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astrophysics

Where can life be?

A connection between the chemical evolution of the Milky Way and the formation of terrestrial planets has been established

Imagine a map of the Milky Way that located the most favorable niches for the formation of planets of terrestrial type, where there is a greater probability for the development of human beings. This may be the outcome of a study on the evolution of our galaxy carried out by Hélio Jaques Rocha-Pinto, of the Astronomy and Geophysics Institute of the University of São Paulo (IAG-USP). In the research, which has just been brought to a conclusion, he established how the chemical elements have distributed themselves in the Galaxy throughout space and time.

The study holds other important results. One of the most significant is the correction on the methodology for the calculation of the ages of stars. He also discovered a type of star that could have the speed of an old star and the activity of a young star. He demonstrated that, contrary to what had been thought, the rate of the formation of stars is not constant, but varies periodically. All of his conclusions were reach during the phases of his doctorate and post-doctorate in astronomy, and he has recently been invited to the University of Virginia in the United States for a period of two years as an associate researcher, beginning in November of this year.

The most fascinating study is that involving the predictions for extra-terrestrial life of planets in the Milky Way – a spiral galaxy whose visible disc, with close to 400 billion stars, is so large that light takes 100,000 years to pass from end to end. The hypothesis that planets of the age of the Earth can harbor life, comes from an analogy: the time necessary for life to have evolved on our planet. Since it is calculated that life here came about some 3.8 billion years ago, from organic molecules in a protobiotic state, in other planets of the same age, life could also have developed over a period similar to our own. “Indications of life in very old rocks lead one to believe that life would develop as soon as the necessary conditions come into existence”, says Rocha-Pinto.

“By investigating the chemical composition of the galaxy,” he goes on, “we confirmed that eventually existing planets of terrestrial type would have an average age of 4.9 billion years, approximately the same age as the earth”. This dating of the planets of the terrestrial type – or that is to say, like the Earth and Mars, in contrast to gaseous planets such Jupiter and Saturn – also suggests that, if there are other civilizations in the Milky Way, they should have a level of technology similar to ours. Super evolved civilizations, such as those described in science fiction books and films, would be difficult to find in the Galaxy, since they wouldn’t have had enough time to develop. “On this scale of time of 4.9 billion years”, observed the astronomer, “it is probable that the development of an eventual extra terrestrial civilization would be similar to that of the Earth.”

Based on a speculative mathematical formula, the so called Drake Number – that came from estimates of parameters such as rate of formation of stars, number of habitable planets in each solar like system and average time of life of a civilization capable of communicating through electromagnetic waves -, it has been calculated that there are between 100 and 200 civilizations in the Milky Way.

The search for planets out with the solar system – the exoplanets – is one of the most active activities in astronomy and the results will accumulate rapidly over the next few years: close to 70 have already been identified. However, all of them are gigantic gaseous masses and without the minimum conditions to house any form of life.

What researchers want is to find planets similar to the Earth. This is impossible with the current equipment, but three large pieces of apparatus will be launched into space at the end of the decade or the beginning of the next: the telescopes Corot and Darwin, from the European Space Agency, and the interpherometer Terrestrial Planet Finder from NASA, the United States Space Agency. When these instruments are in operation, the data which Dr. Rocha-Pinto has might assist them in pointing lenses and collector mirrors to the correct targets.

For him, whoever wants to find living organisms – not exotic beings whose existence is mere speculation, but beings somewhat similar to those that inhabit the Earth – should begin in the regions of space rich in the carbon, nitrogen and oxygen, fundamental for the formation of DNA (deoxyribonucleic acid, the carrier of the genetic code present in all living cells), of proteins and other molecules associated with life.

Such elements are not as abundant as hydrogen and helium, already created in the first few minutes of the universe, in “primordial nuclear synthesis”. Carbon, nitrogen, oxygen and others of higher atomic mass are sons of the stars: their complex nuclei were formed from the fusion of simple nuclei in the fervent centers of stars in a process of nuclear fusion – responsible for the light and heat that we receive from the Sun – which is goes on happening in billions of stars, always enriching the cosmos with heavy atoms.

Since the spectral lines of carbon, nitrogen and oxygen are difficult to observe, a strategy for mapping potential niches of life is to search out stars rich in iron. The explanation: in the production line of nuclear fusion, the element iron (atomic mass of 56) forms after carbon (atomic mass 12), nitrogen (atomic mass 14) and oxygen (atomic mass 16). Thus, when there is lots of iron about, it is hoped that these other elements will also be present.

As well, the detection of iron is made easier by the fact that its atom has various layers of electrons: in the end, it is the energy jumps from one layer to another which makes the atom emit the electromagnetic radiation that makes it possible to observe it. For this reason, to speak about the chemical evolution the Galaxy and the prospects of life is to speak about research into the abundance of iron – the so- called metallicity.

Iron markers
“The excitation potential of iron – the energy necessary for its electrons to jump from one layer or to another – is comparable to the energy of the surface of stars similar to that of the Sun, in the order of 5,000 to 6,000 degrees Kelvin (zero on the Kelvin scale or Absolute zero is equal to -273.16o Celsius). For this reason, in the electromagnetic spectrum of these stars, identified as dwarfs G, iron is the element best represented”,justifies Rocha-Pinto. Iron is in truth the characteristic marker for this type of star.

In very hot stars (types O, B and A), iron cannot be detected since its atoms are ionized. In the colder stars (of the types K and M), its presence is masked by spectral lines characteristic of molecular structures. Iron appears as a beacon exactly in the stars which are of interest: the types F and G, which are not so hot or so cold.

“One of the advantages of working with these stars is that they have an extremely long life expectancy. While the giant and very hot type A stars last some 300 million years, the stars of the solar type, the dwarf G, live as long as the Galaxy itself – more than 10 billion years. For this reason, at least as a possibility, we are able to observe all the stars of this kind that have already been formed in the Milky Way.”

These stars are the strongest candidates to bring life to their planets. The moderate rate of nuclear fusion gives them mild temperatures, enough so that living beings can develop nearby, and they also have a life span long enough to allow these organisms to develop. “Furthermore, the radiation from these stars is much less damaging to components essential to life, such as the DNA molecule. The same does not happen with stars of type A, which emit lethal quantities of ultraviolet light”, adds Rocha-Pinto.

New dating
When he began his doctorate in 1996, he was not yet directly involved with astrobiology. His focus was on the increase of metallicity – presence of iron -, in relation to the age of the stars. “Previous researchers”, he says, “had basically investigated stars of type F such as Procion, whose lifespan is considerably long – from 5 to 6 billion years -, but anyhow younger than the Galaxy. For this reason, the older eras of the Milky Way were not well represented in these studies. The ideal would be to research stars of type G such as the Sun. The problem is that the temperature of these stars did not allow a good estimate of their ages. Our contribution was to adopt another form of dating, called chromospheric dating. Thanks to this, we were able to correlate the age and metallicity of a group of 552 stars.”

This preliminary survey made, throughout the post-doctorate period, to become interested in planets of the terrestrial type: “We verified that only 10% of the stars of the generation of the Sun, constituted around 4.6 billion years ago, have metallicity higher than the Sun. This means that the Sun was born with an abundance of iron well above the average. Consequently it is not a typical star. This atypical situation seems to have been decisive for the formation of a terrestrial planet in the zone of habitation and consequent development of living organisms”.

To clear up this point, the researcher recalls the dominant hypothesis surrounding the formation of the planetary systems, which proposes the following chain of sequences. Initially a cloud of gas and cosmic dust contracts through gravitational effects. The concentration then goes on to attract the surrounding matter. This matter doesn’t fall directly onto the protostar: it adjusts itself into the form of a disk in the equatorial plane of the object. With the shrinkage, the cloud begins to gyrate and to takes the shape of a disc.

Heated due to the contraction, the object begins to beam light and on attaining a critical temperature, transforms itself into a star, converting hydrogen into helium through a fusion reaction. While this is happening, the small pieces of matter in the outer region of the disc act as gravitation magnets, accumulating surrounding gas and dust. The aggregation of the matter starts off planetary formation, rocky bodies the size of smaller asteroids. In the end, this planetary matter comes further together and forms the planets.

The small pieces of matter, decisive in the birth of planets, seem to critically depend on the metallicity of the star forming clouds: “Environments very poor in metals can’t form terrestrial planets due to the lack of small grains capable of grouping matter together. On the other hand, the heavily metallic environments tend to generate an excessive quantity of grains, producing jovian planets – similar to Jupiter, with rocky nuclei and enormous gaseous covering – in a region quite close to the star.

These jovian planets not only could migrate to the inner part of the system, destabilizing the orbit of any terrestrial type planet that might exist in the location, but would stop offering a gravitational cover against the penetration of comets – which is done by Jupiter in the case of the Solar System. The result is the very low probability of these systems harboring life within their habitation zone”, concludes Dr. Rocha-Pinto. According to Charley Lineweaver, of the New South Wales University in Australia, the range of metallicity that is favorable for the formation of terrestrial planets goes from 0.5 to 1.2 times that of the Sun.

Rocha-Pinto agrees, but not with the dating of the terrestrial planets of Lineweaver: while the Brazilian calculates the average age of these planets to be 4.9 billion years, for the Australian they are 6.4 billion. The reason is that Lineweaver doesn’t arrive at the number only by looking at data from the Milky Way. “He brought together information relative to a large number of galaxies and established an average value. And clearly this doesn’t take into account the specificity of the Milky Way.

The calculations of Dr. Lineweaver moves the peak of the formation of stars into the past, which results in much older planetary systems. This is not what is happening in our galaxy, which has had various periods of intensification of star formation, coinciding with the eras of the greater approximation of the Magallenic Clouds – two small galaxies of irregular shape which gravitate around the Milky Way. It was exactly on one of those occasions that the solar system was formed.” If Lineweaver’s number were to be correct, there would be the prospect of finding within the Galaxy, those super civilizations described by science fiction writers. With the dating calculated by Rocha-Pinto, it is less probable that this will occur.

With Walter Maciel of USP, and Gustavo Porto de Mello of the Valongo Observatory of the Federal University of Rio de Janeiro, Rocha-Pinto is calculating the amount of iron, nickel, sodium, calcium and silicon in a new sample of 325 stars of the Sun’s type,obtaining data through the National Astrophysics Laboratory of Itajubá. “For there to be life, it is necessary that critical quantities of various components are achieved”, says the researcher. “We want to know how these amounts are distributed throughout space and how they have varied over time. The result will be a kind of graph of the probabilities of life within our Galaxy.”

Correcting the age of the stars

In the Sun, the chromosphere is the corona which shines brilliantly during an eclipse, a rarified zone formed by atoms and electrons which the star is giving off. In stars of the Sun’s type, the temperature in the photosphere (luminous surface) is of 5,600 degrees Kelvin and in the chromosphere goes from 10,000 to 100,000 degrees. In the photosphere, atoms or electrons absorb photons of energy (light particles) and in the chromosphere they are emitted. For this reason, in the electromagnetic spectrum, the photosphere is identified by absorption lines and the chromosphere normally by emission lines. The analysis of these lines permit the estimation of the age of the star, since as it grows old, it gyrates slower and slower, which affects its magnetic field and consequently the temperature of the chromosphere, lessening the intensity of the spectral line emission.

This fact has been known since the 60’s, but, by not taking into consideration the metallicity, the studies erred in the construction of the absorption spectral lines, and in this way, incorrectly evaluated the emission line – since, in the graphs both are superimposed. The result was the incorrect determination of the age of a star. Rocha-Pinto corrected this and recalculated the dating of many stars. “With this, we could establish an adequate correlation between the age of the stars and their metallicity.

Consequently, we arrived at a much more realistic picture of the chemical evolution of the Galaxy.” The study was crowned by the verification that the index of formation of the stars varies within the Galaxy, altering between phases of more and less proliferation. This is probably due to and interaction between the Milky Way and the two Magallenic Clouds, which indicates recent interactions between the two. The periods of greater approximation correspond to an intensification of star formation, both here and there.”

Still on the age of the stars, he discovered that which he called crojoca (from the the Portuguese combination of young chromospherics and old kinematics) a type of star of paradoxical behavior: like the young, the chromosphere has intense activity, but the star goes off at high velocity, much like old stars. It is the result of the fusion of two binary or twin stars that were born close to each other. “This type of star was only found between those of type A, with large mass and short life, where it is called a blue straggler.

The crojocas are stars analogous to those of type G, of small relative mass and of long life. They are not blue, but yellow. “In reality, they are old stars whose chromospheres were rejuvenated by fusion, which makes the star gyrate quicker than that from which it originated. “One piece of data that has matched up with our hypothesis is that the crojocas almost never possess lithium, a characteristic of old stars, because lithium is rapidly burned during the initial phases of the life of a star.”

The Project
Galactic Evolution and the Chromosphere Activity (nº 00/04539-1); Modality Post-Doctorate Scholarship; Coordinator Hélio Jaques Rocha-Pinto – Astronomy and Geophysics Institute of USP; Investment R$ 34,320

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