Exoplanets

Exoplanets or Extra Solar Planets refers to the planetary bodies outside our Solar System. The discovery of the first planets around a Pulsar in 1992 and around a main sequence star different from our Sun in 1995 has opened, after centuries of speculation, a new era in astronomy.

The search for ‘other worlds’ is perhaps one of the oldest well articulated scientific questions. There had been a first attempt in the 17th century by Christiaan Huygens, who tried to see planets around other stars. He quickly realized that this observation was far beyond the capabilities of his modest telescope. After that, for centuries, seeing such planets could only be a dream. A new hope appeared in the early 20th century after the astrometric and spectroscopic detection (explained further in this article) of ‘unseen’ companions to stars (i.e. very faint stars). However, the expected planetary masses were too small to be reached by these methods.

The first important step was made by Frail and Wolszczan when they detected the first planets around a pulsar. It was a great encouragement for planet hunters, although the star itself (a neutron star) was too odd to resemble our Sun. The good news was heard in October 1995, when the Swiss astronomers Mayor and Queloz claimed the detection, thanks to the radial velocity method using a telescope, of a planet around the star 51 Peg, similar to the Sun. This first discovery was followed by many others; since then, more and more planets have been discovered (by radial velocity measurements of the parent star) each year and there are at present 66 of them. Furthermore, for one of the stars (upsilon Andromedae), three planets have been detected. In 2001, a two-planet system was detected (around the star HD 82943) in which one orbital period is nearly exactly twice as long as the other. HD 74156 is another two-planet system with a Jupiter-like planet and a more massive planet farther out. A giant planet moving in an orbit around a Sun-like central star (HD 168443) already known to possess a planet has been detected, as well as a planet with the most elongated orbit detected so far (HD 80606), moving between 5 and 127 million kilometers from the central star.

To investigate these planetary systems, astronomers use several methods of detection and observation. Extra-solar planets can be detected by either direct or indirect methods. Each detection method is characterized by some observable traits which are related to the intrinsic physical parameters of the planet. These parameters are their mass MP, radius RP, temperature TP, and distance from the parent star, orbital period P, rightness LP and distance D from the solar system. The potential success of a given approach depends naturally on its instrumental limitations. The approaches include methods such as:

• Direct imaging
• Dynamical perturbation of the star by the planet
• Planetary transits
• Gravitational lensing
  

Questions to Ponder:

• How are planets detected around other stars?
• How can we estimate the number of Earth-like planets in the Galaxy?
• What kinds of instruments are used to detect extra-solar planets?
• Why is our solar system so different from all the others we've found?