Spitzer Observations of GJ3470b: a Very Low-density Neptune-size Planet Orbiting a Metal-rich M dwarf

E-print/Working paper (2013)

We present Spitzer/IRAC 4.5-micron transit photometry of GJ3470b, a Neptune-size planet orbiting a M1.5 dwarf star with a 3.3-day period recently discovered in the course of the HARPS M-dwarf survey. We ... [more ▼]

We present Spitzer/IRAC 4.5-micron transit photometry of GJ3470b, a Neptune-size planet orbiting a M1.5 dwarf star with a 3.3-day period recently discovered in the course of the HARPS M-dwarf survey. We refine the stellar parameters by employing purely empirical mass-luminosity and surface brightness relations constrained by our updated value for the mean stellar density, and additional information from new near-infrared spectroscopic observations. We derive a stellar mass of M_star = 0.539+0.047-0.043 M_sun and a radius of R_star = 0.568+0.037-0.031 R_sun. We determine the host star of GJ3470b to be metal-rich, with a metallicity of [Fe/H] = +0.20 +/- 0.10 and an effective temperature of Teff = 3600 +/- 100 K. The revised stellar parameters yield a planetary radius R_pl = 4.83+0.22-0.21 R_Earth that is 13 percent larger than the value previously reported in the literature. We find a planetary mass M_pl = 13.9+1.5-1.4 M_Earth that translates to a very low planetary density, rho_pl = 0.72+0.13-0.12 gcm-3, which is 33% smaller than the original value. With a mean density half of that of GJ436b, GJ3470b is an example of a very low-density low-mass planet, similar to Kepler-11d, Kepler-11e, and Kepler-18c but orbiting a much brighter nearby star that is more conducive to follow-up studies. [less ▲]

Secondary eclipse scanning of HD189733b: The perspectives of mapping distant worlds

in Astronomy and Astrophysics (2012), 548

Context. Mapping the brightness distribution of exoplanets is the next frontier for exoplanet infrared photometry studies. For tidally-locked hot Jupiters that transit and are eclipsed by their host star ... [more ▼]

Context. Mapping the brightness distribution of exoplanets is the next frontier for exoplanet infrared photometry studies. For tidally-locked hot Jupiters that transit and are eclipsed by their host star with non-zero impact parameter, the first steps are now possible. Aims. The aim is to use eclipse scanning from occultation ingress/egress and phase curve measurements to constrain exoplanet large-scale brightness structure. Methods. We use archived Spitzer/IRAC 8 {\mu}m data of HD189733 in a global MCMC procedure encompassing six transits, eight secondary eclipses, and a phase curve in a two-step analysis. The first step derives the planet-star system parameters. The second step investigates the structure found in eclipse scanning, using the previous planet-star system parameter derivation as Gaussian priors. Results. We find a 5-sigma deviation from the expected occultation ingress/egress shape for a uniform brightness disk, and demonstrate that this is dominated by large-scale brightness structure and not an occultation timing offset due to a non-zero eccentricity. Our analysis yields a 2D brightness temperature distribution showing a large-scale asymmetric hot spot whose finer structure is limited by the data quality and planet orbit geometry. We also present an improved upper limit for eccentricity, e<0.0081 (95% confidence). Conclusions. Reanalysis of archived HD 189733 data revealed brightness structure by using global analysis that mitigated systematics. Future eclipse scanning observations of the same exoplanet at other wavelengths will probe different atmosphere layers, ultimately generating a large-scale 3D map. [less ▲]

Discovery and Atmospheric Characterization of Giant Planet Kepler-12b: An Inflated Radius Outlier

in Astrophysical Journal. Supplement Series (2011), 197

We report the discovery of planet Kepler-12b (KOI-20), which at 1.695 ± 0.030 R [SUB]J[/SUB] is among the handful of planets with super-inflated radii above 1.65 R [SUB]J[/SUB]. Orbiting its slightly ... [more ▼]

We report the discovery of planet Kepler-12b (KOI-20), which at 1.695 ± 0.030 R [SUB]J[/SUB] is among the handful of planets with super-inflated radii above 1.65 R [SUB]J[/SUB]. Orbiting its slightly evolved G0 host with a 4.438 day period, this 0.431 ± 0.041 M [SUB]J[/SUB] planet is the least irradiated within this largest-planet-radius group, which has important implications for planetary physics. The planet's inflated radius and low mass lead to a very low density of 0.111 ± 0.010 g cm[SUP]-3[/SUP]. We detect the occultation of the planet at a significance of 3.7σ in the Kepler bandpass. This yields a geometric albedo of 0.14 ± 0.04; the planetary flux is due to a combination of scattered light and emitted thermal flux. We use multiple observations with Warm Spitzer to detect the occultation at 7σ and 4σ in the 3.6 and 4.5 μm bandpasses, respectively. The occultation photometry timing is consistent with a circular orbit at e < 0.01 (1σ) and e < 0.09 (3σ). The occultation detections across the three bands favor an atmospheric model with no dayside temperature inversion. The Kepler occultation detection provides significant leverage, but conclusions regarding temperature structure are preliminary, given our ignorance of opacity sources at optical wavelengths in hot Jupiter atmospheres. If Kepler-12b and HD 209458b, which intercept similar incident stellar fluxes, have the same heavy-element masses, the interior energy source needed to explain the large radius of Kepler-12b is three times larger than that of HD 209458b. This may suggest that more than one radius-inflation mechanism is at work for Kepler-12b or that it is less heavy-element rich than other transiting planets. [less ▲]

The Hot-Jupiter Kepler-17b: Discovery, Obliquity from Stroboscopic Starspots, and Atmospheric Characterization

in Astrophysical Journal. Supplement Series (2011), 197

This paper reports the discovery and characterization of the transiting hot giant exoplanet Kepler-17b. The planet has an orbital period of 1.486 days, and radial velocity measurements from the Hobby ... [more ▼]

This paper reports the discovery and characterization of the transiting hot giant exoplanet Kepler-17b. The planet has an orbital period of 1.486 days, and radial velocity measurements from the Hobby-Eberly Telescope show a Doppler signal of 419.5[SUP]+13.3[/SUP] [SUB]-15.6[/SUB] m s[SUP]-1[/SUP]. From a transit-based estimate of the host star's mean density, combined with an estimate of the stellar effective temperature T [SUB]eff[/SUB] = 5630 ± 100 from high-resolution spectra, we infer a stellar host mass of 1.06 ± 0.07 M [SUB]&sun;[/SUB] and a stellar radius of 1.02 ± 0.03 R [SUB]&sun;[/SUB]. We estimate the planet mass and radius to be M [SUB]P[/SUB] = 2.45 ± 0.11 M [SUB]J[/SUB] and R [SUB]P[/SUB] = 1.31 ± 0.02 R [SUB]J[/SUB]. The host star is active, with dark spots that are frequently occulted by the planet. The continuous monitoring of the star reveals a stellar rotation period of 11.89 days, eight times the planet's orbital period; this period ratio produces stroboscopic effects on the occulted starspots. The temporal pattern of these spot-crossing events shows that the planet's orbit is prograde and the star's obliquity is smaller than 15°. We detected planetary occultations of Kepler-17b with both the Kepler and Spitzer Space Telescopes. We use these observations to constrain the eccentricity, e, and find that it is consistent with a circular orbit (e < 0.011). The brightness temperatures of the planet's infrared bandpasses are T_{3.6\, {\mu m}} = 1880 ± 100 K and T_{4.5\, {\mu m}} = 1770 ± 150 K. We measure the optical geometric albedo A[SUB]g[/SUB] in the Kepler bandpass and find A[SUB]g[/SUB] = 0.10 ± 0.02. The observations are best described by atmospheric models for which most of the incident energy is re-radiated away from the day side. [less ▲]

The astrobiology primer: An outline of general knowledge - Version 1, 2006

in Astrobiology (2006), 6(5), 735-813