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Earth’s exoplanet ‘siblings’ can be different

The finding of numerous exoplants, planets outside of our own solar system or extra-solar planets, have made astrophysics once again a hot topic.  A week does not seem to go by without a new discovery of an exoplanet by NASA’s Hubble telescope.  At the same time intense activity is taking place to understand the nature of these exoplanets.

The interest from professionals and public is understandable.  this forms part of one of the ‘big’ questions; “Are we alone in the universe?”

Finding a variety of planetary systems enables scientist help test theories and models of planetary formation.  One such model looks at the stars of these planetary systems.

The study of the the abundance of elements in the photosphere of stars that host planets is the key to understanding how protoplanets form.  The photosphere is the visible surface of a star.  Since a star is a ball of hot gas this is not a solid surface but actually a layer about 100km thick (very thin compared to the 700,000km radius of the Sun).  It is found that the ratios of elemental abundance in the photosphere is a good indication of the chemistry of any planets that form around it.

It also helps to model which protoplanetary clouds evolve planets and which do not. These studies have important implications for models of giant planet formation and evolution.  They also help us to investigate the internal and atmospheric structure and composition of extrasolar planets.

An international team of researchers has discovered that the chemical structure of Earth-like planets can be very different from the bulk composition of Earth.

They have presented results of simulations of terrestrial planet formation.  Their results have looked at three extrasolar planetary systems.  These systems have photospheres with Mg/Si  values less than 1.0.

Theoretical studies suggest that carbon/silicon (C/O) and magnesium/silicon (Mg/Si), are the most important elemental ratios in determining the mineralogy of terrestrial planets.  The ratios 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.

This resulting bulk chemical is expected to have a dramatic effect on the existence and formation of the biospheres and life on Earth-like planets.

The Earth’s upper mantle has an Mg/Si atomic ratio (1.27) which is also similar to Venus. These characteristics seem to predominate throughout the inner solar system.

During formation of the terrestrial planets, elements more volatile than silicon were depleted and may have been transported outwards to recondense in the lower-temperature environment of the outer asteroid belt. Some Si may also have been lost in this manner, although not enough to alter planetary Mg/Si ratios. However, recondensation of this Si on the relatively small mass of dust particles in the asteroid belt would have caused a substantial enrichment of Si relative to Mg.  It thus seems likely that it is the Mg/Si ratio of the inner planets ( ∼ 1.27), which is more representative of the solar nebula value.

An analogous process of radial chemical fractionation may also have occurred in the outer solar nebula, with volatile elements and silicon lost from the growing giant planets being recondensed onto cosmic interplanetary dust particles and cometary bodies further out from the Sun.

In 2010 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, rocky siblings of Earth, were found to form in all the simulations with a wide variety of chemical compositions.   So these planets might be very different from Earth.

A first detailed and uniform study of C, O, Mg and Si abundances was also carried out in 2010.  This was 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.  This 2010 study looked at 100 stars with detected planets and 270 stars without detected planets.  The majority of this data came from from the homogeneous high-quality European Southern Observatory HARPS studies.

In 2009 the HARPS team announced the discovery of the lightest exoplanet so far, Gliese 581e.  As well as the first exoplanet, Gliese 581d, to exist in the habitable zone.  A zone around its host star where surface water could exist.

Mineralogical ratios quite different from those in the Sun were found.  Showing that there is a wide variety of planetary systems which are unlike the Solar System.  Many planetary-host stars had a Mg/Si value lower than 1.  Suggesting that their planets will have a high Si content to form species such as MgSiO3.  The amount of radioactive and some refractory elements (especially Si) can have important implications for planetary processes like plate tectonics, atmospheric composition and volcanism.

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).

From these earlier studies the present authors believe 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.

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).

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 approximately half the distance of Earth to the Sun (~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 half-earth distances 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 (Iota Horologii, HD19994) and those with C/O > 0.8 (55Cnc) have been found.

These results highlight planets built 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!

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