If a massive star in the birth environment of the Sun –a yellow G2 dwarf star, one of a trillion stars in the Milky Way–had not injected radioactive elements into the early solar system, our home planet could be a hostile ocean world covered in global ice sheets. Some astronomers now think that every Sun-like star has at least one ‘Earth like’ planet in the so-called ‘habitable zone’ around a star, the region where liquid water can exist.
When our proto-Sun formed, a supernova occurred in the cosmic neighborhood. Radioactive elements, including aluminium-26, were fused in this dying massive star and got injected into our young solar system, either from its excessive stellar winds or via the supernova ejecta after the explosion
This very large star, perhaps 30 times as massive as our Sun, exploded, blowing away its outer layers, including aluminium-26, about a million years before the final explosion of the remaining core of the star. This initial outburst would have been enough to trigger the collapse of the knot in the giant molecular cloud from which the Solar System formed. After some million years, the star went supernova, showering its cosmic surroundings,
The supernova explosion must have occurred within a tenth of a light year of the forming Solar System, observes John Gribbin in Alone in the Universe -Why Our Planet Is Unique, when the Sun was less than 2 million years old. No other supernova has ever exploded in such close proximity to the Sun – if it had, life on Earth would have been exterminated. It cannot be a coincidence that this seemingly unlikely event occurred where the Solar System was forming, and the natural explanation is that both the Sun and the supernova were members of a cluster of stars that formed together in the same gas cloud, and have since gone their separate ways.
“The results of our simulations suggest that there are two qualitatively different types of planetary systems,” said Tim Lichtenberg of the National Centre of Competence in Research Planets in Switzerland. “There are those similar to our solar system, whose planets have little water, and those in which primarily ocean worlds are created because no massive star was around when their host system formed.”
Lichtenberg and colleagues, including University of Michigan astronomer Michael Meyer, were initially intrigued by the role the potential presence of a massive star played on the formation of a planet. Meyer said the simulations help solve some questions, while raising others.
“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 said. “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.”
Researchers say while water covers more than two-thirds of the surface of Earth, in astronomical terms, the inner terrestrial planets of our solar system are 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, the mantle is covered by a deep, global ocean and an impenetrable layer of ice on the ocean floor. This prevents geochemical processes, such as the carbon cycle on Earth, that stabilize the climate and create surface conditions conducive to life as we know it.
The researchers 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.
As these planetesimals are heated 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 internal heating may have happened shortly after the birth of our solar system 4.6 billion years ago, as primeval traces in meteorites suggest, and may still be ongoing in numerous places.
The researchers say 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.
They 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.”
Their study appears in Nature Astronomy. Other researchers include those from the Swiss Federal Institute of Technology, University of Bayreuth and University of Bern.
A 2012 study published by University of Chicago researchers challenged the notion that the force of an exploding star prompted the formation of the solar system. In this study, published online last month in Earth and Planetary Science Letters, authors Haolan Tang and Nicolas Dauphas found the radioactive isotope iron 60—the telltale sign of an exploding star—low in abundance and well mixed in solar system material. As cosmochemists, they look for remnants of stellar explosions in meteorites to help determine the conditions under which the solar system formed. Some remnants are radioactive isotopes: unstable, energetic atoms that decay over time. Scientists in the past decade have found high amounts of the radioactive isotope iron 60 in early solar system materials.
“If you have iron 60 in high abundance in the solar system, that’s a ‘smoking gun’—evidence for the presence of a supernova,” said Dauphas, professor in geophysical sciences. Iron 60 can only originate from a supernova, so scientists have tried to explain this apparent abundance by suggesting that a supernova occurred nearby, spreading the isotope through the explosion.
But Tang and Dauphas discovered that levels of iron 60 were uniform and low in early solar system material. They arrived at these conclusions by testing meteorite samples. To measure iron 60’s abundance, they looked at the same materials that previous researchers had worked on, but used a different, more precise approach that yielded evidence of very low iron 60.
The Daily Galaxy, Max Goldberg via Planets