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Post Info TOPIC: GJ 436


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Posts: 131433
Date:
GJ 436b
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Title: A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b
Author: David Ehrenreich, Vincent Bourrier, Peter J. Wheatley, Alain Lecavelier des Etangs, Guillaume Hébrard, Stéphane Udry, Xavier Bonfils, Xavier Delfosse, Jean-Michel Désert, David K. Sing, Alfred Vidal-Madjar

Exoplanets orbiting close to their parent stars could lose some fraction of their atmospheres because of the extreme irradiation. Atmospheric mass loss primarily affects low-mass exoplanets, leading to suggest that hot rocky planets might have begun as Neptune-like, but subsequently lost all of their atmospheres; however, no confident measurements have hitherto been available. The signature of this loss could be observed in the ultraviolet spectrum, when the planet and its escaping atmosphere transit the star, giving rise to deeper and longer transit signatures than in the optical spectrum. Here we report that in the ultraviolet the Neptune-mass exoplanet GJ 436b (also known as Gliese 436b) has transit depths of 56.3 ± 3.5% (1 sigma), far beyond the 0.69% optical transit depth. The ultraviolet transits repeatedly start ~2 h before, and end >3 h after the ~1 h optical transit, which is substantially different from one previous claim (based on an inaccurate ephemeris). We infer from this that the planet is surrounded and trailed by a large exospheric cloud composed mainly of hydrogen atoms. We estimate a mass-loss rate in the range of ~10^8-10^9 g/s, which today is far too small to deplete the atmosphere of a Neptune-like planet in the lifetime of the parent star, but would have been much greater in the past.

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Hubble Sees a 'Behemoth' Bleeding Atmosphere Around a Warm Neptune-Sized Exoplanet

Astronomers using NASA's Hubble Space Telescope have discovered an immense cloud of hydrogen dubbed "The Behemoth" bleeding off a planet orbiting a nearby star. The enormous, comet-like feature is about 50 times the size of the parent star. The hydrogen is evaporating from a warm, Neptune-sized planet, due to extreme radiation from the star.
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Posts: 131433
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RE: GJ 436
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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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Posts: 131433
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GJ 436b
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Title: Compositional diversity in the atmospheres of hot Neptunes, with application to GJ 436b
Authors: Julianne I. Moses, Michael R. Line, Channon Visscher, Molly R. Richardson, Nadine Nettelmann, Jonathan J. Fortney, Kevin B. Stevenson, Nikku Madhusudhan

Neptune-sized extrasolar planets that orbit relatively close to their host stars -- often called "hot Neptunes" -- are common within the known population of exoplanets and planetary candidates. Similar to our own Uranus and Neptune, inefficient accretion of nebular gas is expected produce hot Neptunes whose masses are dominated by elements heavier than hydrogen and helium. At high atmospheric metallicities of 10-10,000x solar, hot Neptunes will exhibit an interesting continuum of atmospheric compositions, ranging from more Neptune-like, H2-dominated atmospheres to more Venus-like, CO2-dominated atmospheres. We explore the predicted equilibrium and disequilibrium chemistry of generic hot Neptunes and find that the atmospheric composition varies strongly as a function of temperature and bulk atmospheric properties such as metallicity and the C/O ratio. Relatively exotic H2O, CO, CO2, and even O2-dominated atmospheres are possible for hot Neptunes. We apply our models to the case of GJ 436b, where we find that a CO-rich, CH4-poor atmosphere can be a natural consequence of a very high atmospheric metallicity. From comparisons of our results with Spitzer eclipse data for GJ 436b, we conclude that although the spectral fit from the high-metallicity forward models is not quite as good as the fit obtained from pure retrieval methods, the atmospheric composition predicted by these forward models is more physically and chemically plausible. High-metallicity atmospheres (orders of magnitude in excess of solar) should therefore be considered as a possibility for GJ 436b and other hot Neptunes.

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RE: GJ 436
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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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GJ 436c and GJ 436d
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Title: Two nearby sub-Earth-sized exoplanet candidates in the GJ 436 system
Authors: Kevin B. Stevenson, Joseph Harrington, Nate B. Lust, Nikole K. Lewis, Guillaume Montagnier, Julianne I. Moses, Channon Visscher, Jasmina Blecic, Ryan A. Hardy, Patricio Cubillos, Christopher J. Campo

We report the detection of UCF-1.01, a strong exoplanet candidate with a radius 0.66 ± 0.04 times that of Earth (Earth radii). 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 Earth radii exoplanet candidate (labelled 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-1.01's equilibrium temperature (T_{eq}, where emitted and absorbed radiation balance for an equivalent blackbody) is 860 K, making the planet unlikely to harbour 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.

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RE: GJ 436
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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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UCF 1.01
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UCF Discovers Exoplanet Neighbour -- University's First Planet

The University of Central Florida has detected what could be its first planet, only two-thirds the size of Earth and located right around the corner, cosmically speaking, at a mere 33-light years away.
The exoplanet candidate called UCF 1.01, is close to its star, so close it goes around the star in 1.4 days. The planets surface likely reaches temperatures of more than 1,000 degrees Fahrenheit. The discoverers believe that it has no atmosphere, is only two-thirds the gravity of Earth and that its surface may be volcanic or molten.

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GJ 436
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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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RE: GJ 436b
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A Neptune-sized exoplanet orbiting a small star about 33 light years away could be a key stepping stone on the path to making sense of an Earth twin.
The finding is the latest advance in the quest to measure Earth-like planets that could possibly host signs of life, which researchers expect to find in the next few years.

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