Astronomy Assignment #7
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Complete the assigned problems from the text listed below and address the Instructor Assigned Topic.
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state your answer in a complete sentence. Failure to complete all three of these tasks will result in less than full
credit awarded. The Instructor assigned topic must be typed.
Text Problems:
Review Questions
1. What are two signatures of an exoplanet in the starlight?
The two signatures of an exoplanet in the starlight are (1) a regular oscillation in the
wavelength of spectral lines from the star that indicates the star is wobbling due to an
unseen companion, and (2) a periodic dimming of the star as a planet transits across its
surface.
2. Why is it better to search for exoplanets in the infrared, rather than the optical band?
Stars emit most of their energy in the visible portion of the spectrum because of their
relatively high temperatures. However, planets that are much cooler emit most of their
energy in the infrared. So a star will appear dimmer and its nearby planet will appear
brighter in the infrared.
3. What sort of exoplanets are the star wobble methods best suited to find? Why?
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It seems to me that any star of not too great a mass would be good candidate for
investigating with the Radial Velocity Method. Very massive stars will not wobble much
and it will be hard to detect a wobble. However, for lower mass stars the wobble will be
more pronounced from a giant planet and should make for easier detection.
4. What exoplanet properties and orbit properties can you find with the star wobble
methods?
The Radial Velocity Method can determine (1) the period of the planets orbit, (2) the
semi-major axis of the planets orbit, (3) the eccentricity of the planet’s orbit, and (4) a
minimum mass of the planet.
5. What exoplanet detection methods could detect Earth-mass or Earth-size exoplanets?
Why would the other methods not be able to find small exoplanets like the Earth?
The Transit method has sufficient sensitivity to detect very small planets from the very
small dimming of star light as the planets transits across the stars surface.
It appears that the Radial Velocity Method may never be able to detect Earth mass
planets around other stars because the wobble caused by these orbiting planets is too
small to appear in the stars spectrum due to its convective and turbulent outer layers that
broaden the spectral lines excessively.
6. What exoplanet detection methods can give us the diameter, density, and maybe
composition of an exoplanet?
The Transit Method can yield a measure of the diameter (from the % dimming of the
star). I am not sure how the Transit Method yields up an estimate for the mass of the
planet which is what would be needed to estimate the density of the planet. Of course, if
you could estimate the density then you could take a pretty good stab at composition.
7. If you wanted to search for exoplanets around a particular star, which method(s) should
you use? Why is that?
The Transit method depends on the orbital plane of the planetary system to be very
precisely aligned with the Earth. This is not likely. So the Transit method would not be a
good way to find planets around a particular star.
Rather, the Radial Velocity Method which can detect the wobble of a star even when the
orbital plane of the planet is significantly misaligned with the Earth. This is the best
starting method for looking planets around a particular star.
8. What challenges to the standard condensation model do the other planetary systems give?
What is a likely explanation?
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Over half the exoplanets known have Jovian mass planets in Terrestrial planet locations.
This is not predicted by the condensation model. There are two simple (an therefore
probably not completely correct) mechanism that might account for this giant planet
migration.
To account for giant planets in very close circular orbits one might hypothesize that the tTauri winds of the star were delayed and that the planets orbited in the nebula for a
longer than “normal” time. During that time a slight drag on the plnets from the soalr
nebula caused them to slowly spiral in close to the star – always staying in a nearly
circular orbit. When the star finally did turn on, the t-Tauri winds cleared the nebula and
giant planet migration stopped.
To account for giant planets in very close non-circular high-eccentricity orbits one might
hypothesize that two Jovain planets in the outer part of the nebula had a close encounter –
not a collision, but a near miss. Simulations indicate that two jovian planets approaching
within 1 AU of each other would cause one of the planets to be ejected from the system
and the other planet to be throw into a close non-circular orbit.
9. What would be a good way to search for Earth-like exoplanets around other stars? How
could you tell if life was probably present on an exoplanet?
NASA’s proposed Terrestrial Planet Finder Mission is designed to search for Earth-like
planets and obtain a spectrum that can reveal the gasses in its atmosphere. Unfortunately
the TPF mission was permanently cancelled in 2011.
If an Earth-sized planet had spectral lines of molecular oxygen (O2) then it may well
harbor photosynthesizing life forms. Molecular oxygen (O2) did not arise in the Earth’s
atmosphere until the appearance of blue-green algae about 2.45 Billion years ago.
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