Lec 20 and 21 Flashcards
(33 cards)
why is it challenging to learn about exoplanets?
exoplanets = planets outside our solar system
The nebular theory of solar system formation, well established decades ago, made such systems seem even more likely, because it predicts that planet formation should be a natural part of the star formation process
direct imaging process
-block out light of star, makes it easier to see planets
indirect detection
There are two major
indirect approaches to finding and studying exoplanets:
1. Observing the motion of a star to detect the subtle gravitational effects of orbiting planets, which can itself be done in two ways:
i. The astrometric method, which looks for change in a star’s precise position in the sky.
ii.The Doppler method, in which we measure how the Doppler shift of a star’s spectrum changes with time.
- Observing changes in a star’s brightness that occur when one of its planets passes in front of the star as viewed from Earth; this is known as the transit method.
How can a star’s motion reveal the presence of planets?
The first two indirect methods (astrometric and Doppler) both rely on measuring how a star’s motion is affected by its planets.
Although we usually think of a star as remaining still while planets orbit around it, that is only approximately correct.
In reality, all the objects in a star system,
including the star itself, orbit the system’s “balance point,” or center of mass
why don’t we notice the sun’s motion?
What we usually think of as Jupiter’s 12-year orbit around the Sun is really a 12-year orbit around the center of mass.
Because the Sun and Jupiter are always on opposite sides of the center of mass (that’s what makes it a “center”), the Sun must orbit this point with the same 12-year period.
The Sun’s orbit traces out only a small ellipse with each 12-year period, because the Sun’s average orbital distance is barely larger than its own radius; that is why we generally don’t notice the Sun’s motion
astrometric method
The first method used to search for the gravitational tugs of planets on stars, called the astrometric method (astrometry means “measurement of the stars”), uses
very precise measurements of stellar positions in the sky to look for motion
-if a star “wobbles” gradually around its avg position(centre of mass), we must be observing influence of unseen planets
2 difficulties of astrometric method
The first difficulty stems from the fact that we are searching for changes in position that are very small even for nearby stars, and these changes become smaller for more distant stars
For example, from a distance of 10 light-years, a Jupiter-size planet in a Jupiter-like orbit (5 AU from a Sun-like star) would cause its star to move slowly over a side-to-side angular distance of only about 0.003 arcsecond
To understand the second difficulty, consider what would happen if we could move Jupiter into an orbit farther from the Sun
–this larger orbit would increase the angular extent of the Sun’s side-to-side motion as seen from a distance (because moving Jupiter outward would also move the center of mass outward from the Sun), but Kepler’s third law
tells us that this move would also increase Jupiter’s orbital period.
As a result, it would take a much longer time for alien astronomers to recognize Jupiter’s effect.
For example, if Jupiter moved to Neptune’s distance from the Sun, at which the orbital period is 165 years, it might take a century
or more of patient observation to be confident that the stellar motion was following an orbital pattern
doppler method
The second method used to search for
the gravitational tugs of orbiting planets is the Doppler method (sometimes called the radial velocity method), which searches a star’s spectrum for Doppler shifts
that change with time in a way that indicates orbital movement.
Recall that the Doppler effect causes a blueshift when a star is moving toward us and a redshift when it is moving away from us, so alternating blueshifts and redshifts (relative
to a star’s average Doppler shift) indicate orbital motion around a center of mass
limitations of doppler method
1) Like the astrometric method, the Doppler method searches for gravitational tugs from orbiting planets and is therefore better for finding massive planets like Jupiter than small planets like Earth,
2) The Doppler method is best suited to identifying these planets when they orbit relatively close to their star (in contrast to the astrometric method, which works better for planets farther from their star)
Close-in planets are easier to detect both because being closer means a stronger gravitational tug and hence a greater velocity for the star (which is easier to measure) and because closer-in planets have shorter orbital periods that allow their orbits to be observed in shorter amounts of time
3) Doppler method requires a fairly large telescope (and long exposure time) in order to obtain spectra with high enough resolution to reveal very small Doppler shifts, which limits the number of stars that can be studied with this method.
how can changes in a star’s brightness reveal the presence of planets?
By the transit method
-result is a transit, in which the planet appears to move across the face of the star
transit method
A transiting planet will block a little
of its star’s light as it passes across the line of sight from us to the star, and the transit method searches for these temporary dimmings of a star
The larger the planet, the more dimming it will cause. Some transiting planets also undergo a measurable eclipse as the planet goes behind the star.
Eclipses also cause a small dip in a system’s brightness, because during the eclipse we see light only from the star rather than from both the star and the planet.
why are eclipse observations easily accomplished in a system’s infrared brightness?
Eclipse observations are more easily accomplished in the infrared, because planets contribute a greater proportion of a system’s infrared brightness than visible-light brightness.
To be confident that the transit method has detected a planet:
Scientists generally have two requirements:
(1) The suspected transit event must be observed to occur at least three times with a regular period, indicating that the same planet is passing in front of the star repeatedly with each orbit
(2) Follow-up observations with another method, such as the Doppler method, should reveal the same planet
advantages of transit method
Reveals planets smaller than Earth
Although these planets cause a small dimming of their star, current instruments have the sensitivity to detect tiny changes
In a multiplanet system, the astrometric and Doppler methods can detect the existence of small planets only as small effects added to the much larger gravitational effects of larger planets in the system.
In contrast, a small planet leaves a distinct signature with the transit method, because planets with different periods will only rarely transit in front of a star at the same time.
limitations of transit method
Works only for the approx. 1% of star systems that by chance have their planet orbits5 aligned “just right” so that they pass in front of their star as seen from Earth
In terms of identifying exoplanets, modern telescopes can be used to monitor large numbers of stars in search of transits
gravitational tugs
We can detect a planet by observing the small orbital motion of its star as both the star and its planet orbit their mutual center of mass.
The star’s orbital period is the same as that of its planet, and the star’s orbital speed depends on the planet’s distance and mass.
Any additional planets around the star will produce additional features in the star’s orbital motion.
Doppler method
As a star moves alternately toward and away from us around the center of mass, we can detect its motion by observing alternating
Doppler shifts in the star’s spectrum: a
blueshift as the star APPROACHES and a
redshift as it RECEDES.
Astrometric method
A star’s orbit around the center of mass leads
to tiny changes in the star’s position in
the sky.
The GAIA mission is expected to discover many new planets with this method.
transit method
If a planet’s orbital plane happens to lie along our line of sight, the planet will transit in front of its star once each orbit, causing a dip in the star’s visible-light brightness.
An eclipse may occur half an orbit later, during which the system’s infrared brightness will
decline because the planet’s contribution is blocked by the star
direct detection
In principle, the best way to learn about an extrasolar planet is to observe
directly either the visible starlight it reflects or the infrared light it emits.
Current technology is capable of direct detection in some cases, but only with very low resolution.
what properties of exoplanets can we measure?
we can determine such planetary characteristics as orbital period and distance, orbital eccentricity,
mass, size, density, and even a little bit about a planet’s atmospheric composition and temperature
orbital period and distance
-astrometric method allows us to observe the star’s orbit motion around the system’s center of mass –> means we know star’s orbital period, planet’s orbital period must be same
Doppler method:
-a detected planet’s orbital period is the time b/w peaks in the star’s velocity curve
Transit method:
-the orbital period is the time b/w repeated transits
once we know planet’s orbital period, we can determine its avg orbital distance (semimajor axis) with Newton’s version of kepler’s 3rd law
orbital eccentricity
all planetary orbits are ellipses, but they vary in eccentricity, which’s a measure of how ‘stretched out’ they are
The planets in our solar system all have nearly circular orbits (low eccentricity), which means that their actual distances from the Sun are always relatively close to their average distances.
Planets with higher eccentricity swing in close to their star on one side of their orbit and go much farther from their star on the other side.
We can determine eccentricity with both the astrometric and the Doppler method, though most measurements to date come from Doppler data.
A planet with a perfectly circular orbit travels at a constant speed around its star, so its
velocity curve is perfectly symmetric.
Any asymmetry in the Doppler curve tells us that the planet is moving with varying speed and therefore must have a more eccentric
elliptical orbit
planetary mass
Both the astrometric and the Doppler method measures motions caused by the gravitational tug of an orbiting planet, so both can allow us to estimate planetary masses
For a given orbital distance, a more massive planet will cause its star to move at higher velocity around the center of mass
