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‘Search for Other Worlds - Science fiction 20 years ago, reality today’ TABLE OF CONTENTS WHAT ARE EXTRASOLAR PLANETS?........................................................................................................3 WHY SEARCH FOR EXTRASOLAR PLANETS?..........................................................................................3 HOW DO ASTRONOMERS SEARCH FOR AND DETECT PLANETS?....................................................4 How to find exoplanets, and what we can learn about them............................................................................4 Basic Observational Techniques..................................................................................................................................4 Astrometry...................................................................................................................................................................5 Radial Velocity or Doppler Method...................................................................................................................5 Transit Method...........................................................................................................................................................5 Gravitational Microlensing ...................................................................................................................................6 Pulsar Timing..............................................................................................................................................................6 Eclipsing Binary.........................................................................................................................................................6 Circumstellar Disks ..................................................................................................................................................7 Coronagraphy .............................................................................................................................................................7 Space Based Technology..................................................................................................................................................7 Convection Rotation and Planetary Transit (COROT) ..............................................................................7 Microvariability and Oscillations of Stars (MOST) .....................................................................................8 Kepler Mission............................................................................................................................................................8 Space Based Technology – Future Missions...........................................................................................................9 PEGASE..........................................................................................................................................................................9 Space Interferometry Mission .............................................................................................................................9 New Worlds Mission................................................................................................................................................9 Terrestrial Planet Finder (TPF) ....................................................................................................................... 10 Darwin Mission ....................................................................................................................................................... 10 Gaia Mission ............................................................................................................................................................. 10 Planetary Transits and Oscillations of Stars (PLATO) ........................................................................... 10 Transiting Exoplanet Survey Satellite (TESS) ........................................................................................... 10 EXTRASOLAR PLANETARY SYSTEMS .................................................................................................... 11 Systems ................................................................................................................................................................................. 11 Properties ............................................................................................................................................................................ 11 Orbital characteristics.......................................................................................................................................... 11 Mass and Size........................................................................................................................................................... 12 Characteristics......................................................................................................................................................... 12 Atmospheres ............................................................................................................................................................ 12 How do discovered systems relate to our understanding based on our knowledge of our own system .................................................................................................................................................................................... 12 NEAR AND LONG TERM PROSPECTS FOR FINDING TERRESTRIALLIKE PLANETS................ 13 BIBLIOGRAPHY............................................................................................................................................. 14 www.astrochix.com 2 WHAT ARE EXTRASOLAR PLANETS? 1. An extrasolar (or exoplanet) planet is a planet outside of our own solar system. As of 31 May 2010, astronomers have made confirmed detections of 455 such planets (ExtraSolarWeb1). Most planets discovered so far are gas giants, however noting that the more massive planets have been easier to detect with current technology it’s thought that this represents a sampling bias which in the future may mean that the smaller planets discovered might be found to outnumber gas giants. It is now believed that a substantial number of stars have planets, meaning that millions of them must exist in our own galaxy, let alone those that exist outside of our galaxy. WHY SEARCH FOR EXTRASOLAR PLANETS? 2. For centuries, human beings have wondered ‘Are we alone?’ They have speculated that other worlds must exist and that some would harbor other forms of life. In our lifetime, advances in science and technology have enabled us to be able ponder an answer to the question. 3. Dominican Monk Giordano Bruno wrote, in 1584 there were countless suns and countless earths all rotating around their suns. Although he was not the first to imagine that other worlds existed his words are remembered. At the beginning of the 20th century astronomer Edwin Hubble realised that the small fuzzy nebulae in his telescope were in fact neighboring galaxies. Along with this discovery came the realization that other galaxies had hundreds of millions of stars, and those stars may very well have their own orbiting planets. 4. In the mid 1980s researchers from the University of Arizona observed a disk of dust around the star Beta Pictoris, the first evidence of a young planetary system in the making. Despite a number of suggested observed planets, no claims were verified until the 1990s. This delay was largely due to the inability of technology to observe the planets, due to their small size against their host star and the enormous distance involved. 5. The first confirmed discovery was in 1994, when Dr. Alexander Wolszczan reported that he had proof of extrasolar planetary systems. However, Wolszczan had discovered two or three planet-sized objects orbiting a pulsar, rather than a normal star. Observing regular variations in the pulsar’s rapidly pulsed radio signal, indicating the planets’ complex gravitational effects on the dead star, made this discovery possible. The origins of the pulsar planets are still under debate, but any planets orbiting a pulsar could not support any sort of life, as we know it. 6. The first discovery of a planet orbiting a star similar to the sun came in 1995, with the announcement of a rapidly orbiting planet, located very close to the star 51 Pegasi. The planet was at least half the mass of Jupiter, and was observed indirectly, using the radial velocity method. Shortly afterwards two more planets were discovered. The end of the 20th century dozens realised the discovery of many more planets. 7. Since the first discoveries, there has been a dramatic increase in the discovery of exoplanets. The sudden increase in discoveries is attributed to the following issues: (NasaWeb2) • Significant improvements in spectrometers, instruments that separate starlight into its component colors for analysis. www.astrochix.com 3 Better electronic sensors that record the incoming starlight collected by telescope optics. The development of computer software that can reliably discern fluctuations in starlight and the motion induced by the gravitational pull of unseen companions. The maturation of these technologies has led to intensified searches and data gathering. • • • HOW DO ASTRONOMERS SEARCH FOR AND DETECT PLANETS? How to find exoplanets, and what we can learn about them 8. Pointing a large telescope at stars and making observations might appear that the most obvious way to find an exoplanet. However, this direct method of observation is not generally possible for a number of reasons; stars are too far from us, they are bright and obscure any planets that might be orbiting the star, the angular separation between star and planet is very small and lastly the optical limit of telescopes (diffraction limit) is such that the bright stellar images obscure any planetary images. 9. Only the very largest ground based telescopes are able to observe exoplanets, but to have any hope of observing a planet, it must be well separated from the star. As direct methods are generally ineffective, other observational techniques must be used. Of the 453 exoplanets discovered so far, the vast majority of planets were observed by the radial velocity method and other indirect methods. The chart below compares the success of the detection methods used to date. Chart 0 Exoplanets discovered to March 2010 (data from www.exoplanet.eu and www.nasa.gov) 10. This chart shows that the number of planets discovered has increased each year, and that the radial velocity and transit methods are clearly the most successful. Basic Observational Techniques 11. Although detection can be difficult, there are a variety of technologies and strategies that can make observation of exoplanets easier. Each has it’s own method of www.astrochix.com 4 detection and strengths and weaknesses. summarised below. The main observational techniques are Astrometry 12. Astrometry is the measurement of stellar positions on the celestial sphere. This method consists of precisely measuring a star's position in the sky, and then making observations of the movement of the star over time. If the star has an orbiting planet or planets, then the gravitational influence of the objects will cause the star to move in a tiny circular or elliptical orbit around the common center of mass. Finding Earth-mass planets by astrometry requires sub-microarcsecond precision. Because the motion of the star is so small, this method has not yet been very productive at detecting exoplanets. ‘In the near term, astrometric accuracy from ground-based telescopes is expected to improve to better than 100 µas on large telescopes as the various sources of wavefront error are understood.’ (Lunine 2009) At that level of precision, groundbased astrometry will concern itself chiefly with finding giant planets and determining their masses and orbits. Radial Velocity or Doppler Method 13. As a star moves in its orbit around a system's center of mass, its velocity changes. Variations in the star's radial velocity can be deduced from displacements in the star's spectral lines due to the Doppler effect. If the motion is towards the observer the received wavelengths are shorter than those emitted by the source, and longer if the motion is away from the observer. ‘Whereas for the astrometric method the orientation of the star’s orbit is immaterial, in the radial velocity method the orientation has to be other than face-on, otherwise there is no variation in the radial velocity’. (Jones 2008) Extremely small radial-velocity variations can be detected, down to roughly 1 m/s. The Anglo-Australian Planet Search is a long-term program that searches for giant planets ‘around more than 240 nearby solar type stars’ (AngloAusWeb1) and to date has discovered more than 20 exoplanets using the Doppler method. This has been by far the most productive method of discovering exoplanets, with ‘90% of all the known planets outside out solar system have been found by that technique.’ (Lunine 2009) Transit Method 14. The transit method measures the faint dip in brightness of a star when a planet transits the star. As the exoplanet transits in front of its parent star, then the observed brightness of the star drops by a small amount, as shown in Figure 1. ‘In the past few years, the transit method of planet detection has emerged as one of the prevailing techniques to search for exoplanets. The tiny amount of stellar light blocked by a planet when it crosses (transits) in front of its host star provides information about the size of the planet.’ (Pin-Gao 2010). Figure 1: Transit Method 15. The amount by which the star dims depends on its size and on the size of the planet. ‘When an exoplanet’s orbit is presented to us sufficiently close to edge-on, then the planet will pass between us and its star.’ (Jones, 2008) A local example of this phenomenon was the transit of Venus across the face of the Sun on 08 June 2004. This has been the second most productive method of detection, though confirmation from www.astrochix.com 5 another method is usually considered necessary. 16. Dips in apparent brightness can arise from events other than a planetary transit, such as a grazing transit by a fainter companion star, however these events can be identified from the shape of the light curve. Hans Deeg notes that ‘with transits we can learn much more about the planets than with any other method to find planets…it’s the only method currently where we can measure the size of the planet’s fairly reliably.’ (Matson 2010) Gravitational Microlensing 17. Gravitational Microlensing happens when the gravitational field of a star acts like a lens and magnifies the light of a background star. When the alignment is exact you might think that the background star would be hidden from view, however the gravitational field of the foreground star bends towards us the light from the background star that passes near it. ‘So far there are 13 secure detections of planets by microlensing. Eight have been published: five Jovian, two somewhat smaller than Neptune, and one 3M⊕ planet.’ (Lunine 2009) This method has the advantage of being very sensitive to planets at large angular separations from the parent stars. Another advantage is that the intensity of the planetary deviation does not depend on the planet mass as strongly as effects in other techniques do. This makes microlensing well suited to finding low-mass planets. One major disadvantage is that the event can’t be repeated, as the alignment is unlikely to occur again. Also the planets tend to be very distant so the other methods are unable to confirm the observations. Pulsar Timing 18. A pulsar emits radio waves extremely regularly as it rotates. Slight anomalies in the timing of its observed radio pulses can be used to track changes in the pulsar’s motion caused by the presence of planets. The presence of a planet orbiting a star affects the timing of the regular signals emitted by the star itself. This phenomenon can be used to detect planets around a pulsar. This method is very sensitive and is capable of detecting of a very small mass. In 1992 Wolszczan and Frail used this method to discover the first exoplanet around the pulsar PSR 1257+12 (Wolszczan 1992). Unfortunately pulsars are pretty rare, so this method is not going produce a large number of exoplanet discoveries. Also, life it’s unlikely that life could not survive on planets orbiting pulsars since high-energy radiation there is intense. Eclipsing Binary 19. When a double star (binary) system is aligned such that the stars pass in front of each other in their orbits, the system is called an eclipsing binary star system. If a planet has a large orbit that carries it around both members of an eclipsing double star system, then the planet can be detected through small variations in the timing of the stars' eclipses of each other. CM Draconis eclipse minimum times have been monitored with high precision between 1994 and 1999 (Deeg 2000); periodic deviations of minimum times from a linear ephemeris may indicate the presence of an orbiting third body. Recent work by Morales et al (2008) leads them to believe that there may be a third body in the system and that further measurements will provide ‘a better basis for investigating the possible presence of a third body in the system.’ www.astrochix.com 6 Circumstellar Disks 20. Disks of space dust surround many stars, and this dust can be detected because it absorbs ordinary starlight and re-emits it as infrared radiation. Features in dust disks may suggest the presence of planets. Scientists believe that dust is generated by collisions of small objects, including comets and/or asteroids. Radiation pressure from stars will push dust particles out into stellar space; therefore any detection of dust around a star indicates the possibility of recent collisions and other objects. A 2005 study of the Eridani Debris Disk confirmed that the ‘structure within the dust ring suggests perturbations by a planet orbiting at tens of AU’ (Greaves et al 2005). Further observations will shed light on the final stages of planet formation, and provide more Figure 2: Image of Fomalhaut b information about the existence of possible planets. Coronagraphy 21. A coronagraph is an object which can be attached to a telescope, which is designed to block out the direct light from a star so that nearby objects, which otherwise would be hidden in the star's bright glare, can be observed. In the past coronagraphs have been developed to view the corona of the Sun, but a new version of similar instruments are being used to find extrasolar planets around nearby stars. Coronagraphs can be attached to either ground based or space based telescopes. While stellar and solar coronagraphs are similar in concept, they are quite different in design. This is so that observations can be made of exoplanets which are much more distant than our own sun. 22. In 2004 the high-resolution camera on the Hubble space telescope produced the first-ever resolved visible-light image of the region around Fomalhaut, at Figure 2. The image showed a ring of protoplanetary debris approximately 21.5 billion miles across. Authors of the study noted that ‘Hubble observations were incredibly demanding. Fomalhaut b is 1 billion times fainter than the star’ (NasaWeb6). 23. A stellar coronagraph concept is currently being studied to fly on the Terrestrial Planet Finder mission. On ground-based telescopes, a stellar coronagraph can be combined with adaptive optics to search for planets around nearby stars. Space Based Technology 24. Many extrasolar planet candidates have been found using ground-based telescopes. However, many of the methods can work more effectively with space-based telescopes that avoid atmospheric haze and turbulence. COROT and Kepler are two active space missions dedicated to extrasolar planet search. Hubble and MOST have found or confirmed a few planets, however neither of those observatories are dedicated to the search for exoplanets. There are also several planned or proposed space missions geared towards exoplanet observation. Below are summaries of COROT, MOST and Kepler, three of the most successful space based missions currently in operation. Convection Rotation and Planetary Transit (COROT) 25. Convection Rotation and planetary Transit was launched in 2006 and is dedicated to extrasolar planet detection. COROT is the first mission capable of detecting rocky planets several times larger than Earth, around nearby stars. It consists of a 30www.astrochix.com 7 centimetre telescope; which it will use to monitor the changes in a star’s brightness that happens when a planet crosses the front of the sun, the transit method. ‘From the ground, the only planets detected around other stars have been giant gaseous worlds (Jupiter-like planets), over 10 times the diameter of the Earth. Not affected by the distorting effects of the atmosphere, COROT will be the first spacecraft capable of finding worlds made of rocks.’ (ESAWeb1) 26. COROT detected its first extrasolar planet, COROT 1b in May 2007, and has gone on to discover 9 exoplanets. The mission was originally scheduled to end 2.5 years from launch but was extended to 2013. Below is a table listing the exoplanets discovered by COROT to date. Table 1: Exoplanets discovered by COROT Planet Constellation Mass (MJ) Radius (RJ) Year COROT 1b Monoceros 1.03 1.49 2007 COROT 2b Serpens 3.31 1.465 2007 COROT 3b Aquila 21.6 1.01 2008 COROT 4b Monoceros 0.72 1.19 2008 COROT 5b Monoceros 0.459 1.28 2008 COROT 6b Monoceros 3.3 1.16 2009 COROT 7b Monoceros 0.0151 0.150 2009 COROT 8b Unknown COROT 9b Serpens 2010 0.84 1.05 2010 Data gathered from http://smsc.cnes.fr/COROT/index.htm Microvariability and Oscillations of Stars (MOST) 27. Launched in June 2003, the MOST mission is the Canadian Space Agency's first space telescope. MOST is a small space telescope, about the size of a suitcase, and orbits the earth at about 800 kilometers. MOST is the first spacecraft dedicated to the study of asteroseismology. MOST was one of the first satellites used to observe exoplanets. Most ‘spends up to seven weeks at a time observing a star and watching the fluctuations in its brightness.’ (NasaWeb7) MOST uses the transit method, to measure the drop in brightness as a planet transits its star. Kepler Mission 28. The Kepler Mission launched in March 2009, is designed to discover exoplanets using the transit method. Kepler uses a photometer to monitor the brightness of over 145,000 main sequence stars in a fixed field of view. Kepler is designed to look for planets 30 to 600 times less massive than the standard of exoplanet currently discovered, closer to the order of Earth's mass using the transit method. 29. The first significant results were announced on 4 January 2010. ‘The first six weeks of observations recorded by the space faring telescope, combined with followup studies from the ground, have revealed five previously unknown extrasolar planets—one body roughly the size of Neptune and four low-density versions of Jupiter. All reside within roasting distance of their parent stars’ (Cowen 2010). www.astrochix.com 8 Table 2: Exoplanets discovered by Kepler Planet Constellation Mass (MJ) Radius (RJ) Year Kepler 1 Kepler 2 Planets Kepler 1-3 discovered by ground based telescopes, but are now within Kepler’s field of view. Kepler 3 Kepler 4b Draco .077 .357 2010 Kepler 5b Cygnus 2.114 1.431 2010 Kepler 6 Cygnus .669 1.323 2010 Kepler 7 Lyra .433 1.478 2010 Kepler 8 Lyra .603 1.419 2010 Data gathered from http://kepler.nasa.gov/Mission/discoveries/ Space Based Technology – Future Missions PEGASE 30. The proposal for this mission is to build a double-aperture interferometer composed of three free-flying satellites. ‘Its main scientific goals are the spectroscopic study of hot giant extra-solar planets, brown dwarfs and circumstellar disks in the nearinfrared domain (1.5 to 6 µm), with a possible extension towards the visible wavelengths’ (Absil et al 2005). One of the main goals of the mission is the study of Hot Jupiters and brown dwarfs. The mission is scheduled to launch in 2012. Space Interferometry Mission 31. One of the main goals of the Space Interferometry Mission (SIM) is the hunt for earth-sized planets orbiting the habitable zone of nearly by stars. Another goal of the SIM is to construct a map of the Milky Way Galaxy. The spacecraft will use optical interferometry to accomplish these and other scientific goals. The SIM will perform its search for nearby, Earth-like planets by looking for the ‘wobble’ in the parent’s motion as the planet orbits. Measurements of over ‘1300 distant stars’ (NasaWeb4) will be taken, and will be up to 100 times better than previous measurements taken. It’s anticipated that the launch will take place in 2015/16. New Worlds Mission 32. The New Worlds Mission aims to discover terrestrial exoplanets, and will be the first space telescope to use a starshade or occulter, to find exoplanets. Once a planet is detected the system would map the planetary systems and separate the light from the parent star to take high quality images. 33. The basic building premise of the mission is a starshade and a collector that function together as a single pinhole camera. ‘The starshade serves as the pinhole for the camera, though the entire shade will be a kilometer (0.6 miles) or more in diameter, with a hole about 10 meters (10 yards) across punched in the center.’ (ExtraSolarWeb5). The collector holds a telescope, and is separated from the starshade by at least 200,000km. A series of images of the planetary system would all measure the planetary orbits, and the brightness and broadband colors of the planets giving us information about the basic nature of each planet. The project was given extra funding www.astrochix.com 9 in 2008 for further study, but to date there is no planned launch date. Terrestrial Planet Finder (TPF) 34. The Terrestrial Planet Finder comprises two complementary observatories that will study all aspects of planets outside our solar system: including formation, development, size, number and habitability. ‘TPF will take the form of two separate and complementary observatories: a coronagraph operating at visible wavelengths and a large-baseline interferometer operating in the infrared.’ (NasaWeb5) The TPF observatories will measure the size, temperature, and placement of planets as small as the Earth in the habitable zones of other solar systems. TPF's spectroscopy instruments will provide information on atmospheric conditions, and allow scientists to determine whether a planet does, or could someday support life. As of June 2008 there has been no funding allocated, and the mission remains without a launch date. Darwin Mission 35. Darwin was a proposed European Space Agency (ESA) mission, which would have involved a constellation of four to nine craft designed to directly detect exoplanets. An early design proposed telescopes, each three to four meters in diameter, flying in formation as an astronomical interferometer. The proposal has not progressed since 2007, and appears unlikely to receive further funding. It’s considered that the design was similar to that of the Terrestrial Planet Finder with a similar design and scientific aims. Gaia Mission 36. The Gaia mission will examine up to a thousand million stars in our Galaxy, and monitor each of the stars about ‘70 times during a five-year period, precisely charting their positions, distances, movements, and changes in brightness’ (EsaWeb3). ‘Gaia’s main contribution to exoplanet science will be its unbiased census of planetary systems orbiting hundreds of thousands nearby (d < 200 pc), relatively bright (V ≤ 13) stars across all spectral types, screened with constant astrometric sensitivity’ (Lattanzi 2010). 37. Mission planners anticipate that every day Gaia will discover ‘on average, 10 stars possessing planets, 10 stars exploding in other galaxies, 30 ‘failed stars’ known as brown dwarfs, and numerous distant quasars, which are powered by giant black holes’ (EsaWeb3). It will do this by watching for tiny movements in the star’s position caused by the minute gravitational pull of the planet on the star. At this stage the mission is funded and launch is planned for 2012. Planetary Transits and Oscillations of Stars (PLATO) 38. PLATO is a proposed space observatory that will use a group of photometers to discover exoplanets. ‘The mission will have a nominal lifetime of six years, divided into three phases. The first two phases will be used for long-duration observations, with each observation focusing on a part of the sky that is expected to contain a high density of cool dwarf stars’ (EsaWeb4) The last phase will include observation of interesting targets for several months each. The spacecraft is planned for launch in 2017. Transiting Exoplanet Survey Satellite (TESS) 39. TESS is a proposed space telescope developed by MIT. The spacecraft would be fitted with up to 9 large angle telescopes and CCD detectors. The mission would www.astrochix.com 10 conduct a two-year long all sky program for exoplanets transiting nearby bright stars. The survey would focus on G and K spectral type stars brighter than 12 magnitudes. At this stage there has been no funding for this project and no launch date although it’s been suggested launch could occur in 2017. EXTRASOLAR PLANETARY SYSTEMS Systems 40. To date there have been 48 multiple extrasolar planetary systems discovered (ExtraSolarWeb1). Astronomers have discovered planets orbiting a substantial number of stars examined. However, the total fraction of stars with planets is uncertain. The fraction of stars with smaller or more distant planets remains difficult to estimate. Extrapolation does suggest that small planets are more common than giant planets. 41. It is now thought that a substantial number of stars have planetary systems, meaning that billions of them must exist in our galaxy alone. The total number of exoplanets must be very large, as our own galaxy has at least 100 billion stars, it may contain as many as hundreds of billions of planets. There are also planets that orbit brown dwarfs and free-floating planets that don’t orbit a host star. 42. Most of the exoplanets discovered so far are gas giants, and thought to resemble Jupiter. This is due to a sampling bias, in that planets of a larger mass have been easier to detect with current technology. However, recently several relatively lightweight exoplanets, only a few times more massive than Earth, have been detected and it’s been suggested that these will eventually be found to outnumber giant planets. Properties Orbital characteristics Figure 3: Exoplanet Semi-Major Axis compared to Planet Eccentricity. 43. Most known exoplanets are discovered using indirect methods, and therefore the type of information about the physical characteristics of the planet available is limited. Research so far indicates that many exoplanets have orbits with very small semimajor axes, and are much closer to their parent planet than any planet is in our own system. ‘About 30% of exoplanets have an orbital separation a less than 0.1 AU’ (Baraffe 2010). It’s suspected that is due to observational selection as the radial-velocity method is most sensitive to planets with small orbits. 44. It also appears that planets on large orbits may be more common than ones on small orbits. Cumming et al (2008) ‘estimate that 17–19% of stars have a gas giant planet within 20 AU… extrapolating to low masses gives 11% of stars with an Earth mass planet or larger within 1 AU.’ 45. Most exoplanets with short orbital periods have near-circular orbits of very low eccentricity; by contrast, most known exoplanets with longer orbital periods have quite eccentric orbits, see Figure 3. The prevalence of elliptical orbits is a major puzzle, since current theories of planetary formation strongly suggest planets should form with www.astrochix.com 11 circular orbits. 46. ‘A number of planetary transit surveys have revealed that most of these exoplanets belong to the special category of 'hot Jupiters', so named because, in contrast to Jupiter, these gas-giant planets orbit their stars just a few stellar radii away from them, and so are scorched by the stars' intense radiation’ (Pin-Gao 2010). 47. By examining astrometric and radial velocity measurements, it has been found that, exoplanets appear to not orbit their star in the same orbital plane, and have varied inclinations. In addition several hot Jupiters have been found with retrograde orbits that raises questions about the formation of planetary systems. Mass and Size 48. The vast majority of exoplanets discovered to date have high masses, some considerable more massive than Jupiter. Although this is in large part due to the fact that current detection methods are more likely to discover massive planets. This makes statistical analysis difficult, but it appears that lower-mass planets are actually more common than higher-mass ones, at least within a broad mass range that includes all giant planets. The number of exoplanets discovered span a wide range of masses from a few Earth masses to a few tens of Jupiter masses. ‘The realm of terrestrial exoplanets starts to open its doors with the lightest known exoplanet, GJ 581e, detected by radial velocity and having a mass M. sin i = 1.9 M⊕’ (Baraffe 2010). Characteristics 49. Most known exoplanets orbit stars similar to our Sun, spectral categories, F, G or K, and appear to be mainly composed of hydrogen and helium. Another compelling property of exoplanetary systems is the ‘correlation between planet-host star metallicity and frequency of planets. The probability of finding giant planets is a strong function of the parent star metallicity, indicating that an environment enriched in heavy material favors planet formation’ (Baraffe 2010). Stars of higher metallicity are much more likely to have planets, and the planets they have tend to be more massive than those of lowermetallicity stars. Atmospheres 50. Spectroscopic measurements can be used to study a transiting planet's atmospheric composition. Elements including water vapor, sodium vapor, methane, and carbon dioxide have been detected in the atmospheres of various exoplanets. This technique might disclose atmospheric characteristics that suggest the presence of life on an exoplanet, but to date no discovery has been made. ‘Most exoplanets yet detected through radial velocity and photometric transits are giant planets, characterised by the presence of a gaseous envelope mostly made of hydrogen and helium’ (Baraffe 2010) How do discovered systems relate to our understanding based on our knowledge of our own system 51. Initially astronomers made an assumption that all other planetary systems must be like our own. Then as we started to make observations of exoplanets it was discovered that most of the planets were in fact large gas giants, with small circular orbits and it was then assumed that this was the norm. Then more recently it was realised that www.astrochix.com 12 observational bias was indicating that planetary systems had larger planets, and that in fact it seemed more likely that there were a large number of terrestrial rocky planets that first thought. Now we know that that exoplanets have a wide variety of eccentric orbits, much more so that our own solar system. Each exoplanet discovered brings us closer to understanding our own solar system, and determining if we are within the standard planetary model or if we are an exception to the standard. NEAR AND LONG TERM PROSPECTS FOR FINDING TERRESTRIAL-LIKE PLANETS 52. As of April 2010 Gliese 581 d appears to be the best candidate for a terrestrial exoplanet orbiting within a habitable zone around its host star. The ‘super-Earth’ Gl5181 c is ‘clearly outside the habitable zone, since it is too close to the star’ (von Bloh 2007). In contrast, Gl581 d is a tidally locked habitable super-Earth near the outer edge of the habitable zone. ‘Despite the adverse conditions on this planet, at least some Primitive forms of life may be able to exist on its surface.’ (von Bloh 2007) Taking this into account it appears that Gl581 d is an interesting target for the planned TPF/Darwin missions, which will search for signs of life on exoplanets. 53. Beckwith (2008) notes that ‘If all stars have planetary systems (but only 15% have massive planets), and all planetary systems have at least one Earth-like planet within the habitable zone, and this planet always evolves life to dominate its atmospheric chemistry, then nearly 100% of the suitable stars will have planets indicative of life.’ 54. The coming decades will bring a large number of discoveries and new knowledge in the field of exoplanets. From the observational standpoint, a multitude of missions based on different techniques will provide an enormous amount of data. Current and future missions, like JWST, COROT and Kepler, will significantly increase the number of known planets and transitioning systems. 55. A large number of additional planets will also emerge in the next years from ground-based wide-field surveys. Increasing the number of detections will allow a more comprehensive study of exoplanet physical properties and will either confirm or contradict current observed trends. ‘The discovery of the triple planetary-mass system orbiting HR 8799 has demonstrated the potential of ground-based direct imaging projects. The development of a new generation of adaptive-optics (AO) systems, such as VLT-SPHERE or the Gemini Planet Imager (GPI) augurs well for direct imaging of planets orbiting solar type stars, enabling direct detection of hundreds of warm Jovian planets in the next decade’ (Baraffe 2010). 56. The holy grail of astrobiology, the detection of life outside of Earth takes one step closer to conclusion with the possibility of identifying habitable planets, with the detection of water vapor and signs of chemical disequilibrium in their atmospheres, is given by projects such as DARWIN, TPF and JWST. This could be one of the most significant and stimulating achievements of Science, as it may tell us that we are not alone in the universe. www.astrochix.com 13 BIBLIOGRAPHY Absil, O., L´eger, A., Ollivier, M., Schneider, J., Rouan, D., Leyre, X., Mourard, D., Man, C. N., Rousset, G., Allard, F., Malbet, F., Udry, S., Martin, E., et al. Pegase: a space interferometer for the spectro-photometry of Pegasides, Bulletin de la Soci´e Royale des Sciences de Li`ege - Vol. 74, 2005, pp. 183-190 AngloAusWeb1 http://www.phys.unsw.edu.au/~cgt/planet/AAPS_Home.html Baraffe, I., Chabrier, G., Barman, T., The Physical Properties of Extrasolar Planets, January 2010, http://arxiv.org/abs/1001.3577v1 Beatty, T. G., Seager, S., Transit Probabilities for Stars with Stellar Inclination Constraints, http://arxiv.org/abs/1002.3168v2 Beckwith, S. V. W., Detecting Life Bearing Extrasolar Planets with Space Telescopes, The Astrophysical Journal, 684:1404 Y 1415, 2008 September 10 Brown, R. A., Soummer, R., New Completeness Methods for Estimating Exoplanet Discoveries by Direct Detection, http://arxiv.org/abs/1003.4700v1 Cowen, R., Kepler space telescope finds its first extrasolar planets, Science News, January 30th, 2010; Vol.177 #3 (p. 12) Cumming, A., Butler, P., Marcy G. W., Steven, S. V., Wright, J., Fischer, D. The Keck Planet Search: Delectability And The Minimum Mass And Orbital Period Distribution Of Extrasolar Planets, 2008, http://arXiv.org/abs/0803.3357v1 Deeg, H., Laurance, D. R., Kozhevnikov, J. E. B., Rottler, L., Schneider, J., A Search for Jovianmass planets around CM Draconis using eclipse minima timing, Astronomy & Astrophysics, 2000, (358): L5–L8. EsaWeb1 http://www.esa.int/esaSC/120372_index_0_m.html EsaWeb2 http://www.esa.int/export/esaSC/120377_index_0_m.html EsaWeb3 http://www.esa.int/export/esaSC/SEMZ4E1A6BD_index_0.html EsaWeb4 http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=42280 ExtrasolarWeb1 http://exoplanet.eu accessed 31 May 2010 ExtrasolarWeb2 http://www.planetary.org/exoplanets/list.php ExtrasolarWeb3 http://kepler.nasa.gov/Mission/discoveries/ ExtrasolarWeb4 http://planetquest.jpl.nasa.gov/missions/jwstMission.cfm ExtrasolarWeb5 http://www.nasa.gov/vision/universe/newworlds/new_worlds_imager.html Greaves, J.S., Holland, W. S., Wyatt, M. C., Dent, W. R. F., Robson E. I., Coulson, I. M., Jenness, T., Moriarty-Schieven, G. H., Davis, G. R., Butner, H. M., Gear, W. K., Dominik, C., Walker, H. J., Structure in the Eridani Debris Disk, The Astrophysical Journal, 619:L187-L190, 2005 Jones, B. W., Exoplanets – Search Methods, Discoveries, and Prospects for Astrobiology, International Journal of Astrobiology, 7 (3&4), 279-292 (2008) Lattanzi, M. G., Sozzetti, A., Gaia and the Astrometry of Giant Planets, March 2010, http://arxiv.org/abs/1003.3921v1 Lunine, J., et al, Worlds Beyond: A Strategy for the Detection and Characterisation of Exoplanets. Executive Summary of a Report of the ExoPlanet Taskforce Astronomy and Astrophysics Advisory Committee, Astrobiology, Vol 8, No 5, 2008. www.astrochix.com 14 Lunine, J., et al, The Detection and Characterisation of Exoplanets, Physics.org, May 2009, http://ptonline.aip.org/journals/doc/PHTOADft/vol_62/iss_5/46_1.shtml?type=PTALERT&bypassSSO=1 Makidon, R. B., Sivaramakrishnan, A., Soummer, R., Anderson, J., van der Marel, R., Towards Observing Extrasolar Giant Planet Environments, Space Telescopes and Instrumentation, Vol 7010, 2008 Marcy, G. W., Butler, P. R., Detection of Extrasolar Giant Planets, Annu. Rev. Astron. Astrophys, 1998, 36: 57-97 Mandel, A. M., Expanding http://arxiv.org/abs/0712.2850 and Improving the Search for Habitable Worlds, Marcy, G., Butler, P. R., Fischer, D., Vogt, S., Wright, J. T., Tinney, C. G., Jones, H. R. A., Observed Properties of Exoplanets: Masses, Orbits, and Metallicities, 2005, http://arXiv.org/abs/astro-ph/0505003v2 Matson, J., A Warm Jupiter: A Newfound Exoplanet Bears a Resemblance to the Solar System’s Own Worlds, Scientific American, March 17, 2010 Morales, J. C., Ribas, I., Jordi, C., Torres, G., Gallardo, J., Guinan, E. F., Charbonneau, D., Wolf, M., Latham, D. W., Anglada-Escude, G., Bradstreet, D. H., Everett, M. E., O’Donovan, F. T., Mandushev, G., Mathieu, R. D., Absolute Properties of the low-mass eclipsing binary CM Draconis, 2008, http://fr.arXiv.org/abs/0810.1541v1 NasaWeb1 http://planetquest.jpl.nasa.gov/ NasaWeb2 http://planetquest.jpl.nasa.gov/science/science_index.cfm NasaWeb3 http://kepler.nasa.gov/ NasaWeb4 http://planetquest.jpl.nasa.gov/SIM/scienceMotivations/scienceOverview/ NasaWeb5 http://planetquest.jpl.nasa.gov/TPF/tpf_index.cfm NasaWeb6 http://www.nasa.gov/mission_pages/hubble/science/fomalhaut.html NasaWeb7 http://planetquest.jpl.nasa.gov/missions/mostMission.cfm Pin-Gao, G., Extrasolar Planets: Larger than they ought to be, Nature 465, 300–301, May 2010 Wolszczan, A., Frail, D. A., A Planetary System around the millisecond pulsar PSR1257+12, Nature, 355, 1992, 145-147 Von Bloh, W., Bounama, C., Cuntz, M., Franck, S., The habitability of super – earths in Gliese 581, 2007, www.arxiv.org/abs/0705.3758, www.astrochix.com 15