Without ancient star, Earth could be an icy, watery wasteland

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Earth’s solid surface and mild climate may be partly due to a massive star in the birth environment of the sun, according to new research.

Without the star’s injecting radioactive elements into the early solar system, our home planet could be a hostile ocean world covered in global ice sheets, the researchers say.

“So, it seems we were just extraordinarily lucky. Were we? Or are there systematic effects at play that distinguish solar system-like planetary systems from others?” asks Tim Lichtenberg of the National Centre of Competence in Research PlanetS in Switzerland, who started the research as a doctoral thesis at the institutes of Astronomy and Geophysics at ETH Zurich.

Drier climes

The role the potential presence of a massive star played on the formation of a planet intrigued Lichtenberg and colleagues. The simulations help solve some questions, while raising others, says Michael Meyer, an astronomer from the University of Michigan.

“It is great to know that radioactive elements can help make a wet system drier and to have an explanation as to why planets within the same system would share similar properties,” Meyer says.

“But radioactive heating may not be enough. How can we explain our Earth, which is very dry, indeed, compared to planets formed in our models? Perhaps having Jupiter where it is was also important in keeping most icy bodies out of the inner solar system.”

If the water content of a rocky planet is significantly greater than on Earth, a deep, global ocean covers the silicate mantle and an impenetrable layer of ice covers the ocean floor.

Water covers more than two thirds of the surface of the Earth, but in astronomical terms the inner terrestrial planets of our solar system appear very dry—fortunately, because too much of a good thing can do more harm than good.

All planets have a core, mantle (inside layer), and crust. If the water content of a rocky planet is significantly greater than on Earth, a deep, global ocean covers the silicate mantle and an impenetrable layer of ice covers the ocean floor. This prevents geochemical processes, such as the carbon cycle on Earth, which stabilize the climate and create surface conditions conducive to life as we know it.

Lichtenberg and his colleagues developed computer models to simulate the formation of planets from their building blocks, the so-called planetesimals—rocky-icy bodies of probably dozens of kilometers in size. During the birth of a planetary system the planetesimals form in a disk of dust and gas around the young star and grow into planetary embryos.

“Current thinking goes that Earth inherited most of its water from such partly water-rich planetesimals,” explains Lichtenberg, who now works as a postdoctoral fellow at the University of Oxford. “But if a terrestrial planet accretes lots of material from beyond the so-called snowline, it receives way too much water.”

As it turns out, however, if these planetesimals get heat from the inside, part of the initial water ice content evaporates and escapes to space before it can be delivered to the planet itself.

This may have happened shortly after the birth of our solar system 4.6 billion years ago and may still be ongoing in numerous places in the galaxy, as primeval traces in meteorites suggest.

Right when the proto-sun formed, a supernova occurred in the cosmic neighborhood. Radioactive elements including Aluminium-26 (Al-26) fused in this dying massive star, which injected them into our young solar system, either from its excessive stellar winds or via the supernova ejecta after the explosion. The decaying Al-26 then heated and dried water-delivering planetesimals from the inside.

2 types of systems

In their computer models, the researchers showed that the radiogenic heating of solar-like or higher Al-26 levels in a forming planetary system systematically dehydrates planetesimals prior to accretion onto planetary embryos.

“The results of our simulations suggest that there are two qualitatively different types of planetary systems,” summarizes Lichtenberg.

“There are those similar to our solar system, whose planets have little water. In contrast, there are those in which primarily ocean worlds are created because no massive star, and so no Al-26, was around when their host system formed. The presence of Al-26 during planetesimal formation can make an order-of-magnitude difference in planetary water budgets between these two species of planetary systems.”

Further questions remain, and future work will, for instance, need to investigate how Al-26 dehydration plays together with the growth of forming giant planets, such as proto-Jupiter in the early solar system.

The quantitative predictions from this work will help near-future space telescopes, dedicated to the hunt for extrasolar planets, to track potential traces and differences in planetary compositions, and refine the predicted implications of the Al-26 dehydration mechanism. And so the researchers are eagerly awaiting the launch of upcoming space missions, with which Earth-sized exoplanets outside our solar system will be observable. These will bring humanity ever-closer to understanding whether our home planet is one of a kind, or if there are “an infinity of worlds of the same kind as our own.”

The research appears in Nature Astronomy Letters.

Source: ETH Zurich, University of Michigan