For a long time astronomers have talked about the nine planets orbiting the Sun. These nine heavenly bodies have always been more special than other objects orbiting the Sun, such as asteroids and comets. The nine planets are larger than others; you can see some of them in the sky even with the naked eye if you know where to look. In September 2003, astronomer Mike Brown of Caltech and his colleagues announced the discovery of a new object in the sky, then named 2003 UB313 Eris, which is 27% larger than Pluto. This made astronomers reconsider the definition of a planet, thereby making Pluto and Eris two of the new category of objects orbiting the Sun dubbed “dwarf planets.”

While astronomers are engaged in the debate on planet definitions, astrophysicists still have not agreed about how planets were created. In fact, the journal Science recently put the birth of planets on the list of the top 125 questions scientists will tackle in the next quarter century.1 The comment ended with: “Planetary systems around other stars should provide clues.”

The reason scientists are interested in extrasolar planets- planets orbiting other stars, or exoplanets in short- is not limited to their curiosity about how planets were created. The second major motive for exoplanet research is the attempt to detect another “habitable” planet. NASA’s Origins Program,2 for example, is attempting to answer the question, “Are there worlds like the Earth around nearby stars? If so, are they habitable, and is life as we know it present there?” This is one of the major questions the new field of astrobiology is striving to answer.

In this article, we give an overview of planets and exoplanets with an emphasis on the critical conditions for life on a planet.

Planets and Exoplanets

The International Astronomical Union’s 2006 definition of a planet states that a planet is a celestial body that (1) is in orbit around a star, (2) has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round shape) but is not itself a star, and (3) has “cleared the neighborhood” around its orbit.

According to this definition, Pluto is indeed not a planet as its “moon” Charon is half the size of Pluto, whereas the moons of all other planets are much smaller than their respective parent planets. In addition, Pluto’s orbit is not as “clean” as the orbits of other planets.

Having introduced the new planet definition, we want to emphasize the first and foremost condition: a planet has to be in orbit around a star-not around the Sun. According to NASA Jet Propulsion Lab’s PlanetQuest website, as of August 2007, about 250 exoplanets in 99 planetary systems have been discovered. (The site exoplanets.org gives 228 planets around nearby stars.)

Discovery of solar planets is not too challenging: you take a clear picture of the same portion of the sky periodically, and compare the successive pictures. If you see an object that changes its position, then you can be sure that it is an object orbiting the Sun. Because stars are very far away compared to bodies orbiting the Sun, they seem stationary relative to us.

To give a sense of how far the stars are from us, think of the nearest star, Proxima Centauri, which is 4.3 light years away. One light year is the distance light travels in one year, which is 5.88 million million miles. If you fly a supersonic jet-a jet that can break the sound barrier (765 mph), such as the SR-713 or the MiG-25R, which can reach three times the speed of sound in the air-you would have to fly for about 1 million years nonstop to arrive at the nearest star. The most distant planet, Uranus, is about 0.002 million million miles away from the Sun, which is more than 12,000 times closer than Proxima Centauri-your trip to Uranus with a supersonic jet will take only about 80 years.

Exoplanets are as far away as stars. Therefore, it is impossible to detect them using the simple picture-the-sky method utilized for the solar planets. Every star with planets in its orbit is affected by the mass of the planets. This causes the star to sway back and forth. With extremely sensitive instruments measuring the Doppler shift in the frequency of light received from the star, the effect of the planet on the star can be detected.4 Another method is called astrometry: precise measurement of the positions of the stars relative to very distant stars, which appear stationary because they are far away. Small movements of the star because of the presence of planets can be detected.5

Direct optical detection of exoplanets is extremely hard, as they do not give off their own light. In the presence of the bright star, the planet becomes totally invisible. There are a few solutions. In the transit method, a planet blocks some of the star’s light as it transits past the star.6 Sensitive instruments can detect such small dips in the brightness of the stars. Also, interferometric detection7 can be used to detect the extremely weak light from the planet. Another optical detection method is called the “choronograph,” which is used to physically block the glare of the parent star, exposing the planet.

How are planets created?

The motion of a planet around the Sun can be described using two conservation laws-conservation of energy and conservation of angular momentum. Based on the understanding of orbital mechanics and the well-known laws of motion (first published in their entirety by Newton), it has become routine to place satellites in orbit around different planets to conduct various studies. Although science has been quite successful in describing planetary motion, we still do not know how planets were created. The conservation laws mentioned above do not determine the number, orbits, rotation directions, sizes, or type-rocky or gas giant-of planets. Initial conditions play a significant role; initial mass distribution around the star, the size of the particles orbiting the star during the early stages of the star’s life, and the initial orbits of these particles-when considered with the laws of motion, conservation laws, and the law of gravitational attraction-result in different planet-creation scenarios.

There are currently two main theories of planet creation, the gravitational instability and core accretion theories. In the gravitational instability theory, planets form during a rapid collapse of a dense cloud. In the core accretion theory, planets start as small rock-ice cores that grow as they gravitationally acquire additional mass.8 By detecting planets recently created around different stars, scientists hope to test these theories.

Search for life on exoplanets

Diverse life forms on the Earth are taken for granted. The average individual does not think much about the inner workings of life and the conditions that make life possible on the Earth.

Earth is a rocky planet that contains heavier elements, such as silicon, iron, and so on. We know that heavier elements were created during the supernova explosions,9 which comprised a few generations of stars, and therefore more than a few billion years. In a galaxy that is very young, one does not expect there will have been enough supernova explosions to produce heavy elements.

In an old galaxy, however, one does not expect to see radioactive elements. Thus, the planets that form in an old galaxy might be as dead as the moon because there will not be enough radioactive fuel. The Milky Way, our galaxy, is neither very young nor very old. Note that ages of galaxies and stars are in the order of billions of years-our sun is estimated to have been created about 4.6 billion years ago, and it is a middle–aged star.

In the Milky Way, our sun is placed at just the right spot, about halfway from the center.10 At the core of our galaxy, the density of stars is so high that they collide with each other. At the outer extremities, at the rim of the galaxy, the star density is too low to generate the heavier elements that make up planets as very few supernova explosions are expected.11

The orbits of all planets are elliptical, but very close to being circles. This is very significant for a planet if life is to prosper. Eccentricity is a measure of the elliptical shape of an orbit. A perfect circular orbit has an eccentricity of 0 (zero), and as the eccentricity comes closer to 1, the orbit becomes like a sausage. The earth’s orbit around the sun has an eccentricity of 0.067, very close to a perfect circle. If the eccentricity were to become 0.3, the average global temperature would become 73 F (23 C), compared with 58 F (14.5 C) on the Earth now, and, “some parts of the African, South American and Australian interiors heat up to 140 F (60 C)” when the Earth passes closest to the Sun,” according to Darren Williams and his colleagues of Pennsylvania State University.12 On an orbit with eccentricity of 0.4, the average temperature would increase to 86 F (30 C). Given the current scientific opinion on global warming and how catastrophic conditions could become because of a few degrees increase due to increasing amount of carbon dioxide in the atmosphere, you can imagine how unbearable the Earth would become for many complex life forms. Therefore, for a planet to bear life on its surface, its orbit must be at an optimum range of distances from the parent star, which is dubbed the “habitable zone.”

All planetary orbits around the Sun-not only that of the Earth-are nearly circular, and they do not cross each other’s orbits. If there were a number of planets with highly eccentric orbits around the Sun, some of them would cross the Earth’s orbit increasing the probability of a collision.

Obviously, a planet with life as we know it on Earth would need to be a rocky planet. In the solar system only four planets (Mercury, Venus, Earth, and Mars) are rocky planets; the other four (Jupiter, Saturn, Uranus, and Neptune) are gas giants.

The existence of gas giants, Jupiter being the largest of all, appears to be very important, too. Meteorite collision is a likely Doomsday scenario for the inhabitants of the Earth. In fact, meteorite collisions are cited as the main cause of the extinction of many species from the face of Earth in its several billion-year history.13 Jupiter is about 5au away from the Sun-1au is the mean Earth–Sun distance, nearly 150 million km-and as the most massive planet it plays a critical role in protecting the Earth from meteorites and comets.

In addition to all these astronomical conditions, the Earth has a magnetic belt that protects it from charged particles ejected from the Sun and other bodies. The Earth has an atmosphere,14 and the presence of water is absolutely critical for life.15

Of the almost three hundred exoplanets so far identified, most of them are gas giants as massive as Jupiter-more than 300 Earth masses. Therefore, scientists do not expect a glimpse of life on them. Recently, Christophe Lovis of the University of Geneva and his colleagues reported three low-mass planets orbiting the nearby star HD 69830, described as “hot-Neptunes” or “super-Earths”, as their masses are from 5–20 times the mass of the Earth. Scientists predict that two of these planets may be rocky planets based on theoretical calculations.16 For more conclusive results, however, telescopes with much higher resolutions are needed. Such telescopes are expected to be operational within a decade.

In conclusion, research interest in exoplanets originates from questions about the mechanism of planet creation, and the attempt to find planets where life can exist as we experience it on our blue planet. We do not know whether we will be able locate other worlds similar to the Earth with their own inhabitants. One thing we know, however, is that life is only possible through a great many critical conditions acting together in stars and planets as well as in cells and molecules. Life is very special indeed. Although the existence of other planetary systems suggests that our solar system is not as unique as once thought, with its “blue” planet -a planet that can support biological life-it still seems absolutely unique. Many scientists think, however, that with better tools and methods it is only a matter of time before we locate an Earth-like exoplanet. Time will prove or disprove their predictions.

Dr. Ertan Salik is an Assistant Prof. of Physics at California State Polytechnic Univ, Pomona. As well as teaching and conducting physics research Dr. Salik is currently involved in many education programs.

Notes

1. Science, Vol. 309, No. 5731, pp. 1–204 (2005).

2. NASA Origins program: http://origins.jpl.nasa.gov and http://origins.stsci.edu/

3. http://www.sr-71.org/ Accessed 2008-07-26.

4. Struve, Otto. “Proposal for a project of high-precision stellar radial velocity work”, The Observatory 72 (1952): 199–200, http://en.wikipedia.org/wiki/Doppler_spectroscopy Accessed 2008-07-26.

5. http://www.planetary.org/explore/topics/extrasolar_planets/extrasolar/astrometry.html Accessed 2008-07-26.

6. Charbonneau, D.; T. Brown; A. Burrows; G. Laughlin (2006). “When Extraslar Planets Transit Their Parent Stars”. Protostars and Planets V, University of Arizona Press.

7. Exoplanet detection using a nulling interferometer, Manuel P. Cagigal and Vidal F. Canales, Optics Express, Vol. 9, No. 1, 2 July 2001.

8. http://planetquest.jpl.nasa.gov/news/giantRockyCore.cfm Accessed 2008-07-26.

9. Gedik, Nuh. “Supernova Explosions and a Miracle of The Qur’an,” The Fountain, April-June 2006.

10. Weed, William Speed. “Circles of Life,” Discovery, November 2002.

11. See Charbonneau 2006.

12. Weed, 2002.

13. Gonullu, Omer Said. “The Message of Meteorites,” The Fountain, January–March 2005.

14. Cakmak, Osman. “A Journey in the Atmosphere,” The Fountain, January–March 2002.

15. Gedik, Nuh. “The Miracles of Water,” The Fountain, January–March 2005.

16. Lovis, Christophe et al. Nature, 441, 305–309 (18 May 2006).

Glory be to Him who created in pairs all things that the earth produces as well as their own kind (selves); and other things of which they have no knowledge. (Va Sin. 36.36)

a) The verse opens with ‘Glory to be to Him’ as if to emphasize that God Himself is beyond possessing a pair, a likeness, or an equal. It goes on to draw attention to the inner meaning of the pair-wise existence of created beings. Pair-wise existence indicates opposition simultaneously with complemantarity, the most obvious case of which is male and female. The scientific definition of pair creation also implies complementary opposites. Thus, hot and cold constitute a pair; in other words, two entities may have almost identical properties and yet be opposed to each other in the nature and effect of those properties.

b) The most interesting aspect of the verse is the examples of pairs it provides. Saying ‘We created beings in pairs’, God gives three examples:

1. ‘Pairs’ that the earth produces,

2. Pairs of selves,

3. Other things created in pairs, of which we have no knowledge.

In order to understand the hidden meaning of this verse, let us first recall the conclusions science has reached in the light of recent research.

The discovery of the positron by the physicist Anderson is regarded by some as a major turning-point in contemporary physics. It was Maurice Dirac who first postulated parity or pair-wise creation, now recognized as a basic principle of modern physics. Every particle in the universe has a certain charge and/or spin. When a particle is formed, its opposite twin or antiparticle is produced together with it, for beings are always created in pairs. The most famous of these are – electron and positron; proton and anti- proton; neutron and anti-neutron, neutrino and anti-neutrino.

It is basic principle of nuclear physics that every particle exists together with its opposite twin – we shall return to this subject in detail below.

Hence, the verse 36 informs us of a basic law of creation in the declaration: ‘We created beings in pairs’.

We may turn now to the general meaning of the verse. With the expression ‘Glory be to God’ in the opening phrase, the verse underlines the fact that God, and God alone, is without equal or likeness. The habit of analogy, of discerning a thing through its opposite, is rooted in our comprehension. But all beings that have opposites or similar partners are mortal or finite. God, on the other hand, is the Glorious: He is without likeness and beyond all. He has, however, created beings in pairs (complementary opposites).

1. Those the earth produces

This part of the verse has been interpreted by some scholars as referring to the plant kingdom. This interpretation is, however, incomplete, for had God wished, He would have specified plants. The meaning certainly includes plants but is not restricted to them.

This first category of pairs delimits the third, It would be wrong, for example, to understand the sentence to mean: ‘I have created many more pairs of which you have no knowledge’ in relation to, for example, electricity or magnetism: those belongs to the pairs produced by the earth.

Pairs produced by the earth, then, maybe listed as follows:

a) Similar pairs that differ in their physical and chemical characteristics, e.g. metals and non-metals,

b) Biologically complementary pairs: the male and female sexes of plants and animals,

c) Physically opposed pairs: e.g. positively and negatively charged ions, and hence the reversal of electric charge (polarity): the two poles of a magnet referred to as North and South, respectively.

d) Phenomena of analysis and synthesis that alter the structure of life in the soil: the vivification of plants by the action of synthesis by bacteria which fix nitrogen: the destructive and dispersive effects of bacteria that break down organic materials and cause decay, and similar complementarities.

2. Pairs ‘of their own kind (selves)’

A range meanings are inherent in this part of the verse:

a) Male and a female (opposite partners).

b) Complementary personality traits (cruel/kind, brave/cowardly, generous/miserly, etc.).

c) Traits which are similar but subject to opposed value judgements, such as recklessness/courage:hypocrisy/ courtesy: simplicity/naivete, etc. Some of these terms denote values which are very near to each other and become negative or positive according to fine judgements about circumstances and/or intentions. For example, hypocrisy is to fake goodness or consideration in order to obtain some favour or advantage unjustly:courtesy may also involve some degree of showing consideration for another individual but in that case with the good intention of winning that person over for a good cause, to become a friend etc.

3. Other pairs ‘of which they have no knowledge’

The law of parity covers all concepts of matter and energy, in the sense that quanta and the systems composed of them manifest themselves as paired or complementary opposites.

a) Processes of energy emission and absorption which physics and astrophysics have, recognized only in recent times, are an example. We have yet to fathom the secret of these processes. However, we do know of black holes where energy is drained and annihilated, and of quasars which concentrate unbelievable amounts of energy. These two regions are places which appear as twins, but exert opposing influence with respect to each other.

b) Another example is forces of attraction and repulsion: in particular, gravity counterbalanced by centrifugal force. If the opposition between these forces did not exist, all planets would either fall into their respective suns or be flung into outer space.

Gravitation and rotational motion together support a dazzling system of equilibrium in the universe. We encounter a mind-boggling harmony when we inspect this equilibrium from the standpoint of our earth and the universe. Besides forming the solar system together with the earth, the neighbouring planets also strike a similar rotational balance with their own satellites. These smaller systems then complete a further revolution around the sun. Thus, the nine planets and their many satellites all revolve around the sun in different orbits. These heavenly objects thereby avoid falling into the sun. In return, the sun balances these planets in their orbits, preventing them from escaping into space. But this is not all. A further and larger rotational motion is simultaneously taking place: the Milky Way galaxy to which the solar system belongs rotates on its axis together with our sun. This journey is completed in about 250 million years for each revolution. Our galaxy in turn is on an immense journey around the centre of the cluster of galaxies to which it belongs. Hence our earth, in the effort to sustain the balance between all these pairs of attractive and repulsive forces, rotates in four different orbits that are finally measurable by a billion years.

Surely every person at some time looks up at the sun and moon and the brilliant stars and asks, who positioned all these so perfectly on the face of the sky’?

People have always marvelled at the stars and planets. But they have not always realized that there is a harmony in their positions and movements, a law and order, as indeed in the whole universe. For example, seen from the perspective of the ancient Greek astronomers, celestial bodies in the universe are aimless objects. That seems to be the implication of the term ‘planet’ which means ‘wandering’. The Greeks may have thought the ‘wandering stars’ or ‘planets’ moved in unstable orbits, more or less randomly.

The ancient astronomers’ judgement was not founded upon the Oneness of the Creator Who orders everything in the universe. Inevitably they did not have a clear grasp of the orderliness of the macro-cosmos and did not seek it.

The Qur’an revealed many centuries ago that it is Allah who created the heavenly bodies and put them into their peculiar orbits. There is nothing in the Islamic teachings that argues the view that phenomena or events are random.

Do they not look at the sky above them, how We have built it and adorned , and there are no flaws in it. (50.6)

We have built above you seven strong (heavens) and placed therein a blazing lamp. (78.12)

The ‘blazing lamp’ referred to is the sun.

People have always been fascinated by the thousands of gleaming lights sprinkled across the night sky. Today many enjoy looking into the heavens and learning about the patterns and positions of the stars, and discovering what stars can tell us about our universe as a whole. From our planet, if very high buildings and city lights permit, we can see about 6,000 stars with the naked eye. They change in colour, size, and brilliance.

We are near enough to one particular star, the sun, to find out many details about what these celestial bodies are made of and how they function. A star is composed of gases and other substances compressed together under the force of gravity. The pressure at the core of a forming star is sufficiently intense to initiate nuclear reactions that begin generating energy. During this process, matter is converted into energy, releasing large quantities of heat and light.

The sun may not catch up the moon, nor may the night outstrip the day. Each one is moving smoothly in its own orbit ( 36.40). Here an essential fact is clearly stated, namely the existence of the solar and lunar orbits. At the time of the Revelation, it was generally believed that the sun orbited a motionless earth. This, the geocentric system, had held sway from the early second century (the time of Ptolemy). It continued to do so until the sixteenth century. Fourteen centuries ago, the Qur’an directed the inhabitants of the Arabian Peninsula and, through them, all of mankind, towards the truth. The demonstration of the existence and details of the solar and lunar orbits is one of the recent achievements of modern astronomy.

Those who do not believe in One Creator maintain that everything comes about by chance. They do not realize that every creature in motion, from minute particles to the planets, displays on itself the stamp of the Eternal and of His Unity. Also, by reason of its movement, each of them, in some sense, takes possession of all the places in which it travels in the name of Unity, thus including them in the property of its Owner. As for those creatures not in motion, each of them, from plants to the fixed stars, is like a seal of Unity that shows the place in which it is situated to be the letter of its Maker. That is to say, each flower and fruit is a stamp and seal of unity that demonstrates, in the name of Unity, that its habitat and native place is the letter of its Maker. What all that inter-connectednes means is that one who does not have all the stars within his command does not have command over a single small particle either.

There are two other verses in the Qur’an about the sun and the moon and their usefulness to human beings, not only as light, but also as points of reference for space and time:

Allah subjected the night and the day for you, the sun and the moon. The stars are in subjection to His Command. Verily in this are signs for people who are wise. (16.12)

Allah is the One Who made the sun a lamp and the moon a light and ordained for it mansions, so that you might know the number of years and the reckoning (of the time).

Allah created this in truth. He explains the signs in detail for people who know (10.5)

The solar system comprises the sun and the nine planets that orbit it. The closest to the sun is the planet Mercury, at an average distance of 58 million km; the farthest, Pluto, is 5,900 million km from the sun. The closer a planet is to the sun, the shorter the time taken to complete its orbit. Thus, Mercury takes only 88 earth days to go round it, while Pluto orbits the sun only once in 248 earth years. Absolute time and distance are nowadays both measured in terms of light speed–a metre, for example, can be defined as the distance the light travels in a certain ‘space’ of time, in fact, 0.000000003335640952 seconds.

It is hard to think of the sun as a passing event. Nevertheless, its ‘term’ is fixed–the Qur’an is explicit on this point: And the sun runs its course for a period fixed for it (36.38). So, how long has the sun left to run? Astronomers nowadays calculate about 4.5 billion more years in its present state. It will still have nearly the same surface temperature (6.000 °C) and yellowish colour that it has now but it will appear about twice as bright because it will be about 60 percent bigger. Its next 4.5 billion years will have begun to take their toll on the sun’s nuclear fuel supply. What then? We don’t really know. Any calculations we make can only be made on the basis of theory.

The sun is full of gases composed of two thousand trillion tons (2×103 kg) of matter,

with the remains of other elements. For every million atoms of hydrogen there are about 85,000 helium atoms and only about 1,000 of any other kind. Pressure from all that mass compressing into the centre of the sun is high enough for the hydrogen atoms to fuse in the core to form helium. This simultaneously creates new energy which keeps the sun from collapsing further and provides the energy that allows it to (or makes it) shine. A series of nuclear fusion reactions, whose end result is the conversion of hydrogen to helium, happen on a vast scale and release very great amounts of energy in the form of heat, light, X-rays and so on. A part of this reaction must be the release of so-called neutrinos. Neutrinos are particles that interact so little with other matter that they can probably float through entire galaxies without being affected. They exist but have no mass nor any other physical property, which is like saying that they simultaneously exist and do not exist: we know they must be around by the way the movement of other (‘real’) particles is affected. If the theory about the way that the sun shines is correct, the sun should be producing about 180×1036 neutrinos each second. Obviously, only a small portion of these neutrinos will come in the earth’s direction.

The sun generates magnetic fields deep in its interior. Through mechanisms not yet fully understood, some of these fields erupt periodically through the sun’s surface, the photosphere. The high temperature and structure of the corona are produced by energy pumped from the photosphere up into the outer layer of the sun’s atmosphere along these magnetic fields.

The sun has been fusing hydrogen into helium throughout its present lifetime of 4.5 billion years, using up less than half of the available hydrogen in its core. By another 4.5 billion years, 90 percent of the available hydrogen in the core will have been converted into helium. Serious questions about the fusion rate in the sun still remain, but according to one theory, the humans of the future will face a sun that is running out of core hydrogen.

When that happens, the gas temperature and pressure will drop and the interior of the sun will collapse under the weight of the surrounding mass. The pressure in the collapsing gas will build up sufficiently for a rind of hydrogen to start burning around the core, now helium. This fusion will provide an outward force on the outermost layers of the sun, pushing them farther out than they are now. The surface of the sun will expand outward until it reaches the orbit of Venus.

Finally, this hydrogen outside the core will run out. The core of the sun will continue to contract, trying to replace the heat no longer generated by hydrogen burning. When the internal temperatures reach 100 million Kelvins, the helium (generated by the hydrogen burning) will itself start to burn. This will happen quickly, forming a carbon-rich core. Around this burned-out core, helium burning will start, and then the rind of hydrogen also will start to burn. The vast energy released by both rinds will push the sun’s outer layers further out until they reach the orbit of Jupiter. Earth will then be ‘inside’ the sun. The temperature on the surface of earth, around 6.5 billion years from now, will be around 30,000 Kelvins, and everything organic will be burned to a crisp.

Intelligent beings on earth 5 or 6 billion years from now, if any, would face the pressure to leave earth and, indeed, the solar system. They would need to have colonized planets around younger (therefore more stable) stars in order to survive. It is likely that humans in the near future will move off the earth in search of mineralogical and economic gain, whereas the future beings of our speculation will move off in order to save the species. The ageing sun will give future life a focus and a goal. And then, if we may be permitted to use the expression, a sort of Doomsday will have happened: certainly, the sun will have run to the end of its appointed (muslaqarr) time.

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