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
than red stars.
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 Laboratoire d'Astrophysique de Grenoble, and his colleagues explore in their research. Montmerle points out that stars do not form one by one in space. Their birthplaces are located in vast star factories known as molecular clouds: huge volumes of gas, mostly hydrogen and helium, but richly laced with complex organic molecules, and ice-covered dust grains. The star formation process in these clouds produces clusters of hundreds to thousands of stars. The best-known and most studied young cluster contains about 2,000 stars that are 2 to 3 million years old. The star cluster is located inside the famous Orion nebula -- a fuzzy bright spot near the "belt" of the Orion constellation.
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
Most astrophysicists agree that turbulence is the agent for star birth (and for molecular cloud support on larger scales). 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. It is this cloudlet that can collapse to form a star. Put another way, molecular clouds have weather, and this leads to situations in which the internal pressure becomes overwhelmed by gravity. Collapse then commences more or less simultaneously at various locations within the cloud.
But what feeds the turbulent energy in molecular clouds? There are several imaginative scenarios, all of which are hotly debated. Turbulence can be linked to a feedback from stellar mass loss: powerful jets from young low-mass stars like the Sun, strong winds from high-mass stars, or even supernova explosions, the endpoint of the evolution of the most massive stars.
Supernova explosions in the Galaxy are absolutely critical in astrobiology. They provide the means by which the chemical elements made inside stars are dispersed in the interstellar medium, and thus ultimately incorporated into the next generation of stars.
Although supernova explosions are not thought to be directly responsible for star formation, “There is increasing evidence from meteorite data that the young solar system was contaminated by a supernova explosion,” says Montmerle. This adds a new twist to the supernova connection to astrobiology.
The asteroid belt is estimated to contain
over 1 million asteroids with diameters
exceeding one kilometer.
Credit: NASA/U. of Arizona
Meteorites are debris from collisions between "planetesimals", kilometer-sized "little planets" that would subsequently aggregate to form full-grown planets. Astronomers prize meteorites because they can be used to reconstruct the earliest history of our solar system. It was discovered in the 60's that the dust grains present in meteorites contained tell-tale radioactive material. Although these radioactive materials have long ago decayed into other products, their decay products are a record of the original composition.
A species of radioactive iron known as iron 60 was discovered only two years ago. This discovery provides new evidence in favour of a supernova explosion in the vicinity of the young 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, but commensurate to the time it took for the protoplanetary disk to condense into planetesimals. 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 Trapezium star cluster in the heart of the Orion Nebula, 1500 light
years from Earth
The presence of iron 60 in meteorites implies that it must have been incorporated into the protoplanetary disk at about the same time a supernova explosion took place in its vicinity.
“The presence of such a supernova tells us that the Sun was very likely born in a huge star cluster with many massive stars, because we know from the observations of young, solar-like stars today that protoplanetary disks last only a few 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 just in time to affect the disk from which the meteorites and planets formed.
“In turn, the presence of massive stars implies the existence of thousands of lower-mass stars. Therefore, we have to think in terms of the solar system’s origin being in a cluster of possibly up to 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 at the same time 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, but they are in fact infrequent, because the timing between the supernova explosion and the disappearance of the protoplanetary disk must be very precise,” Montmerle observes. “The events surrounding the birth of our solar system were admittedly special, but we don't know yet whether this was important, once its survival was somehow secured, for its subsequent evolution."
The apocalyptic origin of the solar system is unlikely to have directly 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 necessarily 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.