3677 life in the universe: extra-solar planets

DEPARTMENT OF PHYSICS AND ASTRONOMY

3677 Life in the Universe:3677 Life in the Universe:Extra-solar planetsExtra-solar planets

Dr. Matt BurleighDr. Matt Burleighwww.star.le.ac.uk/mrb1/lectures.htmlwww.star.le.ac.uk/mrb1/lectures.html

Dr. Matt Burleigh 3677: Life in the Universe


Course outlineCourse outline

• Lecture 1Lecture 1– Definition of a planetDefinition of a planet– A little historyA little history– Pulsar planetsPulsar planets– Doppler “wobble” (radial velocity) techniqueDoppler “wobble” (radial velocity) technique

• Lecture 2Lecture 2– Transiting planetsTransiting planets– Transit search projects Transit search projects – Detecting the atmospheres of transiting planetsDetecting the atmospheres of transiting planets



• Lecture 3Lecture 3– MicrolensingMicrolensing– Direct ImagingDirect Imaging– Planets around evolved starsPlanets around evolved stars

• Lecture 4Lecture 4– Statistics: mass and orbital distributions, incidence Statistics: mass and orbital distributions, incidence

of solar systems, etc.of solar systems, etc.– Hot JupitersHot Jupiters– Super-EarthsSuper-Earths– Planetary formationPlanetary formation– The host starsThe host stars



• Lecture 5Lecture 5– The quest for an Earth-like planetThe quest for an Earth-like planet– Results from the Kepler missionResults from the Kepler mission– Habitable zonesHabitable zones– BiomarkersBiomarkers– Future telescopes and space missionsFuture telescopes and space missions


Useful numbersUseful numbers• RRSunSun = 6.995x10 = 6.995x1088m m

• RRjupjup = 6.9961x10 = 6.9961x1077m ~ 0.1Rm ~ 0.1RSunSun

• RRnep nep = 2.4622x10= 2.4622x1077m ~ 4Rm ~ 4Rearthearth

• RRearth earth = 6.371x10= 6.371x1066m ~ 0.1Rm ~ 0.1Rjup jup ~ 0.01R~ 0.01RSunSun

• MMSunSun= 1.989x10= 1.989x103030kgkg

• MMjupjup= 1.898x10= 1.898x102727kg ~ 0.001Mkg ~ 0.001MSunSun = 317.8M = 317.8Mearthearth

• MMnepnep= 1.02x10= 1.02x102626kg ~ 5x10kg ~ 5x10-5-5MMSunSun ~ 0.05M ~ 0.05Mjupjup = 17.15M = 17.15Mearthearth

• MMearthearth= 5.97x10= 5.97x102424kg = 3x10kg = 3x10-6-6MMSunSun = 3.14x10 = 3.14x10-3-3MMjupjup

• 1AU = 1.496x101AU = 1.496x101111mm• 1 day = 86400s1 day = 86400s


TransitsTransits

• Planets observed at inclinations near 90Planets observed at inclinations near 90o o will transit their host stars will transit their host stars


TransitsTransits

• AssumingAssuming– The whole planet passes in front of the starThe whole planet passes in front of the star– And ignoring limb darkening as negligibleAnd ignoring limb darkening as negligible

• Then the depth of the eclipse is simply the ratio of the planetary and Then the depth of the eclipse is simply the ratio of the planetary and stellar disk areas:stellar disk areas:

• Where Δf is the change in the star’s flux (brightness), RWhere Δf is the change in the star’s flux (brightness), Rpp is the is the planet radius and Rplanet radius and R** the star’s radius the star’s radius


TransitsTransits

• RSun = 6.995x108m Rjup = 6.9961x107m Rearth = 6.371x106m

(note: Rjup~ 0.1RSun & Rearth ~ 0.1Rjup ~ 0.01RSun)

• Jupiter transit: depth = 0.01 = 1%• Earth transit: depth = 8.3x10-5 = 0.0083%

(note: best photometry from ground ~0.1%)

• 55 Cancri R* = 1.15RSun

• Planet 55 Cancri e = 8.3Rearth

• Transit depth = 0.004 = 0.4%


TransitsTransits

• In practice: In practice: • We measure the change in magnitude We measure the change in magnitude m, and obtain the stellar m, and obtain the stellar

radius from the spectral type radius from the spectral type – Hence by converting to flux we can measure the planetHence by converting to flux we can measure the planet’’s radiuss radius– Rem. Rem.

– Thus Thus

• (rem in magnitude system a smaller number means brighter)(rem in magnitude system a smaller number means brighter)


Discovery of first transiting planet: Discovery of first transiting planet: HD209458bHD209458b

• HD209458b was discovered HD209458b was discovered originally via the radial velocity originally via the radial velocity methodmethod– 3.5 day period3.5 day period

• Astronomer Dave Astronomer Dave Charbonneau monitored it with Charbonneau monitored it with a small telescope called a small telescope called STARESTARE– Transit discovered in 1999Transit discovered in 1999


TransitsTransits

Example: first known transiting planet Example: first known transiting planet HD209458bHD209458b m = 0.017 magsm = 0.017 mags

– So (fSo (f* * / f/ ftransittransit) = 1.0158, i.e. ) = 1.0158, i.e. f=1.58%f=1.58%

– From the spectral type (G0) R=1.15RFrom the spectral type (G0) R=1.15Rsunsun

– So using So using f / ff / f** = (R = (Rp p / R/ R**))2 2 and setting and setting

ff**=100%=100%

– Find RFind Rpp=0.145R=0.145Rsunsun

– Since RSince Rsunsun=9.73R=9.73RJ J thenthen

– RRpp = 1.41R = 1.41RJJ


TransitsTransits

• HD209458b more:HD209458b more:– From Doppler wobble method know From Doppler wobble method know MM sin sin ii = 0.62M = 0.62MJJ

– Transiting, hence assume i=90Transiting, hence assume i=90oo, so , so MM=0.62M=0.62MJJ

– Density = 0.29 g/cmDensity = 0.29 g/cm33

• c.f. Saturn 0.69 g/cmc.f. Saturn 0.69 g/cm33 – HD209458b is a gas giant! HD209458b is a gas giant!


The shape of the transit light The shape of the transit light curvecurve

• Ingress and egress affected by stellar limb darkeningIngress and egress affected by stellar limb darkening


TransitsTransits

• For an edge-on orbit, transit duration is given by:For an edge-on orbit, transit duration is given by:

• Where P=period, a=semi-major axis of orbitWhere P=period, a=semi-major axis of orbit

• Example: HD209458bExample: HD209458b–P=3.52475 days = 304538sP=3.52475 days = 304538s–RR**=1.15R=1.15RSunSun = 1.15x6.955x10 = 1.15x6.955x1088mm–a=0.04747AU=7.1x10a=0.04747AU=7.1x1099mm–Δt=10920s=3.03hoursΔt=10920s=3.03hours–Note for Earth (a=1AU) Δt=46668s=12.96hours Note for Earth (a=1AU) Δt=46668s=12.96hours


TransitsTransits

• Probability of transit (for random orbit)Probability of transit (for random orbit)

– For Earth (a=1AU), For Earth (a=1AU), PPtransittransit=0.5%=0.5%

– But for close, But for close, ““hothot”” Jupiters, Jupiters, PPtransittransit==10%10%

– Of course, relative probability of detecting Earths is lower Of course, relative probability of detecting Earths is lower since would have to observe continuously for up to 1 year since would have to observe continuously for up to 1 year

• (See Kepler mission)(See Kepler mission)


Transits: AdvantagesTransits: Advantages

• Easy. Can be done with small, cheap telescopesEasy. Can be done with small, cheap telescopes• Possible to detect low mass planets, including Possible to detect low mass planets, including

““EarthsEarths””, especially from space (Kepler mission), especially from space (Kepler mission)


Transits: DisadvantagesTransits: Disadvantages

• Probability of seeing a transit is lowProbability of seeing a transit is low• Need to observe many stars simultaneously for long Need to observe many stars simultaneously for long

periods of timeperiods of time

• MimicsMimics– Easy to confuse with starspotsEasy to confuse with starspots– Easy to confuse with grazing binary star eclipse Easy to confuse with grazing binary star eclipse – Blended eclipsing binary in a triple system, or merely Blended eclipsing binary in a triple system, or merely

in background in background

• Low mass red dwarfs, brown dwarfs and gas Low mass red dwarfs, brown dwarfs and gas giants have same radii giants have same radii – Needs radial velocity measurements for confirmation, Needs radial velocity measurements for confirmation,

massesmasses


Super WASPSuper WASP

• Wide Angle Search for Planets (by Wide Angle Search for Planets (by transit method)transit method)

• First “telescope” located in La Palma, First “telescope” located in La Palma, second in South Africasecond in South Africa

• ““Telescopes” are 8 x 400mm camera Telescopes” are 8 x 400mm camera lenses with a high grade CCDlenses with a high grade CCD

• Operations started May Operations started May ‘‘0404• Data stored and processed at Data stored and processed at

Leicester Leicester • ~100 new planets detected!~100 new planets detected!• www.superwasp.org


Super WASPSuper WASP

• SuperWASP monitors about 1/4 SuperWASP monitors about 1/4 of the sky from each siteof the sky from each site

• That means millions of stars, That means millions of stars, every night!every night!


Belfast, DLR Berlin, Geneva, Leicester, Warwick, Cambridge

www.ngtransits.org


WASP planets in green


NGTS Prototype La Palma 2010


Early NGTS v SuperWASP Early NGTS v SuperWASP


NGTS sensitivity (planet periods)NGTS sensitivity (planet periods)


NGTS Site: ESO Paranal observatory, Chile

VISTA

VLT

NGTS

Construction 2014Operations 2014-2019

Astronomers’ hotel / baddies’ lair in “Quantum of Solace”


Transmission spectroscopyTransmission spectroscopy• Transiting planets allow us to make measurements of the chemical

composition and physical properties of their atmospheres• Previously we assumed the planet was an opaque disk with a sharp

edge• In reality, it has an atmosphere & the opacity diminishes with height• By observing a transit at a specific wavelength (eg Na, H) can

measure the extra absorption from that element in the planet’s atmosphere

• Very challenging observations: HST, Spitzer, 8m telescopes


Transmission spectroscopyTransmission spectroscopy


Secondary eclipsesSecondary eclipses

• In secondary eclipse the planet passes behind the star

• The drop in combined light is tiny, but measurable with careful observations

• Gives thermal emission and temperature of “day” side of planet


Secondary eclipsesSecondary eclipses

• Note that the secondary eclipse Note that the secondary eclipse depth increases with wavelengthdepth increases with wavelength– Bcse planets are cooler than Bcse planets are cooler than

stars, their emission is stronger stars, their emission is stronger at longer wavelengthsat longer wavelengths


Carbon rich atmosphere in WASP-12b Madhusudhan … Wheatley ... Pollacco & West 2010, Nature

Secondary eclipses


Phase curve & map of HD189733bPhase curve & map of HD189733b

• Equisite observations Equisite observations of the transiting of the transiting planet HD189733b at planet HD189733b at 8 microns with 8 microns with Spitzer reveal the Spitzer reveal the changing brightness changing brightness of the planet as it of the planet as it rotatesrotates

• The hottest point on The hottest point on the “day” side is the “day” side is offset slightly from offset slightly from the expected positionthe expected position– Extreme weather?Extreme weather?


Transit timing variationsTransit timing variations

• The transits of a planet in a The transits of a planet in a Keplarian orbit around its host Keplarian orbit around its host star are exactly periodic. star are exactly periodic.

• However, if a third body is However, if a third body is present in the system, the orbits present in the system, the orbits are not Keplarian, and the time are not Keplarian, and the time between consecutive transits between consecutive transits varies varies

• This offers the possibility of This offers the possibility of detecting non-transiting planets detecting non-transiting planets via photometry. via photometry.

• It is even possible to determine It is even possible to determine the maximum mass of the the maximum mass of the planetsplanets


Transit timing variationsTransit timing variations

• The maximum TTV for an inner planet, due to influence of a more The maximum TTV for an inner planet, due to influence of a more distant second planet, is given by:distant second planet, is given by:

• where Mwhere M22 and P and P22 are the mass and period of the second planet, M are the mass and period of the second planet, M** the the mass of the star, amass of the star, a11 and a and a22 are the semi-major axis of the planets’ are the semi-major axis of the planets’ orbits, and eorbits, and e22 is the eccentricity of the 2 is the eccentricity of the 2ndnd planet’s orbit. planet’s orbit.

• Note that there is no TTV in this instance if the second planet’s orbit is Note that there is no TTV in this instance if the second planet’s orbit is perfectly circular!perfectly circular!

– Q: What is the maximum TTV in the orbit of the inner planet in a system around Q: What is the maximum TTV in the orbit of the inner planet in a system around a solar type star, where an inner, Jovian mass planet orbits at aa solar type star, where an inner, Jovian mass planet orbits at a 11=0.05AU with a =0.05AU with a period of 4 days, and the outer planet has a mass half that of Jupiter (i.e. 0.5x10period of 4 days, and the outer planet has a mass half that of Jupiter (i.e. 0.5x10 --

33MMSunSun) and a period of 100 days for an eccentric orbit with a) and a period of 100 days for an eccentric orbit with a22=0.42AU and =0.42AU and ee22=0.5?=0.5?A: 3.6 secondsA: 3.6 seconds

3677 life in the universe: extra-solar planets

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