Exoplanets

Abstract

We are in the midst of a revolution in our understanding of planets in the Universe. In the last 20 years, 200-times more planets have been discovered beyond our solar system than reside within it. Astronomers are determining their properties, are finding correlations between a star's type and its planet population, and have begun to probe exoplanetary atmospheres. One idea to emerge is that the planets of the solar system are not representative in type, mix, or orbit. Another is that comparative planetology has become a palpable reality. A third is that we are merely on the cusp of a transformation in our conceptions of planet birth and evolution. With all this progress, textbooks are being rewritten yearly. To communicate a sense of the pace and excitement of this new subject, we have assembled a collection of articles by some of the leading researchers in their subfields that summarizes the current state of play across the broadening spectrum of topics relevant to the study of exoplanets.In "Spectra as windows into exoplanet atmospheres," Adam Burrows (1) provides a perspective on exoplanet theory and remote sensing via photometry and spectroscopy. He highlights the need for better spectra and the limitations-despite recent progress-in our current knowledge of exoplanet atmospheres. Importantly, Burrows emphasizes that the true function of the recent past of exoplanet research has been to train the next generation of scientists who will ensure that the foundations of comparative exoplanetolgy in the future will be robust.Yoram Lithwick and Yanqin Wu (2) explore the organization of planetary orbits and systems resulting from chaos in their gravitational dynamics and mutual evolution. Eric Ford (3) also reviews ideas concerning the architectures of exoplanetary systems, and addresses some implications for their formation. In particular, Ford focuses on the creation of multiple planet systems in tight orbits, the excitation of orbital eccentricities, and the usefulness of transit timing variation caused by mutual gravitational interactions in providing constraints on the formation and orbital evolution of planetary systems with lowmass planets.David Spiegel, Jonathan Fortney, and Christophe Sotin (4) delve into the physical structure of exoplanets. Emphasizing that the emerging data suggest that our solar system is in no way typical, these authors review the diversity of possible planet interior structures, from super-Jupiters to planets half the mass of the Earth. Along the way, they discuss the variety of material properties-from those of compressed solids to those of dilute gases-that determine the size and profiles of planets in the galaxy. The relevant equations of state are explored and the possible role of the radius-mass distribution function in informing our understanding of planet formation and evolution is investigated.Christopher McKay (5) reviews the requirements and limits for life and habitability in the context of exoplanets. Using the variety of life on Earth as a guide, McKay emphasizes that te...

Balancing the energy budget of short-period giant planets: evidence for reflective clouds and optical absorbers

“…As a result, the dayside of a hot Jupiter emits somewhat more in the mid-IR-and considerably more in the near-IR-than one would predict based on its equilibrium temperature. In any case, we agree with Burrows (2014) that there are no signs of molecular absorption features in the aggregate spectrum. Since molecules are undoubtedly present in the atmospheres of exoplanets, we conclude that their absorption features are being muted by vertically isothermal atmospheres, optically thick cloud, or both.…”

“…The trend line is shown in green; it suggests that hotter planets have disproportionately hot daysides. wavelengths reported by Burrows (2014). It is worth noting that Figure 4 of Burrows (2014) used data from fewer planets, and was normalized differently: the equilibrium temperature of each planet was divided out, rather than its actual dayside effective temperature.…”

“…wavelengths reported by Burrows (2014). It is worth noting that Figure 4 of Burrows (2014) used data from fewer planets, and was normalized differently: the equilibrium temperature of each planet was divided out, rather than its actual dayside effective temperature. Recall that a planet's equilibrium temperature is what one would expect for a planet with zero Bond albedo and uniform temperature; it is merely a convenient theoretical quantity proportional to the irradiation temperature, T0, that we utilize in this work.…”

THE 4.5 μm FULL-ORBIT PHASE CURVE OF THE HOT JUPITER HD 209458b

“…Although our new measurement does not rule out a dayside temperature inversion, it suggests that an inversion may not be necessary to explain the current 4.5 µm broadband data. New high-precision eclipse measurements in the other Spitzer bandpasses, specifically photometry at 5.8 and 8 µm, and ideally mid-infrared spectroscopic measurements (e.g., Madhusudhan & Seager 2010;Burrows 2014;Line et al 2014) would help to provide a more definitive answer to this question.…”

Photoionization of the CO and NO molecules

“…Molecular absorption is an important process in atmospheric physics and in several astrophysical contexts, such as exoplanetary atmospheres [1][2][3][4], star forming regions [5], and interstellar environments [6]. The most relevant wavelength region for molecular absorption is in the far infra-red bands, where bound-bound absorption usually between ro-vibrational states dominates.…”