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3677 Life in the Universe: Extra-solar planets. Dr. Matt Burleigh www.star.le.ac.uk/mrb1/lectures.html. Course outline. Lecture 1 Definition of a planet A little history Pulsar planets Doppler “ wobble ” (radial velocity) technique Lecture 2 Transiting planets Transit search projects

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3677 life in the universe extra solar planets

3677 Life in the Universe:Extra-solar planets

Dr. Matt Burleigh

www.star.le.ac.uk/mrb1/lectures.html


Course outline
Course outline

  • 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


Course outline1
Course outline

  • 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


Course outline2
Course outline

  • 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


Useful numbers
Useful numbers

  • 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


Exoplanet count 10 11 13 exoplanet eu
Exoplanet count 10/11/13 (exoplanet.eu)

  • 1039 confirmed planets

    • In 787 planetary systems

    • 173 multi-planet systems

    • 873 radial velocity detected planets

    • 425 transiting planets

    • 41 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


Hit 1000 exoplanet mark
Hit 1000 exoplanet mark

Transiting planets in blue


Eccentricity of exoplanet orbits
Eccentricity of exoplanet orbits

Solar systems with highly eccentric planets may be bad news for life


Extra solar planet period distribution
Extra-solar planet period distribution

  • 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


Selection effects
Selection effects

  • 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


Hot jupiter planets
Hot Jupiter” planets

  • 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


Extra solar planet mass distribution
Extra-solar planet mass distribution

  • Mass distribution peaks at 1-2 x mass of Jupiter

  • 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?


A continuum of planet mass
A continuum of planet mass

1’000’000

Red box indicates “Super-Earths”


Transiting planets in blue

Red box indicates “Super-Earths”


Super earths
Super-Earths

  • In the solar system, there is no planet with a mass and radius between that of Earth and Neptune/Uranus

  • But we see many such exoplanets

  • What are they? Gas giants, terrestrial, or something else?


What are exoplanets made of

hydrogen/helium envelope

thin atmosphere

?

ice mantle/volatile envelope

solid core (rocks+metals)

?

What are exoplanetsmade of?

?

?


What are exoplanets made of1

telluric

super-Earths?

gas dwarfs?

mini Neptunes?

ocean planets?

What are exoplanetsmade of?

?

hydrogen/helium envelope

thin atmosphere

ice mantle/volatile envelope

solid core (rocks+metals)


What are exoplanets made of2

telluric

super-Earths?

gas dwarfs?

HD 149026b

mini Neptunes?

ocean planets?

What are exoplanetsmade of?

?

hydrogen/helium envelope

thin atmosphere

ice mantle/volatile envelope

solid core (rocks+metals)


How common are gas giants
How common are gas giants?

  • 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…..?


What about the stars themselves
What about the stars themselves?

  • 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


Metallicity the abundance of elements heavier than he relative to the sun
MetallicityThe abundance of elements heavier than He relative to the Sun

  • 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 lower metallicity stars!


Planet formation scenarios
Planet formation scenarios

  • 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.


Accretion onto cores
Accretion onto cores

  • 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.


Gravitational instability
Gravitational instability

  • 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


Where do the hot jupiters come from
Where do the hot Jupiters come from?

  • 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


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