Astro 118 – Physics of Planetary Systems
Problem Set #9, Fall 2024
Due on Wednesday night midnight, December 4, 2024
1) Below is the first Hubble Space Telescope phase curve observation of an exoplanet, the hot
Jupiter WASP-43b, which shows the secondary eclipse when the planet passes behind the parent
star. The upper-right inset is the transit, which is way off the scale in the phase curve shown.
“Normalized Flux” is the total observed flux (planet’s emission + stellar emission) at this
wavelength, assuming the stellar flux alone is 1.000. Assuming the parent star is 0.7 RSun and a
Teff=4500 K blackbody, determine:
a) the planet’s radius
b) the approximate day-side temperature of the planet. Assume that the Rayleigh-Jeans
approximation holds. The star’s flux is constant over this time interval.
2) Why is obtaining a transit transmission spectrum of an Earth-sized, Earth-temperature
planet around a Sun-sized parent star more challenging than around a parent star smaller
than the Sun?
3) This Hubble detection below in the ultraviolet shows transit of planet HD 209458b in front
of its parent star, at a specific wavelength where neutral atomic H absorbs, the “Lyman
alpha” line at 121.6 nm. Assume the parent star is exactly like the Sun. At most wavelengths
the planet blocks out 1.5% of the parent star’s light. At this Lyman alpha wavelength, the
planet blocks out 15% of the parent star’s light!
a) What radius (in Jupiter radii) are we seeing out to at this UV wavelength?
