Astronomy 1021 Chapter Notes - Chapter 10: Gliese 876, 2M1207, Terrestrial Planet

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16 Aug 2016
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Chapter 10: Other Planetary Systems
Tuesday, January 5, 2016
11:33 AM
The Habitable Zone
The right temperature
oNot too hot not too cold
Liquid water on the surface of the planet
A medium for life to live in/ to transport things in
oEx. Blood stream
Big enough size of a rocky planet
Right air pressure
Extrasolar Planet- a planet that is not orbiting the Sun (exoplanet)
When naming them, we place a lowercase letter after the star name
Lettered in term of when they are discovered
o"b" for the first planet found, "c" for the second…
o"a" is reserved for the star itself
Ex. 51 Peg b
Gliese 876 d is closer to the star than Gliese 876 b; distance to the star doesn't
matter with the letter
DETECTING EXOPLANETS
Direct methods- obtaining an image/spectrum of the planet directly
Indirect methods- establishing the presence of a planet by its effect on the star
DIRECT METHODS
Very difficult
oStars are far away, angular separation very small
oPlanets reflect sunlight
But Sun is about a billion times brighter than Jupiter
Planets are much fainter
oMajor glare of starlight interfering
Can work best if:
oThe planet is fairly far away from the star
oStar is close to us
oPlanet is bright compared to the star
oWe can "mask" or reduce the starlight efficiently without reducing the light from the
planet
Need very sharp eyes, but we can do it
Planets emit primarily at infrared; not very hot
oAt infrared wavelengths, a star is "only" a thousand to a million times bright than the
planet
Young planets tend to be hotter, but still looks bad
2M1207 & 2M1207 b
oBrown dwarf- faint type of star, emits very little visible light
oRelatively bright in the infrared
oAlmost a star
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oVery far from its star
o1700 years orbital period, can't tell if it’s a planet
Starlight can be covered using a coronograph to increase the contrast to make it look less
overwhelming
Q&A: Sun size of a grapefruit, the earth is the size of the tip of a ballpoint, 15 meters away
Q&A: Sun size of grapefruit, nearest star is in San Francesco
INDIRECT METHOD
Almost all extrasolar planets have been discovered by indirect methods
oGravitational tugs
Newton's version of Kepler's laws tell us that two bodies will orbit their center
of mass
Planets are not orbiting the sun but are orbiting the center of mass of the solar
system
Also means the sun is orbiting the center of mass too
oPeriodic dimming of starlight
oKepler's 3rd law tells relationship of orbital period and distance
If period is 12 years, then 5.2 AU of average distance, planet orbit has
eccentricity of 0.05
Q&A: Measuring gravitational tugs of planets help to determine the distance from star, planet's mass,
and its orbital characteristics
The star wobbles reveal presence of planet, and the distance from the star, its mass, and its
orbital characteristics (eccentricity)
More planets = more complex
In principle, wobble can give info of all the planets of the star
INDIRECT METHOD #1
Astrometry- carefully measure the position of stars over time
Complications:
oFarther away the star = smaller its movement
oMore distant planetary orbit mean larger movements (good) , but means longer periods,
takes longer to detect
Q&A: The tool to measure motions of celestial objects is spectroscopy (Doppler effect)
INDIRECT METHOD #2
The Doppler Technique- using spectroscopy to measure changes in the star's radial velocities
Q&A: Object is moving towards us, spectral lines will appear at shorter wavelengths (blueshifted)
Radial velocity- that part of the velocity that is directly toward or away from us and they are small
Q&A: The lowest stellar velocities we could measure with Doppler effect is about 1m/s (walking speed)
Radial velocity curve- the period we see in it tells us the average distance of the planet, the amplitude,
its mass, and the shape give orbital characteristics
Can only observe the motion towards and away from us
Longer distance = longer period
Doppler shifts only reveal part of star's motion directed toward or away from us
Only radial velocity
Ex: If we measure radial velocity of 20m/s for a star, true orbital velocity of star is likely higher
than 20m/s
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We determine planet's mass from radial velocity amplitude, which is only part of full motion
Using the Doppler technique we have a precise determination of the period (and thus the
average distance) and the orbital shape, but the mass of the planet we determine from the
amplitude is the lowest possible mass the planet can have.
oSo the planet’s mass is likely larger than the mass determined from the Doppler
technique.
Technique favours detections of massive planets close to their parent star
oSmall mass= changes in radial velocity too small to measure
oLarge distance= time to complete an orbit takes longer than we've been measuring
oResults in selection effects: properties of planets found by this method are not
representative for the entire population of planets
INDIRECT METHOD #3
Transits and Eclipses: a (fairly small) fraction of planetary orbits are aligned edge-on such that the planet
passes directly in front of the star at regular times. This is called a transit (as we have seen, this
sometimes happens for Mercury and Venus)
When such planets pass behind the star, this is called an eclipse
Transit will block as very small fraction of the starlight
… while an eclipse will block the radiation from the planets (most notably in the infrared)
For transiting planets, the Doppler method measures the actual mass
The actual planet mass will be pretty much exactly equal to the mass we determine, since in this
case, we are in fact measuring the full velocity
Because we know the orbit is edge-on
Since we know mass & size, we know the density
Can also determine a transiting planet's atmospheric composition
2001 confirmed extrasolar planets to date, list of 3704 planet candidates
We know distances and masses for planets found with Doppler method
Most planets detected with Doppler method have greater mass than Jupiter
oSelection effect
oPlanets with smaller masses are harder to detect
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find more resources at oneclass.com