a secondary eclipse of the hot Neptune GJ436b. The observations were obtained using the 8-micron band of the InfraRed Array Camera (IRAC). The data spanning the predicted time of secondary eclipse show a clear flux decrement with the expected shape and duration. The observed eclipse depth of 0.58 mmag allows us to estimate a blackbody brightness temperature of T_p = 717 +- 35 K at 8 microns. We compare this infrared flux measurement to a model of the planetary thermal emission, and show that this model reproduces properly the observed flux decrement. The timing of the secondary eclipse confirms the non-zero orbital eccentricity of the planet, while also increasing its precision (e = 0.14 +- 0.01). Additional new spectroscopic and photometric observations allow us to estimate the rotational period of the star and to assess the potential presence of another planet.
(Abridged) The quiet M2.5 star GJ 436 hosts a warm Neptune that displays an extended atmosphere that dwarfs its own host star. Predictions of atmospheric escape in such planets state that H atoms escape from the upper atmosphere in a collisional regime and that the flow can drag heavier atoms to the upper atmosphere. It is unclear, however, what astrophysical mechanisms drive the process. Our objective is to leverage the extensive coverage of HST/COS observations of the far-ultraviolet (FUV) spectrum of GJ 436 to search for signals of metallic ions in the upper atmosphere of GJ 436 b. We analyzed flux time-series of species present in the FUV spectrum of GJ 436, as well as the Lyman-$alpha$ line. GJ 436 displays FUV flaring events with a rate of $sim$10 d$^{-1}$. There is evidence for a possibly long-lived active region or longitude that modulates the FUV metallic lines of the star with amplitudes up to 20%. Despite the strong geocoronal contamination in the COS spectra, we detected in-transit excess absorption signals of $sim$50% and $sim$30% in the blue and red wings, respectively, of the Lyman-$alpha$ line. We rule out a wide range of excess absorption levels in the metallic lines of the star during the transit. The large atmospheric loss of GJ 436 b observed in Lyman-$alpha$ transmission spectra is stable over the timescale of a few years, and the red wing signal supports the presence of a variable hydrogen absorption source besides the stable exosphere. The previously claimed in-transit absorption in the Si III line is likely an artifact resulting from the stellar magnetic cycle. The non-detection of metallic ions in absorption could indicate that the escape is not hydrodynamic or that the atmospheric mixing is not efficient in dragging metals high enough for sublimation to produce a detectable escape rate of ions to the exosphere.
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