DEPARTMENT OF PHYSICS AND ASTRONOMY 3677 Life in the Universe: Extra-solar planets Dr. Matt Burleigh

Dr. Matt Burleigh 3677: Life in the Universe Course outline Lecture 1Lecture 1 –Definition of a planet –A little history –Pulsar planets –Doppler “wobble” (radial velocity) technique Lecture 2Lecture 2 –Transiting planets –Transit search projects –Detecting the atmospheres of transiting planets: secondary eclipses & transmission spectroscopy –Transit timing variations

Dr. Matt Burleigh 3677: Life in the Universe Course outline Lecture 3Lecture 3 –Microlensing –Direct Imaging –Other methods: astrometry, eclipse timing –Planets around evolved stars Lecture 4Lecture 4 –Statistics: mass and orbital distributions, incidence of solar systems, etc. –Hot Jupiters –Super-Earths –Planetary formation –Planetary atmospheres –The host stars

Dr. Matt Burleigh 3677: Life in the Universe Course outline Lecture 5Lecture 5 –The quest for an Earth-like planet –Habitable zones –Results from the Kepler mission How common are rocky planets?How common are rocky planets? Amazing solar systemsAmazing solar systems –Biomarkers –Future telescopes and space missions

Dr. Matt Burleigh 3677: Life in the Universe Useful numbers R Sun = 6.995x10 8 mR Sun = 6.995x10 8 m R jup = x10 7 m ~ 0.1R SunR jup = x10 7 m ~ 0.1R Sun R nep = x10 7 m ~ 4R earthR nep = x10 7 m ~ 4R earth R earth = 6.371x10 6 m ~ 0.1R jup ~ 0.01R SunR earth = 6.371x10 6 m ~ 0.1R jup ~ 0.01R Sun M Sun = 1.989x10 30 kgM Sun = 1.989x10 30 kg M jup = 1.898x10 27 kg ~ 0.001M Sun = 317.8M earthM jup = 1.898x10 27 kg ~ 0.001M Sun = 317.8M earth M nep = 1.02x10 26 kg ~ 5x10 -5 M Sun ~ 0.05M jup = 17.15M earthM nep = 1.02x10 26 kg ~ 5x10 -5 M Sun ~ 0.05M jup = 17.15M earth M earth = 5.97x10 24 kg = 3x10 -6 M Sun = 3.14x10 -3 M jupM earth = 5.97x10 24 kg = 3x10 -6 M Sun = 3.14x10 -3 M jup 1AU = 1.496x10 11 m1AU = 1.496x10 11 m 1 day = 86400s1 day = 86400s

Dr. Matt Burleigh 3677: Life in the Universe Exoplanet count 18/11/14 (NASA exoplanet archive) 1767 confirmed planets1767 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”Unexpected population with periods of <1 to ~4 days: “hot Jupiters” Planets with orbits like Jupiter discovered (eg 55 Cancri d)Planets with orbits like Jupiter discovered (eg 55 Cancri d) Smallest planets:Smallest planets: –Kepler-20e: 0.87R earth, –Alpha Cen Bb M sin i > 1.1M earth

Dr. Matt Burleigh 3677: Life in the Universe Hit 1000 exoplanet mark Transiting planets in blue

Dr. Matt Burleigh 3677: Life in the Universe

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

Dr. Matt Burleigh 3677: Life in the Universe Extra-solar planet period distribution Notice the “pile-up” at periods of <1 to ~4 days / AUNotice the “pile-up” at periods of <1 to ~4 days / AU The most distant planets discovered by radial velocities so far are at 5-6AUThe most distant planets discovered by radial velocities so far are at 5-6AU Imaging surveys finding very wide (>10AU) orbit planetsImaging surveys finding very wide (>10AU) orbit planets Orange are “hot Jupiters”Orange are “hot Jupiters” Yellow is Jupiter- mass in Jupiter-like orbitsYellow is Jupiter- mass in Jupiter-like orbits

Dr. Matt Burleigh 3677: Life in the Universe Selection effects Astronomical surveys tend to have built in biasesAstronomical surveys tend to have built in biases These “selection effects” must be understood before we can interpret resultsThese “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-inThese 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 JupiterAs 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 limitThe 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

Dr. Matt Burleigh 3677: Life in the Universe “Hot Jupiter” planets Doppler Wobble and transit surveys find many gas giants in orbits of <1 to ~4 daysDoppler 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 themThese survey methods are biased towards finding them –Larger Doppler Wobble signal –Greater probability of transit These planets are heated to >1000 o F on “day” sideThese planets are heated to >1000 o F on “day” side –And are “tidally locked” like the Moon –Causes extreme weather conditions

Dr. Matt Burleigh 3677: Life in the Universe Extra-solar planet mass distribution Lowest mass confirmed planet so far: Alpha Cen Bb M sin i =1.1xM EarthLowest mass confirmed planet so far: Alpha Cen Bb M sin i =1.1xM Earth Super-Jupiters (>few M Jup ) are not commonSuper-Jupiters (>few M Jup ) are not common –Implications for planet formation theories? –Or only exist in number at large separation? –Or exist around massive stars?

Dr. Matt Burleigh 3677: Life in the Universe A continuum of planet mass 1’000’000 Red box indicates “Super-Earths”

Dr. Matt Burleigh 3677: Life in the Universe Transiting planets in blue Red box indicates “Super-Earths”

Dr. Matt Burleigh 3677: Life in the Universe 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?

Dr. Matt Burleigh 3677: Life in the Universe

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

Dr. Matt Burleigh 3677: Life in the Universe ? What are exoplanets made of? ice mantle/volatile envelope thin atmosphere hydrogen/helium envelope solid core (rocks+metals) telluric super-Earths? ocean planets? mini Neptunes? gas dwarfs?

Dr. Matt Burleigh 3677: Life in the Universe ? What are exoplanets made of? ice mantle/volatile envelope thin atmosphere hydrogen/helium envelope solid core (rocks+metals) telluric super-Earths? ocean planets? mini Neptunes? gas dwarfs? HD b

Dr. Matt Burleigh 3677: Life in the Universe How common are gas giants? Radial velocity surveysRadial 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 improvesKnown 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 rareHot Jupiters are easy to discover, but in fact are rare How many have Earths…..?How many have Earths…..?

Dr. Matt Burleigh 3677: Life in the Universe What about the stars themselves? 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 ( 1.5M sun )Now extended to M dwarfs ( 1.5M sun ) –Subgiants are the descendants of A stars Incidence of planets is greatest for late F starsIncidence of planets is greatest for late F stars –F7-9V > GV > KV > MV More massive stars tend to have more massive planetsMore massive stars tend to have more massive planets

Dr. Matt Burleigh 3677: Life in the Universe Metallicity Metallicity The abundance of elements heavier than He relative to the Sun Overall, ~10% of solar-like stars have radial velocity –detected JupitersOverall, ~10% of solar-like stars have radial velocity –detected Jupiters But if we take metallicity into account: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 1/3 rd of the Sun’s metallicity have gas giants Does this result imply that planets more easily form in metal-rich environments?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!

Dr. Matt Burleigh 3677: Life in the Universe 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.

Dr. Matt Burleigh 3677: Life in the Universe 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 10 7 yrs.

Dr. Matt Burleigh 3677: Life in the Universe 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

Dr. Matt Burleigh 3677: Life in the Universe Where do the hot Jupiters come from? No element will condense within ~0.1AU of a star since T>1000KNo 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 formsPlanets 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 ~ AU cannot have formed thereHot Jupiters currently at ~ AU cannot have formed there –Migration: Planets migrate inwards and stop when disk is finally cleared If migration time < disk lifetimeIf migration time < disk lifetime –Planets fall into star –Excess of planets at AU is evidence of a stopping mechanism tides? magnetic cavities? mass transfer?tides? magnetic cavities? mass transfer? Large planets will migrate more slowlyLarge planets will migrate more slowly –Explanation for lack of super-Jupiters in close orbits