Exoplanets part 1 Flashcards
Why are exoplanets difficult to observe?
- Don’t produce any light of their own
- Incredibly distant
- Millions times dimmer than the stars they orbit
What are the two ways to search for exoplanets?
Directly - pictures or spectra for direct evidence
Indirectly - precise measurements of a star’s properties may reveal effects of orbiting planets
When does direct detection work?
Star system is near
Planet is large
Large distance from host tar
Planet is hot
What are the 5 indirect techniques used?
Astronomy
Transits
Doppler shifts
Pulsar timing
Microlensing
What is the astronomy technique?
Observe motion of stars across the sky
What is the basis for the astronomy technique?
All stars exhibit proper motion with most being small (5-10 km/s) compared to galactic rotation of 250km/s.
Nearby stars change their position by a few arcsec per year
How does the astronomy technique work?
If planets going around a star, the stars proper motion changes
What causes modification of stars proper motion?
Gravitational pull of planet
What example can be found of greatest proper motion?
Barnards star has motion of 10 arcsec/yr
Very near to us
What is the Planetary transits technique?
Planets crossing stars and dimming them
How does Planetary transit technique work?
- Earthlike planets dims sunlike star by hundredth of a percent
What can Planetary transits tell us?
- How often this happens give us orbital period
- Duration, depth and shape give indicators of planet/star size ration
How would Planetary transit look like for our solar system?
Jupiter would cause dimming of 1% every 12 years
Transit of Earth would cause 1/12000 drop
What are the problems with using transits?
- signal drop can be very small
- dont know when it will happen
- Some orbital periods are very long
- high rate of false detections
How does the transit method allow a planets atmosphere to be studied?
Analyse light passing through its upper atmosphere and study stellar spectrum
What has the Kelper mission found?
9.6 yrs in orbit
Discovery of 2,600 planets
Observed 500,000 stars
What is the Doppler shift technique?
Looking at stars orbital movement around its centre of mass by studying its spectrum.
How does the Doppler shift technique work?
Small shifts in wavelength are caused by velocity changes of star as planets go around it
Where is the centre of mass between the sun and Earth?
333,000 times closer to the centre of the sun than the Earth.
How does sun and earth orbit each other?
Earth moves at 30km/s around its orbit. Sun moves at 1/330,000 so 10cm/s around its orbit.
Jupiter causes 30m/s
What are the limits for doppler method?
- Best suited for identifying massive planets close to host star
- detection limit is 1m/s
What was the exoplanet discovery in 1995?
51 Pegasi - sunlike star
Dimidium planet - close to star
- surface temp 1200
- mass half of jupiter
What is the Pulsar Timing Technique?
Using cosmic clock to reveal planets?
How has Pulsar Timing worked?
PSR 1257+12 neutron star spinning at 161 times per second
Observe modulation due to presence of planets in this clock frequencty
Limitations to Pulsar Timing
Only find planets around dead stars
How does gravitation microlensing technique work?
Light from a distant object will bend due to the distortion of space
What will a planet do to light beams?
Focus them and cause increase in brightness
What is the chance of planet causing lensing effect annually?
1 in a million
What is the Optical Gravitational Lens experiment in Chile?
Polish project to discover dark matter using microlensing technique
What does OGLE do?
Observes several million stars each night
17 planets discovered
What are the problems with gravitational lensing?
Never predict where it will happen
Can never repeat the experiment
What is Barnards star?
Red dwarf of spectral type M4
What is the first confirmed transiting exoplanet?
HD209458
How was 51 Pegasi found?
Doppler technique
