2. Measuring
The first planet around a sun-like (main sequence) star 51 Pegasi has been detected in 1995 by the Swiss astronomers M. Mayor and D. Queloz (Geneva Obs) employing Precision Doppler radial velocity (RV) technique which is todate the most efficient method for surveying stars. The 100th planet, HD 2039b was confirmed on July 1, 2002 by the California and Anglo Australian planet search teams. Almost all sun-like stars within 100 light-years have been surveyed - approximately 1000.
"Are we alone?" is a frequently asked question. Until now a planet with Earth-like environment has not been detected. Most of the planets own the mass of Jupiter or multiples of it and orbit their sun in distances closer than Mercury's orbit, very much unlike our Solar System.
Illustration: Greg Bacon, STScI
It is therefore reasonable to assume that there is no life in the form we know it.
Probably, large planets near their star is not common for outer worlds since current measuring techniques are not yet precise enough to detect planets with smaller masses, such as Earth's. At least, there is still hope for life outside our Solar System.
Scheduled for launch in 2006, the Kepler Mission will be equipped with a pupose-tailored spaceborne
telescope capable of finding rocky and Earth size planets around other stars.
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Measuring
The Doppler Detection Method
As Jupiter causes the Sun to orbit with it around a common center point of gravity so do large planets around other stars.
The larger the mass of a planet the more the center of masses is displaced from the center of the star causing proportionally larger wobbeling and changes in velocity. If a large planet orbits close to the star it rotates in short periods causing faster wobbles. This stellar wobble, also referred to as "reflex motion" displays a small mirror of the orbit of a planet.
The common center of gravity is always close to the center of the star and often within the star's outer hulls. A star wobbles around this center point in the same period as the planet revolves around the star.
This 'wobbeling' (changes in velocity) of a star can be ascertained by measuring its light over a period of days, months or even years. As a star orbits around the common center it gets once a little closer and once a little further away as seen from the line of sight from Earth. The observed wavelength of the star's light is then shifted to red (preceding) or blue (receding), a phenomena referred to as the Doppler effect. For reference Jupiter introduces a velocity change of 12.5m/sec, Saturn 2.8m/sec causing a slight wobble in our Sun over ca 12 and 30 years, respectively. The inner planets Mars, Venus and Mercury are too small to invoke wobbeling. Current precision of measurement is 3m/sec (as of Mid 2002).
Now, the magnitude of wobbling, thus cyclic red and blue shifts in the star's spectrum allows to compute the companion's orbital period. By its spectrum, the mass of the star can be obtained and with the known cyclic shifts, the companion's orbital distance can be computed. With this known distance and the magnitude of the reflex motion the companion's mass is obtained. Since a star's spectrum can only be measured on the direct line of sight, this mass property is a minimum value (*sin i), because of the unknown inclination of the planet's orbit to the line of sight from Earth.
The radial velocity curve of 51 Pegasi. P is the time between two amplitude peaks, thus the orbit period of the planet. K is the semi-amplitude (velocity * sin i). From this curve can further be calculated the planet's mass, average distance to the star and eccentricity using the third Kepler equation and mass-gravitation laws. The eccentrity is zero if the curve is a 'perfect' sine wave. The sine deforms with increasing eccentricity. The formular for Doppler velocity of the orbit with inclination is...
Photometric observations are made to look for transits and if not found then the inclination is less than 84.3 deg. Further, if no photometric variability larger than 0.0008 magnitudes is detected over the orbital period it is safe to assume that the measured radial velocity variation (applied to find planets) was not caused by starspots or pulsations, increasing the likelihood of an orbiting body.
By one definition, the companion is a planet if its mass is up to about 13 of Jupiter's, beyond it is considered a brown dwarf. This definition is disputed, however, whether the delimiter is mass, energy radiation or other properties, the definition is not the primary issue right now since we are talking about just detected bodies all of which orbit around a star.
The huge distance of other stars and thus the close angular proximity of a planet to it makes it impossible to separate the components photographically also considering that the brightness of the star overcasts the planet.
Illustration: NASA
When the inclination angle to the line of sight is nearly zero then a transit of the planet in front of the star can be observed. In such an event the drop of visual magnitude of the star will allow more precise computations especially to determine the orbit period and the planet's mass. This is a rare case though but could be observed before in front of the star HD 209458. Measuring a transit is another detection method, but requires a vey low inclination.
Planets with larger masses cause stronger wobbling which is easier to measure with given accuracy of measuring equipment. For this reason most of the detected planets came up with multiple masses of Jupiter. As the accuracy of the instruments increases, smaller planets will get into their range and existing data of larger planets can be recomputed to higher precision.
Lick Hamilton Cross-dispersed Echelle Spectrograph
Currently (2002) used instrumentation for the Doppler works include cross-dispersed Echelle Spectrometers performing order filtering or
cross dispersion. During the analysis an entire spectrum is typically split into chunks of ca 2 Angstrom each. The average Doppler error
is up to 3m/s with given telescope aperture and CCD quality, both producing a S/N (signal to noise ratio) among aother factors generating
the error figure.
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Is Life out there?
Possible candidates of life sustaining planets need to be structured and positioned similar to Earth to harbour life as we know it. The right distance from the star and a small orbit eccentricity. A too large gravitation as a result of planet mass could prevent development of higher life forms. Apart from this, water and a stable atmosphere containing oxygen are further conditions. Considering that we have all this in our Earth should let us realize how precious our home planet is and how vulnerable it must be to disturbances in its natural balance.
Now, which of the detected planets owns the basic ccnditions for life? Here is a speculation:
Located deep in the southern hemisphere, Eps Reticuli (HD 27442) is a star of 1.2 Sun masses in a distance of approximately 60 light-years. It shines 4.63 times brighter than our Sun and emits a similar spectrum. The planet's known average distance to the star is 1.2AU (20% further out than Earth) while its orbit eccentricity is only 0.02 (Earth = 0.016). However, the computed mass of the planet counts 1.3 Jupiters and is therefore either very large or very dense while the overall structure is unknown.
The planet is orbiting well within in the "life zone" and hardly changes its distance to the star over an orbit period, a fact which allows assumption of stable temperatures conditions. This does not take into account eventual heat radiations from the planet, such as high volcanic activity.
At the end if the day, the existence of life can only be assumed by known likelihoods. New measurements with more advanced instruments may unveil entirely different circumstances that may favor or exclude the possibility of life.
Nevertheless, why not believing in life out there. Hope combined with the human quest of exploration often did and will do more great
things, and... space is full of stars to look for planets.
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Keck Observatory
One of the observatories where stars are surveyed in an attempt to find planets is the Keck Interferometer located atop the volcano Mauna Kea, "Big Island" of Hawaii at 4150 meters above the Pacific Ocean. Joining two optical and infrared telescopes with primary mirrors 10 meters across, consisting of 36 hexagonal segment mirrors, the presence of planets the size of Uranus can be detected. As one of its ground based operations the Keck Interferometer is owned by NASA.
The relatively thermally stable atmosphere over Mauna Kea offers the best observation conditions for most time of the year.
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Brown Dwarf around HR 7672
News of January 2002
Source: http://www2.keck.hawaii.edu3636/ news/news.html
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