The planets are bound by the gravitation or the force of gravity of the stars they revolve around. The concept of force of gravity was first proposed by Sir Isaac Newton. But the planets follow the Kepler's law of planetary motion.
The planets were thought by Ptolemy to orbit the Earth in deferent and epicycle motions. Though the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. By careful analysis of the observation data, Johannes Kepler found the planets' orbits to be not circular, but elliptical though this experimentation was done in and about 450-500 AD by Aryabhatta. As observational tools improved,astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice-caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.
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| Geocentricism |
With the advancement of technology, the planets of the Solar System were discovered and later Pluto was even made a dwarf planet in the year 2006.
Search for extra-solar planet: The search for any other planet like earth which could support life began in the early 2000s. The concept of another planet in the universe which could support life simply flashed in the mind of scientists since there were billions of trillions of stars in the universe and so there were chances that they might be a planet which supported life. Now the probability of finding a planet with life is 100% but not on all the planets.
Methods of finding a planet: Till date, astronomers have discovered over 200 planets revolving around the stars. So finding it wasn't very easy. They used two main methods of doing it
Doppler Shift
This method has been the most successful.
Why does a planet cause a star to sway? If a star has a single companion, both move in nearly circular orbits around their common center of mass. Even if one body is much smaller, the laws of physics dictate that both will orbit the center of the combined star and planet system. The center of mass is the point at which the two bodies balance each other.
The radial velocity method measures slight changes in a star's velocity as the star and the planet move about their common center of mass. In this case, however, the motion detected is toward the observer and away from the observer. Astronomers can detect these variances by analyzing the spectrum of starlight. In an effect known as Doppler shift, light waves from a star moving toward us are shifted toward the blue end of the spectrum. If the star is moving away, the light waves shift toward the red end of the spectrum.
This happens because the waves become compressed when the star is approaching the observer and spread out when the star is receding. The effect is similar to the change in pitch we hear in a train's whistle as it approaches and passes.
The larger the planet and the closer it is to the host star, the faster the star moves about the center of mass, causing a larger color shift in the spectrum of starlight. That's why many of the first planets discovered are Jupiter-class (300 times as massive as Earth), with orbits very close to their parent stars.
Astrometric Measurement
As with the radial velocity technique, this methods depends on the slight motion of the star caused by the orbiting planet. In this case, however, astronomers are searching for the tiny displacements of the stars on the sky.
The planets of our solar system have this effect on the Sun, producing a to-and-fro motion that could be detected by an observer positioned several light years away.
An important goal of the Space Interferometry Mission is to detect the presence of Earth-size planets orbiting nearby solar type stars via narrow angle astrometry. Similarly, the Keck Interferometry will conduct an astrometric survey of hundreds of stars to search for planets with masses as small as Uranus.
If a planet passes directly between a star and an observer's line of sight, it blocks out a tiny portion of the star's light, thus reducing its apparent brightness.
Sensitive instruments can detect this periodic dip in brightness. From the period and depth of the transits, the orbit and size of the planetary companions can be calculated. Smaller planets will produce a smaller effect, and vice-versa. A terrestrial planet in an Earth-like orbit, for example, would produce a minute dip in stellar brightness that would last just a few hours.
This method derives from one of the insights of Einstein's theory of general relativity: gravity bends space. We normally think of light as traveling in a straight line, but light rays become bent when passing through space that is warped by the presence of a massive object such as a star. This effect has been proven by observations of the Sun's gravitational effect on starlight.
When a planet happens to pass in front of a host star along our line of sight, the planet's gravity will behave like a lens. This focuses the light rays and causes a temporary sharp increase in brightness and change of the apparent position of the star.
Astronomers can use the gravitational microlensing effect to find objects that emit no light or are otherwise undetectable.
Methods of finding a planet: Till date, astronomers have discovered over 200 planets revolving around the stars. So finding it wasn't very easy. They used two main methods of doing it
Doppler Shift
Precise measurement of the velocity or change of position of stars tells us the extent of the star's movement induced by a planet's gravitational tug. From that information, scientists can deduce the planet's mass and orbit.
Why does a planet cause a star to sway? If a star has a single companion, both move in nearly circular orbits around their common center of mass. Even if one body is much smaller, the laws of physics dictate that both will orbit the center of the combined star and planet system. The center of mass is the point at which the two bodies balance each other.
The radial velocity method measures slight changes in a star's velocity as the star and the planet move about their common center of mass. In this case, however, the motion detected is toward the observer and away from the observer. Astronomers can detect these variances by analyzing the spectrum of starlight. In an effect known as Doppler shift, light waves from a star moving toward us are shifted toward the blue end of the spectrum. If the star is moving away, the light waves shift toward the red end of the spectrum.
This happens because the waves become compressed when the star is approaching the observer and spread out when the star is receding. The effect is similar to the change in pitch we hear in a train's whistle as it approaches and passes.
The larger the planet and the closer it is to the host star, the faster the star moves about the center of mass, causing a larger color shift in the spectrum of starlight. That's why many of the first planets discovered are Jupiter-class (300 times as massive as Earth), with orbits very close to their parent stars.
Astrometric Measurement
As with the radial velocity technique, this methods depends on the slight motion of the star caused by the orbiting planet. In this case, however, astronomers are searching for the tiny displacements of the stars on the sky.
An important goal of the Space Interferometry Mission is to detect the presence of Earth-size planets orbiting nearby solar type stars via narrow angle astrometry. Similarly, the Keck Interferometry will conduct an astrometric survey of hundreds of stars to search for planets with masses as small as Uranus.
Transit Method
Sensitive instruments can detect this periodic dip in brightness. From the period and depth of the transits, the orbit and size of the planetary companions can be calculated. Smaller planets will produce a smaller effect, and vice-versa. A terrestrial planet in an Earth-like orbit, for example, would produce a minute dip in stellar brightness that would last just a few hours.
Gravitational Microlensing
This method derives from one of the insights of Einstein's theory of general relativity: gravity bends space. We normally think of light as traveling in a straight line, but light rays become bent when passing through space that is warped by the presence of a massive object such as a star. This effect has been proven by observations of the Sun's gravitational effect on starlight.
When a planet happens to pass in front of a host star along our line of sight, the planet's gravity will behave like a lens. This focuses the light rays and causes a temporary sharp increase in brightness and change of the apparent position of the star.
Astronomers can use the gravitational microlensing effect to find objects that emit no light or are otherwise undetectable.
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