Worlds Beyond Our Own by Sujan Sengupta

Author:Sujan Sengupta
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

Difficult to Detect Extra-Solar Planets Directly

Since the discovery of 51 Pegasi b, a large number of planets and planetary systems outside the solar system have been discovered within a short span of time. But almost all of these planets are discovered by indirect methods. Why is it difficult to detect planets around a distant star directly? There are two main reasons. Firstly, the planets are extremely faint as they do not have their own source of light. They only reflect lights from their parent star. Even a planet as large as Jupiter, orbiting around a Sun-like star at a distance of 1 AU (i.e., at the distance between the Earth and the Sun) would be a billion times fainter than the star in the visible light. In the infrared, the planets are usually brighter but about a million times fainter than the star. Therefore, it’s difficult to detect them, especially when their positions are not fixed but alter rapidly around their parent stars. However, a sufficiently large space telescope can do the job even if they are extremely faint. The main problem in detecting a planet around a distant star is the apparent visual separation between the bright star and the faint planet. It is extremely difficult, many a times impossible to separate a planet from the bright star. This is because of the fact that, for a fixed orbital distance between the star and the planet, the angular separation between them is extremely small to an observer at the Earth. It can be realized from Fig. 5.2. The angle subtended by the planet, the observer at the Earth, and the star reduces as the distance between the star and the Earth increases. The angular separation is proportional to the ratio between the orbital distance of the planet from the star (AO) and the distance of the star from the observer (OB). For example, suppose there is a planet orbiting its parent star at a distance of about 0.1 AU or less. If the star is a few light years away from the Earth, the angular separation between it and the planet would be so small that the planet cannot be resolved from the star by any instrument however large or powerful the telescope is. So due to the limitation of instruments, planets orbiting around a distant star cannot be detected directly. However, a planet orbiting a few AU away from its parent star may be detected directly by blocking the bright light of the star. Very recently, a few young and bright planets far away from their host stars have been directly imaged. If the planet is far away from its parent star, we may see them by blocking the starlight, but if the planet is near to the star then we can only feel them. So the question is how these dim, small planets orbiting very close to their bright and large parent stars far away from the Earth can be detected.

Fig. 5.2Angular separation of the planet A

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