Sunday, November 28

Is our planet exotic? – Extrasolar rock planets in the vicinity are surprisingly different and diverse –

But not “Earth twins”? Many exoplanets may only be Earth-like at first glance, as astronomical observations now suggest. According to this, extrasolar rock planets have a far more exotic and diverse geology and composition than previously assumed. The conditions and chemical cycles on supposed “Earth twins” could therefore be completely different from those on Earth or our neighboring planets, as the astronomers report in the journal “Nature Communications”.

They are roughly the size of the earth, have a stony surface and orbit in the habitable zone of their star: such celestial bodies are typically considered to be earth-like, potentially even life-friendly planets. Some of these “earth twins” even orbit nearby stars like Proxima Centauri, the red dwarf L 98-59 or TRAPPIST-1. How they are composed, what their surface is made of and which rocks shape their geology, has so far been beyond our possibilities of observation.

So far, planetary researchers have assumed that extrasolar rock planets are also made up of a metal core, silicate shell and granitic or basaltic crust – similar to the earth and its neighbors in the solar system. After all, these alien celestial bodies were once formed by accretion of material from the primordial cloud of their star and were probably exposed to similar physical conditions – so the previous assumption.

Planetary remnants around white dwarfs

But now new astronomical observations raise doubts. For their study, Keith Putirka from California State University in Fresno and Siyi Xu from the Gemini Observatory of the NOIRLab in Hawaii chose what at first sight appeared to be an unusual method: In order to track down the composition of extrasolar rock planets, they examined the light of 23 white people Dwarfs within around 4,000 light years of the sun.

The highlight: These white dwarfs are “polluted” – their gas shell contains the remains of planets and asteroids that once orbited these stars. As these stars expanded into red giants, they devoured their planetary satellites before ejecting part of their shell and collapsing their core to form the white dwarf. The earth, too, will be devoured in this way by the dying sun in the distant future.

“Cosmic mass spectrometer”

The remains of matter around such white dwarfs offer astronomers a unique opportunity to obtain information about the composition of the destroyed exoplanets. Typical elements from the crust and mantle of the planets can be detected using spectral analyzes of the light from these white dwarfs.

“The so-called polluted white dwarfs function as cosmic mass spectrometers and provide us with an almost direct analysis of the composition of exoplanets,” explain Putirka and Xu. The researchers compared this spectral data to what the inner planets of the solar system would leave behind in a star if they decay.

Surprisingly different

The surprising result: the elemental distribution of silicon, calcium, magnesium and iron only resembled what one would expect from the relic of an earth-like planet in very few cases. The data also had little in common with the composition known from the earth and its neighboring planets. “They have no counterparts in our solar system,” says Xu.

In many cases, the comparatively high content of magnesium oxide and relatively little silicate and calcium oxide was striking, as Putirka and Xu report. This suggests that this material mainly comes from the mantle of exoplanets. In contrast, no spectral signatures were found for granitic crusts, which were previously considered typical for rocky planets.

Completely new types of rock

The diversity is also astonishing: According to the data, the range of planetary compositions determined from the white dwarfs does not only go far beyond what is found in the solar system. The geology also deviates considerably from what planetary researchers had assumed for stony exoplanets around 4,000 other stars still in their mid-life.

“Most of these exoplanets had rock types that don’t even exist in the solar system,” says Xu. “Some of these exotic rocks lack olivine, but are saturated with quartz. Others lack orthopyroxes and are rich in periclase. ”In order to be able to describe these unusual mineral combinations at all, the researchers sometimes had to create new names, including quartz pyroxenite, periclase dunite or periclase wehrlite.

Different geology and features

This means, however, that the supposedly Earth-like exoplanets could also be significantly more exotic in terms of their geology and environmental conditions than previously assumed: “Some of these exotic rocks could produce a thicker crust or melt at lower temperatures. Others could be less stable and thus facilitate plate tectonics, ”says Putirka.

The interaction with water could also be significantly different from that of terrestrial rocks, as the researchers explain. Among other things, this would influence whether and when an ocean can form on such a planet. The geology of these exoplanets would have a decisive influence on how life-friendly these extrasolar worlds are and what form of extraterrestrial life could arise on them.

Who is the real exotic?

However, the results also raise the question of who the real exotic species are in our cosmic neighborhood: Is earth an exception in the portfolio of rocky planets or is it just one of many possible varieties of planet formation in space? “Such exoplanet studies also confront us with some still unresolved questions about Earth,” write Putirka and Xu.

In fact, even within our solar system, it has not yet been fully clarified why our neighboring planets, despite many similarities, also show some subtle but decisive differences in their composition. Thinking outside the box of our own planetary system could help find some of these answers. (Nature Communications, 2021; doi: 10.1038/s41467-021-26403-8)

Quelle: NOIRLab, W. M. Keck Observatory

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