Exoplanetology is the Art and Science of
Exoplanets and the search for Life on other Worlds. It is formally defined as the study of the characteristics, formation and fate of planets outside our solar system. The discovery of extrasolar planets or exoplanets in transit in front of their parent starhas led to the birth of this burgeoning new field in science. Through December 2007, these varying techniques have been used to discover 270 planets outside of our solar system. [ [http://exoplanet.eu/catalog.php Interactive Extra-solar Planets Catalog] , The Extrasolar Planets Encyclopaedia. updated December 20, 2007. Accessed January 7, 2008.]
The current primary methods of Exoplanetology hinges upon the detection of exoplanets. Also termed as "planet-hunting", the techniques outlined below are the current methods employed by the science of exoplanetology.
Established detection methods
Astrometry is the oldest search method for extrasolar planets. It consists of precisely measuring a star's position in the sky and observing how that position changes over time. If the star has a planet, then the gravitational influence of the planet will cause the star itself to move in a tiny circular or elliptical orbit. Effectively, star and planet each orbit around their mutual center of mass (
barycenter), as explained by solutions to the two-body problem. Since the star is much more massive, its orbit will be much smaller.cite web | first= Amir | last= Alexander| title =Space Topics: Extrasolar Planets Astrometry: The Past and Future of Planet Hunting | url=http://www.planetary.org/explore/topics/extrasolar_planets/extrasolar/astrometry.html | publisher=The Planetary Society |accessdate= 2006-09-10 ]
During the 1950s and 1960s, claims were made for the discovery of planets around more than ten stars using this method. Astronomers now generally regard those claims as erroneous. Unfortunately, the changes in stellar position are so small that even the best ground-based telescopes cannot produce precise enough measurements. In 2002, however, the
Hubble Space Telescopedid succeed in using astrometry to characterize a previously discovered planet around the star Gliese 876. [cite journal | author=G. F. Benedict; B. E. McArthur; T. Forveille; X. Delfosse; E. Nelan; R. P. Butler; W. Spiesman; G. Marcy; B. Goldman; C. Perrier; W. H. Jefferys; M. Mayor | title=A Mass for the Extrasolar Planet Gliese 876b Determined from Hubble Space Telescope Fine Guidance Sensor 3 Astrometry and High-Precision Radial Velocities | journal=The Astrophysical Journal | volume=581 | year=2002 | pages=L115 – L118 | url=http://www.journals.uchicago.edu/doi/full/10.1086/346073 | doi=10.1086/346073 ] Future space-based observatories such as NASA's Space Interferometry Missionmay succeed in uncovering large numbers of new planets via astrometry, but for the time being it remains a minor method of planetary detection.
One potential advantage of the astrometric method is that it is most sensitive to planets with large orbits. This makes it complementary to other methods that are most sensitive to planets with small orbits. However, very long observation times will be required — years, and possibly decades, as planets far enough from their star to allow detection via astrometry also take a long time to complete an orbit.
Like the astrometric method, the radial-velocity method uses the fact that a star with a planet will move in its own small orbit in response to the planet's gravity. The goal now is to measure variations in the speed with which the star moves toward or away from Earth. In other words, the variations are in the radial velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent star's
spectral lines due to the Doppler effect.
The velocity of the star around the
barycenteris much smaller than that of the planet because the radius of its orbit around the center of mass is so small. Velocity variations down to 1 m/s can be detected with modern spectrometers, such as the HARPS ( High Accuracy Radial Velocity Planet Searcher) spectrometer at the ESO3.6 meter telescope in La Silla Observatory, Chile, or the HIRES spectrometer at the Keck telescopes.
This has been by far the most productive technique used by planet hunters. It is also known as Doppler spectroscopy. The method is distance independent, but requires high
signal-to-noise ratios to achieve high precision, and so is generally only used for relatively nearby stars out to about 160 light-years from Earth. It easily finds massive planets that are close to stars, but detection of those orbiting at great distances requires many years of observation. Planets with orbits perpendicular to the line of sight from Earth produce smaller wobbles, and are thus more difficult to detect. One of the main disadvantages of the radial-velocity method is that it can only estimate a planet's minimum mass. Usually the true masswill be within 20% of this minimum value, but if the planet's orbit is almost perpendicular to the line of sight, then the true mass will be much higher.
The radial-velocity method can be used to confirm findings made by using the transit method. When both methods are used in combination, then the planet's true mass can be estimated.
pulsaris a neutron star: the small, ultradense remnant of a star that has exploded as a supernova. Pulsars emit radio waves extremely regularly as they rotate. Because the intrinsic rotation of a pulsar is so regular, slight anomalies in the timing of its observed radio pulses can be used to track the pulsar's motion. Like an ordinary star, a pulsar will move in its own small orbit if it has a planet. Calculations based on pulse-timing observations can then reveal the parameters of that orbit. cite paper | author = Townsend, Rich | title = The Search for Extrasolar Planets (Lecture) | publisher = Department of Physics & Astronomy, Astrophysics Group, University College, London | date = 27 January, 2003 | url = http://www.star.ucl.ac.uk/~rhdt/diploma/lecture_2/ | accessdate = 2006-09-10 ]
This method was not originally designed for the detection of planets. But it is so sensitive that it is capable of detecting planets far smaller than any other method can, down to less than a tenth the mass of Earth. It is also capable of detecting mutual gravitational perturbations between the various members of a planetary system, thereby revealing further information about those planets and their orbital parameters.
The main drawback of the pulsar-timing method is that pulsars are relatively rare, so it is unlikely that a large number of planets will be found this way. Also, life as we know it could not survive on planets orbiting pulsars since high-energy radiation there is extremely intense.
Aleksander Wolszczanand Dale Frailused this method to discover planets around the pulsar PSR 1257+12.cite paper | author = A. Wolszczan and D. A. Frail | title = A planetary system around the millisecond pulsar PSR1257+12 | publisher = Nature 355 p. 145-147 | date = 9 January, 1992 | url = http://www.nature.com/physics/looking-back/wolszczan/index.html | accessdate = 2007-04-30 ] Their discovery was quickly confirmed; making it the first confirmation of planets outside our Solar System.
While the above methods provide information about a planet's mass, this method can determine the radius of a planet. If a planet crosses (transits) in front of its parent star's disk, then the observed visual brightness of the star drops a small amount. The amount the star dims depends on its size and on the size of the planet. For example, in the case of
HD 209458, the star dims 1.7%.
This method has two major disadvantages. First of all, planetary transits are only observable for planets whose orbits happen to be perfectly aligned from astronomers' vantage point. About 10% of planets with small orbits have such alignment, and the fraction is far smaller for planets with larger orbits. However, because transit surveys can scan large areas of the sky at once, the probability of finding extrasolar planets could potentially exceed that of the radial-velocity method [cite journal | author=Hidas, M. G.; Ashley, M. C. B.; Webb, et al. | title=The University of New South Wales Extrasolar Planet Search: methods and first results from a field centred on NGC 6633 | journal=Monthly Notices of the Royal Astronomical Society | year=2005 | volume=360 | issue=2 | pages=703 – 717 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005MNRAS.360..703H&db_key=AST&data_type=HTML&format=&high=44bf31ad8525219 | doi=10.1111/j.1365-2966.2005.09061.x] .
Secondly, the method suffers from a high rate of false detections. A transit detection requires additional confirmation, typically from the radial-velocity method. [cite journal | author=O'Donovan, F.; Charbonneau, D.; Torres, G.; et al. | title=Rejecting Astrophysical False Positives from the TrES Transiting Planet Survey: The Example of GSC 03885-00829 | journal=The Astrophysical Journal | year=2006 | volume=644 | issue= | pages=1237 – 1245 | url=http://www.journals.uchicago.edu/doi/full/10.1086/503740 | doi=10.1086/503740 ]
The main advantage of the transit method is that the size of the planet can be determined from the lightcurve. When combined with the radial velocity method (which determines the planet's mass) one can determine the density of the planet, and hence learn something about the planet's physical structure. The nine planets that have been studied by both methods are by far the best-characterized of all known exoplanets.cite conference | first=D. | last=Charbonneau | coauthors=T. Brown; A. Burrows; G. Laughlin | title=When Extrasolar Planets Transit Their Parent Stars | booktitle=Protostars and Planets V | publisher=University of Arizona Press | year=2006 | url=http://fr.arxiv.org/abs/astro-ph/0603376]
The transit method also makes it possible to study the atmosphere of the transiting planet. When the planet transits the star, light from the star passes through the upper atmosphere of the planet. By studying the high-resolution stellar spectrum carefully, one can detect elements present in the planet's atmosphere. A planetary atmosphere (and planet for that matter) could also be detected by measuring the polarisation of the starlight as it passed through or is reflected off of the planet's atmosphere. Additionally, the secondary eclipse (when the planet is blocked by its star) allows direct measurement of the planet's radiation. If the star's photometric intensity during the secondary eclipse is subtracted from its intensity before or after, only the signal caused by the planet remains. It is then possible to measure the planet's temperature and even to detect possible signs of cloud formations on it. In March 2005, two groups of scientists carried out measurements using this technique with the
Spitzer Space Telescope. The two teams, from the Harvard-Smithsonian Center for Astrophysics, led by David Charbonneau, and the Goddard Space Flight Center, led by L. D. Deming, studied the planets TrES-1and HD 209458brespectively. The measurements revealed the planets' temperatures: 1,060 K (790°C) for TrES-1 and about 1,130 K (860°C) for HD 209458b. [cite journal | author=Charbonneau, D.; Allen, L.; Megeath, S.; Torres, G.; Alonso, R.; Brown, T.; Gilliland, R.; Latham, D.; Mandushev, G.; O'Donovan, F; Sozzetti, A.; | title=Detection of Thermal Emission from an Extrasolar Planet | journal=The Astrophysical Journal | year=2005 | volume=626 | issue=1 | pages=523 – 529 | url=http://www.journals.uchicago.edu/doi/full/10.1086/429991 | doi=10.1086/429991 ] cite journal | author=Deming, D.; Seager, S.; Richardson, J.; Harrington, J. | title=Infrared radiation from an extrasolar planet | journal=Nature | year=2005 | volume=434 | issue= | pages=740 – 743 | url=http://www.obspm.fr/encycl/papers/nature03507.pdf | doi=10.1038/nature03507|format=PDF]
Gravitational microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. This effect occurs only when the two stars are almost exactly aligned. Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other. More than a thousand such events have been observed over the past ten years. If the foreground lensing star has a planet, then that planet's own gravitational field can make a detectable contribution to the lensing effect. Since that requires a highly improbable alignment, a very large number of distant stars must be continuously monitored in order to detect planetary microlensing contributions at a reasonable rate. This method is most fruitful for planets between earth and the center of the galaxy, as the galactic center provides a large number of background stars.
In 1991, astronomers Shude Mao and
Bohdan Paczyńskiof Princeton Universityfirst proposed using gravitational microlensing to look for exoplanets. Successes with the method date back to 2002, when a group of Polish astronomers (Andrzej Udalski, Marcin Kubiak and Michał Szymański from Warsaw, and Bohdan Paczyński) during project OGLE (the Optical Gravitational Lensing Experiment) developed a workable technique. During one month they found several possible planets, though limitations in the observations prevented clear confirmation. Since then, four confirmed extrasolar planets have been detected using microlensing. As of 2006this is the only method capable of detecting planets of Earthlike mass around ordinary main-sequencestars.cite journal | author=J.-P. Beaulieu; D.P. Bennett; P. Fouque; A. Williams; M. Dominik; U.G. Jorgensen; D. Kubas; A. Cassan; C. Coutures; J. Greenhill; K. Hill; J. Menzies; P.D. Sackett; M. Albrow; S. Brillant; J.A.R. Caldwell; J.J. Calitz; K.H. Cook; E. Corrales; M. Desort; S. Dieters; D. Dominis; J. Donatowicz; M. Hoffman; S. Kane; J.-B. Marquette; R. Martin; P. Meintjes; K. Pollard; K. Sahu; C. Vinter; J. Wambsganss; K. Woller; K. Horne; I. Steele; D. Bramich; M. Burgdorf; C. Snodgrass; M. Bode; A. Udalski; M. Szymanski; M. Kubiak; T. Wieckowski; G. Pietrzynski; I. Soszynski; O. Szewczyk; L. Wyrzykowski; B. Paczynski | title=Discovery of a Cool Planet of 5.5 Earth Masses Through Gravitational Microlensing | journal=Nature | year=2006 | volume=439 | issue= | pages=437 – 440 | url=http://www.nature.com/nature/journal/v439/n7075/full/nature04441.html | doi=10.1038/nature04441]
A notable disadvantage of the method is that the lensing cannot be repeated because the chance alignment never occurs again. Also, the detected planets will tend to be several kiloparsecs away, so follow-up observations with other methods are usually impossible. However, if enough background stars can be observed with enough accuracy then the method should eventually reveal how common earth-like planets are in the galaxy.
Observations are usually performed using networks of
robotic telescopes. In addition to the NASA/ National Science Foundation-funded OGLE, the Microlensing Observations in Astrophysics(MOA) group is working to perfect this approach.
The PLANET (
Probing Lensing Anomalies NETwork)/RoboNet project is even more ambitious. It allows nearly continuous round-the-clock coverage by a world-spanning telescope network, providing the opportunity to pick up microlensing contributions from planets with masses as low as Earth. This strategy was successful in detecting the first low-mass planet on a wide orbit, designated OGLE-2005-BLG-390Lb.cite journal | author=J.-P. Beaulieu; D.P. Bennett; P. Fouque; A. Williams; M. Dominik; U.G. Jorgensen; D. Kubas; A. Cassan; C. Coutures; J. Greenhill; K. Hill; J. Menzies; P.D. Sackett; M. Albrow; S. Brillant; J.A.R. Caldwell; J.J. Calitz; K.H. Cook; E. Corrales; M. Desort; S. Dieters; D. Dominis; J. Donatowicz; M. Hoffman; S. Kane; J.-B. Marquette; R. Martin; P. Meintjes; K. Pollard; K. Sahu; C. Vinter; J. Wambsganss; K. Woller; K. Horne; I. Steele; D. Bramich; M. Burgdorf; C. Snodgrass; M. Bode; A. Udalski; M. Szymanski; M. Kubiak; T. Wieckowski; G. Pietrzynski; I. Soszynski; O. Szewczyk; L. Wyrzykowski; B. Paczynski | title=Discovery of a Cool Planet of 5.5 Earth Masses Through Gravitational Microlensing | journal=Nature | year=2006 | volume=439 | issue= | pages=437 – 440 | url=http://www.nature.com/nature/journal/v439/n7075/full/nature04441.html | doi=10.1038/nature04441]
radiation. Even if the dust particles have a total mass well less than that of Earth, they can still have a large enough total surface area that they outshine their parent star in infrared wavelengths.cite journal | author=J.S. Greaves; M.C. Wyatt; W.S. Holland; W.F.R. Dent | title=The debris disk around tau Ceti: a massive analogue to the Kuiper Belt | journal=Monthly Notices of the Royal Astronomical Society | volume=351 | year=2004 | pages=L54 – L58 | doi=10.1111/j.1365-2966.2004.07957.x]
Hubble Space Telescopeis capable of observing dust disks with its NICMOS (Near Infrared Camera and Multi-Object Spectrometer) instrument. Even better images have now been taken by its sister instrument, the Spitzer Space Telescope, which can see far deeper into infraredwavelengths than the Hubble can. Dust disks have now been found around more than 15% of nearby sunlike stars.cite conference | first=J.S. | last=Greaves | coauthors=M.C. Wyatt; W.S. Holland; W.F.R. Dent | title=Submillimetre Images of the Closest Debris Disks | booktitle=Scientific Frontiers in Research on Extrasolar Planets | publisher=Astronomical Society of the Pacific | year=2003 | pages=239 – 244]
The dust is believed to be generated by collisions among comets and asteroids. Radiation pressure from the star will push the dust particles away into interstellar space over a relatively short timescale. Therefore, the detection of dust indicates continual replenishment by new collisions, and provides strong indirect evidence of the presence of small bodies like comets and
asteroids that orbit the parent star.cite conference | first=J.S. | last=Greaves | coauthors=M.C. Wyatt; W.S. Holland; W.F.R. Dent | title=Submillimetre Images of the Closest Debris Disks | booktitle=Scientific Frontiers in Research on Extrasolar Planets | publisher=Astronomical Society of the Pacific | year=2003 | pages=239 – 244] For example, the dust disk around the star tau Cetiindicates that that star has a population of objects analogous to our own Solar System's Kuiper Belt, but at least ten times thicker.]
More speculatively, features in dust disks sometimes suggest the presence of full-sized planets. Some disks have a central cavity, meaning that they are really ring-shaped. The central cavity may be caused by a planet "clearing out" the dust inside its orbit. Other disks contain clumps that may be caused by the gravitational influence of a planet. Both these kinds of features are present in the dust disk around
epsilon Eridani, hinting at the presence of a planet with an orbital radius of around 40 AU (in addition to the inner planet detected through the radial-velocity method).cite journal | author=J.S. Greaves; W.S. Holland; M.C. Wyatt; W.R.F. Dent; E.I. Robson; I.M. Coulson; T. Jenness; G.H. Moriarty-Schieven; G.R. Davis; H.M. Butner; W.K. Gear; C. Dominik; H. J. Walker | title=Structure in the Epsilon Eridani Debris Disk | journal=The Astrophysical Journal | volume=619 | year=2005 | pages=L187 – L190 | url=http://www.journals.uchicago.edu/doi/full/10.1086/428348 | doi=10.1086/428348 ]
As mentioned previously, planets are extremely faint light sources compared to stars and what little light comes from them tends to be lost in the glare from their parent star. So in general, it is very difficult to detect them directly. In certain cases, however, current telescopes may be capable of directly imaging planets. Projects to equip the current generation of telescopes with new, planet-imaging-capable instruments are underway at the
Gemini telescope(GPI), the VLT( SPHERE), and the Subaru telescope(HiCiao). Specifically, this may be possible when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and young (so that it is hot and emits intense infrared radiation).
In July 2004, a group of astronomers used the
European Southern Observatory's Very Large Telescopearray in Chile to produce an image of 2M1207b, a companion to the brown dwarf2M1207. [cite journal | author=G. Chauvin; A.M. Lagrange; C. Dumas; B. Zuckerman; D. Mouillet; I. Song; J.-L. Beuzit; P. Lowrance | title=A giant planet candidate near a young brown dwarf | journal=Astronomy & Astrophysics | year=2004 | volume=425 | issue= | pages=L29 – L32 | url=http://www.edpsciences.org/articles/aa/abs/2004/38/aagg222/aagg222.html | doi=10.1051/0004-6361:200400056] In December 2005, the planetary status of the companion was confirmed. [cite web | title= Yes, it is the Image of an Exoplanet (Press Release) | work=ESO website | date= April 30, 2005| url=http://www.eso.org/outreach/press-rel/pr-2005/pr-12-05-p2.html | accessdate=2006-09-10 ] The planet is believed to be several times more massive than Jupiter and to have an orbital radius greater than 40 AU.
Three other possible exoplanets have now been directly imaged:
GQ Lupi b, AB Pictoris b, and SCR 1845 b. [cite journal | author=R. Neuhauser; E. W. Guenther; G. Wuchterl; M. Mugrauer; A. Bedalov; P.H. Hauschildt | title=Evidence for a co-moving sub-stellar companion of GQ Lup | journal=Astronomy & Astrophysics | year=2005 | volume=435 | issue= | pages=L13 – L16 | url=http://www.edpsciences.org/articles/aa/abs/2005/19/aagj061/aagj061.html | doi=10.1051/0004-6361:200500104] As of March 2006 none have been confirmed as planets; instead, they might themselves be small brown dwarfs. [cite web | title=Is this a Brown Dwarf or an Exoplanet? | date= April 7, 2005| work=ESO Website | url=http://www.eso.org/outreach/press-rel/pr-2005/pr-09-05.html | accessdate=2006-07-04] [cite journal | author=M. Janson; W. Brandner; T. Henning; H. Zinnecker | title=Early ComeOn+ adaptive optics observation of GQ Lupi and its substellar companion | journal=Astronomy & Astrophysics | year=2005 | volume=453 | pages=609 – 614 | url=http://www.edpsciences.org/articles/aa/abs/2006/26/aa4475-05/aa4475-05.html | doi=10.1051/0004-6361:20054475]
This is one of the popular choices for Amateur Exoplanetology, more commonly called as Differential Photometry, and most often used with a CCD camera for hunting exoplanets. It involves plotting the measured changes in the brightness of an star over time to produce the
light curve, whereby the presence of an exoplanet is inferred.
* [http://www.exoplanetology.com/ Official Exoplanetology Website]
* [http://exoplanetology.blogspot.com/ Official Exoplanetology Blog]
* [http://planetquest.jpl.nasa.gov/ NASA's PlanetQuest]
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