Educación religiosa escolar en perspectiva del pluralismo religioso

The most recent six months in exoplanets have marked an acceleration toward the goal of identifying a transiting terrestrial planet in the habitable zone of its host star. Kepler 22b is the first super Earth with a measured radius to reside in the habitable zone of a sun-like star outside of the Solar System, though it is likely too large, at 2.4 R⊕, to have a rocky composition (Borucki et al. 2011) The Kepler-20 (Fressin et al. 2012) and KOI 961 (Muirhead et al. 2012) exoplanetary systems each comprise multiple planets that are Earth-sized or smaller, with KOI 961.03 as small as Mars. However, though these planets are likely more terrestrial in composition than Kepler-22b, they orbit too close to their host stars to be habitable. The most recent release of Kepler exoplanetary candidates (Batalha et al. 2012) contains 10 members, though as-yet unvalidated as authentic planets, which are have planetary radii <2 R⊕ and with equilibrium temperatures between 185 and 303 K, which range is a generous definition of the habitable zone proposed by Kasting (2012). Our characterization and forthcoming validation of of KOI 1361.01, a 2 R⊕-sized planet candidate in the habitable zone of its M star and described in Chapter 5, will be another member of this group of planets that edge closer to the “habitable and terrestrial” marker.

transmission spectroscopy, though favored by the small size of the host star, will be untenable even for JWST– the types of atmospheric studies lavished on GJ 1214 are enabled by its lying so close to the sun. At 30 pc, it is an order of magnitude closer than KOI 1361.01. This comparison speaks to the necessity of identifying nearby and small stellar hosts to terrestrial planets, for the purpose of addressing questions of habitability with atmospheric characterization. While Kepler will identify perhaps hundreds of super-Earth candidates in the habitable zones of their host stars, the mission is statistical in nature– these candidates’ host stars will not be bright enough for the follow-up required to search for biomarkers (the chemical signatures of life, at least as we understand it on Earth) in the planetary atmospheres. For this reason, we must turn to dedicated searches of nearby stars for the possibility of detecting a rocky world in the habitable zone of its star, which also lies within our solar neighborhood and is therefore more amenable to atmospheric follow-up. There are four such efforts underway at present: the MEarth Observatory (Irwin et al. 2009), the Transiting Exoplanet Survey Satellite (TESS), Project MINERVA, and ExoplanetSAT (Smith et al. 2010), only one of which (MEarth) is currently operational. The project designs are varied: MEarth is a ground-based network of small telescopes gathering photometry of nearby, high-proper motion M dwarfs, TESS is a proposed NASA Small Explorer spacecraft that will monitor very nearby and bright G and K-type stars, as well as the M dwarfs within 30 pc, Project MINERVA will be a ground-based combined radial-velocity and photometric effort that will monitor the quietest and brightest FGK stars already identified as such by the

California Planet Survey, and the ExoplanetSAT design envisions a flotilla of small photometers, each less than 10 kg, which will fly in a low-Earth orbit and also monitor the nearest and brightest stars for transits.

1.4.2

The Challenges of Low-Mass Stellar Characterization

As mentioned above in §1.2.2, the results to date from the Kepler Mission point to the increased frequency of small (2–4 R⊕) planets around smaller stars. Though Kepler is not yet complete for “Earth-sized” transit candidates < 1.25 R⊕, if this relationship holds for smaller planets, then the nearest terrestrial and potentially habitable planet to the Solar System likely orbits an M star. However, the traditional procedures for deriving the physical parameters of exoplanet host stars, such as mass and radius (and by extension, the physical parameters of the planets themselves) are robust only where stellar evolution models are trustworthy: namely, outside of the M dwarf regime (Teff >4000 K, M! >0.5 M"). Models for the atmospheres of low-mass stars differ substantially, and constraints on the radii of M dwarfs from eclipsing binaries are inconsistent with modeled radii (Ribas 2006; Torres 2011). While a subset of the nearest M dwarfs will be close enough for direct interferometric measurement of the radius (Berger et al. 2006) or for parallax measurements, which directly constrain the luminosity of the star (S´egransan et al. 2003), our characterization of many other M dwarfs is reliant on (as yet uncertain) stellar models. Herein lies a tremendous challenge, since the ability to characterize the radius of the host star is the limiting factor toward measuring the planetary radius. For example, an error bar of 0.5 R⊕ for

a 1.5 R⊕ planet at 500 K encloses both the possibility of a substantial hydrogen and helium envelope (comprising 0.1% the mass of the planet) and a composition of ice and heavy elements with no substantial hydrogen/helium envelope (Rogers et al. 2011), with the latter considered more favorable for habitability. Measuring the equilibrium temperature of a planet to deduce whether its surface could support the existence of liquid water also depends critically on knowledge of the stellar temperature.

However, there exist promising inroads in the spectral constraints on M dwarfs from near-infrared spectral features and colors. In K-band wavelengths, Covey et al. (2010) demonstrated the ability to derive effective temperature from some continuum regions of the spectra, while Rojas-Ayala et al. (2010) identified absorption features in sodium and calcium that enable constraints on the stellar metallicity. Muirhead et al. (2012) combined these techniques to measure the effective temperatures and metallicities of 84 Kepler Objects of Interest orbiting low-mass stars, and concluded that the stellar temperatures were in most cases several hundred Kelvin cooler than what had been inferred from the Kepler Input Catalog. However, a physical interpretation of these revised temperatures and metallicities relies upon trustworthy stellar evolutionary models. Cooler stars translate in a general sense to smaller radii, which is especially compelling in the case of the Kepler candidates, since a smaller stellar radius translates to a smaller planetary radius. In Chapter 6, I compare the known exoplanets with Rp < 3R⊕ and dynamically measured masses (9 such planets are currently compiled in the literature). Though this relationship is undersampled, smaller

planets in this radius range have higher densities, and so a smaller planetary radii may also imply a more terrestrial composition. For this reason, careful treatment of stellar characterization of the cool KOIs is imperative. I plan to undertake such a study as a postdoctoral fellow at the University of Washington, combining infrared spectroscopy (for measurements of stellar temperature and metallicity as described above) and optical spectroscopy (for corroboration of stellar type, in addition to probing stellar activity via the presence of Hα in emission or absorption), enabled by instruments at the Apache Point Observatory. I am hopeful that an increased expertise in M dwarfs will be a highly valued trait in an exoplanetary scientist in the next 5 years, since our nearest neighboring habitable exoplanet likely orbits such a star.

A Search for Additional Planets

in the NASA

EPOXI Observations

of the Exoplanet System GJ 436

S. Ballard, J. L. Christiansen, D. Charbonneau, D. Deming, M. J. Holman, D. Fabrycky, M. F. A’Hearn, D. D. Wellnitz, R. K. Barry, M. J. Kuchner, T. A. Livengood, T. Hewagama, J. M. Sunshine, D. L. Hampton, C. M. Lisse, S. Seager, J. F. Veverka The Astrophysical Journal, Vol. 716, pp. 1047-1059, 2010

Abstract

We present time series photometry of the M dwarf transiting exoplanet system GJ 436 obtained with the EPOCh (Extrasolar Planet Observation and

Characterization) component of the NASA EPOXI mission. We conduct a search of the high-precision time series for additional planets around GJ 436, which could be revealed either directly through their photometric transits, or indirectly through the variations these second planets induce on the transits of the previously known planet. In the case of GJ 436, the presence of a second planet is perhaps indicated by the residual orbital eccentricity of the known hot Neptune companion. We find no candidate transits with significance higher than our detection limit. From Monte Carlo tests of the time series, we rule out transiting planets larger than 1.5 R⊕ interior to GJ 436b with 95% confidence, and larger than 1.25 R⊕ with 80% confidence. Assuming coplanarity of additional planets with the orbit of GJ 436b, we cannot expect that putative planets with orbital periods longer than about 3.4 days will transit. However, if such a planet were to transit, we rule out planets larger than 2.0 R⊕ with orbital periods less than 8.5 days with 95% confidence. We also place dynamical constraints on additional bodies in the GJ 436 system, independent of radial velocity measurements. Our analysis should serve as a useful guide for similar analyses of transiting exoplanets for which radial velocity measurements are not available, such as those discovered by the Kepler mission. From the lack of observed secular perturbations, we set upper limits on the mass of a second planet as small as 10 M⊕ in coplanar orbits and 1 M⊕ in non-coplanar orbits close to GJ 436b. We present refined estimates of the system parameters for GJ 436. We find P = 2.64389579 ± 0.00000080 d, R! _{= 0.437 ± 0.016 R}", and Rp _{= 3.880 ± 0.147 R}⊕. We also report a sinusoidal modulation in the GJ 436 light curve that we attribute to star spots. This

signal is best fit by a period of 9.01 days, although the duration of the EPOCh observations may not have been long enough to resolve the full rotation period of the star.