* Astronomy

Title: A Hubble Space Telescope Search for a Sub-Earth-Sized Exoplanet in the GJ 436 System
Author: Kevin B. Stevenson, Jacob L. Bean, Daniel Fabrycky, Laura Kreidberg

The detection of small planets orbiting nearby stars is an important step towards the identification of Earth twins. In previous work using the Spitzer Space Telescope, we found evidence to support at least one sub-Earth-sized exoplanet orbiting the nearby mid-M dwarf star GJ 436. As a follow up, here we used the Hubble Space Telescope to investigate the existence of one of these candidate planets, UCF-1.01, by searching for two transit signals as it passed in front of its host star. Interpretation of the data hinges critically on correctly modelling and removing the WFC3 instrument systematics from the light curves. Building on previous HST work, we demonstrate that WFC3 analyses need to explore the use of a quadratic function to fit a visit-long time-dependent systematic. This is important for establishing absolute transit and eclipse depths in the white light curves of all transiting systems. The work presented here exemplifies this point by putatively detecting the primary transit of UCF-1.01 with the use of a linear trend. However, using a quadratic trend, we achieve a better fit to the white light curves and a reduced transit depth that is inconsistent with previous Spitzer measurements. Furthermore, quadratic trends with or without a transit model component produce comparable fits to the available data. Using extant WFC3 transit light curves for GJ436b, we further validate the quadratic model component by achieving photon-limited model fit residuals and consistent transit depths over multiple epochs. We conclude that, when we fit for a quadratic trend, our new data contradict the prediction of a sub-Earth-sized planet orbiting GJ 436 with the size, period, and ephemeris posited from the Spitzer data by a margin of 3.1{\sigma}.

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Title: Dynamical evolution of the Gliese 436 planetary system - Kozai migration as a potential source for Gliese 436b's eccentricity
Authors: Hervé Beust (IPAG), Xavier Bonfils (IPAG), Guillaume Montagnier (OHP), Xavier Delfosse (IPAG), Thierry Forveille (IPAG)

The close-in planet orbiting GJ 436 presents a puzzling orbital eccentricity considering its very short orbital period. Given the age of the system, this planet should have been tidally circularised a long time ago. Many attempts to explain this were proposed in recent years, either involving abnormally weak tides, or the perturbing action of a distant companion. We address here the latter issue based on Kozai migration. We propose that GJ 436b was formerly located further away from the star and that it underwent a migration induced by a massive, inclined perturber via Kozai mechanism. In this context, the perturbations by the companion trigger high amplitude variations to GJ 436b that cause tides to act at periastron. Then the orbit tidally shrinks to reach its present day location. We numerically integrate the 3-body system including tides and General Relativity correction. We first show that starting from the present-day location of GJ 436b inevitably leads to damping the Kozai oscillations and to rapidly circularising the planet. Conversely, starting from 5-10 times further away allows the onset of Kozai cycles. The tides act in peak eccentricity phases and reduce the semi-major axis of the planet. The net result is an evolution characterised by two phases: a first one with Kozai cycles and a slowly shrinking semi-major axis, and a second one once the planet gets out of the Kozai resonance characterised by a more rapid decrease. The timescale of this process appears in most cases much longer than the standard circularisation time of the planet by a factor larger than 50. This model can provide a solution to the eccentricity paradox of GJ 436b. Depending on the various orbital configurations, it can take several Gyrs to GJ 436b to achieve a full orbital decrease and circularisation. According to this scenario, we could be witnessing today the second phase of the scenario where the semi-major axis is already reduced while the eccentricity is still significant. We then explore the parameter space and derive in which conditions this model can be realistic given the age of the system. This yields constraints on the characteristics of the putative companion.

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NASA'S Spitzer Finds Evidence for an Exoplanet Smaller than Earth

Astronomers using NASA's Spitzer Space Telescope have detected what they believe is a planet two-thirds the size of Earth. The exoplanet candidate, called UCF-1.01, is located a mere 33 light-years away, making it possibly the nearest world to our solar system that is smaller than our home planet.
Exoplanets circle stars beyond our sun. Only a handful smaller than Earth have been found so far. Spitzer has performed transit studies on known exoplanets, but UCF-1.01 is the first ever identified with the telescope, pointing to a possible role for Spitzer in helping discover potentially habitable, terrestrial-sized worlds.
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Title: The GJ 436 System: Directly Determined Astrophysical Parameters of an M-Dwarf and Implications for the Transiting Hot Neptune
Authors: Kaspar von Braun (1,11), Tabetha S. Boyajian (2,3), Stephen R. Kane (1,11), Leslie Hebb (9), Gerard T. van Belle (4), Chris Farrington (6), David R. Ciardi (1,11), Heather A. Knutson (11), Theo A. ten Brummelaar (6), Mercedes Lopez-Morales (5,8), Harold A. McAlister (2), Gail Schaefer (6), Stephen Ridgway (7), Andrew Collier Cameron (10), P. J. Goldfinger (6), Nils H. Turner (6), Laszlo Sturmann (6), Judit Sturmann (6) ((1) NExScI, (2) Georgia State U, (3) Hubble Fellow, (4) Lowell Observatory, (5) CSIC-IEEC, (6) The CHARA Array, (7) NOAO, (8) CIW - DTM, (9) Vanderbilt U., (10) St. Andrews, (11) Caltech)

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 ± 0.018 solar radii) and effective temperature (T_{EFF} = 3416 ± 54 K). We combine our stellar parameters with literature time-series data, which allows us to calculate physical and orbital system parameters, including GJ 436's stellar mass (M_{star} = 0.472^{+ 0.0636}_{- 0.0566} solar masses), planetary radius (R_{p} = 0.370^{+ 0.0149}_{- 0.0145} Jupiter radii), planetary mass (M_{p} = 0.075^{+ 0.0076}_{- 0.0072} Jupiter masses), implying a mean planetary density of rho_{p} = 1.48^{+ 0.116}_{- 0.103} 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 Spitzer monitoring observations.

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