“Appearances are a glimpse of the unseen.”
Anaxagoras

Throughout the ages, human beings have looked at the sky with admiration. The star-studded sky on summer nights reminds us how enormous our universe is and how small we are in that immensity. NASA (National Aeronautics and Space Administration) has been scanning the universe with giant telescopes for decades. The fact that the starlight we see in the sky are their images from four years ago (that is, the stars are so far away that their light takes four years to reach us), should give us an idea of the vast size of the universe. The question, then, comes to mind: how can we have information about the places these telescopes cannot observe? The answer is that we get precise information about these galaxies, the planets, and even their moons, which are beyond the reach of our telescopes thanks to Mathematics and Mathematical thinking. These tools have been used for centuries and will continue to be used in the future. With mathematics, we can obtain accurate results with heuristic methods without directly seeing or observing the object.

Mathematics enables us to understand invisible structures with high-level thinking, allowing a meaningful system to emerge from paths that previously seemed random. Using our minds to “see” when we lack physical vision fosters abstract thinking, leading to extraordinary advances in human civilization. Let me clarify this by analyzing the three-prisoner problem.

In ancient times, three prisoners were brought to a room, blindfolded. The prisoners were told to line up behind each other, the tallest in the back, the shortest in the front, all facing the wall. Then, the guards put a hat on each of the prisoner’s head. They told them that the hats were selected from a total of five hats, three white and two black, but they did not tell them which color the one on their head was. The prisoners were expected to guess the color.

When the blindfolds are removed, the tallest man in the back could see the hats of the other two, the one in the middle could see the one on the shortest man, but the shortest man could see nothing but the wall. None of the prisoners could see the hat on his own head. The remaining two hats were hidden away.

The tallest man was asked to guess first. If he guessed the color of the hat correctly, he would be free. If not, he would die. Another option was to say, “I have no guess,” but for that he would have to live the rest of his life in the dungeon. The tallest man said, “No answer! I have no guess!” The man in the middle was asked if he knew; he also said, “No answer!” But the man at the front said he solved the puzzle and guessed correctly the color of the hat on his head: white. How was this possible although all he could see was nothing but the wall?

What’s lovely about this problem is that the person with zero visual information at the start becomes the first person to know their own hat color.

Let’s begin by looking at the possible combinations of hats for the prisoners. It can be solved using the process of elimination.

There are only seven possible combinations of hats on the three people:

• WWW
• WWB
• WBW
• WBB
• BWW
• BWB
• BBW

[W= white, B= black, and the order from last to first.]

The tallest man could have deduced his hat’s color if the other two had black hats – since there were a total of two black hats, the tallest one would surely have a white one. Since he chose not to guess, we can eliminate WBB. The knowledge that WBB has been eliminated is now part of the common knowledge held by all three people.

The prisoner in the middle also chose not to answer. If he saw a black hat in front of him, he could have deduced he was NOT wearing a black hat, since if he did have a black hat, then the person on the back would have guessed that his own hat was white. Since this did not happen, then we can eliminate all combinations with a B in the third place. We are left with WWW, WBW, BWW, and BBW. In all these combinations, the first person (the one in the front) is wearing a white hat. This is how he deduces the right color. The other prisoners could not know what color their hat was, for the remaining options included the chances that it could be either black or white.

This ancient three-prisoner problem is a fascinating example of mathematical thinking in action, showcasing the power of abstract thinking to unravel complex puzzles. Just as blindfolded prisoners use logic and deduction to determine the colors of their hats, we can strongly argue for the existence and qualities of many physical entities beyond our perceptions. For instance, most of the exoplanets (any planet beyond our solar system) have been discovered not with direct sighting, but through indirect methods, like the transit photometry by which we measure the dimming of a star’s light when a planet orbiting it partially blocks it. Another indirect method is the radial velocity which describes a planet’s gravity pull of the star either toward us or away from us and thus shifting the wavelength of its light (Doppler effect). And a third method is the microlensing method, when a star passes by another star, its gravity bends the light, making the other star brighter. If the passing star has a planet, this bend is even bigger, and it increases the star’s brightness.

This means that although we do not have a direct image of most of the exoplanets, we still know they exist. This falsifies the argument “I do not believe what I do not see.”

Rational methods and logic enable humans to see the invisible and know the unknowable. When multiple ways are used together, we can learn the vital statistics of whole planetary systems without directly imagining the planets themselves. The best example is the TRAPPIST-1 system about 40 light-years away, where seven roughly Earth-sized planets orbit a small, red star. Think that 1 light year is 9,461 trillion kilometers, then 40 light years are 3784.4 trillion kilometers away from the Earth. In addition, an astronomical unit (AU) is the equivalent of 150 million km / 93 million mi, and the Sun is 1 AU away from Earth. In light-years, the Sun is 0.00001581 light-years away, while in light minutes, the Sun is 8.20 light minutes away, or 500 light-seconds away from Earth. Just to give an idea about how far this system is, it would take 6,912 years of non-stop flight for the Parker Solar Probe, the fastest spacecraft ever built, at the speed of 430,000 mi / 692,017 km per hour, to arrive at the TRAPPIST-1. The seven known exoplanets that revolve around the TRAPPIST-1 planets have been examined with ground and space telescopes. These observations revealed their diameters and the subtle gravitational influence these closely packed planets have upon each other. From this, scientists determined each planet’s mass. We also know how much energy their star radiates onto these planets’ surfaces, allowing scientists to estimate their temperatures. We can even make reasonable estimates of the light level and guess the color of the sky if we were standing on one of them. And while much remains unknown about these seven worlds, including whether they possess atmospheres or oceans, ice sheets or glaciers, they have become the best-known solar system apart from our own.

“Appearances are a glimpse of the unseen,” says Anaxagoras (d. 428 BC), one of the earliest philosophers of natural science. One of my math teachers once said that what we know is perhaps the size of a small island in an ocean of unknowns.

If you think of what we know as a circle, the more you know, the wider the circle around it and the greater its relationship with the unknown. It is like the more you know, the more you do not. The larger the island of the known in the ocean of the unexplored, the longer the coast of the unknown. “Ignorance is bliss,” some say, perhaps in contrast to the fear of being lost in this ever-growing ocean of the unknown. The seen are the forerunners of the unseen, just like an iceberg. Much more of an iceberg stays underwater than what meets the eye. We only see one-thirtieth of the iceberg. Canny inferences can be more accurate than physical observations. Mathematical thinking shows us all aspects of the iceberg.

As we continue to push the boundaries of knowledge, fueled by curiosity and guided by mathematics, we unlock new insights into the universe’s mysteries. By expanding our understanding of the cosmos through abstract reasoning, we embark on a journey of exploration that promises to reveal the secrets of existence and inspire future generations to reach for the stars.

 “Those who believe in the unseen…” (al-Baqarah 2:3)

Rewinding the clock of human skin cells

Gill et al. Multi-omic rejuvenation of human cells by maturation phase transient reprogramming. eLife, April 2022.

The cells in our bodies perform their functions slower and begin to dysfunction as we get older. When skin cells age, they produce less collagen which leads to wrinkles form and cuts are slower to heal. Scientists have recently discovered a secret to younger skin. A new technique developed for rejuvenating skin cells can rewind the biological clock of patients by almost 30 years. This new approach utilized the revolutionary scientific method used to generate stem cells, originally developed by a Nobel laurate scientist named Yamanaka in 2007. Yamanaka’s molecular approach essentially erases cellular identity to convert any cell in the body to a stem cell. For this study, researchers used the Yamanaka’s molecular approach for a shorter period to make human skin cells younger without erasing the identity of skin cells. Yamanaka’s process of stem cell reprogramming takes around 50 days by activating 4 transcription factors in somatic cells. In this study, scientists followed the same protocol but only for 13 days. The scientists have chosen a common type of skin cell called fibroblasts. They collected fibroblasts from three old donors (average age of 50), applied their molecular reprogramming paradigm and investigated their age-related biological changes. Strikingly, the rejuvenated 50-years-old skin cells looked chemically and genetically as 20-years-old skin cells that are collected from younger donors. Moreover, when they tested the function of rejuvenated fibroblasts in wound healing assay, they observed that the rejuvenated cells behaved the similar to younger cells, as they moved fast and filled the gap in healing wounds. This work holds great potential for regenerative medicine and could be used to repair damaged cells and tissues in many different diseases. The long-term aim of regenerative medicine is to extend the human health span, rather than the lifespan. Very first potential application of this approach could be to rejuvenate skin in older people in parts of the body where they have been cut or burned.

Microplastics found in human blood

Leslie et al. Discovery and quantification of plastic particle pollution in human blood. Environment International, May 2022.

Vast amounts of plastic waste are disposed into environment each year. It is estimated that 188 million tons of plastic was dumped in 2016 and this number will double over the next 20 years to become 380 million tons in 2040. Smaller plastic fragments that can never fully break down are called microplastics. They have been found in oceans, forests, drinking water and even feces of babies and adults. Now, scientists have found them in human blood. The new study looked at the blood samples of 22 people and found that 17 of them (77%) had microplastics in their blood. The participants had an average of 1.6 micrograms of plastic polymers in every milliliter of blood. That plastic concentration is equivalent to a teaspoon of plastic in 10 large bathtubs of water. In the light of these numbers, scientists estimate that average person eats approximately 5 grams of microplastic every week, which corresponds to the amount in a credit card. Polyethylene terephthalate (PET), used in water bottles and other food and beverage containers, was the most common type of plastic found in human blood. Polystyrene, used in packaging like Styrofoam, was the second most common type of plastic. While finding plastic in our blood is frightening, it is not actually that surprising. We eat, drink, and even breathe microplastics every day. It is still unclear what that means for our health. Studies on common chemicals in plastics have linked them to increased risk of cancer, fertility, and developmental problems. However, most of these studies has focused on single molecules or additives like BPA, rather than the plastic polymers. More research is needed to tell us what health complications could occur through these daily plastic exposures.

5,000 exoplanets uncovered and counting

Cosmic Milestone: NASA Confirms 5,000 Exoplanets. NASA’s Exoplanet Exploration Program, March 2022. https://exoplanets.nasa.gov

The universe is full of many other worlds, orbiting their own suns. For almost 4 centuries now astronomers have been looking through telescopes at distant stars and dreaming about the planets hiding behind. In January 1992, astronomers at NASA discovered two objects that changed our understanding of space completely. They discovered two exoplanets, also known as extrasolar planets, which were whirling a star 2,300 light-years away. After 30 years of exploration, now NASA has confirmed the existence of 5,005 exoplanets; over 8,700 still stay as candidates as of March 21^st, 2022. Among the 5,005 exoplanets, 4,900 are located within a several thousand light-years away from us. The farthest exoplanet discovered so far is 27,727 light years away while the closest is only 4 light years away. Exoplanets come in different sizes and content. Some are smaller than Mercury, others are more than double the size of Jupiter. Some are freezing-cold, others are boiling hot. Some are rocky with surface and others are completely gaseous. Astronomers can now confidently say that our Milky Way galaxy have likely more planets than stars. They have also found some rocky exoplanets about the size of Earth, that reside in the potentially habitable zones and that has the right conditions for liquid water. But no one has yet discovered evidence of life in another planet’s atmosphere yet. So far, Earth is a quite rare planet with the right conditions for human life. However, one should remember that living organisms can adapt to variety of life conditions that are regarded as very harsh such as temperature, radiation, and acidity. It is still possible that life might have started on other worlds and adapted to quite different conditions. With the recently launched James Webb Space Telescope, astronomers expect not only to discover more exoplanets but also inform us more about them. According to NASA, there are likely as 100 to 200 billion exoplanets left to discover in our Milky Way galaxy.

Hsueh MF et al. Analysis of “old” proteins unmasks dynamic gradient of cartilage turnover in human limbs. Science Advances, October 2019.

Humans may not be able to regrow amputated limbs, but a recent study showed that damaged cartilage may regrow through a process similar to that of animals such as salamanders and zebrafish. Scientists collected 18 specimens of joint tissue from the hips, knees, or ankles of patients who underwent surgery. They then placed the tissue in a mass spectrometer and measured the age of the cartilage proteins in the sample. These analyses showed that the age of cartilage largely depended on where it resided in the body. Cartilage in ankles is young, middle-aged in the knee, and old in the hips. This correlation between the age of human cartilage and its location in the body suggests that limb repair occurs in humans in a similar fashion to certain animals in which tissue regeneration takes place at the furthest tips such as the ends of legs or tails. This finding also helps to explain why injuries to people’s knees and, especially, hips take a long time to recover and often develop into arthritis, while ankle injuries heal quicker and less often become severely arthritic. The researchers further identified the molecules that are instrumental in the regulation of this region-specific regeneration process. They are called microRNAs and, not surprisingly, are present at very high levels in animals that are known for limb, fin, or tail repair including salamanders, zebrafish, and lizards. Scientists believe that these regulator microRNAs can be utilized in the regeneration of degenerated cartilage of an arthritic joint to reverse arthritis. Regeneration of part or all of an injured human limb may even be possible by finding components salamanders have and we don’t. Finally, it is also possible that this could be a fundamental mechanism of repair that could be applied to many tissues, not just cartilage, which might open up many new avenues in the regenerative medicine.

The universe might have many Earth-like exoplanets

Doyle AE et al. Oxygen fugacities of extrasolar rocks: Evidence for an Earth-like geochemistry of exoplanets. Science, October 2019.

New astrophysical and geochemical evidence suggests that Earth may not be that unique, and Earth-like planets may be common in the universe. All of the planets in our solar system orbit around the Sun. Planets that orbit around other stars are called exoplanets.  The first exoplanets were discovered in the early 1990s. Since then, thousands of exoplanets have been revealed with over 4,000 confirmed and a further 4,495 potential candidates. There have been major efforts to narrow down the exoplanets that may have properties similar to Earth with conditions suitable for life. This includes being a rocky planet that is not too hot or cold so that liquid water can exist. When searching for exoplanets that are similar to Earth, astronomers typically look for worlds in orbit around a type of star called a red dwarf or an M-dwarf. These types of stars are somewhat similar to our sun and make up about 70% of the stars in our galaxy. However, a new study shows that rocky exoplanets in orbit around a different type of star, a white dwarf, can have interiors that are surprisingly similar to our planet. White dwarf stars are dense remains of normal stars that have exhausted their nuclear fuel. These stars are typically composed of light elements such as hydrogen and helium, but in some cases they attract heavier elements such as magnesium, iron, and oxygen in their atmospheres due to their extreme gravity. These heavy elements are thought to be introduced when a rocky exoplanet crashes into a star, which gives astronomers evidence of what the exoplanets were like before they were destroyed. In this recent study, scientists looked at six white dwarfs located 200 to 665 light-years from Earth and rocks from the planets that once orbited it. Their analyses showed that five out of the six white dwarfs had sucked up fragments whose chemical composition is similar to rocks on Earth, Venus, and Mars. While the conditions suitable for life depend upon many additional factors, this study points towards the idea that many rocky planets are likely very familiar in terms of their general composition and, therefore, structure and behavior. This study also made a substantial leap forward in being able to make inferences for bodies outside of our own solar system and indicates that it is very likely that there are truly Earth analogs out there.

Artificial leaf points to a sustainable path to carbon-neutral fuels

Andrei V et al. Bias-free solar syngas production by integrating a molecular cobalt catalyst with perovskite–BiVO4 tandems. Nature Materials, October 2019.

An artificial leaf from which a “clean” fuel alternative to petrol could be produced has been developed. Synthetic gas can be obtained from the lead by using only sunlight, carbon dioxide, and water.

Synthetic gas, also called syngas, is typically a mixture of carbon monoxide and hydrogen. It is largely produced by exposing fossil fuels such as coal or natural gas to high temperature steam and pressure, and the process releases carbon dioxide. Syngas is broadly used in a wide range of commodities including fuels, plastics, and fertilizers. While the utilization of fossil fuels has enabled large-scale industrial development in human history, the burning of fossil fuels is the largest source of emissions of carbon dioxide, which is one of the greenhouse gases that contributes to global warming. For decades scientists have been trying to discover new ways to produce syngas in order to close the global carbon cycle and to establish a sustainable chemical and fuel industry. In a recent study, researchers got inspired by leaves. These perfect little machines use sunlight to convert carbon dioxide and water into fuel for plants through photosynthesis. An artificial leaf has been designed to have two light absorbers, similar to the molecules in plants that harvest sunlight, and a catalyst made from the naturally abundant element cobalt. When the leaf is immersed in water, one light absorber uses the catalyst to produce oxygen and the other one carries out the chemical reaction that reduces carbon dioxide and water into carbon monoxide and hydrogen, thus forming the syngas mixture. The scientists are now searching for ways to use their technology to produce a sustainable liquid syngas that could serve as an alternative to petrol. Although major efforts to generate renewable energy sources are being made, the development of synthetic petrol is critical as electricity can currently fulfill about 25% of our total global energy demand. There is a huge demand for liquid fuels to power heavy transport, shipping, and aviation sustainably.

A planet that revolves around another star outside our solar system is called an Exoplanet or Extra solar planet. Ongoing studies involving this field are carried out via simultaneous ground and space based missions and observations. Scientists are searching a small portion of the Milky Way galaxy, approximately 3000 light years away, by using ground and space telescopes, along with various other astronomic methods (1). Despite all this technology, the observation area is too big when compared to the size of the object of interest.

It has been calculated that the Milky Way, a disc shaped galaxy, consists of 200 billion stars spread over a diameter of nearly 100,000 light years and a thickness of 1000 light years. When we consider the amount of stars in a single galaxy, and the fact that there are between a hundred billion and one trillion galaxies in the universe, the number of possible exoplanets is likely much larger than those we currently know of.

Classification of exoplanets

Exoplanets are classified according to their physical, chemical, and other characteristics, along with their diameter and mass: Jupiter like; greater than Jupiter; Earth like; greater than Earth

Classifications according to surface and atmospheric temperatures are as follows: Hotter than Jupiter; colder than Neptune; colder than Jupiter; small blue dots or twin Earths.

The presences of free-floating planets which have lost their parent stars because of different formation processes or other factors have also been discovered.

One of the common features of the exoplanets currently discovered is their short distance to the star they revolve around, which is usually less than half the distance between the Earth and the Sun. The known exoplanets are also defined by their faster revolutions in much shorter periods. Therefore, larger planets that are closer to their stars can be observed easily. When these planets are passing in front of their stars, a decrease in the brightness of the star is detected via spectrometers (Figure 1).

Figure 1. Passing of a planet in front of a star and a spectrum of this event.

Radial velocity, one of the methods used to discover exoplanets, relies on the observations of a star’s kinetic fluctuations. The proximity and size of a revolving planet leads to slight changes in location and velocity of a host star. As a result of this, the star gets closer to earth and then becomes more distant, which is observed as the Doppler shift of spectral line color waves. 75 % of all known planets have been discovered using this method (Figure 2).

Figure 2. Doppler shift – radial velocity

Earth-like planets or habitable places

In an official NASA report in December 2011, the discovery of an Earth-like planet was announced for the first time. This planet, named Kepler 22b, is 600 light years away and remains the most similar one to Earth among the known heavenly bodies. The distance of Kepler 22b to its star shows a high possibility for the presence of a habitable zone.

So what does this mean?

Earth is such a special home for us humans that everything here has been assigned to serve us with delicate calculations. Factors such as the Earth’s mass, gravity, distance to the Sun, rotational and revolution velocity, chemistry, thickness of the atmosphere, magnetic shield, hydrosphere/land ratio, ecological balances, and average temperature are all perfect for biological life.

Earth revolves in such a region and position that a majority of the planetary water is in a liquid state and is not ice or vapor.Thedistance of the habitable zone to our Sun is between 135,000,000 – 225,000,000 km. Earth revolves at a 150,000,000 km distance to the Sun. The value of a habitable zone for each planet depends on the diameter, mass, heat and radiation strength of the host star. In other words, aside from the similarity of an exoplanet to Earth, a classification of its host star with in terms of size and age is also important.

Kepler 22b owns the title as the first planet to match the criteria above with its following features:

• Has a radius 2.4 times bigger than Earth
• Revolution time is 290 days (365 for Earth)
• 15% closer to its star compared to the Earth-Sun distance
• The size and surface temperature of Kepler 22b’s host star is very similar to that of the Sun’s
• The surface temperature of the planet is 22 C
• The size of the habitable zone for Kepler 22bis 133,500,000 – 240,000,000 km (Figure 3).

Figure 3. Comparison of the solar systems of Kepler 22b and Earth.

Aside from these similarities, it is noteworthy to report the problems that scientists encountered regarding Kepler 22b:

• The unknown presence of water on the surface
• No information on the gaseous contents of the atmosphere.
• The gravitational force is 2.5 times greater than on Earth.
• Rocks constitute the surface instead of soil.

The hardest part is that Kepler 22bremains 600 light years away from us. This means it would take us 11 billions years to get there with today’s fastest spacecrafts. Who knows when we will be able to decrease this time with the advent of superior technology.

What do religious scholar say about life in outer space?

Among His manifest signs is the creation of the heavens and the earth, and that He has dispersed in both of them living creatures. And He has full power to gather them together when He wills. (Ash-Shura 42:29)

While interpreting the Qur’anic verse above, Fethullah Gülen notes the following:

“Since the earliest times, this verse has been taken as a proof for the view that there are living creatures, whether resembling human beings or not, in the places other than the earth. This view may be true. The second part of the verse, ‘He has full power to gather them together when He wills,’ has been understood that these creatures and human beings will possibly come together either in this world or in that of the other creatures. … there may be earth-like globes in the heaven where creatures resembling earthly ones live.” (Gülen 2012, 272-273)

“Perhaps people will not be able to reach those places individually or as a whole generation, but this can be achieved by mankind as a species. In other words, when the Divine Will manifests itself in that direction, humans here can encounter those other life forms.” (Gülen 2007, 232)

This commentary reflects what Bediuzzaman Said Nursi had said decades ago:

“The earth, although much smaller than other heavenly bodies, is so densely inhabited by living creatures that even its grossest and most rotten parts are full of living things, such as micro-organisms. This shows that those infinite firmaments, with their numerous stars and constellations, are inhabited by conscious, living beings …” (Nursi 2010, 530-531) 29th Word, First Aim, First Fundamental)

Nebiyev is a professor of physics in Azerbaijan.

References

• http://kepler.nasa.gov/
• http://planetquest.jpl.nasa.gov/
• http://en.wikipedia.org/wiki/Habitable_zone
• Gülen, M. Fethullah. 2007. Kendi iklimimiz, Istanbul, Nil Yayinlari.
• Gülen. M. Fethullah. 2012. Reflections on the Qur’an: Commentaries on Selected Verses, NJ: Tughra Books.
• Nursi, Bediuzzaman Said. 2010. The Words, (29th Word) NJ: The Light, Inc.
• Chris Kitchin Exoplanets: Finding, Exploring, and Understanding Alien Worlds- (www.springer.com/series/6960)-2012
• Mercy, G., P. Butler et al. 2005. “Observed Properties of Exoplanets: Masses, Orbits, and Metallicitie”.. Progress of Theoretical Physics Supplement, Vol. 158, No. 24-42.

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).