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Earth's chemical composition is unusual for a habitable planet

September 27, 2015

Is Earth an outsider? In certain respects, the answer can be Yes. Our planet is accompanied by an extraordinarily large moon, which is indispensable for the stabilization of Earth's axis. On Earth there are oceans - large enough to preserve life but not too much water to flood the continents entirely. The magnetic field of Earth is also much stronger than on the other three terrestrial planets. Now the results of a new study suggest that Earth is also unusual among the other habitable planets of the galaxy.

Vardan Adibekyan of the Institute of Astrophysics and Space Sciences in Portugal and colleagues investigated stars similar in mass and radius to our Sun and having planets within the habitable zone. They found these stars contained significantly less iron and other metals than those of which only uninhabitable worlds are known.

According to the authors, the planets of a star are made of the same material as the star. Changes in the metal content of the star will affect the composition of its planets. Stars with a metal content comparable to the Sun typically have massive Super-Earths with dense atmospheres where there is probably no life. Also, these planets are closer to their star and are thus significantly hotter than Earth. For a habitable planet, Earth contains unusually large amounts of iron and other metals.

Stars with a planet within the habitable zone are poorer in metals and contain more magnesium than silicon and more silicon than iron, compared to the composition of Earth. Adibekyan and colleagues suspect that these stars have formed in an earlier epoch than our solar system, when the gas in the galaxy was not yet so much enriched with metals.

The authors examined a total of 45 planets in 20 star systems. 15 of these are planets within the habitable zone. Only for 3 planets the metal contents of their parent stars was higher than in our Sun, the other stars exhibited lower values. Only one planet around a Sun-like star has a metal content comparable to the Sun: Kepler-452b.

Although the results are still preliminary because of the small sample size, they may show an interesting trend. Most habitable planets should thus be older than Earth, because they formed in earlier epochs. As the galaxy evolved, the frequency of habitable planets should drop, as the more metal-rich stars seem to produce predominantly Super-Earths on tight orbits. Among the known habitable planets, Earth with its high metal content seems to actually stand out.

Habitable planets may be more alien than previously thought. They may be less rich in metals than Earth.

V. Adibekyan, P. Figueira, N. C. Santos (2015): Which type of planets do we expect to observe in
the Habitable Zone? Eingereicht bei Origins of Life and Evolution of Biospheres.

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