Lecture Six Flashcards
What defines a planet?
1) A planet is a celestial body that
(a) is in orbit around the sun.
(b) has sufficient mass for its self-gravity to overcome ridged body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
(c) has cleared the neighbourhood around its orbit.
2) A “dwarf planet” is a celestial body that
(a) is in orbit around the sun
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
(c) has not cleared the neighbourhood around its orbit.
(d) is not a satellite.
3) All other objects except satellites, orbiting the sun shall be referred to collectively as “small solar system bodies”
Define an extrasolar planet.
Any planet which orbits a star that is not out sun (Sol) shall be called an extrasolar planet, or exoplanet for short.
What problems are we faced with when looking for extrasolar planets?
Planets don’t generate their own light.
Extrasolar planets are a long way away from the Earth.
The stars they orbit around are bright enough to make visual detection of the small amount of their light that a planet reflects extremely difficult.
How was the first extrasolar planet discovered and when?
1994 - planets were discovered by measuring the variations in light from a pulsar (a rotating dead neutron star). They indicated two or three planets orbiting it.
When and how was 51 Pegasi B discovered?
1995 - Planet detected orbiting the sun like star 51 Pegasi.
Named 51 Pegasi B.
About 47% the size of Jupiter.
Has a pull on its star of 51m/s.
Detected by the gravitational pull it had on its star.
Since the 27th of February this year, how many exoplanets were known.
More than 1,766 confirmed.
At least 484 multiple planet systems.
How is the Doppler Shift used to find an exoplanet?
As a planet orbits its star it drags the star slightly closer towards it.
The stars movement causes slight red-shift and blue-shift int he light as the light from that star moves towards us.
A large planet close to the star gives the easiest motion to detect.
Thats why so many early exoplanets found have been hot Jupiter’s.
How is astrometric movement used to find an exoplanet?
As a planet orbits its star it drags the star slightly towards it.
This causes the star to move slightly from out point of view against the rest of the background.
How is gravitational microlensing used to find an exoplanet?
Light rays from a star beyond the target star are bent by the gravity of the target star, and also by an orbiting planet.
This causes the light from that distant star to temporarily increase in brightness, and the star appears to change its position.
How is the transit method used to find exoplanets?
As a planet orbits its star it blocks part of the stars light that would normally reach an observer.
This causes the light from that star to slightly dip in intensity.
This is the method used by the Kepler Space Telescope.
How is direct detection used to find exoplanets?
It is now possible to detect light from planets orbiting other stars.
NASA has indefinitely delayed an orbiting telescope that would be abel to see them in better resolution, as a consequence of budges re-prioritisation D’:.
Is our solar system normal?
Planets found so far have all shapes of orbits.
Nearly circular orbits are not always the case.
But new Kepler planetary systems are more like ours.
Is there bias in planet hunting?
It is easier to find massive planets, closer to host star due to faster and clearer data.
Early data is very biased.
Can we live on another planet?
Planet needs to be in habitable zone of its star.
Distance where temperature would allow liquid water to exist on the surface.
3 new habitable zone ‘super-earths’ discovered around red dwarf stars 15-80 light years away. (Discovered that every red dwarf star has at least one planet).
What have we recently found out about the star in our milky way?
~15-20% of stars in the milky way are sun like and have earth sized planets in habitable zones.
