Question

1. Can a star have a Jupiter-size plan in orbit at a distance similar to Earth yet not exhibit a Doppler shift in its light? Explain your answer.

2. You observe a star for a few years yet do not detect any Doppler shifting of its light. Give three reasons (other than any that may have been stated in the previous question) why it may still have planets orbiting around it.

3. Around what types of stars would it be easiest to detect transiting planets? For what types of planets would it be easiest to detect transits?

Answer 1

1. As we know Doppler spectroscopy is used to measure the shift in the wavelngth in the light emitted by the star. It relies on the fact that the star does not remain stationary when orbited by the companion planet (e.g. Jupitar). But it is assumed that star itself moves in circle or ellipse because of planet's gravitational pull. So when observed from distances, causes changes in the normal spectrum of the star.

Here it is assumed that no Doppler shift occured, it may be becasue the Gravitational pull is not enough to cause variation in the spectrum of the star. It was observed earlier the error in the measurement is nearly 1000m/s. And planet like Jupitar causes only change the velocity of Star(e.g.Sun) b 12m/s, making it nearly impossible to detect or shift in wavelength of the star.

2. Four possible reasons which may result in no Doppler shift:

1. The planet may no be line of sight of the observer and the Star

2. Size of the planet i.e. mass of the planet is very small compare to parent star.

3. Temperature of the planet.

4. Sensitivity of the instrument used to detect the doppler shift

3. Stars which are not variable in nature makes it easy to detect. Stars which have gas envelope and can expand and contract are called Variable stars, makes it difficult to detect variation in wavelength due to planet becasue of it intrinsic variablility.

It is easy to detect a massive planet and close to the planet star ie. its orbit radius should be small. Massive planet will apply large gravitational force, which in turn change the radial velocity of the star with large degree and make it detectable. These type of planet are called Hot Jupitars.