Lecture 10 - Exoplanets Flashcards
What are exoplanets? how do they compare to the planets in our solar system?
planets outside our solar system that orbit other stars
diff properties than planets in our solar system, but many are earth-like
why are exoplanets so hard to detect? (2)
- planets are very tiny
- the stars are ~1 billion times brighter than the light reflected by any orbiting planet –> starlight overwhelms any planetary light
what is the Contrast Ratio Problem?
what reduces the problem?
stars way brighter than planets
if we use infrared light, we can detect planets because planets emit their own infrared light and stars are usually dimmer in the infrared
what are the 2 general ways of detecting exoplanets?
- direct
- indirect
how are exoplanets directly detected? is this common?
detected with images or spectra –> only some found this way
how are exoplanets indirectly detected?
inferring existence or properties without actually seeing it
what are the 2 general methods for indirect detection?
- observe changes in star’s brightness when a planet passes in front of it
- observe motion of a star to detect gravitational effects of its orbiting planets
what is 1 degree equal to?
60 arcminutes
what is 1 arcminute equal to?
60 arcseconds
what is the equation for angular size in degrees?
how does angular size change with distance?
angular size = (physical size/distance) * (360 deg/2pi radians)
angular size decreases with distance
what is the diffraction limit? how can we overcome it?
the diffraction of light waves limits the resolution of an optical system
we cannot overcome it –> it is nature’s limit
what equation is used to describe the diffraction limit?
angle in radians = 1.22 (wavelength/optical system’s diameter)
what is atmospheric distortion? how does distortion change with increased exposure?
blurring will distort the images so it is impossible to see very small separations
more distortion with increased exposure
how can we solve atmospheric blurring?
telescopes use a LASER GUIDE STAR to determine the effect of atmospheric blurring
what are 2 types of laser guide stars we can use?
- NATURAL STAR but may not be bright enough
- artificial guide by shining a laser into the atmosphere and light reflects back to the telescope
how does the laser guide star fix atmospheric blurring? what is the name of this technique?
laser guide star bounces light off a thin, deformable mirror that can be adjusted thousands of times a second to reposition the images to the center and reduce blurring –> therefore allows NEAR DIFFRACTION-LIMITED imaging from the ground
called ADAPTIVE OPTICS
what solves the contrast ratio problem?
how does it work?
what happens to faint planets?
CORONOGRAPH –> blocks all light from the star, acting like an artificial eclipse
faint planets become visible
what are the 2 types of planets that are best to detect with the coronograph?
- planets that are far from the star
- large planets that reflect a lot of light
why is it easy to detect large planets that reflect a lot of light with the coronograph?
lower contrast ratios
what are the 2 types of direct detection of exoplanets?
what problems do they solve?
what 2 types of planets are they best for?
- adaptive optics –> solves atmospheric blurring
- coronograph –> solves contrast ratio problem
best for:
1. planets further away
2. large planets
THEREFORE, will be some small planets close to the star that are less detectable
what is a planet’s transit?
when a planet moves across the face of a star
how can we determine the composition of a planet’s atmosphere? why?
change in spectrum during transit
because, during transit, the planet will block starlight at ALL wavelengths, but its atmosphere can absorb some wavelengths
what are the 3 types of indirect detection of exoplanets?
- Transit Method
- Radial Velocity Method
- Gravitational Microlensing
what is the transit method?
why do we have to do repeated measurements?
what telescope uses the transit method?
as a planet transits, it will block some of the star’s light so we can measure the change in brightness which corresponds to the size of the planet
stars exhibit intrinsic variations in brightness so must do repeated measurements to detect dips in brightness that repeat with a regular period
Kepler
what is the difference btwn eclipse and transit?
in eclipse, the two objects are similar in size so we only see light from the star and the whole system’s brightness is reduced
in transit, the passing object is much smaller so we see light from both the star and planet
what type of planets is best detected with the transit method?
close-in planets with short orbital periods before it finds orbital planets –> less time is needed to observe repeated transits
what type of measurements do we use with the transit method?
SPECTROSCOPY –> compare the spectrum of starlight before and during transit
(also lets us determine planetary atmospheric composition)
what are the 2 limitations of the transit method?
- only works for planetary systems where planets orbit edge-on to Earth –> some planets will not be detected
- best for planets with shorter orbital periods –> takes shorter time to detect
how does Kepler overcome the requirement for planets to orbit edge-on to earth?
observe large numbers of stars so even a small fraction can represent large numbers
what is the advantage of the transit method?
what are the 2 things that allow this method?
can detect very small planets
- can measure dimmings <0.01% of a star’s brightness
- small planets can leave a distinct signature because each planet orbiting a star will not orbit the star at the same time
what detection biases exist for exoplanets found using the transit method?
tend to be large and near the star
what exoplanet property can we measure most easily with the transit method?
planet size
if an alien was discovering the existence of Earth by the transit method, where on the celestial sphere would the alien’s exoplanetary system be?
along the celestial equator
what are 4 surprising characteristics of exoplanets found by Kepler?
- exoplanets can have highly elliptical orbits –> harder for life to grow bc would be very far from sun during winter
- large diversity in size and density
- some are HOT JUPITERS that orbit very close to their sun
- could be 40 billion rocky Earth-size exoplanets
describe stellar wobble
the exoplanet orbits the star around a center of mass and the star ALSO ORBITS around the same center of mass that is very near or inside a star
what are the 2 ways you can measure stellar wobble?
- DIRECTLY using micro-arcsecond angular resolution (more difficult)
- SPECTROSCOPICALLY using stellar spectrum (most common)
- describe the stellar wobble of the sun with Jupiter
- why does it appear that Jupiter orbits the sun?
- why don’t we see the sun rotate?
- how can we use this to deduce the existence of Jupiter without seeing Jupiter?
- the center of mass for the sun and Jupiter is located near the SOLAR PHOTOSPHERE –> both orbit this center of mass on opposite sides
- even though the center of mass is not in the center of the sun, it appears that Jupiter orbits the sun because the center of mass is so close to the sun
- we don’t see the sun rotate bc the distance to the center of mass is so similar to its own radius
- can deduce the existence of Jupiter by detecting the movement of the sun and understanding that Jupiter is causing a gravitational tug to change the sun’s position
do all planets have a wobble with the sun, or just Jupiter?
what can we learn about a planet from its stellar wobble?
all planets have a wobble –> therefore, each planet exerts a tiny gravitational tug on the sun to change its position
can deduce:
1. the existence of a planet
2. its mass
how can we determine a planet’s existence based on the sun’s orbit?
if a planet is more massive, its center of mass is further from the sun’s center SO the sun would have a larger orbit and faster orbital speed
what is the Doppler Method?
gravitational tugs on the sun can be detected by looking at changing doppler shifts in its spectrum
describe the doppler shift with sound waves
stationary source –> sound waves go all ways equally
moving –> sound waves catch up to each other
describe the doppler shift with light waves when a star goes around its orbit
how does the speed of light change?
in its orbit:
- star moves TOWARDS us –> catches up to light waves and wavelength is smaller and BLUE
- star moves AWAY from us –> wavelength is larger and RED
speed of light stays the same!!
what is the equation that represents the doppler shift?
(delta wavelength/wavelength) = (star’s velocity/c)
what 2 things does the doppler shift allow us to measure and what do they indicate?
- measures period of the star’s motion –> indicates orbital period of the planet
- measures planet’s mass –> more massive planet has greater gravitational effect on the star so the star orbits faster
how are stellar radial velocity and mass related?
if a planet has more mass, it will have a greater gravitational effect on the star to make it move more quickly
what does stellar radial velocity indicate about mass?
we can INFER the MINIMUM MASS
why can we only infer the mass?
we can’t actually measure the mass because we only have the radial component of velocity (i.e. velocity directly towards/away from us)
what is the typical radial velocity of exoplanets?
what is the typical radial velocity for binary stars?
exoplanets: tens of m/s
binary stars: km/s
OVERALL, the doppler method is:
good for 2 things:
bad for 3 things:
GOOD FOR:
1. big exoplanets in circular orbit
2. inferring mass
BAD FOR:
1. exoplanets in distant orbits
2. small exoplanets
3. measuring size
what are the 2 reasons that larger exoplanets are better detected using the wobble/stellar radial velocity methods?
- more mass = greater tug on star = greater effect on light (blue/red shift)
- more mass = greater tug on star = greater velocity (easier to measure)
what kind of telescope is used for detecting the doppler shift? why?
large telescope with long exposure time –> allows you to obtain spectra with high enough resolution to see small doppler shifts
if you found a star with the same mass as the sun moving back and forth with a period of 16 months, what would you conclude?
orbits at >1 AU bc orbit is proportional to its distance from the sun
what is gravitational microlensing?
an object lies behind a massive object with a strong gravitational field acting as a gravitational lens
this causes the light to bend around the massive object and distort the image like a lens would do
what are 3 types of gravitational lenses?
- large galaxy
- cluster of galaxies
- one star (microlensing)
what does Einstein’s theory state that relates gravitational microlensing?
Einstein states that space-time is curved due to MASS and dark matter which then bends light
but this is a description, not an explanation
what is gravitational microlensing?
the distant object may appear brighter as the lens object passes in front of it
how do heavier objects affect microlensing?
heavier objects cause MORE DISTORTION in space time and greater curves of light
therefore, the duration of the microlensing event can indicate the object’s mass (since we cannot see the lens object)
Newton vs Einstein gravity
Newton: gravity is a mysterious “action at a distance”
Einstein: removed the mystery, showing that gravity arises from spacetime curvature
how can we use gravitational microlensing to detect exoplanets?
light from a background star will bend the light around the planet in front of it
