observations of extrasolar planets20planets%201.pdf• the earth is visible in reflected light from...

Observations of extrasolar planets

1Wednesday, November 9, 2011

Mercury

Venusradar image from Magellan (vertical scale exaggerated 10 X)

Mars4Wednesday, November 9, 2011

Jupiter

Saturn

Uranus and Neptune

we need to look out about 10 parsecs

or 2 million astronomical units

to examine 100 stars for planets

• the Earth is visible in reflected light from the Sun

• the total light reflected by Earth is about 10-9 of the light emitted by the Sun

• Jupiter is bigger but more distant so only about 4X brighter than Earth

These would be easy to see with modern telescopes at 10 parsecs if the star were not right beside them

(imagine looking at a lighthouse 1000 km away and trying to detect a firefly flying 1 meter from the light)

With current technology direct imaging requires one or more of:

• planet with a large orbital radius (e.g. 100 AU vs. 30 AU for Neptune)

• observations in the infrared (for a black body in thermal equilibrium at 100 AU around the Sun, emission peaks at 100 microns)

• young planet (Jupiter’s internal luminosity falls at 1/age)

why is finding planets hard?

• star is at 7.7 parsecs• planet has 110 AU orbital radius, 900 yr orbital period• 100-300 Myr stellar age

HR 8799

• star is at 39 parsecs • planets have masses of 7-10 Jupiter masses and projected separations of 14-68 AU• 30-160 Myr age

Marois et al. (2008, 2010)

• current accuracy: velocity of 1 meter/sec (3 parts in 109)

• cross-correlation uses all lines in spectrum

• high S/N (bright stars, big telescopes)

• iodine absorption cell• old stars (less rotation, less activity)• G stars

• Jupiter: orbital period of 12 yr, reflex velocity of Sun 13 meter/sec

• Earth: orbital period of 1 yr, reflex velocity of Sun 0.1 meter/sec

Given the star mass M (known from spectral type), radial-velocity observations yield:

• orbital period P• semi-major axis a• combination of planet mass m & inclination I, m sin I• orbit eccentricity e

• the star is similar to the Sun and 11 parsecs away

• the planet orbits once every 15.8 days, has a mass 4 X that of Jupiter (if edge-on orbit), and is 0.11 AU from the star 100 meters/second

HD 82943

planet 1:

m sin I = 1.84MJ

P = 435 de = 0.18 ± 0.04

planet 2:

m sin I = 1.85MJ

P = 219 de = 0.38 ± 0.01(Mayor et al. 2003)

Timing

• works for pulsars, pulsating stars, eclipsing binaries

• observed period is P=P0(1+v/c) so

Pulsar planets

• three planets discovered orbiting PSR B1257+12 by Wolszczan & Frail (1992)

• orbital parameters can be determined far more accurately than for radial-velocity measurements of nearby stars

• two planets near 3:2 resonance which enhances mutual perturbations, so these can be measured

• remarkably similar to the inner solar system

Konacki & Wolszczan (2003)

(a) no planets

(b) three planets

(c) three planets + mutual interactions

Konacki & Wolszczan (2003)

Transit of Venus

• next June 6, 2012, 22:10 UTC at Tokyo

Jupiter’s satellite Io

Transit searches

Why are these so hard?

• probability that a given planet will transit is small, ~ rstar/a (only 0.5% at a = 1 AU)

• transit duration is short, ~(rstar/a)P/π

• transit depth is small, <1% *

• confusion from grazing eclipsing binary stars

• star spots, stellar pulsations, stellar flares

• incomplete sampling (daytime, weather, observing schedules, etc.)*

* much easier in space

Kepler (NASA)• launch March 6 2009

• stare 24/7 for five years at a single patch of sky

• monitor 200,000 stars for transits

• ppm precision

transit: planet moves in front of the star; U-shape because of limb darkening in staroccultation: planet moves behind the star; square shape

• orbital period P = 5.2 days• two curious features:

• sinusoidal brightness variations at fundamental and first harmonic • transit (U shape) is shallower than occultation (square well)

van Kerkwijk et al. (2010)

• orbital period P = 5.2 days• two curious features:

• sinusoidal brightness variations at fundamental and first harmonic • transit (U shape) is shallower than occultation (square well)

• both can be explained if the companion is a white dwarf rather than a planet:

• occultation is deeper because the white dwarf is hotter than the primary (T=13,000 K vs. 9,400 K) • first harmonic due to tidal distortion of the primary by the white dwarf• fundamental due to Doppler boosting • white dwarf has mass 0.22±0.03 M⊙; radius 0.043±0.004 R⊙

• P/2 variations due to tidal distortion of the primary star by the white dwarf• P variations due to Doppler boosting orbital velocity to 1 km/s

van Kerkwijk et al. (2010)

Struve (1952)

Gravitational lensing

• a particle traveling at high speed v past a mass M with impact parameter b suffers angular deflection α =2GM/v2b

• in general relativity, deflection of a photon is obtained by replacing v by c and multiplying by 2:

• three effects: position shift, image splitting, image magnification

Gravitational lensing

the gravitational field from the lensing star:

- splits image into two- magnifies one image and demagnifies the other- if source, lens and observer are exactly in line the image appears as an Einstein ring

lensing massEinstein ring

source track

Gravitational microlensing

Consider a source star near the center of the Galaxy, lensed by an intervening star at half that distance. Then θE=0.001 arcsec ~ 4 AU.

• image splitting or shift is impossible to see

• image magnification is easy to see

• time required to transit Einstein ring ~DLθE/v~0.2 yr, for v~100 km/s

• substantial magnification if and only if impact parameter less than Einstein radius

• chance that any given star is microlensed is only ~10-6

Mao et al. (2002)33Wednesday, November 9, 2011

Gravitational microlensing of planets

• Einstein radius scales as M1/2 so cross-section and expected duration scale as M1/2~ 0.03 for Jupiter, i.e. duration ~ 1 day for Jupiter, ~1 hour for Earth

• image magnification is the same

• Einstein ring radius ~ typical planet orbital radius

Beaulieu et al. (2006): 5.5 (+5.5/-2.7) MEarth, 2.6(+1.5/-0.6) AU orbit, 0.22(+0.21/-0.11) MSun, DL=6.6±1.1 kpc

daysGaudi et al. (2008)

Gaudi et al. (2008)

1% of Einstein radius or 15 µas

star path

source size

• two planets, b and c• can detect orbital motion of Earth and planet c• mb = 0.71 ± 0.08 MJupiter, mc = 0.27 ± 0.03 MJupiter

• assuming coplanar, circular orbits ab = 2.3 ± 0.2 AU, ac = 4.6 ± 0.5 AU• distance 1.49 ± 0.13 kpc• M*=0.50 ± 0.05 M⊙

Gaudi et al. (2008)

Astrometry

the Sun’s motion as seen from

why this is hard:

• typical motions << 0.001 arcsec ~ 10-8 radians

• not many nearby stars

• confusion from outer planets: maximum radial velocity is (m/M)(GM/a)1/2 but maximum wobble is (m/M)a

space missions:

• GAIA (ESA) • launch 2013 • every Jupiter analog within 50 pc• 104 - 5 × 104 planets•also transits via photometry

the current track record:

• imaging: 26• radial velocity: 644• transits: 185

• gravitational lensing: 13• timing: 12

• astrometry: 0

see http://www.exoplanet.eu, http://exoplanets.org/

figure from Seager (2011)

Current record-holders (RV surveys)

• smallest semi-major axis a = 0.0143 AU = 3.06 RSun

• largest semi-major axis a=11.6 AU (Jupiter = 5.2 AU)

• biggest eccentricity e = 0.94

• smallest eccentricity e = 0

• smallest mass 0.0061 MJupiter = 1.9 MEarth

Mercury Venus Earth Mars Jupitersolar radius 0.00465 AU

GJ 1214 b, a=0.0143 AU

undercounted because of selection effects

“hot Jupiters”

Grether & Lineweaver (2006)

hydrogen burning limit

deuterium burning limit

tidal circularization

Ribas & Miralda-Escudé (2007)

binary stars

massive planets (M > 4 Jupiter masses)

planets (M < 4 Jupiter masses)

eccentricity distribution of massive planets is similar to that of binary stars

Johnson (2007)

high-mass stars are more likely to host planets

(from J. Winn)50Wednesday, November 9, 2011

(from J. Winn)

obliquity = angle between spin angular momentum of star and orbital angular momentum of planetRossiter-McLaughlin measures angle between projection of spin angular momentum of star and orbital angular momentum of planet on the sky plane

HAT-P-30b 7

s−1]

−2 0 2Time from midtransit [hr]

50 60 70 80 90 100! [deg]

s−1]

Fig. 4.— Rossiter-McLaughlin effect for HAT-P-30 Left.—Apparent radial velocity variation on the night of 2011 Feb 21, spanninga transit. The top panel shows the observed RVs. The bottom panel shows the residuals between the data and the best-fitting model.Right.—Joint constraints on λ and v sin i!. The contours represent 68.3%, 95.4%, and 99.73% confidence limits. The marginalized posteriorprobability distributions are shown on the sides of the contour plot.

HATNet operations have been funded by NASA grantsNNG04GN74G, NNX08AF23G and SAO IR&D grants.GT acknowledges partial support from NASA grantNNX09AF59G. We acknowledge partial support alsofrom the Kepler Mission under NASA CooperativeAgreement NCC2-1390 (D.W.L., PI). G.K. thanks the

Hungarian Scientific Research Foundation (OTKA) forsupport through grant K-81373. This research hasmade use of Keck telescope time granted through NASA(N167Hr). Based in part on data collected at SubaruTelescope, which is operated by the National Astronom-ical Observatory of Japan.

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HAT P-7bλ = 183±9°Winn et al. (2009)HAT P-30b

λ = 74 ± 9°Winn et al. (2009)

HAT P-14bλ = 189 ± 5°Winn et al. (2011)

Seager & Deming (2010)

detection of CH4, H2O, Na, CO, CO2

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