Chemical Clues On Formation of Planetary Systems: Earth 'Siblings' Can Be Different

An international team of researchers, with the participation of IAC astronomers, has discovered that the chemical structure of Earth-like planets can be very different from the bulk composition of Earth. This may have a dramatic effect on the existence and formation of the biospheres and life on Earth-like planets.

The study of the photospheric stellar abundances of the planet-host stars is the key to understanding how protoplanets form, as well as which protoplanetary clouds evolve planets and which do not. These studies, which have important implications for models of giant planet formation and evolution, also help us to investigate the internal and atmospheric structure and composition of extrasolar planets..
Theoretical studies suggest that C/O and Mg/Si, are the most important elemental ratios in determining the mineralogy of terrestrial planets, and they can give us information about the composition of these planets. The C/O ratio controls the distribution of Si among carbide and oxide species, while Mg/Si gives information on the silicate mineralogy. In 2010 Bond et al. (2010b) carried out the first numerical simulations of planet formation in which the chemical composition of the proto-planetary cloud was taken as an input parameter. Terrestrial planets were found to form in all the simulations with a wide variety of chemical compositions so these planets might be very different from Earth.
Delgado Mena et al. (2010) have carried out the first detailed and uniform study of C, O, Mg and Si abundances for 61 stars with detected planets and 270 stars without detected planets from the homogeneous high-quality unbiased HARPS GTO sample. They found mineralogical ratios quite different from those in the Sun, showing that there is a wide variety of planetary systems which are unlike the Solar System. Many planetary-host stars present a Mg/Si value lower than 1, so their planets will have a high Si content to form species such as MgSiO3. This type of composition can have important implications for planetary processes like plate tectonics, atmospheric composition and volcanism.
'There could be billions of Earth-like planets in the Universe but a great majority of them may have a totally different internal and atmospheric structure. Building planets in chemically non-solar environments (which are very common in the Universe) may lead to the formation of strange worlds, very different from the Earth! The amount of radioactive and some refractory elements (especially Si) may have drastic implications for planetary processes such as plate tectonics and volcanic activity,' concludes Garik Israelian.
The latest numerical simulations have shown that a wide range of extrasolar terrestrial planet bulk compositions are likely to exist. Planets simulated as forming around stars with Mg/Si ratios less than 1 are found to be Mg-depleted (compared to Earth), consisting of silicate species such as pyroxene and various types of feldspars. Planetary carbon abundances also vary in accordance with the host stars' C/O ratio. The predicted abundances are in keeping with observations of polluted white dwarfs (expected to have accreted their inner planets during their previous red giant stage).
'The observed variations in the key C/O and Mg/Si ratios for known planetary host stars implies that a wide variety of extrasolar terrestrial planet compositions are likely to exist, ranging from relatively "Earth-like" planets to those that are dominated by C, such as graphite and carbide phases (e.g. SiC, TiC),' Delgado Mena stresses.
The results of Delgado Mena et al. (2010) were used in this study as they are the first to determine the abundance of all of the required elements in a completely internally consistent manner, using high quality spectra and an identical approach for all stars and elements, for a large sample of both host and non-host stars.
The chemical and dynamical simulations were combined by assuming that each embryo retains the composition of its formation location and contributes the same composition to the simulated terrestrial planet. The innermost terrestrial planets (located within ?0.5 AU from the host star) contain a significant amount of the refractory elements Al and Ca (?47% of the planetary mass). Planets forming beyond ?0.5 AU from the host star contain steadily less Al and Ca with increasing distance. One planetary system, 55 Cnc, has a C/O ratio above 1 (C/O = 1.12). This system produced carbon-enriched "Earth-like" planets. All of the terrestrial planets considered in this work have compositions dominated by O, Fe, Mg and Si, most of these elements being delivered in the form of silicates or metals (in the case of iron). However, important differences between those planets forming in systems with C/O < 0.8 (HD17051, HD19994) and those with C/O > 0.8 (55Cnc) have been found.
'We are working hard to decrease abundance measurement errors and make the results of theoretical models and numerical simulations more reliable,' comments González Hernández, 'There is much work to be done'.

Putting the Squeeze On Planets Outside Our Solar System

Just as graphite can transform into diamond under high pressure, liquid magmas may similarly undergo major transformations at the pressures and temperatures that exist deep inside Earth-like planets.
Using high-powered lasers, scientists at Lawrence Livermore National Laboratory and collaborators discovered that molten magnesium silicate undergoes a phase change in the liquid state, abruptly transforming to a more dense liquid with increasing pressure. The research provides insight into planet formation.
"Phase changes between different types of melts have not been taken into account in planetary evolution models," said lead scientist Dylan Spaulding, a University of California, Berkeley graduate student who conducted most of his thesis work at the Laboratory's Jupiter Laser Facility. "But they could have played an important role during Earth's formation and may indicate that extra-solar 'Super-Earth' planets are structured differently from Earth."
Melts play a key role in planetary evolution. The team said that pressure-induced liquid-liquid phase separation in silicate magmas may represent an important mechanism for global-scale chemical differentiation and also may influence the thermal transport and convective processes that govern the formation of a mantle and core early in planetary history. Liquid-liquid phase separation is similar to the difference between oil and vinegar -- they want to separate because they have different densities. In the new research, however, the researchers noticed a sudden change between liquid states of silicate magma that displayed different physical properties even though they both have the same composition when high pressure and temperatures were applied.
The team used LLNL's Janus laser and OMEGA at the University of Rochester to conduct the experiments to achieve the extreme temperatures and pressures that exist in the interiors of exoplanets -- those objects outside our solar system.
In each experiment, a powerful laser pulse generated a shock wave while it traveled through the sample. By looking for changes in the velocity of the shock and the temperature of the sample, the team was able to identify discontinuities that signaled a phase change in the material.
"In this case, the decay in shock-velocity and thermal emission both reverse themselves during the same brief time interval," Spaulding said.
The team concluded that a liquid-liquid phase transition in a silicate composition similar to what would be found in terrestrial planetary mantles could help explain the thermal-chemical evolution of exoplanet interiors.
The research appears in the Feb. 10 edition of the journal, Physical Review Letters.
Other LLNL authors include Jon Eggert, Peter Celliers, Damien Hicks, Gilbert Collins and Ray Smith. Other collaborators include UC Berkeley, the Carnegie Institution of Washington and Howard University.
The work was funded by the National Nuclear Security Administration, the National Science Foundation and the University of California.
The above story is reprinted from materials provided by DOE/Lawrence Livermore National Laboratory. 

Scientists Help Define Structure of Exoplanets

Using models similar to those used in weapons research, scientists may soon know more about exoplanets, those objects beyond the realm of our solar system.
In a new study, Lawrence Livermore National Laboratory scientists and collaborators came up with new methods for deriving and testing the equation of state (EOS) of matter in exoplanets and figured out the mass-radius and mass-pressure relations for materials relevant to planetary interiors.
Astronomers started detecting exoplanets 18 years ago and more than 700 have been found so far, the vast majority within the last two years. Interest is now growing in the structure and atmospheres of these worlds.
New equation-of-state work helps interpret the structure of exoplanets. As there is a minimal amount of data in each exoplanet observation, interpretation of their composition and structure depends largely on comparing their mass and radius with the composition expected given the distance from their parent star. The makeup implies a mass-radius relation, which relies heavily on EOS calculated from electronic structure theory and measured experimentally on Earth.
In the new research, lead Laboratory scientist Damian Swift, along with LLNL colleagues Jon Eggert, Damien Hicks, Sebastien Hamel, Kyle Caspersen, Eric Schwegler and Rip Collins, compared their modeling results with the observed masses and radii of exoplanets. Their results broadly support recent assumptions about the structures of exoplanets but can now take advantage of the accurate EOS models and data produced at Livermore.
"Current theoretical techniques for calculating electronic structures can predict EOS relevant to planetary interiors," Swift said. "But we still need experimental validation of these calculations; something that can now be done at the National Ignition Facility (NIF)."
LLNL's National Ignition Facility is the world's largest laser designed to perform research on national security, fusion experimentation and basic science, such as astrophysics.
The team made specific predictions for notable exoplanets having earth-like, rocky, icy compositions, with planetary center pressures ranging from 8 to 19,000 Mbar (8 million to 1.9 billion atmospheres of pressure).
"We have a project to measure material properties up to billions of atmospheres on NIF. We will eventually exceed the highest pressures investigated in the very small number of previous experiments using underground nuclear tests, which reached far above pressures that can be explored with other techniques currently available," Swift said.
Placing constraints on the structure of exoplanets requires accurate information about the compressibility of relevant compositions of matter, including iron alloys, silicates, and ices, under extreme conditions of pressure and temperature.
"This sets the record straight and presents a survey of exoplanet structure information using material properties generated for, and validated using, experimental capabilities at the national labs," Swift said.
Other collaborators include the University of Rostock and the University of Edinburgh. The research recently appeared in The Astrophysical Journal.
The research was funded by LLNL's Laboratory Research and Development program. LDRD is used to fund creative basic and applied research activities in areas aligned with the Lab's principal missions.

The above story is reprinted from materials provided by DOE/Lawrence Livermore National Laboratory.