HW5 Q2, 3, 5, 6 (Danny LeBrun)

3.
--What were two early surprising features of exoplanetary systems?
     Super Earths are one of the early features of exoplanet systems which are characterized as being between the size of Earth and Neptune. From interstellar observations such as Kepler it's been shown that they are quite common in these systems, yet within our own solar system we don't contain one. We'd vainly expect other systems to be similar to ours, however such is not the case as you can see.
Another feature is the existence of Hot Jupiters where Jupiter sized planets orbit very close to their host star and thus cause them to be quite hot. These are found to be common in exoplanetary systems also, however again we do not find one in our own solar system.

--What correlation of planet size with occurrence have Kepler results demonstrated?
     Kepler showed a correlation between planetary size and abundance. It demonstrated that there are mainly 3 types of planets: Hot and Cold Jupiters, as well as Super Earths. Cold Jupiters aren't super common, but they are found, whereas Hot Jupiters and Super Earths are extremely common in the observations made by Kepler.

--How is the Habitable Zone around a star defined?
     The Habitable Zone is known as a range of distances from the host star that a planet can potentially have liquid water exist on its surface.

5.
--What are two properties of a proto-planetary disk that determine the kinds of planets that
will form.
     The temperature of the proto-planetary disk is important because it determines how gaseous the formed planets can become. The mass of the proto-planetary disk is also important due to the fact it provides information on how many planets could possibly form and to what size they could form to. With more mass, more massive planets can be born.
--Describe the three basic types of planetary migration.
     A planet orbiting a disk gravitationally perturbs the gas in its vicinity which launches density waves at orbital radii where the gas is in resonance with the planet. This leads to 2 of the 3 types of planetary migration.
     Low mass planets (<10Earth masses) undergo Type-1 migration where the migration rate is proportional to the planet's mass and the movement itself weakly alters the density of the disk. Radial displacement is comparable to the thickness of the gas disk. The interaction of the gas disk interior to the orbit of the planet adds angular momentum to the planet, while interaction with the exterior removes angular momentum. These changes lead to an inward or outward motion of the planet within the disk.
     Type-2 migration is noted for more massive planets where they strongly perturb the gas disk. This leads to exchange of angular momentum between the planet and the disk which thus repels gas from the orbit creating a gap within the disk. This gap causes the angular momentum transport processes (that evolve the disk over time) within the disk to attempt to correct for the missing mass density, thus forcing the migration of the planet. The motion of the planet is locked to the viscous evolution of the disk.
     Type-3 migration is for extreme cases where a planet contains a relatively fast radial motion which displaces gas in its co-orbital region. This causes a difference in the density between the gas leading and trailing the planet. This difference leads to the planetary migration.
     Another interesting form of migration is by interaction with other bodies within the vicinity of the disk. This form is what scientists expect occurred to Jupiter for it to be created so quickly.

6.
--Briefly explain what is meant by an “inversion layer” and why it is important for exoplanet
atmospheres.
     Through atmospheric physics we expect the temperature on a planet to decrease with altitude. However, in inversion layers temperature increases with altitude, hence the inversion terminology. A certain composition of the atmosphere causes this to occur and if determined can help denote characteristics of exoplanets. In fact, molecules that absorb UV radiation have been known to compile these layers which increases their interest in terms of exoplanets since UV radiation is a main antagonist to building life.
--Consider the atmospheric pressure at the summit of Mount Everest (elevation of 8.8 km
above sea level). The Earth’s atmospheric scale height is 8 km. Calculate the altitude needed to
reach the same atmospheric pressure on a super-Earth with a = 0.2 AU, Mp = 5M⊕, Rp = 1.2R⊕,
and is orbiting a solar analog. Assume the surface pressure on the planet is the same as the
surface pressure on Earth
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