13:
- CHAPTER Why is it so difficult to detect planets around other stars?
=Distance and Brightness difference: a sun-like star is about a billion times brighter than
the sunlight reflected from planets. Even best telescopes blur the light from stars and the
glare of scattered starlight would overwhelm blips of planetary light.
-measureable properties: orbital period, distance, shape, planet size, mass, density,
composition
-orbits of extrasolar planets: mist of detected planets have orbits smaller than Jupiters;
Planets at greater distances are harder to detect with Doppler; most of detected planets
have greater mass than Jupiter; planets with smaller masses harder to detect
1]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 stars orbital
period is same as that of planet and stars orbital speed depends on planets distance as
mass.
2] Doppler as a star moves alt. toward and away from us around center of mass, we can
detect its motion by observing alt. Doppler shifts in stars spectrum: blue approaches, red
recedes
3] astronometric : a stars orbit around the center of mass leads to tiny changes in stars
position in sk. As we improve our ability to measure these tiny changes, we should
discover many more extrasolar planets.
4] transits and eclipses: if a planets orbital plane happens to lie in line of sight, the planet
will transit in front of its star once each orbit, while being eclipsed behind its star half an
orbit later. The amount of starlight blocked by transiting planet can tell us planets size,
and changes in the spectrum can tell us about the planets atmosphere.
5] direct detection: in principle, the best way to learn about extrasolar planet is to observe
directly either the visible starlight it reflects or the infared light that it emits.
-one in ten stars examined so far have turned out to have planets; others may still have
smaller planets that current techniques cannot detect
- transit missions: NASAs Keplers mission is scheduled to begin looking for transiting
planets in 2008, designed to measure .008% decline in brightness when an Earth-mass
planet ellipses sun-like star
-How do we detect planets around other stars? How many extra-solar planets are
currently known? What are hot Jupiters, and why do they present a challenge for the
nebular hypothesis for the of formation planetary systems?
=astrometric- we can detect planets by measuring change in stars position on sky; tiny
motions are difficult to measure; the farther a star is, the smaller side-to-side movement
will appear; easier to detect larger movements
=Doppler- measuring a stars Doppler shift can tell us if motion toward and away from
us; looks for the gravitational influence of a planet on a star
= hot Jupiters: Jupiter-like mass but much higher temp; Hot Jupiters, like so many of the
known exoplanets, should not exist so close to their stars according to our model of
formation, so how did they form? Although it is not known for sure, one possibility is that
these hot Jupiters formed farther from their parent star and migrated inward to a smaller,
more eccentric orbit.
=mass of planet that causes star to move around systems center of mass, so for a given
orbital distance, a more massive planet will cause faster stellar motion; any asymmetry in
Doppler curve tells us planet is moving with varying speed and must have more eccentric
elliptical orbit.
= Doppler shifts of star 51 Pegasi indirectly reveal a planet with 4-day orbital period,
short periods means small orbital distance, first extrasolar planet to be discovered
= Transits and Eclipses: a transit is when a planet crosses in front of a star; resulting
eclipse reduces stars apparent brightness and tells us radius; no orbital tilt: accurate
measurement of planet mass
= well above 250 planets
- orbital period: astrometric, Doppler, transits
- orbital distance: once we know orbital period, we can calc distance by Newtons version
of Keplers third law
- orbital shape: need data spanning entire orbit to determine circle or ellipse; ast. And
Doppler yes but not transits alone
- mass: orbital period
- size: transit (radius)
- density- size and mass
- composition: spectra, transits, ellipses, direct detections
-nebular theory predicts Jupiter like plants shouldnt form outside frostline (<<5AU);
planetary migration or gravitational encounters may explain: a young planets motion
can create waves in a planet-forming disk; models show that matter in these waves can
tug on a planet, causing its orbit to migrate inward; close gravitational encounters
between two massive plants can eject one planet while flinging other in highly elliptical
orbit; multiple close encounters with small planetesimals can also cause inward migration