Homework 5, Astr 305V, due: November 12
The
link included HERE
gives you an up-to-date view of the extra-solar planets discovered
thus far. If you look down this table, you will generally find the
masses of the detected planets expressed in units of the mass of
Jupiter (e.g. "10.5 Jupiters" for the first entry means
that the planet likely has 10.5 times the mass of Jupiter). If you
click on the star name (e.g. "11 Umi" in the first entry)
you will get information on the star compared to the Sun, in terms of
color and the luminosity and size of the star. You can also click on
the individually detected planets for each star and learn about the
properties of the orbit of the planet detected, if known at this
point.
a. You will note that most of the planets are expressed in Jupiter masses, and only a few in Earth masses. Explain why for the two main detection techniques that we have discussed in class (transits and the Doppler shift), massive planets like Jupiter are much easier to find than Earth mass planets. The reason is different for each of the two techniques, so be careful to answer the question completely.
b. Consider the stars HD69830 and Gliese581. These have 2 or more planets with masses expressed in Earth masses which means that for these stars we could detect low mass planets. What are the properties of these stars compared to the Sun? Are these stars more or less massive than the Sun? What about their temperatures?
Would it be easier to detect low mass planets like Earth around low mass stars or around high mass stars, using the Doppler method? Explain.
c. Now look at the orbits of the detected planets (click on the planet name) in comparison to the orbits of the planets in the solar system. What do you notice in terms of the distance of the planets to their stars, and the orbital periods compared to e.g. the Earth's orbit or Mercury's orbit? What you discover is an observational bias due to our limited technical capibilities: we can so far only detect these low mass planets in the orbits shown, but not in e.g. orbits like the Earth's orbit around the Sun. Why would this be?