Question

Describe the observational biases inherent to using the direct detection method of finding exoplanets.

Describe how you can learn about a planet's composition by combining both the radial velocity and transit detection methods.

Why have astronomers found more Jupiter-sized planets at a distance of 1 AU around other stars than Earth-sized planets using the radial velocity method?

Answer 1

Direct detection of exoplanets are relatively harder,because planets reflect very little starlight and the light is overwhelmed by the starlight itself. So,they are not very much identifiable under visible light. Because of this reason they are detected in infrared radiation. If the planets is large enough,widely separated from the parent star and close enough to the solar system,then it emits large portion of its emission spectrum in the infrared range. We can take infrared images to directly detect these exoplanets.

In transit detection method we observe the dimming of stars spectrum because of the obstruction of a planetary body. When we see this dimming happening at regular intervals and in similar amounts,then we can definitively say that this is because of the orbital motion of a planet. We can determine the size of the planets from the amount of dimming. Of the dimming is in great amount then the size of the star will be bigger. From photometry we can determine the size of the host star. Then from the dimming we can determine the size of the planet with respect to the size of the star. Spectroscopic methods can give us the mass of the planet. Using these two informations we can determine the density of the planet.

Radial velocity method can in this case help us with providing the true mass of the planet. This true mass then can be used to determine the true density of the planet.

From spectroscopic measurements absorption spectrum of the planet can be derived that will give the contribution of different gases present in the planet's atmosphere that have absorbed the starlight to produce the absorption spectrum. Using the density and absorption spectrum we can determine the planets composition.

At this distance from the star, planets are detected using mainly two methods. Transit method and the doppler shift method. When the planet is large and of the size of Jupiter or more,then only the planet causes greater amount of transit for the stars light. These planets also cause greater amount of Doppler shift of the stars spectrum because of the greater attraction of these plants of large size. So,these are easily detectable to the observers. This is the reason, why these large planets are more frequently found at 1AU distance from the star.

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