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. Read more (942kb, PDF)
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. Read more (359kb, PDF)
Title: Extrasolar Planets Orbiting Active StarsAuthors: Jörg WeingrillNew 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.Read more (9725kb, PDF)
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?
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.
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.
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.