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TOPIC: Terrestrial Exoplanets


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 Three Earthlike planets identified by Cornell astronomers

Cornell astronomers, using data from the NASA Kepler Mission, have identified three Earthlike planets orbiting their own suns, all of which could be hospitable to life.
The three planets orbit within their host stars' "habitable zones" -- the orbital distance in which liquid water could exist, and the sweet spot for determining whether life could be possible. The host stars -- KOI (Kepler Object of Interest) 463.01, KOI 812.03 and KOI 854.01 -- are located in areas of the sky between the constellations Cygnus and Lyra, in the range of a few hundred to a few thousand light years away.

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Title: Theoretical Spectra of Terrestrial Exoplanet Surfaces
Authors: Renyu Hu, Bethany L. Ehlmann, Sara Seager

We investigate spectra of airless rocky exoplanets with a theoretical framework that self-consistently treats reflection and thermal emission. We find that a silicate surface on an exoplanet is spectroscopically detectable via prominent Si-O features in the thermal emission bands of 7 - 13 µm and 15 - 25 µ m. The variation of brightness temperature due to the silicate features can be up to 20 K for an airless Earth analogue, and the silicate features are wide enough to be distinguished from atmospheric features with relatively high-resolution spectra. The surface characterization thus provides a method to unambiguously identify a rocky exoplanet. Furthermore, identification of specific rocky surface types is possible with the planet's reflectance spectrum in near-infrared broad bands. A key parameter to observe is the difference between K band and J band geometric albedos (A_g (K)-A_g (J)): A_g (K)-A_g (J) > 0.2 indicates that more than half of the planet's surface has abundant mafic minerals, such as olivine and pyroxene, in other words primary crust from a magma ocean or high-temperature lavas; A_g (K)-A_g (J) < -0.09 indicates that more than half of the planet's surface is covered or partially covered by water ice or hydrated silicates, implying extant or past water on its surface. Also, surface water ice can be specifically distinguished by an H-band geometric albedo lower than the J-band geometric albedo. The surface features can be distinguished from possible atmospheric features with molecule identification of atmospheric species by transmission spectroscopy. We therefore propose that mid-infrared spectroscopy of exoplanets may detect rocky surfaces, and near-infrared spectrophotometry may identify ultramafic surfaces, hydrated surfaces and water ice.

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Title: Resolving the terrestrial planet forming regions of HD113766 and HD172555 with MIDI
Authors: R. Smith, M.C. Wyatt, C.A. Haniff

We present new MIDI interferometric and VISIR spectroscopic observations of HD113766 and HD172555. Additionally we present VISIR 11um and 18um imaging observations of HD113766. These sources represent the youngest (16Myr and 12Myr old respectively) debris disc hosts with emission on <1AU (>35mas). When combined with limits from TReCS imaging the dust at ~10um is constrained to lie somewhere in the region 1-8AU. Observations at ~18um reveal extended disc emission which could originate from the outer edge of a broad disc, the inner parts of which are also detected but not resolved at 10um, or from a spatially distinct component. These observations provide the most accurate direct measurements of the location of dust at 1-8AU that might originate from the collisions expected during terrestrial planet formation. These observations provide valuable constraints for models of the composition of discs at this epoch and provide a foundation for future studies to examine in more detail the morphology of debris discs.

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Title: Extrasolar Planets Orbiting Active Stars
Authors: Jörg Weingrill

New discoveries of transiting extrasolar planets are reported weekly. Ground based surveys as well as space borne observatories like CoRoT and Kepler are responsible for filling the statistical voids of planets on distant stellar systems.
I want to discuss the stellar activity and its impact on the discovery of extrasolar planets. Up to now the discovery of small rocky planets called "Super-Earths" like CoRoT-7b and Kepler-10b are the only exceptions. The question arises, why among over 500 detected and verified planets the amount of smaller planets is strikingly low. An explanation besides that the verification of small planets is an intriguing task, is the high level of stellar activity that has been observed.
Stellar activity can be observed at different time-scales from long term irradiance variations similar to the well known solar cycle, over stellar rotation in the regime of days, down to the observations of acoustic modes in the domain of minutes. But also non periodic events like flares or the activity signal of the granulation can prevent the detection of a transiting Earth sized planet.
I will describe methods to detect transit-like signals in stellar photometric data, the influences introduced by the star, the observer and their impact on the success. Finally different mathematical models and approximations of transit signals will be examined on their sensibility of stellar activity.
I present a statistical overview of stellar activity in the CoRoT dataset. The influence of stellar activity will be analysed on different transiting planets: CoRoT-2b, CoRoT-4b und CoRoT-6b.
Stellar activity can prevent the successful detection of a transiting planet, where CoRoT-7b marks the borderline. Future missions like Plato will be required to provide long-term observations with mmag precision to overcome the limitations set by active stars in our Galactic neighbourhood.

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Title: The Occurrence Rate of Earth analogue Planets Orbiting Sunlike Stars
Authors: Joseph Catanzarite, Michael Shao (Jet Propulsion Laboratory, California Institute of Technology)

Kepler is a space telescope that searches Sun-like stars for planets. Its major goal is to determine {\eta}_Earth, the fraction of Sunlike stars that have planets like Earth. When a planet 'transits' or moves in front of a star, Kepler can measure the concomitant dimming of the starlight. From analysis of the first four months of those measurements for over 150,000 stars, Kepler's science team has determined sizes, surface temperatures, orbit sizes and periods for over a thousand new planet candidates. Here, we show that 1.4% to 2.7% of stars like the Sun are expected to have Earth analogue planets, based on the Kepler data release of Feb 2011. The estimate will improve when it is based on the full 3.5 to 6 year Kepler data set. Accurate knowledge of {\eta}_Earth is necessary to plan future missions that will image and take spectra of Earthlike planets. Our result that Earths are relatively scarce means that a substantial effort will be needed to identify suitable target stars prior to these future missions.

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Title: Colours of a Second Earth II: Effects of Clouds on Photometric Characterisation of Earth-like Exoplanets
Authors: Yuka Fujii, Hajime Kawahara, Yasushi Suto, Satoru Fukuda, Teruyuki Nakajima, Timothy A. Livengood, Edwin L. Turner

As a test-bed for future investigations of directly imaged terrestrial exoplanets, we present the recovery of the surface components of the Earth from multi-band diurnal light curves obtained with the EPOXI spacecraft. We find that the presence and longitudinal distribution of ocean, soil and vegetation are reasonably well reproduced by fitting the observed colour variations with a simplified model composed of a priori known albedo spectra of ocean, soil, vegetation, snow and clouds. The effect of atmosphere, including clouds, on light scattered from surface components is modelled using a radiative transfer code. The required noise levels for future observations of exoplanets are also determined. Our model-dependent approach allows us to infer the presence of major elements of the planet (in the case of the Earth, clouds and ocean) with observations having S/N \gtrsim 10 in most cases and with high confidence if S/N \gtrsim 20. In addition, S/N \gtrsim 100 enables us to detect the presence of components other than ocean and clouds in a fairly model-independent way. Degradation of our inversion procedure produced by cloud cover is also quantified. While cloud cover significantly dilutes the magnitude of colour variations compared to the cloudless case, the pattern of colour changes remains. Therefore, the possibility of investigating surface features through light curve fitting remains even for exoplanets with cloud cover similar to the Earth's.

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The hunt is on for a distant planet similar to our own. Astronomers should decide just how similar it needs to be, before the candidates start pouring in.

The search for a second Earth has long enthralled readers of science fiction. What rich and varied life could it contain? What would such a discovery mean for humanity's own place in the Universe? How many similar planets are out there? The question is more than a philosophical puzzle, and it comes with a hard scientific edge that should be considered sooner rather than later. As the search for planets beyond the Solar System widens and public interest in the quest grows, at which point should astronomers declare the hunt for another Earth a success?
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The possibility of discovering a planet that is small, cool, rocky, orbiting a sunlike star and able to host life - an Earth twin, in other words - has made the search for planets outside of our solar system, or exoplanets, one of the hottest research areas in physical science. This three-part series explores MIT researchers' roles in the quest to find an Earth twin and the effort to make sense of the 500 exoplanets that have been discovered since 1995.
In September, researchers announced the discovery of Gliese 581g, a rocky planet with a mass that is just three to four times that of Earth. If the discovery is confirmed with independent data, it could be the closest that planetary scientists have come to finding a planet outside the solar system that resembles our own. Although other planets with nearly the same mass as Earth have been discovered, Gliese 581g is the smallest planet that is also in the "Goldilocks zone," or at a distance from its host star to make the planet's temperature cool enough for liquid water to exist on its surface.

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Many scientists speculate that our galaxy could be full of places like Pandora from the movie "Avatar" -- Earth-like worlds in solar systems besides our own.
That doesn't mean such worlds have been easy to find, however. Of the 400-plus planets so far discovered, none could support life as we know it on Earth.

"The problem with finding Earth-like planets is that their host stars can emit 10 million times more infrared light than the planet itself. And because planets like ours are small and orbit very close to their respective stars, it makes Earths almost impossible to see" - Stefan Martin, an engineer at NASA's Jet Propulsion Laboratory, Pasadena, California.

Together with A.J. Booth (formerly at JPL and now at Sigma Space Corp., Lanham, Md.), Martin may have developed a way to make this almost impossible feat a reality.

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Watery, rocky planets are commonplace in the Milky Way

Astronomers have found evidence that rocky planets are commonplace in the Milky Way, or the Galaxy.
Dr. Jay Farihi, Leicester University scientist and lead author, surveyed white dwarfs, the compact remnants of stars that were once like our Sun, and found that many show signs of contamination by heavier elements and possibly even water, improving the prospects for extraterrestrial life.

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