ASK most astronomers about the origins of the solar system, and they’ll have a tedious tale to tell. For decades, the received wisdom has been that our sun formed in a quiet corner of a dull and quiescent cloud of molecules – a galactic suburbia. There, with the minimum of fuss and in splendid isolation from other stars, the solar system simply congealed out of the surrounding gas and dust.
Astronomers backed the quiet-birth hypothesis because they believed that the slow congealing of a sun and planets would be disturbed by any violent events in the neighbourhood. But it seems they were wrong. “Everything has changed,” says Jeff Hester, an astronomer at Arizona State University in Tempe.
Pinning down how the solar system really formed requires complex detective work. The clues are hidden in the chemical make-up of meteorites, the builders’ rubble left over from the construction of the solar system some 4.5 billion years ago. Meteorites provide a fossil record of the conditions that existed at that time. “The trick is in knowing how to read them,” Hester says.
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There are many kinds of meteorites. Some are primitive aggregates of nebular dust. Others are formed from fragments of asteroids complex enough to have melted and differentiated into a core and mantle. Where and when the meteorites were created can be deduced from the various minerals they comprise, each of which forms at a particular temperature and location in the solar nebula. To complicate things, some also contain various kinds of inclusions – chemicals or isotopes in particular abundances. In short, the process of analysing meteorite samples to solve the mystery of the solar system’s birth is a task worthy of Hercule Poirot.
Perhaps that is why it has taken so long to uncover the truth: after all, we have had circumstantial evidence of a violent birth for decades. In the 1970s, a team led by Typhoon Lee, then at the California Institute of Technology in Pasadena, discovered that meteorite samples once contained aluminium-26. They don’t any longer because aluminium-26 is unstable, with a half-life of only about 720,000 years, and disintegrates into lighter atomic nuclei. This decay has, however, left a characteristic pattern of disintegration products – the fingerprint of aluminium-26.
The discovery that aluminium-26 existed in the early solar system sparked excitement because astronomers thought it could be made only inside a star tens of times as massive as the sun. So the only way aluminium-26 could have found its way into local meteorites would be if a high-mass star of this kind had been in the vicinity of the forming sun and planets, and exploded as a so-called core-collapse supernova, contaminating the solar nebula with the rare isotope. The evidence seemed to imply that the sun’s birthplace was a violent and interesting place, somewhere like the great nebula in Orion or the Triffid or Eagle nebulae where young, high-mass stars burn with prodigious luminosity.
However, another explanation soon arose. Astrophysicists realised that aluminium-26 could have come from heavy atomic nuclei floating around in the early solar system. When these nuclei were hit by protons released by flares in the young sun, they would have shattered into smaller nuclei – a process called spallation. A common spallation product when heavy nuclei and protons collide is aluminium-26. “The aluminium-26 in meteorites could not, after all, be taken as proof that the sun was born among massive stars,” Hester says.
Change and decay
Then, in the 1990s, came the discovery that meteorites also contained the decay products of another short-lived atomic nucleus – beryllium-10, which can be made only by spallation. With both isotopes satisfactorily explained by a mundane solar mechanism, astronomers were faced with an obvious answer: the sun was born among low-mass stars in a dull environment. They also had perfect examples of sites that looked right, places like Taurus-Auriga, a cold, quiescent molecular cloud only 450 light years away, where low-mass, low-luminosity stars could form in relative isolation. It was perfect – a watertight case, until now.
Hester thinks the case for a quiet birth was settled more by prejudice and expedience than hard fact. “Taurus-Auriga, being relatively close, was very well studied,” Hester says. “It’s a bit like a drunk looking for his lost keys under a street light, not because it is where he expects to find them, but because that’s where the light is.”
A discovery last year changed everything. After years of painstaking work, Gary Huss and Shogo Tachibana of Arizona State University in Tempe (Tachibana is now at the University of Tokyo) found the decay products of yet another short-lived nucleus, iron-60, in the most common type of meteorites, chondrites. Iron-60 is a rare neutron-rich nucleus with a half-life of about 1.5 million years. It decays into nickel-60.
Huss had been searching for clear evidence of iron-60 in chondrites for several years. It was only when Tachibana proposed that they look at troilite (iron sulphide) that their fortunes turned. Troilite has very high iron-to-nickel ratios and thus should have shown an anomaly in the amount of nickel-60 if iron-60 had once been present. Tachibana set to work. “He found some suitable primitive meteorites and identified some troilite grains. We developed an experimental protocol, and he tried the measurements,” Huss says. Tachibana found nothing conclusive at first, but eventually “with a lot more work, he found evidence in several troilite grains from two different meteorites, and we reported the discovery”, Huss says.
Although the fingerprint of iron-60 was extremely difficult to find, the detective work was worth the effort. Iron-60 is created when the more common isotope, iron-56, repeatedly captures neutrons. There are only two environments where it can capture enough extra neutrons to become iron-60. One is inside a very old red-giant star, the other is during the cataclysmic explosion of a giant star – a supernova.
Red giants are many billions of years old, and move away from star-forming regions during their lifetime; the likelihood of the sun being born in the vicinity of such an ancient star is negligible. So it must be the other option: the iron-60 had to have been formed during a supernova blast. There is even evidence that links the formation of the solar system to a specific explosion. Meteorites either contain a particular abundance of iron-60 or have none at all. “If several supernovae injected iron-60 into the solar nebula, one after another, we would expect to see a continuum of abundances of iron-60 between zero and some maximum,” Hester says.
The final piece of the puzzle fell into place when Steve Desch, also at Arizona State University, found an alternative explanation for the presence of beryllium-10 in meteorites (Astrophysical Journal, vol 602, p 528). Desch and his colleagues showed that the amount of this isotope in the early solar system is just what would be expected from its direct capture from the cosmic rays of the galaxy itself.
“The supernova blast was close – maybe no more than a light year away”
“Beryllium-10, unlike iron-60, is found in all meteoritic inclusions,” Hester notes. “And this is exactly what we would expect if this stuff was in cosmic rays that were continually raining down on the solar nebula from deep space.”
Hester pulled all this evidence together into a new history of the solar system (Science, vol 304, p 1116). It suggests that rather than being born into the sleepy suburbs of the galaxy the solar system’s neighbourhood regularly played host to the galactic equivalent of an inner-city riot. The region was crammed with massive, super-luminous stars, each pumping the energy of a million suns into a cloud of interstellar gas and dust. Every now and then one of the stars would reach the end of its life and blow up. The ensuing cataclysm was tremendous. For a brief period the dying star blazed brighter than a galaxy of 100 billion ordinary stars.
Squeezing stars
The presence of giant stars would certainly make a huge difference to any planets forming in the region. The amount of matter pressing down on the core of a massive star squeezes the material and makes it very hot. This has a disproportionate effect on the light output of the star because its nuclear reactions are enormously sensitive to temperature. A star 60 times as massive as the sun is not 60 times as luminous, it is about 800,000 times as luminous. So massive stars pump enormous amounts of energy into a molecular cloud, creating powerful shock fronts which criss-cross the cloud, slamming into each other, and generating great turbulence. Because these stars have high surface temperatures – up to 50,000 °C – they also generate copious ultraviolet radiation, which is capable of ionising the surrounding gas. As an ionisation front races outwards from the star at thousands of kilometres per second, it eats into the molecular gas, carving out an immense and ever-growing cavity. Temperatures in this region may reach 20,000 °C.
The ionisation front, and the shock wave associated with it, would have triggered the birth of the sun. “It compressed a globule of the molecular gas, causing it to begin shrinking under its own gravity,” Hester says.
But the trauma did not end with searing ultraviolet radiation and multiple shock fronts from the massive stars. It was followed by the supernova blast. Hester reckons the explosion was close – maybe no more than a light year away. For evidence of this he points to the Kuiper belt, the region of icy debris left over from the birth of the planets that now extends outwards from the orbit of Neptune. Around stars in Taurus-Auriga, he says, proto-planetary discs typically span hundreds of astronomical units (1 AU is the average distance between the sun and the Earth). But our disc seems to have been eaten away by something. “There is evidence that the Kuiper belt is truncated to about 50 AU from the sun,” Hester says. “This truncation may very well be a fossil record of the supernova blast wave that slammed into the embryonic solar system.”
So 4.5 billion years ago the solar nebula was being baked with radiation. Then within a century of the supernova explosion the shell of debris, expanding at 30,000 kilometres per second and carrying with it iron-60, aluminium-26 and other short-lived nuclei, slammed into the embryonic solar system. “It may appear remarkable that the planetary disc survived the onslaught,” Hester says. “However, being far denser than typical interstellar material, the disc was quite robust.”
Giant planets
The story pleases Alan Boss of the Carnegie Institution’s department of terrestrial magnetism in Washington DC. Boss was the first to propose the idea that the solar system formed in a region of high-mass stars, where supernovae might appear. In 2002, he and his colleagues George Wetherill and Nader Haghighipour pointed out that this provided a natural explanation for the formation of the solar system’s giant planets (Icarus, vol 156, p 291). They argued that the ultraviolet radiation from the nearby massive stars would photoevaporate the disc gas outside Saturn’s orbit. This same radiation would also photoevaporate the gaseous envelopes of the two outermost gas giant protoplanets, stripping them down to their ice and rock cores with a thin gaseous envelope, exactly as seen in Uranus and Neptune today. “Jupiter is inside the critical radius outside of which the gas stripping occurs, and so does not lose any gas,” Boss says. That would account for its present structure: mostly gas with a small ice and rock core. According to Boss, the iron-60 evidence is another good reason to argue that the sun and planets formed in a region with high-mass stars. “I am violently in agreement with Hester,” he says.
“Suddenly, the search for extraterrestrial life is looking a lot more optimistic”
Charles Lada of the Harvard-Smithsonian Center for Astrophysics agrees with Boss and Hester but for a different reason. “The consensus among those working in the area of star and planet formation is that about 90 per cent of stars form in clusters containing at least 100 stars,” he says. “My estimate is that maybe 25 per cent of these form in clusters with so many stars – greater than 500 – that they might possess the relatively rare high-mass stars that ultimately go supernova. So there is a good chance that the sun formed in a rich cluster such as the Eagle nebula.”
Not everyone is entirely convinced that Hester has it right. “Ideas change slowly,” Huss says. “I would not say that Jeff’s article and our discovery have caused everyone to switch to his model. However, the astrophysics and cosmochemistry communities are now talking [about it] more than ever before.” This should lead to a more detailed and widely accepted story of the formation of our solar system, he says.
Witch’s brew
And the story has more than historical relevance. In Hester’s model, the intense ultraviolet radiation from nearby massive stars may have energised chemical reactions in the witch’s brew of molecules swirling around the newborn sun. It could be that the chemicals necessary for the creation of life formed this way. “At this time, much of this is speculation,” Hester stresses. “But the point is, we are now starting to see ways in which the ability of Earth to sustain life may be tied directly to the larger interstellar environment which gave birth to the sun and solar system.”
And this may boost the prospect of finding extraterrestrial life. According to Hester, the planetesimals that came together to form the Earth should have given our planet 20 to 30 times more water than it actually has. “This anomaly can be explained by the tremendous heat liberated by the decay of aluminium-26,” Hester says. So we know what to look for: if a proto-planetary disc is contaminated with too great a quantity of short-lived nuclei, the energy they liberate when they decay will completely bake out planetesimals. Any terrestrial-type planets that subsequently form will therefore be waterless. Hester believes it should be possible to look at star-forming regions with massive stars and simply count the number of embryonic stars at a distance from massive stars that would allow them to retain some water and form Earth-like planets.
Suddenly, the search for extraterrestrial life is looking a lot more optimistic. We know that around 90 per cent of stars form in regions of high-mass star formation. “If that is where the solar system formed, then the chances of there being planetary systems similar to our own jump by a factor of about 10 compared to the alternative,” Boss says. That makes it 10 times more likely that we’ll find a habitable planet in the neighbourhood.
