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Diagram showing how a smaller object (such as an extrasolar planet) orbiting a larger object (such as a star) could produce changes in position and velocity of the latter as they orbit their common center of mass (red cross).
Diagram showing how a smaller object (such as an extrasolar planet) orbiting a larger object (such as a star) could produce changes in position and velocity of the latter as they orbit their common center of mass (red cross). An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312 A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth

Doppler spectroscopy, also known as radial velocity measurement, is a spectroscopic method for finding extrasolar planets. Radial velocity is the Velocity of an object in the direction of the line of sight (i Spectroscopy was originally the study of the interaction between Radiation and Matter as a function of Wavelength (λ An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312 It involves the observation of Doppler shifts in the spectrum of the star around which the planet orbits. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for A spectrum (plural spectra or spectrums) is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is

It is extremely difficult to observe extrasolar planets directly because they are very faint at interstellar distances, although the first claims of direct observations were made in 2004 and 2005. As a result, planets outside of our solar system are usually discovered using indirect methods, through the effect of the planet on an object that is easier to observe, such as the parent star. The Solar System consists of the Sun and those celestial objects bound to it by Gravity. Successful methods include Doppler spectroscopy, astrometry, pulsar timing, transits, and Gravitational microlensing. Astrometry is the branch of Astronomy that relates to precise measurements and explanations of the positions and movements of Stars and other celestial bodies Pulsars are highly magnetized rotating Neutron stars that emit a beam of Electromagnetic radiation in the form of radio waves The term transit or astronomical transit has three meanings in astronomy A transit is the astronomical event that occurs when one Gravitational microlensing is an astronomical phenomenon due to the Gravitational lens effect Almost all of the known extrasolar planets have been discovered using Doppler spectroscopy.

Contents

History

Otto Struve proposed in 1952 the use of powerful spectrographs to detect distant planets. Not to be confused with his grandfather Otto Wilhelm von Struve (1819 &ndash 1905 ---- Otto Struve ( August 12 1897 – A spectrometer is an Optical instrument used to measure properties of Light over a specific portion of the Electromagnetic spectrum, typically used He described how a very large planet, as large as Jupiter, for example, would cause its parent star to wobble slightly as the two objects orbit around their center of mass. [1] He predicted that the small Doppler shifts to the light emitted by the star, caused by its continuously varying radial velocity, would be detectable by the most sensitive spectrographs as tiny red shifts and blue shifts in the star's emission. In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by Blue shift is the shortening of a transmitted signal's Wavelength, and/or an increase in its Frequency, due to the Doppler Effect, which indicates that However, the technology of the time produced radial velocity measurements with errors of 1,000 m/s or more, making them useless for the detection of orbiting planets. [2] The expected changes in radial velocity are very small – Jupiter causes the Sun to change velocity by about 13 m/s over a period of 12 years, and the Earth's effect is only 0. The Sun (Sol is the Star at the center of the Solar System. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 1 m/s over a period of 1 year – so long-term observations by instruments with a very high resolution are required. Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged [2][3]

Advances in spectrometer technology and observational techniques in the 1980s and 1990s produced instruments capable of detecting the first of many new extrasolar planets. 51 Pegasi b, the first extrasolar planet to be detected, was discovered in October 1995 using Doppler spectroscopy. 51 Pegasi b, also unofficially named Bellerophon and or abbreviated as 51 Peg b, is an Extrasolar planet approximately 50 Light-years [4] Since that date, nearly 300 exoplanet candidates have been identified, and most have been detected by Doppler search programs based at the Keck, Lick, and Anglo-Australian Observatories (respectively, the California, Carnegie and Anglo-Australian planet searches), and teams based at the Geneva Extrasolar Planet Search. The W M Keck Observatory is a two-telescope Astronomical observatory at the 4145 meter (13600 ft summit of Mauna Kea in Hawai'i. The Lick Observatory is an astronomical Observatory, owned and operated by the University of California. The Anglo-Australian Observatory (AAO is an Optical astronomy Observatory with its headquarters in suburban Sydney, Australia. The Geneva Extrasolar Planet Search is a variety of observational programs run by M [4]

Procedure

A series of observations is made of the spectrum of light emitted by a star. Periodic variations in the star's spectrum may be detected, with the wavelength of characteristic spectral lines in the spectrum increasing and decreasing regularly over a period of time. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared These variations may be indicative of the radial velocity of the star being altered by the presence of planet orbiting the star, causing Doppler shifts in the light emitted by the star.

If an extrasolar planet is detected, its mass can be determined from the changes in the star's radial velocity. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object A graph of measured radial velocity versus time will give a characteristic curve (sine curve in the case of a circular orbit), and the amplitude of the curve will allow the planet's mass to be calculated.

Example

The graph to the right illustrates the sine curve created using Doppler spectroscopy to observe the radial velocity of an imaginary star which is being orbited by a planet in a circular orbit. Observations of a real star would produce a similar graph, although eccentricity in the orbit will distort the curve and complicate the calculations below. In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape

This theoretical star's velocity shows a periodic variance of ±1 m/s, suggesting an orbiting mass that is creating a gravitational pull on this star. Using Kepler's third law of planetary motion, the observed period of the planet's orbit around the star (equal to the period of the observed variations in the star's spectrum) can be used to determine the planet's distance from the star (r) using the following equation:

r^3=\frac{GM_{star}}{4\pi^2}P_{star}^2\,

where:

Having determined r, the velocity of the planet around the star can be calculated using Newton's law of gravitation, and the orbit equation:

V_{PL}=\sqrt{GM_{star}/r}\,

where VPL is the velocity of planet. Johannes Kepler (ˈkɛplɚ ( December 27 1571 &ndash November 15 1630) was a German Mathematician, Astronomer In Astronomy, Kepler's Laws of Planetary Motion are three mathematical laws that describe the motion of Planets in the Solar System. The gravitational constant, denoted G, is a Physical constant involved in the calculation of the gravitational attraction between objects with mass The newton (symbol N) is the SI derived unit of Force, named after Isaac Newton in recognition of his work on Classical Gravitation is a natural Phenomenon by which objects with Mass attract one another In Astrodynamics an Orbit equation defines the path of Orbiting body m_2\\! around Central body m_1\\! relative to

The mass of the planet can then be found from the calculated velocity of the planet:

M_{PL}=\frac{M_{star}V_{star}}{V_{PL}}\,

where Vstar is the observed velocity of parent star. The observed Doppler velocity, K = Vstarsin(i), where i is the inclination of the planet's orbit to the line perpendicular to the line of sight. Inclination in general is the Angle between a Reference plane and another plane or axis of direction

Thus, assuming a value for the inclination of the planet's orbit and for the mass of the star, the observed changes in the radial velocity of the star can be used to calculate the mass of the extrasolar planet.

Problems

The major problem with Doppler spectroscopy is that it can only measure movement along the line of sight, and so depends on a measurement (or estimate) of the inclination of the planet's orbit to determine the planet's mass. If the orbital plane of the planet happens to line up with the line of sight of the observer, then the measured variation in the star's radial velocity is the true value. However, if the orbital plane is tilted away from the line of sight, then the true effect of the planet on the motion of the star will be greater than the measured variation in the star's radial velocity, which is only the component along the line of sight. As a result, the planet's true mass will be higher than expected. The term true mass is synonymous with the term Mass, but is used in Astronomy to differentiate the measured mass of a planet from the lower limit of mass usually obtained

To correct for this effect, and so determine the true mass of an extrasolar planet, radial velocity measurements must be combined with astrometric observations, which track the movement of the star across the plane of the sky, perpendicular to the line of sight. Astrometry is the branch of Astronomy that relates to precise measurements and explanations of the positions and movements of Stars and other celestial bodies Astrometric measurements allows researchers to check whether objects that appear to be high mass planets are more likely to be brown dwarfs. Brown dwarfs are sub- stellar objects with a mass below that necessary to maintain Hydrogen -burning Nuclear fusion reactions in their cores as do stars [2]

A further problem is that the gas envelope around certain types of stars can expand and contract, and some stars are variable. For the astronomical object see Variable star. Variable Star is a 2006 novel written by Spider Robinson This method is unsuitable for finding planets around these types of stars, as changes in the stellar emission spectrum caused by the intrinsic variability of the star can swamp the small effect caused by a planet.

The method is best at detecting very massive objects close to the parent star - so-called "hot Jupiters" - which have the greatest gravitational effect on the parent star, and so caused the largest changes in its radial velocity. Hot Jupiters (also called roasters, epistellar jovians, pegasids or pegasean planets) are a class of Extrasolar planets whose Observation of many separate spectral lines and many orbital periods allows the signal to noise ratio of observations to be increased, increasing the chance of observing smaller and more distant planets, but planets like the Earth remain undetectable with current instruments. Signal-to-noise ratio (often abbreviated SNR or S/N) is an Electrical engineering concept also used in other fields (such as scientific Measurements

Footnotes

  1. ^ Otto Struve, "Proposal for a project of high-precision stellar radial velocity work" The Observatory 72 (1952): 199-200. Not to be confused with his grandfather Otto Wilhelm von Struve (1819 &ndash 1905 ---- Otto Struve ( August 12 1897 –
  2. ^ a b c David Darling, "radial velocity method" The Encyclopedia of Astrobiology, Astronomy, and Space Flight, April 27, 2007. David Darling (born July 29, 1953 in Glossop) is an English Astronomer and full-time freelance science
  3. ^ "Doppler spectroscopy and astrometry - Theory and practice of planetary orbit measurements", Astronomy 497: Astronomy of Extrasolar Planets, Alex Wolszczan, Department of Astronomy and Astrophysics, Penn State University, Spring 2006. Aleksander Wolszczan (alɛk'sandɛr 'vɔlʂt͡ʂan) ( Apr 29 1946 in Szczecinek, Poland) is a Polish astronomer. The Pennsylvania State University (commonly known as Penn State) is a state-related, land-grant, space grant public research University (PDF)
  4. ^ a b R. P. Butler, et al. Paul Butler is an Astronomer who searches for Extrasolar planets He and Geoffrey Marcy were the first to discover Extrasolar planets orbiting "Catalog of Nearby Exoplanets" Astrophysical Journal. The Astrophysical Journal (abbreviated to ApJ or Astrophys J) is a Scientific journal covering Astronomy and Astrophysics. Vol. 646 (2006):2-3, 25-33. (PDF)

References

See also

External links

Any Planet is an extremely faint light source compared to its parent Star. Systemic is a research project designed to search data for Extrasolar planets using amateur Astronomers The project is an example
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