Transit Searches: Results

I. Results from individual transit searche programs

II. Interesting cases

III. Spectroscopic Transits

IV. In-transit spectroscopy

V. Radiated light (secondary transits)

CoRoT results will be presented during the Space Missions lecture

The first time I gave this lecture (2003) there was one transiting extrasolar planet

There are now 52 transiting extrasolar planets

First ones were detected by doing follow-up photometry of radial velocity planets. Now transit searches are discovering exoplanets

Radial Velocity Curve for HD 209458

Period = 3.5 days

Msini = 0.63 MJup

Charbonneau et al. (2000): The observations that started it all:

• Proof that RV variations are due to planet

• Mass = 0,63 MJupiter

• Radius = 1,35 RJupiter

• Density = 0,38 g cm–3

Transit detection was made with the 10 cm STARE Telescope

A light curve taken by amateur astronomers…

..and by the Profis ( Hubble Space Telescope).

Successful Transit Search Programs

• OGLE: Optical Gravitational Lens Experiment (http://www.astrouw.edu.pl/~ogle/)

• 1.3m telescope looking into the galactic bulge

• Mosaic of 8 CCDs: 35‘ x 35‘ field

• Typical magnitude: V = 15-19

• Designed for Gravitational Microlensing

• First planet discovered with the transit method

• 7 Transiting planets discovered so far

The first planet found with the transit method

Konacki et al.

Until this discovery radial velocity surveys only found planets with periods no shorter than 3 days. About ½ of the OGLE planets have periods less than 2 days.

M = 1.03 MJup

R = 1.36 Rjup

Period = 3.7 days

Successful Transit Search Programs

• WASP: Wide Angle Search For Planets (http://www.superwasp.org). Also known as SuperWASP

• Array of 8 Wide Field Cameras

• Field of View: 7.8o x 7.8o

• 13.7 arcseconds/pixel

• Typical magnitude: V = 9-13

• 15 transiting planets discovered so far

CoordinatesRA 00:20:40.07 Dec +31:59:23.7

Constellation Pegasus

Apparent Visual Magnitude 11.79

Distance from Earth 1234 Light Years

WASP-1 Spectral Type F7V

WASP-1 Photospheric Temperature

6200 K

WASP-1b Radius 1.39 Jupiter Radii

WASP-1b Mass 0.85 Jupiter Masses

Orbital Distance 0.0378 AU

Orbital Period 2.52 Earth Days

Atmospheric Temperature 1800 K

Mid-point of Transit 2453151.4860 HJD

Successful Transit Search Programs

• TrES: Trans-atlantic Exoplanet Survey (STARE is a member of the network http://www.hao.ucar.edu/public/research/stare/)

• Three 10cm telescopes located at Lowell Observtory, Mount Palomar and the Canary Islands

• 6.9 square degrees

• 4 Planets discovered

Successful Transit Search Programs

• HATNet: Hungarian-made Automated Telescope (http://www.cfa.harvard.edu/~gbakos/HAT/

• Six 11cm telescopes located at two sites: Arizona and Hawaii

• 8 x 8 square degrees

• 8 Planets discovered

Follow-up with larger telescope

HAT 1b

Special Transits: GJ 436

Host Star:

Mass = 0.4 Mּס (M2.5 V)

Butler et al. 2004

„Photometric transits of the planet across the star are ruled out for gas giant compositions and are also unlikely for solid compositions“

Special Transits: GJ 436

Butler et al. 2004

The First Transiting Hot Neptune

Gillon et al. 2007

Star  

Stellar mass [  Mּס  ] 0.44 (  ± 0.04)  

Planet  

Period [days] 2.64385 ±   0.00009  

Eccentricity 0.16 ±   0.02  

Orbital inclination 86.5   0.2

Planet mass [ ME  ] 22.6 ±   1.9

Planet radius [ RE  ] 3.95 +0.41-0.28

Special Transits: GJ 436

Mean density = 1.95 gm cm–3, in between Neptune (1.58) and Uranus (2.3)

Mean density is first hints of the internal composition

M = 3.11 MJup

Special Transits: HD 17156

Probability of a transit ~ 3%

R = 0.96 RJup

Barbieri et al. 2007

Mean density = 4.88 gm/cm3

Mean M2 star ≈ 4.3 gm/cm3

Companion is probably more like a brown dwarf

Rp = 0.7 RJup

Mp = 0.36 MJup

Sato et al. 2005

Special Transits: HD 149026

Mean density = 2.8 gm/cm3

~70 Mearth core mass is difficult to form with gravitational instability

HD 209458b and HAT-P-1b have anomalously large radii that still cannot be explained by planetary structure and evolution models

Mass Radius Relationship

Mazeh et al. 2008 found a mass-period relationship for transiting planets. Suggest this is evidence of evaporation, only the most massive planets can survive

Results from the Rossiter-McClaughlin Effect

The RM effect causes a distortion in the radial velocity curve during a transit whose strength depends on the radius of the planet and the rotation rate of the star

So far all transiting planets for which an RM effect has been measured has shown prograde orbits

What about misalignment of the spin axis?

HD 147506

Best candidate for misalignment is HD 147506 because of the high eccentricity

Two possible explanations for the high eccentricities seen in exoplanet orbits:

• Scattering by multiple giant planets

• Kozai mechanism

Planet-Planet Interactions

Initially you have two giant planets in circular orbits

These interact gravitationally. One is ejected and the remaining planet is in an eccentric orbit

Recall that there are no massive planets in circular orbits

This mechanism has been invoked to explain the „massive eccentrics“

Kozai Mechanism

Two stars are in long period orbits around each other.

A planet is in a shorter period orbit around one star.

If the orbit of the planet is inclined, the outer planet can „pump up“ the eccentricity of the planet. Planets can go from circular to eccentric orbits.

This was first investigated by Kozai who showed that satellites in orbit around the Earth can have their orbital eccentricity changed by the gravitational influence of the Moon

Kozai Mechanism

The Kozai mechanism has been used to explain the high orbital eccentricity of 16 Cyg B, a planet in a binary system

Winn et al. 2007: HD 147506b (alias HAT-P-2b)

If either mechanism is at work, then we should expect that planets in eccentric orbits not have the spin axis aligned with the stellar rotation. This can be checked with transiting planets in eccentric orbits

Spin axes are aligned within 14 degrees (error of measurement). No support for Kozai mechanism or scattering

What about HD 17156?

Narita et al. (2007) reported a large (62 ± 25 degree) misalignment between planet orbit and star spin axes!

Cochran et al. 2008: = 9.3 ± 9.3 degrees → No misalignment!

In-transit Spectroscopy

• Take a spectrum of the star during the out-of-transit time

• Take a spectrum of the star during the transit

• Subtract the two and what remains is the spectrum of the planet atmosphere

In practice this is very difficult. One requires high signal-to-noise ratio data (≈ 1000) which means repeated measurements that have to be co-added.

Problem: In transit spectra can only be made during transits (infrequent) and only for about 3 hours!

Fig. 1.— Flux of HD 209458 a (upper curve) and the transmitted flux through the planet’s transparent atmosphere (lower curve). Superimposed on the transmitted flux are the planetary absorption features, including the He i triplet line at 1083 nm. The other bound-bound lines are alkali metal lines (see Fig. 2 for details). The H2O and CH4 molecular absorption dominates in the infrared. The dotted line is a blackbody of 1350 K representative of the CEGP’s thermal emission, but the thermal emission can be larger than a blackbody blueward of 2000 nm.

From The Astrophysical Journal 537(2):916–921.© 2000 by The American Astronomical Society.For permission to reuse, contact journalpermissions@press.uchicago.edu.

In-transit Spectroscopy

Sasselov & Seager 2004

Fig. 2.—Upper plot: The normalized in-transit minus out-of-transit spectra, i.e., percent occulted area of the star. In this model the cloud base is at bar. Rayleigh scattering is important in the UV. Lower plot: A model with cloud base at 0.2 bar. The stellar flux passes through higher pressures, densities, and temperatures of the planet atmosphere compared to the model in the upper plot. In addition, a larger transparent atmosphere makes the line depth larger. Observations will constrain the cloud depth. See text for discussion.

From The Astrophysical Journal 537(2):916–921.© 2000 by The American Astronomical Society.For permission to reuse, contact journalpermissions@press.uchicago.edu.

Fig. 4.—Top: Unbinned time series nNa (Fig. 2, top panel). Bottom: These data binned in time (each point is the median value in each bin). There are 10 bins, with roughly equal numbers of observations per bin (42). The error bars indicate the estimated standard deviation of the median. The solid curve is a model for the difference of two transit curves (described in § 3), scaled to the observed offset in the mean during transit, ΔnNa = −2.32 × 10−4.

From The Astrophysical Journal 568(1):377–384.© 2002 by The American Astronomical Society.For permission to reuse, contact journalpermissions@press.uchicago.edu.

Charbonneau et al. 2001

Redfield et al. 2007 Sodium

Calcium

An element not expected to show excess absorption shows none

Vidal-Majar et al. 2003

HD 209458 shows excess abroption in Hydrogen Lyman

Evidence for an evaporating atmosphere of Hydrogen?

Picture of the geocorona taken by the Apollo astronauts

Fig. 3.— Comparison between Lyα line profiles in and out of transit period. The sky background spectral window is indicated by two dashed vertical lines. (a) The in-transit line profile (thin solid line) is accumulated for the time period starting 3900 s before TCT and ending 3900 s after it. To correct for the 8.9% obscuration derived in this study, the corresponding intensity is scaled by 1.098. ∼The resulting line profile (dotted curve) properly recovers the unperturbed line profile (histogram). (b) The first in-transit line profile, B1 (thin solid line), was accumulated over the time period starting 4000 s before the TCT and ending 600 s after it. The second in-transit line profile, B2 (dotted line), was accumulated over the time period starting 1800 s before TCT and ending 3900 s after it. ∼ ∼ ∼

From The Astrophysical Journal Letters 671(1):L61–L64.© 2007 by The American Astronomical Society.For permission to reuse, contact journalpermissions@press.uchicago.edu.

Ben-Jaffel 2007

Or not?

Fig. 1.— Observed HD 209458 Lyα profiles as observed by VM03 before and during the planetary transit. The BJ07 reanalysis of nearly the same data set produced a similar Lyα line profile. The two vertical dashed lines define the limits and of the line core where H i planetary absorption takes place. In VM03 as well as in BJ07, the central part of the line (noted “Geo”) possibly perturbed by the Earth geocoronal emission is omitted from the analysis. The line wings are used by VM03 as a flux reference to correct for the stellar Lyα intrinsic variations.

From The Astrophysical Journal Letters 676(1):L57–L60.© 2008 by The American Astronomical Society.For permission to reuse, contact journalpermissions@press.uchicago.edu.

Vidal-Majar et al. 2008

Claim is that difference is due to different wavelength range used to calculate absorption depth

Secondary Transits: The Planet Albedo

The planet reflects light, so one should see a modulation in the light curve, plus an eclipse of the planet

Secondary Transits: The Planet Albedo

The planet reflects light, so one should see a modulation in the light curve, plus an eclipse of the planet

Rowe et al. 2008

Albedo < 0.12

Jupiter: 0.5

Fig. 7.— (a) Spherical albedo of a class III clear EGP. In addition to the isolated (thin curve) and modified (thick curve) T-P profile models, the dashed curve depicts what the albedo would look like in the absence of the alkali metals. (b) Spherical albedo of a class IV roaster. Theoretical albedo spectra of isolated (thin curve) and modified (thick curve) T-P profile class IV models are depicted.

Sudarsky et al. 2005.

900 < T < 1500 K

Fig. 8.— Spherical albedo of a class V roaster. A silicate layer high in the atmosphere results in a much higher albedo than a class IV roaster. No ionization is assumed in this model.

From The Astrophysical Journal 538(2):885–903.© 2000 by The American Astronomical Society.For permission to reuse, contact journalpermissions@press.uchicago.edu.

T > 1500 K

Better upper limits will be found by CoRoT. Kepler may be able to detect the second transit.

Secondary Transits with Kepler

For a short period giant in a 4 day orbit Kepler will observe more than 250 transits. It will be able to detect secondary transits (eclipses) for Albedos as low as 0.08

Secondary Transits: Infrared Measurements with Spitzer

The „hot Jupiters“ have temperatures of ~ 1000 K. The radiated light can be much higher than the reflected light:

Reflected light = Lstar4d2 =

AR2

4d2

A = geometric albedo, R = planet radius, d = distance from star

For A = 0.1, d=0.05 AU, R = 1 RJup

Reflected light ≈ 10–5

AR2Lstar 1

Secondary Transits: Infrared Measurements with Spitzer

In radiated light however, for a planet with Teff ≈ 1000 K:

Flux from planet = 2hc2/–5

ehc/kTp

–12Rp

2

Flux from star = 2hc2/–5

ehc/kT* –1

2R*2 Only looking at half

the star

Fp/F* = ehc/kT

* –1 Rp

2

ehc/kTp –1 R*

2

Fp/F* ≈ 0.0016

For a 1.5 RJup planet with Tp = 1000 K around a solar-type star (5800 K) at 8 m:

Spitzer is a 0.85m telescope that can measure infrared radiation between 3 and 180 m

HD 209458 secondary transit (eclipse) at 24 m

Teff = 1130 K

Fig. 3.— Solid black line shows the Sudarsky et al. (2003) model hot Jupiter spectrum divided by the stellar model spectrum (see text for details). The open diamonds show the predicted flux ratios for this model integrated over the four IRAC bandpasses (which are shown in gray and renormalized for clarity). The observed eclipse depths at 4.5 and 8.0 μm are overplotted as black diamonds. No parameters have been adjusted to the model to improve the fit. The dotted line shows the best-fit blackbody spectrum (corresponding to a temperature of 1060 K), divided by the model stellar spectrum. Although the Sudarsky et al. (2003) model prediction is roughly consistent with the observations at 8.0 μm, the model overpredicts the planetary flux at 4.5 μm. The prediction of a relatively large flux ratio at 3.6 μm should be readily testable with additional IRAC observations.

From The Astrophysical Journal 626(1):523–529.© 2005 by The American Astronomical Society.For permission to reuse, contact journalpermissions@press.uchicago.edu.

Spitzer Measurements of Radiated Light at 8 m of HD 189733

Knutson et al. 2007

Tmax = 1211 K

Tmin = 973 K

Spitzer Measurements of Radiated Light at 8 m of HD 189733

Primary Secondary

Predicted time of secondary transit is off by 120 s → eccentricity?

Brightest point is shifted by 16 degrees from the sub-stellar point

GJ 836 Spitzer measurements

Radius = 4.33 ± 0.18 RE

Tp = 712 K

Eccentricity = 0.15

Summary

1. 52 Transiting planets have been discovered so far. This is the Golden Era of transit detections

2. In 5 years more transiting planets than non-transiting planets will be known. My guess: 500

3. The measurement of the mean density is putting constraints on planet formation and structure theories

4. In-transit spectroscopy is yielding the first chemical composition of an extrasolar planet

5. Albedo measurements are placing contraints on atmospheric models

6. First indication of exoplanet „weather“

7. We are actually measuring the phyisical properties of the planets themselves: exoplanetary science

These are exciting times!