Lecture 1 Lecture 1 - Definition of a planet
- A little history
- Pulsar planets
- Doppler “wobble” (radial velocity) technique
Lecture 2 - Transiting planets
- Transit search projects
- Detecting the atmospheres of transiting planets: secondary eclipses & transmission spectroscopy
- Transit timing variations
Lecture 3 Lecture 3 - Microlensing
- Direct Imaging
- Other methods: astrometry, eclipse timing
- Planets around evolved stars
Lecture 4 - Statistics: mass and orbital distributions, incidence of solar systems, etc.
- Hot Jupiters
- Super-Earths
- Planetary formation
- Planetary atmospheres
- The host stars
Lecture 5 Lecture 5 - The quest for an Earth-like planet
- Habitable zones
- Results from the Kepler mission
- How common are rocky planets?
- Amazing solar systems
- Biomarkers
- Future telescopes and space missions
RSun = 6.995x108m RSun = 6.995x108m Rjup = 6.9961x107m ~ 0.1RSun Rnep = 2.4622x107m ~ 4Rearth Rearth = 6.371x106m ~ 0.1Rjup ~ 0.01RSun MSun= 1.989x1030kg Mjup= 1.898x1027kg ~ 0.001MSun = 317.8Mearth Mnep= 1.02x1026kg ~ 5x10-5MSun ~ 0.05Mjup = 17.15Mearth Mearth= 5.97x1024kg = 3x10-6MSun = 3.14x10-3Mjup 1AU = 1.496x1011m 1 day = 86400s
1767 confirmed planets 1767 confirmed planets - In 1160 planetary systems
- 471 multi-planet systems
- 517 radial velocity detected planets
- 1153 transiting planets
- 35 directly imaged
- “Confirmed” = have “measured” masses
Unexpected population with periods of <1 to ~4 days: “hot Jupiters” Planets with orbits like Jupiter discovered (eg 55 Cancri d) Smallest planets: - Kepler-20e: 0.87Rearth ,
- Alpha Cen Bb M sin i > 1.1Mearth
Notice the “pile-up” at periods of <1 to ~4 days / 0.04-0.05AU Notice the “pile-up” at periods of <1 to ~4 days / 0.04-0.05AU The most distant planets discovered by radial velocities so far are at 5-6AU Imaging surveys finding very wide (>10AU) orbit planets Orange are “hot Jupiters” Yellow is Jupiter-mass in Jupiter-like orbits
Astronomical surveys tend to have built in biases Astronomical surveys tend to have built in biases These “selection effects” must be understood before we can interpret results - The Doppler Wobble method is most sensitive to massive, close-in planets, as is the Transit method
- Imaging surveys sensitive to massive planets in very wide orbits (>10AU)
These methods are not yet sensitive to planets as small as Earth, even close-in As orbital period increases, the Doppler Wobble method becomes insensitive to planets less massive than Jupiter The length of time that the DW surveys have been active (since 1989) sets the upper orbital period limit - But imaging surveys can find the widest planets
Doppler Wobble and transit surveys find many gas giants in orbits of <1 to ~4 days Doppler Wobble and transit surveys find many gas giants in orbits of <1 to ~4 days - cf Mercury’s orbit is 80 days
- Larger Doppler Wobble signal
- Greater probability of transit
These planets are heated to >1000oF on “day” side - And are “tidally locked” like the Moon
- Causes extreme weather conditions
Lowest mass confirmed planet so far: Alpha Cen Bb M sin i =1.1xMEarth Lowest mass confirmed planet so far: Alpha Cen Bb M sin i =1.1xMEarth Super-Jupiters (>few MJup) are not common - Implications for planet formation theories?
- Or only exist in number at large separation?
- Or exist around massive stars?
Radial velocity surveys - ~10% of FGK stars have gas giants between 0.02AU and 5AU
- At least 20% have gas giants in wider orbits
- Known population will grow as radial velocity surveys cover longer periods, & direct imaging improves
- <0.1% have Hot Jupiters
- Hot Jupiters are easy to discover, but in fact are rare
How many have Earths…..?
Surveys began by targeting sun-like stars (spectral types F, G and K) Surveys began by targeting sun-like stars (spectral types F, G and K) Now extended to M dwarfs (<1 Msun) and subgiants (>1.5Msun) - Subgiants are the descendants of A stars
Incidence of planets is greatest for late F stars
Overall, ~10% of solar-like stars have radial velocity –detected Jupiters Overall, ~10% of solar-like stars have radial velocity –detected Jupiters But if we take metallicity into account: - >20% of stars with 3x the metal content of the Sun have gas giants
- ~3% of stars with 1/3rd of the Sun’s metallicity have gas giants
Does this result imply that planets more easily form in metal-rich environments? - Possibly true for gas giants
- But Kepler results suggest super-Earths & terrestrial planets equally common around stars of all metallicities!
There are two main models which have been proposed to There are two main models which have been proposed to describe the formation of the extra-solar planets: - (I) Planets form from dust which agglomerates into cores which then accrete gas from a disc.
- (II) A gravitational instability in a protostellar disc creates a number of giant planets.
Both models have trouble reproducing both the observed distribution of extra-solar planets and the solar-system.
Planetary cores form through the agglomeration of dust into grains, pebbles, rocks and planetesimals within a gaseous disc Planetary cores form through the agglomeration of dust into grains, pebbles, rocks and planetesimals within a gaseous disc At the smallest scale (<1 cm) cohesion occurs by non-gravitational forces e.g. chemical processes. On the largest scale (>1 km) gravitational attraction will dominate. On intermediate scales the process is poorly understood. These planetesimals coalesce to form planetary cores The most massive cores accrete gas to form the giant planets Planet formation occurs over 107 yrs.
A gravitational instability requires a sudden change in disc properties on a timescale less than the dynamical timescale of the disc. A gravitational instability requires a sudden change in disc properties on a timescale less than the dynamical timescale of the disc. Planet formation occurs on a timescale of 1000 yrs. A number of planets in eccentric orbits may be formed. Sudden change in disc properties could be achieved by cooling or by a dynamical interaction. Simulations show a large number of planets form from a single disc. Only produces gaseous planets – rocky (terrestrial) planets are not formed. Is not applicable to the solar system. Could explain the directly imaged HR8799 system
No element will condense within ~0.1AU of a star since T>1000K No element will condense within ~0.1AU of a star since T>1000K Planets most likely form beyond the “ice-line”, the distance at which ice forms - More solids available for building planets
- Distance depends on mass and conditions of proto-planetary disk, but generally >1AU
Hot Jupiters currently at ~0.03-0.04AU cannot have formed there If migration time < disk lifetime - Planets fall into star
- Excess of planets at 0.03-0.04AU is evidence of a stopping mechanism
- tides? magnetic cavities? mass transfer?
Large planets will migrate more slowly - Explanation for lack of super-Jupiters in close orbits
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