Extrasolar planets detection

Radial velocity or Doppler method as we have seen above, a star will move under the gravitational attraction of a planet. This motion about the center of gravity of the system can be detected by small Doppler shifts of the star s spectral lines (see Fig. 6.2). Note that only the radial velocity component, that is the velocity that is directed toward (or away) from the observer causes a Doppler shift which varies periodically. This method has been so far the most promising one and most of the extrasolar planets detected so far were found from these Doppler shifts. The inclination / of the orbital plane with the sky is unknown, therefore, we measure the velocity... 

How could extrasolar planets be detected In recent years, astronomers have developed four different methods of finding non-emitting objects (planets) and even, to some extent, characterizing them. These are ... 

The transit method requires that the central star, the planet and the observer are connected by a line of sight. The dark planet passes across the light source and thus diminishes its light intensity to some extent. Observation is only possible when observer, star and planet are in a favourable position, i.e., the planet lies between the star and the observer. In spite of this requirement, the method permits the discovery of planets of about the size of the Earth information is also available on the size, mass and density of the planet as well as on its orbit. Because of its limits of applicability, this method is not often used. In the case of the star OGLE-TR-56, it was possible to detect an extrasolar planet, the orbit of which is very close to its sun only a twentieth of the distance of Mercury away from it. The temperature of the planet was determined to be around 1,900 K its diameter is about 1.3 times larger than that of Jupiter, its density about 500 kg/m3 (Brown, 2003 Konacki, 2003). 

The masses of the planets so far discovered vary between about 0.02 and 18 Jupiter masses. There are also very large variations in the values of the semi-major axis of the planetary orbits. If the first two methods for the discovery of extrasolar planets are compared (Doppler and transit methods), Doyle et al. (2000) point out the following facts around 40,000 photons are required to determine the transit of an extrasolar planet across the star HD 209548 using a photometer. But detection of the same system using variations in radial velocity requires 10 million photons. 

The detection of small extrasolar planets (of around the size of the Earth) will be done by registering the infrared light which they emit. Interference filters will blot out the light emitted by the star in question. Because of the huge distance from the Earth, effects due to its atmosphere and its IR radiation will not interfere. Darwin is intended not only to discover planets but to analyse their atmospheres for possible signs of life. 

These detection techniques have found heavy planets with up to 400 Earth-mass but are unlikely to detect Earth-mass planets because the Doppler shift is too small. The first extrasolar planet to be discovered by the Doppler variation technique was 51-Pegasi, with the results shown in Figure 7.9. Precise radial... 

Direct detection of extrasolar planets requires a body with a luminosity some 109 times smaller than its parent star to be detected. The wavelength maximum... 

Scientists have recently gained their first glimpse of a planet outside our solar system. The first extrasolar planet was detected in 1996 by the wobble that it transmits to its mother star as it circulates in orbit. But in 1999 astronomers were able to detect the light reflected from such a planet. It was slightly blue. 

The detection of light from an extrasolar planet was reported by A. C. Cameron, K. Horne, A. Penny, and D. James, Probable Detection of Starlight Reflected from the Giant Planet Orbiting t Bootis , Nature, 402 (1999), 751. 

Some astronomers believe that the growing population of extrasolar planets may be misleading. Nearly half of the so-called planets recently discovered orbiting around other stars may actually be brown dwarfs. The standard method for detecting extrasolar planets can only detect the minimum mass of an orbiting object. The actual mass may be much greater. [For more information, see Ron Cowen, Are most extrasolar planets hefty impostors Science News 158(18) 227 (October 28, 2000).]... 

Finally, the committee calls attention to the importance of using remote sensing to detect and characterize extrasolar planets that could support alternative carbon-based life. In addition to looking for evidence of water in... 

Pegasi (Michel Mayor and Didier Queloz) Mayor and Queloz detect a planet orbiting another normal star, the first extrasolar planet (exoplanet) to be found. As of June, 2009, 353 exoplanets were known. 

The following methods are applied in order to detect extrasolar planets ... 

Extrasolar Planets Found by Different Detection Methods... 

Before the first extrasolar planets were detected, the observational evidence of dust around stars was a first hint to the existence of extrasolar planetary systems. 

The detection of atmospheres on extrasolar planet is a very difficult task. 71% of the Earth is covered by oceans but up to now it is the only planet with water in liquid form on its surface. Venus might have had water on its early history, on Mars water may exist in a frozen state near the surface and climatic changes have occurred and formed river-like structures that are observed on its surface. There exists the possibility to find condensed water in the atmospheres of Jupiter and Saturn and in deeper layers of Uranus and Neptune. Subsurface oceans may exist on several satellites of the giant planets. But how can we detect water on extrasolar planets, how can we detect whether these objects have even an atmosphere ... 

The detection and observation of extrasolar planets will be one of the key projects in astrophysics in the next decades. Several instruments will be available to observe the faint signatures of planets and to study by spectroscopy their atmosphere and to detect water therein. 

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