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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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Stellar 'pollution' may be remains of watery planets

A generation of planets may now be no more than a whiff of pollution in the atmospheres of their dead parent stars. If so, it would suggest that rocky planets are common, and hints that most such planets have water.
White dwarfs - the dense remnants of ordinary stars - usually have very pure atmospheres dominated by the lightweight elements hydrogen and helium; heavier elements tend to sink into a star's interior. But about 20 per cent of white dwarfs are tainted by traces of heavier elements.

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Rocky planets 'are commonplace' in our Galaxy

An international team of astronomers have discovered compelling evidence that rocky planets are commonplace in our Galaxy. Leicester University scientist and lead researcher Dr Jay Farihi 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. On Tuesday 13th April Dr Farihi will present his results at the RAS National Astronomy Meeting (NAM 2010) in Glasgow.
White dwarf stars are the endpoint of stellar evolution for the vast majority (>90%) of all stars in the Milky Way, including our Sun.  Because they should have essentially pure hydrogen or pure helium atmospheres, if heavier elements (in astronomy described as metals, examples including calcium, magnesium and iron) are found then these must be external pollutants. For decades, it was believed that the interstellar medium, the tenuous gas between the stars, was the source of metals in these polluted white dwarfs.
Farihi and his team used data from the Sloan Digital Sky Survey (SDSS), a project that aims to survey the sky in infrared light, imaging more than 100 million objects and following up 1 million of these by obtaining their spectrum (dispersing the light by colour).
By examining the positions, motions and spectra of the white dwarfs identified in the SDSS, Farihi and his team show that this is no longer a viable theory. Instead, rocky planetary debris is almost certainly the culprit in most or all cases.
The new work indicates that at least 3% and perhaps as much as 20% of all white dwarfs are contaminated in this way, with the debris most likely in the form of rocky minor planets with a total mass of about that of a 140 km diameter asteroid.
This implies that a similar proportion of stars like our Sun, as well as stars that are a little more massive like Vega and Fomalhaut, build terrestrial planetary systems.  Astronomers are thus playing the role of celestial archaeologists by studying the 'ruins' of rocky planets and or their building blocks.

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It seems increasingly likely that, as they stare at the heavens, astronomers are going to find an Earth out there, or at least something that they can plausibly claim is a rocky planet where water could splash at the surface and -- who knows? -- harbour some kind of life. But it's also clear that, when they make their big discovery, the astronomers might want to hire movie director James Cameron to help with the special effects.
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Super-Earth 'began as gas giant'

The smallest-known planet outside our Solar System, Corot-7b, probably began as a Saturn-sized "gas giant" planet, say researchers.
They say the planet, which orbits at one-sixtieth the distance from the Earth to the Sun, has had much of its mass boiled away by the star's heat.
The team also suggests that, if its orbit is not exactly circular, Corot-7b is a hotbed of volcanic activity.

The research was presented at the 215th American Astronomical Society meeting.
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