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Title: Kepler-11 is a Solar Twin: Revising the Masses and Radii of Benchmark Planets Via Precise Stellar Characterization
Author: Megan Bedell, Jacob L. Bean, Jorge Melendez, Sean M. Mills, Daniel C. Fabrycky, Fabricio C. Freitas, Ivan Ramirez, Martin Asplund, Fan Liu, David Yong

The six planets of the Kepler-11 system are the archetypal example of a population of surprisingly low-density transiting planets revealed by the Kepler mission. We have determined the fundamental parameters and chemical composition of the Kepler-11 host star to unprecedented precision using an extremely high quality spectrum from Keck-HIRES (R \simeq 67,000, S/N per pixel \simeq 260 at 600 nm). Contrary to previously published results, our spectroscopic constraints indicate that Kepler-11 is a young main-sequence solar twin. The revised stellar parameters raise the densities of the Kepler-11 planets by about 30%, making them more typical of the emerging class of "puffy" close-in exoplanets. We obtain photospheric abundances of 22 elements and find that Kepler-11 has an abundance pattern similar to that of the Sun with a slightly higher overall metallicity. We additionally analyse the Kepler lightcurves using a photodynamical model and discuss the tension between spectroscopic and transit/TTV-based stellar density estimates.

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Title: Stability of the Kepler-11 System and its Origin
Author: Nikhil Mahajan (Toronto), Yanqin Wu (Toronto)

A significant fraction of Kepler systems are closely-packed, largely coplanar and circular. We study the stability of a 6-planet system, Kepler-11, to gain insights on the dynamics and formation history of such systems. Using a technique called `frequency maps' as fast indicators for long-term stability, we explore the stability of Kepler-11 system by analyzing the neighbourhood space around its orbital parameters. Frequency maps provide a visual representation of chaos and stability, and their dependence on orbital parameters. We find that the current system is stable, but lies within a few percent of several dynamically dangerous 2-body mean-motion resonances. Planet eccentricities are restricted below a small value, ~0.04, for long-term stability, but planet masses can be more than twice their reported values (thus, allowing for the possibility of mass-loss by past photoevaporation). Based on our frequency maps, we speculate on the origin for instability in closely-packed systems. We then proceed to investigate how the system can have been assembled. The stability constraints on Kepler-11 (mainly, eccentricity constraints) suggest that if the system were assembled in-situ, a dissipation mechanism must have been at work to neutralize eccentricity excitation. On the other hand, if migration was responsible for assembling the planets, there has to be little differential migration among the planets, to avoid them either getting trapped into mean motion resonances, or crashing into each other.

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Title: All Six Planets Known to Orbit Kepler-11 Have Low Densities
Authors: Jack J. Lissauer, Daniel Jontof-Hutter, Jason F. Rowe, Daniel C. Fabrycky, Eric D. Lopez, Eric Agol, Geoffrey W. Marcy, Katherine M. Deck, Debra A. Fischer, Jonathan J. Fortney, Steve B. Howell, Howard Isaacson, Jon M. Jenkins, Rea Kolbl, Dimitar Sasselov, Donald R. Short, William F. Welsh

The Kepler-11 planetary system contains six transiting planets ranging in size from 1.8 to 4.2 times the radius of Earth. Five of these planets orbit in a tightly-packed configuration with periods between 10 and 47 days. We perform a dynamical analysis of the system based upon transit timing variations observed in more than three years of \ik photometric data. Stellar parameters are derived using a combination of spectral classification and constraints on the star's density derived from transit profiles together with planetary eccentricity vectors provided by our dynamical study. Combining masses of the planets relative to the star from our dynamical study and radii of the planets relative to the star from transit depths together with deduced stellar properties yields measurements of the radii of all six planets, masses of the five inner planets, and an upper bound to the mass of the outermost planet, whose orbital period is 118 days. We find mass-radius combinations for all six planets that imply that substantial fractions of their volumes are occupied by constituents that are less dense than rock. The Kepler-11 system contains the lowest mass exoplanets for which both mass and radius have been measured.

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Title: A dynamical analysis of the Kepler-11 planetary system
Authors: Cezary Migaszewski, Mariusz Slonina, Krzysztof Gozdziewski

The Kepler-11 star hosts at least six transiting super-Earth planets detected through the precision photometric observations of the KEPLER mission (Lissauer et al.). In this paper, we re-analyse the available KEPLER data, using the direct N-body approach rather than an indirect TTV method in the discovery paper. The orbital modelling in the realm of the direct approach relies on the whole data set, rather than the times of mid-transits only. Most of the results in the original paper are confirmed and extended. We constrained the mass of the outermost planet g to less than 30 Earth masses. The mutual inclinations between orbits b and c as well as between orbits d and e are determined with a good precision, in the range of [1,5] degrees. Having several solutions to four qualitative orbital models of the Kepler-11 system, we analyse its global dynamics with the help of dynamical maps. They reveal very complex structure of the phase space with narrow regions of regular motion. The dynamics are governed by a dense net of three- and four-body mean motion resonances, forming the Arnold web. Overlapping of these resonances is a main source of instability. We found that the Kepler-11 system may be long-term stable only in particular multiple resonant configurations with small relative inclinations. The mass-radius data derived for all companions reveal a clear anti-correlation between the mean density of the planets with their distance from the star. It may reflect the formation and early evolution history of the system.

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All six planets transit the star; this means that all six planets' orbits appear to cross in front of their star as viewed from the Earth's perspective. Their inclinations relative to Earth's line of sight, or how far above or below the plane of sight they are, vary by a little more than a degree. This allows direct measurements of the planets' periods and relative diameters (compared to the host star) by monitoring each planet's transit of the star. Simulations suggest that the mean mutual inclinations of the planetary orbits are about 1, meaning the system is probably more coplanar (flatter) than the Solar System, where the corresponding figure is 2.3.
The estimated masses of planets b - f fall in the range between those of Earth and Neptune. Their estimated densities, all lower than that of Earth, imply that none of them have an Earth-like composition; a significant hydrogen atmosphere is indicated for planets d, e and perhaps f, while b and c probably contain substantial amounts of ices and either hydrogen and helium or both.

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Title: A Closely-Packed System of Low-Mass, Low-Density Planets Transiting Kepler-11
Authors: Jack J. Lissauer, Daniel C. Fabrycky, Eric B. Ford, William J. Borucki, Francois Fressin, Geoffrey W. Marcy, Jerome A. Orosz, Jason F. Rowe, Guillermo Torres, William F. Welsh, Natalie M. Batalha, Stephen T. Bryson, Lars A. Buchhave, Douglas A. Caldwell, Joshua A. Carter, David Charbonneau, Jessie L. Christiansen, William D. Cochran, Jean-Michel Desert, Edward W. Dunham, Michael N. Fanelli, Jonathan J. Fortney, Thomas N. Gautier III, John C. Geary, Ronald L. Gilliland, Michael R. Haas, Jennifer R. Hall, Matthew J. Holman, David G. Koch, David W. Latham, Eric Lopez, Sean McCauliff, Neil Miller, Robert C. Morehead, Elisa V. Quintana, Darin Ragozzine, Dimitar Sasselov, Donald R. Short, Jason H. Steffen

When an extrasolar planet passes in front of its star (transits), its radius can be measured from the decrease in starlight and its orbital period from the time between transits. Multiple planets transiting the same star reveal more: period ratios determine stability and dynamics, mutual gravitational interactions reflect planet masses and orbital shapes, and the fraction of transiting planets observed as multiples has implications for the planarity of planetary systems. But few stars have more than one known transiting planet, and none has more than three. Here we report Kepler spacecraft observations of a single Sun-like star that reveal six transiting planets, five with orbital periods between 10 and 47 days plus a sixth one with a longer period. The five inner planets are among the smallest whose masses and sizes have both been measured, and these measurements imply substantial envelopes of light gases. The degree of coplanarity and proximity of the planetary orbits imply energy dissipation near the end of planet formation.

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Six Small Planets Orbiting a Sun-Like Star Amaze Astronomers

A remarkable planetary system discovered by NASA's Kepler mission has six planets around a Sun-like star, including five small planets in tightly packed orbits. Astronomers at the University of California, Santa Cruz, and their coauthors analyzed the orbital dynamics of the system, determined the sizes and masses of the planets, and figured out their likely compositions -- all based on Kepler's measurements of the changing brightness of the host star (called Kepler-11) as the planets passed in front of it.
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NASA Finds Earth-Size Planet Candidates In Habitable Zone, Six Planet System

NASA's Kepler mission has discovered its first Earth-size planet candidates and its first candidates in the habitable zone, a region where liquid water could exist on a planet's surface. Five of the potential planets are near Earth-size and orbit in the habitable zone of smaller, cooler stars than our sun.
Candidates require follow-up observations to verify they are actual planets. Kepler also found six confirmed planets orbiting a sun-like star, Kepler-11. This is the largest group of transiting planets orbiting a single star yet discovered outside our solar system.

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NASA's Kepler Spacecraft Discovers Extraordinary New Planetary System

Scientists using NASA's Kepler, a space telescope, recently discovered six planets made of a mix of rock and gases orbiting a single sun-like star, known as Kepler-11, which is located approximately 2,000 light years from Earth.

"The Kepler-11 planetary system is amazing. It's amazingly compact, it's amazingly flat, there's an amazingly large number of big planets orbiting close to their star - we didn't know such systems could even exist" - Jack Lissauer, a planetary scientist and a Kepler science team member at NASA's Ames Research Center, Moffett Field, Calif.

In other words, Kepler-11 has the fullest, most compact planetary system yet discovered beyond our own.

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