The Diversity of Extrasolar Terrestrial Planets

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The Diversity of Extrasolar Terrestrial Planets

Post by Sirius_Alpha on 25th January 2010, 2:19 am

The Diversity of Extrasolar Terrestrial Planets
http://arxiv.org/abs/1001.3901

Abstract wrote:Extrasolar planetary host stars are enriched in key planet-building elements. These enrichments have the potential to drastically alter the building blocks available for terrestrial planet formation. Here we report on the combination of dynamical models of late-stage terrestrial planet formation within known extrasolar planetary systems with chemical equilibrium models of the composition of solid material within the disk. This allows us to constrain the bulk elemental composition of extrasolar terrestrial planets. A wide variety of resulting planetary compositions exist, ranging from those that are essentially "Earth-like", containing metallic Fe and Mg-silicates, to those that are dominated by graphite and SiC. This implies that a diverse range of terrestrial planets are likely to exist within extrasolar planetary systems.

A short, but very interesting paper. Well worth reading!

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Re: The Diversity of Extrasolar Terrestrial Planets

Post by Lazarus on 25th January 2010, 4:13 pm

Be nice to know where exactly they took the numbers from: several of those papers quote various different numbers for the same abundances. Depending on which ones I take, the C/O ratio can come out very different!

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Re: The Diversity of Extrasolar Terrestrial Planets

Post by Ryag Han on 26th January 2010, 2:06 pm

terrestrial planets have a higher density than gas giants.
if a star has a low metallicity (enrichment in element heavier than hydrogen )then it might be possible for a terrestrial planet not to
form in the first place.if it dose form,it will probably be of low density.

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Re: The Diversity of Extrasolar Terrestrial Planets

Post by Sirius_Alpha on 26th January 2010, 2:22 pm

It does not seem that terrestrial planets have as strong a metallicity dependence that gas giants do.

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Re: The Diversity of Extrasolar Terrestrial Planets

Post by Ryag Han on 26th January 2010, 7:02 pm

maybe,there's no way of knowing for sure just by speculating.
planets formation might vary greatly and have different factors
that don't appear at other planets.
hell,there are more terrestrial planets in our solar system
than gas giants,and the sun is an average star.
what about pulsar planets? most of them are super-earths.
how could they form when there in no accretion disk?
the formation of a pulsar is not exactly peaceful for a planet
to have any chance to form around it,but yet IT DOSE...and
i am getting away from the subject...

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Re: The Diversity of Extrasolar Terrestrial Planets

Post by Sirius_Alpha on 26th January 2010, 7:16 pm

Ryag Han wrote:maybe,there's no way of knowing for sure just by speculating.
Gas giant planets are preferentially found around stars with higher metallicities. This is one reason red dwarfs seem to not have as many. Yet red dwarfs commonly have terrestrial and Neptune-mass planets. As such, it does not seem that the metal abundance is as important for the creation of low-mass planets.

Ryag Han wrote:what about pulsar planets? most of them are super-earths. how could they form when there in no accretion disk?
They likely formed from the debris after the pulsar was created.

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Re: The Diversity of Extrasolar Terrestrial Planets

Post by Lazarus on 27th January 2010, 3:21 pm

Sirius_Alpha wrote:Gas giant planets are preferentially found around stars with higher metallicities. This is one reason red dwarfs seem to not have as many.
Red dwarfs are not intrinsically metal poor. It also seems that planet occurrence is a function of mass: low mass stars seem to end up with lower numbers of (detectable) planets, while the planet fractions for stars more massive than the Sun appears to be quite high.

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Re: The Diversity of Extrasolar Terrestrial Planets

Post by Stalker on 27th January 2010, 4:42 pm

It would be interessant to have a table with the abundance of elements named in the paper for every exoplanet host star. It would be possible to envisage compositions of superearths.

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Re: The Diversity of Extrasolar Terrestrial Planets

Post by Sirius_Alpha on 27th January 2010, 8:16 pm

Lazarus wrote:Red dwarfs are not intrinsically metal poor.

D:

That's what I grew up believing. Red dwarfs, being old, were formed in a much younger Universe that lacked the metal abundance that is enjoyed today. Was I mistaken? Or did that idea get ruled out and I failed to notice?

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Re: The Diversity of Extrasolar Terrestrial Planets

Post by Stalker on 28th January 2010, 3:28 am

The red dwarfs are statistically poorer in metals because there are red dwarfs of everything ages. There are young and old, while for other stars, there are only young stars.

But I think that exoplanets is just discovered around young red dwarfs because they could not form around old.

for examples, GJ1214, VB10 and GJ 876 are younger than the Sun, GJ584 isnot a lot older.

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Re: The Diversity of Extrasolar Terrestrial Planets

Post by Lazarus on 29th January 2010, 5:14 pm

Yes thanks for pointing that out, the issue of the survival of lower mass stars for long periods of time means the population should contain more lower-metallicity stars from way back when. The mass below which the star lifetime is longer than the age of the universe is somewhere around 0.8-0.9 solar masses, so if the low abundance was due to significant contamination by very old, very low metallicity stars then it should probably affect all stars later than G. This doesn't explain the continuing decrease of planet occurrence to lower masses... this agrees with predictions that the lower-mass discs around such stars should be less efficient at forming giant planets.

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The Compositional Diversity of Extrasolar Terrestrial Planets

Post by Mongo on 8th April 2010, 9:28 am

The Compositional Diversity of Extrasolar Terrestrial Planets: I. In-Situ Simulations

Abstract: Extrasolar planet host stars have been found to be enriched in key planet-building elements. These enrichments have the potential to drastically alter the composition of material available for terrestrial planet formation. Here we report on the combination of dynamical models of late-stage terrestrial planet formation within known extrasolar planetary systems with chemical equilibrium models of the composition of solid material within the disk. This allows us to determine the bulk elemental composition of simulated extrasolar terrestrial planets. A wide variety of resulting planetary compositions are found, ranging from those that are essentially "Earth-like", containing metallic Fe and Mg-silicates, to those that are dominated by graphite and SiC. This shows that a diverse range of terrestrial planets may exist within extrasolar planetary systems.

From the paper:

Of the 60 systems shown, 21 have C/O values above 0.8, implying that carbide minerals are important planet building materials in potentially more than 30% of known planetary systems. This implies that a similar fraction of protoplanetary disks should contain high abundances of carbonaceous grains. As comets represent some of the most primitive material within our planetary system, it is likely that a similar mass fraction will apply to the protoplanetary nebula. Furthermore, infrared spectral features at 3.43 and 3.53 μm observed in 4% of protoplanetary disks have been identified as being produced by nano-diamonds (Acke & van den Ancker 2006). Such high abundances of carbon-rich grains in nascent planetary systems is inconceivable if they have primary mineralogy similar to our Solar System, thus implying that C-rich planetary systems may be more common than previously thought. The idea of C-rich planets is not new (Kuchner & Seager 2005) but the potential prevalence of these bodies has not been previously recognized, nor have specific systems been identified as likely C-rich planetary hosts. These data clearly demonstrate that there are a significant number of systems in which terrestrial planets could have compositions vastly different to any body observed in our Solar System.

Ten known extrasolar planetary systems spanning the entire compositional spectrum of observed planetary systems were selected for this study. This wide range was purposefully chosen to explore the full diversity of possible extrasolar terrestrial planets.

Two very distinct types of condensation sequence are produced for the systems studied here - those resembling the Solar condensation sequence (HD27442, HD72659 and HD213240) and those in which carbide phases are present within the disk (55Cnc, Gl777, HD4203, HD17051, HD19994, HD108874 and HD177830). The C enrichment can further be classified as being low (0.78<C/O<1.0), in which C and carbide phases are present within a spatially narrow region of the disk at temperatures below 1800 K (55Cnc, Gl777, HD17051 and HD177830), and high (C/O>1.0) where a broader carbide-dominated region is stable for temperatures below 2300 K and thus extends into the innermost reaches of the disk (HD4203, HD19994 and HD108874).

The compositional differences between these different classes of systems result in significantly different compositions of the terrestrial planets that form in those systems, and the characteristics of those planets will be discussed in the following sections.

The interior mineralogy and structure of one of our model planets orbiting HD72659 is similar to Earth. It contains a pyroxene and feldspathic dominated crust (133 km deep) overlying an olivine mantle (985 km deep) with an Fe-Ni-S core (radius  4930 km). The crust is thicker than seen on Earth as we are currently neglecting density and phase changes. Given its structure and comparable mineralogy, we would expect to observe planetary processes similar to those seen on Earth. The planets location within the habitable zone of the host star suggests that a liquid water ocean is feasible, provided sufficient hydrous material can be delivered. Melting conditions and magma compositions are expected to be comparable and it is feasible that a liquid core would develop, resulting in the production of a magnetic dynamo. In general, based on their mass and composition, the terrestrial planets of HD72659 are likely to have structures and mineral assemblages similar to those observed in our system.

The simulated planet for HD177830 (the system with the highest Mg/Si value) is depleted in Si, relative to the Earth, resulting in high spinel and olivine content in the mantle (resembling that of type I kimberlites) and a thicker mellite and calcium dominated crust than found for HD72659 (309 km deep). The core would produce a considerable amount of heat via potential energy release during differentiation, potentially producing melts with compositions similar to komatiite (dominated by olivine with trace amounts of pyroxene and plagioclase). Volcanic eruptions would be comparable to basaltic flows observed on Earth due to the low silica content of the melt. However, given the thickness of the crust, extrusive volcanism and plate tectonics are unlikely to occur as high stress levels would be required to fracture the crust. Producing and sustaining such stresses would be challenging. Therefore, it is questionable whether or not a planet with this composition and structure would be tectonically active for long periods of time. Given the similar composition and size of the core compared to Earth, a magnetic dynamo is still expected to be produced within the core.

Finally, carbide planets are expected to form around HD108874. The resulting composition and structure is unlike any known planet. Its small size, refractory composition and possible lack of radioactive elements (due to the potential absence of phosphate species, common hosts for U and Th, and possible lack of feldspar and carbonates, the common host of K) will inhibit long-term geologic activity due to the difficulty of melting the mantle. Only large amounts of heat due to core formation and/or tidal heating would be able to provide the required mantle heating. Once all the primordial heat has been removed, it is unlikely that the mantle would remain molten on geologic timescales. Until that time, given the buoyancy of molten carbon, volcanic eruptions would be expected to be highly enriched in C. The core is also expected to be molten, thus making it likely that a magnetic dynamo would be produced (Gaidos & Selsis 2007). Note that this assumes that sufficient heat is initially available to melt the body and allow for differentiation and core formation to occur in the first place. In essence, although initially molten and probably active, old carbide planets of this type would be geologically dead.

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Re: The Diversity of Extrasolar Terrestrial Planets

Post by Sirius_Alpha on 8th April 2010, 10:12 am

Merged with this topic, as it's very, very similar.
(not a duplicate thread as I thought)

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Re: The Diversity of Extrasolar Terrestrial Planets

Post by Lazarus on 28th September 2010, 4:32 pm

More on the Mg/Si and C/O ratios of stars with and without planets.

No evidence for different planetary system architecture at high C/O, where the concept of the "tar line" might be more relevant than the "ice line".

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