3677 Life in the Universe: Extra-solar planets

1. 3677 Life in the Universe:Extra-solar planets Dr. Matt Burleigh www.star.le.ac.uk/mrb1/lectures.html

3. 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

4. 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

5. 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

6. 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

7. 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

8. Hit 1000 exoplanet mark Transiting planets in blue

10. Eccentricity of exoplanet orbits Solar systems with highly eccentric planets may be bad news for life

11. 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

12. 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

13. “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

14. 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?

15. A continuum of planet mass 1’000’000 Red box indicates “Super-Earths”

16. Transiting planets in blue Red box indicates “Super-Earths”

17. 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?

19. hydrogen/helium envelope thin atmosphere ? ice mantle/volatile envelope solid core (rocks+metals) ? What are exoplanetsmade of? ? ?

20. 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)

21. 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)

22. 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…..?

23. 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

24. 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!

25. 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.

26. 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.

27. 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

28. 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