Nov. 18th - Exoplanets Flashcards
Why is it so challenging to learn about exoplanets?
Detecting exoplanets poses a huge technological challenge - WHY?
What methods (2) combat this?
- Recall that detecting planets around even the nearest other stars is equivalent to looking for objects the size of ball points or marbles from a distance of thousands of kilometers away
- Astronomers have nevertheless found a number of ways to meet this technological challenge. If we strip away the details, however, we can group all the detection methods into two general categories:
1. direct detection, in which we obtain images (and/or spectra) of the object
2.indirect detection, in which we infer the object’s existence without actually seeing it.
Direct detection:
Planet Dimness - How to avoid
- The scale issues alone would make it challenging to take a picture of an exoplanet, but these issues are further compounded by the fact that **stars are much brighter than planets. **
- Problem is somewhat lessened if we observe in the infrared
Direct Detection
Planet Dimness - Why is it efficient to observe in the infrared?
…because planets emit their own infrared light and stars are usually dimmer in the infrared
Direct Detection
The key to meeting the challenge of direct imaging is to…
…block out the light of the star itself, making it easier (though still quite difficult!) to see the planets.
* Astronomers do this by using an instrument known as a coronagraph, which is placed in front of the telescope to block out the star. (The basic idea is the same as what you do when you hold up your hand to shade your eyes from strong sunlight)
Direct Detection
Why does direct imaging remain limited?
this means that direct imaging is limited to exoplanets that are fairly bright—which means fairly large, more massive than Jupiter—and orbit relatively far from their host star
Indirect Detection
2 major indirect approaches to finding and studying exoplanets
(These approaches are considered to be indirect because we discover the planets by observing their stars without actually seeing the planets themselves):
-
Observing the motion of a star to detect the subtle gravitational effects of orbiting planets, which can itself be done in two ways:
* The astrometric method, which looks for change in a star’s precise position in the sky.
* 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?
General answer
Star orbiting around the system’s “balance point” or center of mass
How can a star’s motion reveal the presence of planets?
Star orbiting around the system’s “balance point” or center of mass
Jupiter example
- Jupiter’s 12 year orbit appears to be around the sun
- 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
- A more massive planet at the same distance would pull the center of mass farther from the Sun’s center, giving the Sun a larger orbit. Because the Sun’s period around the center would still be 12 years, the larger orbit would mean a faster orbital speed around the center of mass.
Indirect Detection
The Astrometric Method
- 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 average position (the center of mass), we must be observing the influence of unseen planets.
Indirect Detection
The Astrometric Method - Difficulties
- 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.
- 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 (wobble around the centre of mass) 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: P^2 = a^3 (period of a planet’s orbit = semi major axis of the orbit)
As a result, it would take a much longer time for alien astronomers to recognize Jupiter’s effect.
Indirect Detection
The Doppler Method
- 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
How can the Doppler Method tell us about planetary mass?
Hot Jupiters
- The Doppler data also allow us to determine the planet’s approximate mass, because a more massive planet has a greater gravitational effect on the star (for a given orbital distance) and therefore causes the star to move at higher speeds around the system’s center of mass
- Scientists therefore refer to this planet as a hot Jupiter, because it has a Jupiter-like mass but a much higher surface temperature.
The second general approach to indirect detection of planets relies on searching for slight changes in a star’s brightness caused by orbiting planets.
The 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.
How is the Transit Method validated?
2 ways (Confirmed & Candidate planets)
- 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
- Follow-up observations with another method, such as the Doppler method, should reveal the same planet
Typically, planets found by the transit method are counted as “candidate” planets once they have made the minimum of three transits specified in the first requirement, but are deemed “confirmed” planets only after follow-up observations
Advantages and Limitations of the Transit Method
Limits
- The major limitation of the transit method is that it works only for the approximately 1% of star systems that by chance have their planet orbits aligned “just right” so that they pass in front of their star as seen from Earth.
- However, in terms of identifying exoplanets, that limitation can be made up for by the fact that modern telescopes can be used to monitor large numbers of stars in search of transits
