Definition of a planet a little history

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

• These survey methods are biased towards finding them

• 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

• 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

• F7-9V > GV > KV > MV

• More massive stars tend to have more massive planets

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

• Migration: Planets migrate inwards and stop when disk is finally cleared

• 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