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Caltech-led astronomers estimate that at least 100 billion planets populate the galaxy

Look up at the night sky and you'll see stars, sure. But you're also seeing planets - billions and billions of them. At least.
That's the conclusion of a new study by astronomers at the California Institute of Technology (Caltech) that provides yet more evidence that planetary systems are the cosmic norm. The team made their estimate while analysing planets orbiting a star called Kepler-32 - planets that are representative, they say, of the vast majority in the galaxy and thus serve as a perfect case study for understanding how most planets form.

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Title: Finding Exoplanets Orbiting Young Active Stars. I. Technique
Authors: V. E. Moulds, C. A. Watson, X. Bonfils, S. P. Littlefair, E. K. Simpson

Stellar activity such as starspots can induce radial velocity variations that can mask or even mimic the RV signature of orbiting exoplanets.

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Title: Flow of Planets Raises Short Period Fall Off
Authors: Stuart F. Taylor

After finding more planets than expected at the shortest period, there has been an effort to explain their numbers by weak tidal friction. However, we find that the strength of tidal dissipation that would produce the occurence distribution found from Kepler planet candidates is different for giant versus medium radii planets. This discrepancy can be resolved if there is a "flow" of the largest planets regularly arriving such that they go through a "hot Jupiter" stage. We also show a correlation of higher stellar Fe/H with higher eccentricity of giant planets that may be from smaller planets having been sent into the star by the migration of the larger planet. This disruption of the orbits of medium and smaller planets could account for the lower occurrence of "hot Neptune" medium radius planets.

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Title: Transit Timing Observations from Kepler: VII. Confirmation of 27 planets in 13 multiplanet systems via Transit Timing Variations and orbital stability
Authors: Jason H. Steffen (1), Daniel C. Fabrycky (2,3), Eric Agol (4), Eric B. Ford (5), Robert C. Morehead (5,6), William D. Cochran (7), Jack J. Lissauer (8), Elisabeth R. Adams (9), William J. Borucki (8), Steve Bryson (8), Douglas A. Caldwell (10), Andrea Dupree (9), Jon M. Jenkins (8), Paul Robertson (7), Jason F. Rowe (8), Shawn Seader (8), Susan Thompson (8), Joseph D. Twicken (8) ((1) Fermilab Centre for Particle Astrophysics, Batavia, IL, (2) UCO/Lick Observatory, University of California, Santa Cruz, CA, USA, (3) Hubble Fellow, (4) Department of Astronomy, University of Washington, Seattle, WA, (5) Astronomy Department, University of Florida, Gainesville, FL, USA, (6) National Science Foundation Graduate Research Fellow, (7) McDonald Observatory, The University of Texas, Austin, TX, USA, (8) NASA Ames Research Centre, Moffett Field, CA, USA, (9) Harvard-Smithsonian Centre for Astrophysics, Cambridge, MA, USA, (10) SETI Institute, Mountain View, CA, USA)

We confirm 27 planets in 13 planetary systems by showing the existence of statistically significant anti-correlated transit timing variations (TTVs), which demonstrates that the planet candidates are in the same system, and long-term dynamical stability, which places limits on the masses of the candidates---showing that they are planetary. %This overall method of planet confirmation was first applied to kepler systems 23 through 32. All of these newly confirmed planetary systems have orbital periods that place them near first-order mean motion resonances (MMRs), including 6 systems near the 2:1 MMR, 5 near 3:2, and one each near 4:3, 5:4, and 6:5. In addition, several unconfirmed planet candidates exist in some systems (that cannot be confirmed with this method at this time). A few of these candidates would also be near first order MMRs with either the confirmed planets or with other candidates. One system of particular interest, Kepler-56 (KOI-1241), is a pair of planets orbiting a 12th magnitude, giant star with radius over three times that of the Sun and effective temperature of 4900 K---among the largest stars known to host a transiting exoplanetary system.

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 The Art of Exoplanets

While astronomers have identified over 500 planets around other stars, they're all too small and distant to fill even a single pixel in our most powerful telescopes. That's why science must rely on art to help us imagine these strange new worlds.



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Title: Tidal evolution of exo-planetary systems: WASP-50, GJ 1214 and CoRoT-7
Authors: Dong Yao (1,2), Ji Jianghui (1) ((1) Purple Mountain Observatory, (2) Graduate School of Chinese Academy of Sciences)

We perform numerical simulations to investigate tidal evolution of two single-planet systems, that is, WASP-50 and GJ 1214 and a two-planet system CoRoT-7. The results of orbital evolution show that tidal decay and circularisation may play a significant role in shaping their final orbits, which is related to the initial orbital data in the simulations. For GJ 1214 system, different cases of initial eccentricity are also considered as only an upper limit of its eccentricity (0.27) is shown, and the outcome suggests a possible maximum initial eccentricity (0.4) in the adopted dynamical model. Moreover, additional runs with alternative values of dissipation factor Q^\prime_1 are carried out to explore tidal evolution for GJ 1214b, and these results further indicate that the real Q^\prime_1 of GJ 1214b may be much larger than its typical value, which may reasonably suggest that GJ 1214b bears a present-day larger eccentricity, undergoing tidal circularisation at a slow rate. For the CoRoT-7 system, tidal forces make two planets migrating towards their host star as well as producing tidal circularisation, and in this process tidal effects and mutual gravitational interactions are coupled with each other. Various scenarios of the initial eccentricity of the outer planet have also been done to investigate final planetary configuration. Tidal decay arising from stellar tides may still work for each system as the eccentricity decreases to zero, and this is in association with the remaining lifetime of each planet used to predict its future.

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Title: Comparing HARPS and Kepler surveys: On the alignment of multiple-planet systems
Authors: P. Figueira, M. Marmier, G. Boué, C. Lovis, N. C. Santos, M. Montalto, S. Udry, F. Pepe, M. Mayor

Aims. We study a subset of the planetary population characterized both by HARPS and Kepler surveys. We compare the statistical properties of planets in systems with m.sin i >5-10 M_Earth and R>2 R_Earth. If we assume that the underlying population has the same characteristics, the different detection sensitivity to the orbital inclination relative to the line of sight allows us to probe the planets' mutual inclination.
Methods. We considered the frequency of systems with one, two and three planets as dictated by HARPS data. We used Kepler's planetary period and host mass and radii distributions (corrected from detection bias) to model planetary systems in a simple yet physically plausible way. We then varied the mutual inclination between planets in a system according to different prescriptions (completely aligned, Rayleigh distributions and isotropic) and compared the transit frequencies with one, two or three planets with those measured by Kepler.
Results. The results show that the two datasets are compatible, a remarkable result especially because there are no tunable knobs other than the assumed inclination distribution. For m.sin i cutoffs of 7-10 M_Earth, which are those expected to correspond to the radius cutoff of 2 R_Earth, we conclude that the results are better described by a Rayleigh distribution with mode of 1 deg or smaller. We show that the best-fit scenario only becomes a Rayleigh distribution with mode of 5 deg if we assume a rather extreme mass-radius relationship for the planetary population.
Conclusions. These results have important consequences for our understanding of the role of several proposed formation and evolution mechanisms. They confirm that planets are likely to have been formed in a disk and show that most planetary systems evolve quietly without strong angular momentum exchanges .

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Title: One or more bound planets per Milky Way star from microlensing observations
Authors: A. Cassan, D. Kubas, J.-P. Beaulieu, M. Dominik, K. Horne, J. Greenhill, J. Wambsganss, J. Menzies, A. Williams, U.G. Jorgensen, A.Udalski, D.P. Bennett, M.D. Albrow, V. Batista, S. Brillant, J.A.R. Caldwell, A. Cole, Ch. Coutures, K.H. Cook, S. Dieters, D. Dominis Prester, J. Donatowicz, P. Fouque, K. Hill, N. Kains, S. Kane, J.-B. Marquette, R. Martin, K.R. Pollard, K.C. Sahu, C. Vinter, D. Warren, B.Watson, M.Zub, T. Sumi, M.K. Szymanski, M. Kubiak, R. Poleski, I. Soszynski, K. Ulaczyk, G. Pietrzynski, L. Wyrzykowski

Most known extrasolar planets (exoplanets) have been discovered using the radial velocity^{\bf 1,2} or transit^{\bf 3} methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17--30% (refs 4, 5) of solar-like stars host a planet. Gravitational microlensing^{\bf 6
m{\bf -}\bf 9}, on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing^{\bf 10}. These planets are at least as numerous as the stars in the Milky Way^{\bf 10}. Here we report a statistical analysis of microlensing data (gathered in 2002--07) that reveals the fraction of bound planets 0.5--10 AU (Sun--Earth distance) from their stars. We find that 17_{\bf -9}^{\bf +6}% of stars host Jupiter-mass planets (0.3--10 \MJ, where \MJ {\bf = 318} \Mearth and \Mearth is Earth's mass). Cool Neptunes (10--30 \Mearth) and super-Earths (5--10 \Mearth) are even more common: their respective abundances per star are 52_{\bf -29}^{\bf +22}% and 62_{\bf -37}^{\bf +35}%. We conclude that stars are orbited by planets as a rule, rather than the exception.

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Title: The McDonald Observatory Planet Search: New Long-Period Giant Planets, and Two Interacting Jupiters in the HD 155358 System 
Authors: Paul Robertson, Michael Endl, William D. Cochran, Phillip J. MacQueen, Robert A. Wittenmyer, J. Horner, Erik J. Brugamyer, Attila E. Simon, Stuart I. Barnes, Caroline Caldwell 

We present high-precision radial velocity (RV) observations of four solar-type (F7-G5) stars - HD 79498, HD 155358, HD 197037, and HD 220773 - taken as part of the McDonald Observatory Planet Search Program. For each of these stars, we see evidence of Keplerian motion caused by the presence of one or more gas giant planets in long-period orbits. We derive orbital parameters for each system, and note the properties (composition, activity, etc.) of the host stars. While we have previously announced the two-gas-giant HD 155358 system, we now report a shorter period for planet c. This new period is consistent with the planets being trapped in mutual 2:1 mean-motion resonance. We therefore perform an in-depth stability analysis, placing additional constraints on the orbital parameters of the planets. These results demonstrate the excellent long-term RV stability of the spectrometers on both the Harlan J. Smith 2.7 m telescope and the Hobby-Eberly telescope. 

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NASA's Kepler Announces 11 Planetary Systems Hosting 26 Planets
 
NASA's Kepler mission has discovered 11 new planetary systems hosting 26 confirmed planets. These discoveries nearly double the number of verified planets and triple the number of stars known to have more than one planet that transits, or passes in front of, the star. Such systems will help astronomers better understand how planets form.
The planets orbit close to their host stars and range in size from 1.5 times the radius of Earth to larger than Jupiter. Fifteen are between Earth and Neptune in size. Further observations will be required to determine which are rocky like Earth and which have thick gaseous atmospheres like Neptune. The planets orbit their host star once every six to 143 days. All are closer to their host star than Venus is to our sun.

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