Introduction

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).