Homework 05
Please answer at least 4 of the following 5 questions correctly for full
credit:
Question 1
Between Mars and Jupiter we have the Asteroid Belt, and beyond Neptune we have
the Kuiper Belt.
Are the objects in each belt made from the same materials? If
your answer is no, please explain why there should be a difference.
Answer
No, the objects are not made from the same materials. The Asteroid Belt objects
are rocky like the inner planets, whereas the Kuiper Belt objects are icy like
the comets. That difference exists because the solar wind from the young sun
expelled all the volatile elements from the inner solar system when it was
formed - the same reason that has the inner planets being rocky and the outer
ones gaseous/icy.
Question 2
Gravity wants to pull everything together as closely as it can. What prevents
all the dust in the proto-planetary cloud from just falling into the central
star but instead shapes it to be spread out in a disk?
Answer
The conservation of angular momentum forces the contracting cloud to
settle into a disk rather than to have it collapse into its gravitational
center.
Question 3
Two collections of pictures are shown on the right hand side of slide 12 of my
lecture 15
(see PDF). Explain why the proto-planetary disk in the
lower collection is bright while it is a dark band in the upper collection.
Answer
In the upper collection the dust has not fully settled into the disk yet, but
already is much concentrated near the eventual disk. So the thick dust near
the disk is blocking the light from the central star. That starlight is seen
as scattered of the thin dust that is still settling into the disk from
above and below.
In the lower collection the dust has completely settled into a thin disk,
and now the starlight can be reflected of the top and bottom of the disk.
That is why now it appears bright, reflecting starlight from its surface
rather than absorbing it in its bulk.
Question 4
Same two collections of pictures as in Question 3: Which pictures were taken
earlier in the
evolution of a proto-planetary disk: the ones in the upper or the ones in the
lower collection?
Answer
The pictures in the upper collection were taken earlier: The dust cloud has
not yet completely collabsed into a disk, and the star's radiation has not
yet cleared out the space above and below the disk.
Question 5
How does our method of finding planets through the motion of their host star
affect the size of the planets that we discover?
Answer
By selecting planets that are very massive and close to their parent star.
Only if they fulfill both conditions simultaneously can we see them by their
gravitational effect on the star: the star moves on an orbit around the
common center of gravity rather than staying at one of the foci of the
planets elliptical orbit. This is one example for a selection bias.
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Last modified: Thu Feb 28 14:53:38 MST 2008