Summary: Astrobiologists have a strong interest in understanding the conditions on the early Earth, but the record for the first 700 million years of Earth history is gone. The stages that made our planet fit for life are not recorded in the rocks we have today.
Astrobiology is all about origins: the origin of the universe; the origin of its atoms and molecules; the origin of the stars and their planets; the origin of life. We inhabit a universe that is fit for life. That statement leads to the anthropic principle, a concept in philosophy that says the universe has the properties we observe because it contains life.
Half a century ago the British cosmologist Fred Hoyle used the anthropic principle to predict that the nucleus of carbon must have an enhanced energy state. This was the first example of an astronomer making an important discovery as a result of looking at a problem from an astrobiology "origins" viewpoint.
Any special circumstances concerning the origin of our solar system will profoundly influence theories and speculations about the ubiquity of life in the universe. The famous Drake equation makes explicit provision for calculating the likelihood that stars that will have planetary systems suitable for life.
| Blue stars in the Pleiades.
These stars produce
more UV radiation
Credit: DSS and LTImage
We know that our solar system has at least one planet with life – Earth. So perhaps solar systems that formed in ways similar to our own will also have the potential for life. But this invites the question: how normal was the formation of our solar system?
That’s a conundrum Thierry Montmerle, a high-energy astrophysicist at the Université Joseph Fourier in Grenoble, explores in his research. Thierry points out that stars do not form one by one in space. Their birthplaces are in vast star factories known as molecular clouds: huge volumes of gas, mostly hydrogen and helium, but richly laced with complex organic molecules, dust, and ice. The star formation process in these clouds produces clusters of stars. The best-known young cluster, and certainly the most striking to the naked eye, is the Pleiades or Seven Sisters in the constellation of Taurus (the Bull), which is just 100 million years old.
Paradoxically, molecular clouds do not naturally tend to form stars. Gravity’s urge to collapse a cloud is balanced by its internal pressure. Most of the time the cloud is too warm for star formation because at higher temperatures the outwards push of the pressure is greater than the inwards tug of gravity. However, stars do form, so what triggers that birth?
| Spectacular gas remnants
from exploding star.
Image Credit: Hubble
Astrophysicists agree that turbulence is the agent of star birth. Turbulence is a stirring process in which eddies move energy from the large scale, the cloud itself, to the small scale, a cloudlet less than a light year in size. Putting it another way, molecular clouds have weather, and this leads to situations in which the internal pressure becomes overwhelmed by gravity and collapse commences.
But what drives turbulence in molecular clouds? There are several imaginative scenarios, all of which are hotly debated. Montmerle thinks one potential driver is a supernova, the catastrophic explosion of a giant star at the end of its life. Supernova explosions are absolutely critical in astrobiology because they provide the means by which the chemical elements made inside a star are seeded back into molecular clouds: all atoms apart from hydrogen and helium have come from supernova explosions.
Meteorites formed before there were planets, so astronomers prize them because they can be used to reconstruct the earliest history of our solar system. The dust grains from which meteorites formed contained radioactive material. Although the radioactive materials have long ago decayed into other products, those decay products are a record of the original composition.
The asteroid belt is estimated to contain
over 1 million asteroids with diameters
exceeding one kilometer.
Credit: NASA/U. of Arizona
A species of radioactive iron known as iron 60 provides the evidence in favour of a supernova explosion being the trigger for the formation of the solar system. Nine-tenths of the iron in the universe is iron 56: its nucleus has 26 protons and 30 neutrons. Iron 60, on the other hand, has 34 neutrons, and those four extra neutrons mean that whereas iron 56 is stable, iron 60 undergoes radioactive decay. One of its neutrons becomes a proton, and the decay product is nickel 60. The half-life of the decay process is 1.5 million years. In cosmic terms, that half-life is the blink of an eye. Since the solar system is thought to be 4.6 billion years old, all the iron 60 has long vanished from meteorites. But it has left behind ghosts, the nuclei of nickel 60, which tell us how much iron 60 the meteorite once had.
The iron 60 in meteorites must have been made at about the same time as the solar nebula began to condense, since the only site for making iron 60 is inside a giant star that exploded as a supernova. Montmerle’s thesis is that the explosive thrust of the supernova shattered the equilibrium of the molecular cloud, causing the turbulence that marks the onset of star formation. The debris of that explosion settled onto dust grains and then became incorporated into meteorites when the solar system formed around the infant Sun.
“The Sun was likely born in a huge star cluster with many massive stars, because the proto planetary disk lasted only a couple of million years,” says Montmerle. The most massive stars have the shortest lifetimes, and Montmerle says that only these short-lived stars – stars of 40 solar masses or more - could have generated supernovae in time to affect the disk from which the meteorites and planets formed.
“Furthermore, we have to think in terms of the solar system’s origin being a cluster of 100,000 stars, which is enormous,” adds Montmerle. Because the parent star cluster was so large, the environmental conditions in the nascent solar system were diabolical. Numerous massive stars were pouring out high-energy x-rays, and this fierce onslaught, combined with the supernova explosions of about 10% of all the stars in the cluster, would have altered the formation processes of the solar system.
“These events are spectacular and infrequent,” Montmerle observes. “The birth of our solar system was somewhat special.”
The apocalyptic origin of the solar system is unlikely to have affected the emergence of life on Earth, which happened long after the birth pangs. Of course, there are plenty of locations in our Galaxy where solar-type stars have been made, and their planets did not have to survive such fierce initial conditions.
Astronomy has much to offer astrobiology in terms of explaining the formation of stars and their planets. What the astronomers are saying in this case is that our solar system had a violent origin of a kind that is not typical. That extra dose of iron 60 in meteorites speaks volumes about our cosmic origins.
By Simon Mitton
This feature draws on the following review
Solar system formation and early evolution: the first 100 million years, by T. Montmerle, J-C. Augereau, M. Chaussidon, M. Gounelle, B. Marty, and A. Morbidelli, in Earth, Moon, and Planets (2006) vol 98: pp 299 - 312.