Distant exoplanets not so different to Earth

Analysis finds terrestrial geochemistry and geophysics is far from unique. Barry Keily reports.

There might be some exoplanets out there that are similar to Earth. 
Lev Savitskiy

At least some exoplanets are geophysically and geochemically similar to Earth, researchers have found.

A team led by Alexandra Doyle of the University of California, Los Angeles, made the finding after analysing the atmospheres of six dead stars which had sustained impacts from asteroids or mini-planets falling out of orbit and smashing into them.

Such dead stars, known as White Dwarfs, are the remnant cores left behind after the ageing stars eject their hydrogen-rich outer layers. The result is a body about half the mass of the Sun and roughly the size of the Earth.

The surfaces of White Dwarfs comprise leftover hydrogen and helium. Any heavier elements, such as magnesium, iron and oxygen, are pulled towards the centre by gravity. However, spectrographic analysis reveals that in 50% of dwarfs with an effective temperature of around 25,000 degrees Celsius such elements remain as traces in the atmosphere. They are a tell-tale sign that at some point, rocky bodies smashed into them.

By analysing the relative frequencies of these added elements, Doyle and colleagues were able to assess the effect that oxygen previously played in the formation of the now destroyed baby planets, known as planetesimals.

The degree of oxidisation – known as fugacity – in rocks is a key indicator of the geochemical processes that formed them. For rocky bodies in the solar system – Earth and Mars, for instance – fugacity is approximately five orders of magnitude higher than that in the hydrogen-rich gas of the sun.

The results showed very similar measures – indicating that the geochemistry of the Solar System is by no means unique, and may, indeed, be very common in the universe.

“Our data indicate that rocky exoplanets constructed from these planetesimals should be geophysically and geochemically similar to rocky planets in the Solar System, including Earth,” they conclude.

For those playing at home, the White Dwarfs investigated are classified as: SDSS J104341.53+085558.2, SDSS J122859.92+104033.0, SBSS 1536+520, GD 40, SDSS J073842.56+183509.6, and LBQS 1145+0145.

The research is published in the journal Science.

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  1. https://science.sciencemag.org/cgi/doi/10.1126/science.aax3901
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