* Astronomy

Title: Alien Maps of an Ocean-Bearing World
Authors: N.B. Cowan, E. Agol, V.S. Meadows, T. Robinson (University of Washington), T.A. Livengood, D. Deming (NASA Goddard), C.M. Lisse (Johns Hopkins), M.F. A'Hearn, D.D. Wellnitz (University of Maryland), S. Seager (MIT), D. Charbonneau (Harvard)

To simulate the kinds of observations that will eventually be obtained for exoplanets, the Deep Impact spacecraft obtained light curves of Earth at seven wavebands spanning 300-1000 nm as part of the EPOXI mission of opportunity. In this paper we analyse disc-integrated light curves, treating Earth as if it were an exoplanet, to determine if we can detect the presence of oceans and continents. We present two observations each spanning one day, taken at gibbous phases. The rotation of the planet leads to diurnal albedo variations of 15-30%, with the largest relative changes occurring at the reddest wavelengths. To characterize these variations in an unbiased manner we carry out a principal component analysis of the multi-band light curves; this analysis reveals that 98% of the diurnal colour changes of Earth are due to only 2 dominant eigencolours. We use the time-variations of these two eigencolours to construct longitudinal maps of the Earth, treating it as a non-uniform Lambert sphere. We find that the spectral and spatial distributions of the eigencolours correspond to cloud-free continents and oceans; this despite the fact that our observations were taken on days with typical cloud cover. We also find that the near-infrared wavebands are particularly useful in distinguishing between land and water. Based on this experiment we conclude that it should be possible to infer the existence of water oceans on exoplanets with time-resolved broadband observations taken by a large space-based coronagraphic telescope.

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Title: A Super-Earth caught in a trap
Authors: Edyta Podlewska, Ewa Szuszkiewicz (Institute of Physics and CASA*, University of Szczecin, Poland)

This paper is an extension of the work done by Pierens & Nelson (2008) in which they have investigated the behaviour of a two-planet system embedded in a protoplanetary disc. They have put a Jupiter mass gas giant on the internal orbit and a lower mass planet on the external one. We consider here a similar problem taking into account a gas giant with masses in the range of 0.5 to 1 Jupiter mass and a Super-Earth as the outermost planet. By changing disc parameters and planet masses we have succeeded in getting the convergent migration which allows for the possibility of their resonant locking. However, in the case in which the gas giant has the mass of Jupiter, before any mean motion first order commensurability could be achieved, the Super-Earth is caught in a trap when it is very close to the edge of the gap opened by the giant planet. This confirms the result obtained by Pierens & Nelson (2008) in their simulations. Additionally, we have found that, in a very thin disc, an apsidal resonance is observed in the system if the Super-Earth is captured in the trap. Moreover, the eccentricity of the small planet remains low, while that of the gas giant increases slightly due to the imbalance between Lindblad and corotational resonances. We have also studied analogous systems in which the gas giant is allowed to take Sub-Jupiter masses. In this case, after performing an extensive survey over all possible parameters, we have succeeded in getting the 1:2 mean motion resonant configuration only in a disc with low aspect ratio and low surface density. However, the resonance is maintained just for few thousand orbits. Thus, we conclude that for typical protoplanetary discs the mean motion commensurabilities are rare if the Super-Earth is located on the external orbit relative to the gas giant.

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U.S. astronomers say some of the hundreds of planets that once orbited stars outside our solar system may have fallen into their stars and no longer exist.
University of Washington astronomer Rory Barnes says recent computer modelling has provided the first evidence gravitational forces might pull a planet into its parent star.

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Lightest exoplanet yet discovered
Well-known exoplanet researcher Michel Mayor today announced the discovery of the lightest exoplanet found so far. The planet, "e", in the famous system Gliese 581, is only about twice the mass of our Earth. The team also refined the orbit of the planet Gliese 581 d, first discovered in 2007, placing it well within the habitable zone, where liquid water oceans could exist. These amazing discoveries are the outcome of more than four years of observations using the most successful low-mass-exoplanet hunter in the world, the HARPS spectrograph attached to the 3.6-metre ESO telescope at La Silla, Chile.

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