Characterization of Planets: Mass and Radius (Transits Results Part I)

I. Results from individual transit search programsII. Interesting casesIII. Global PropertiesIV. Spectroscopic Transits

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

There are now 59 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:

• 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).

HD 209458b has a radius larger than expected.

HD 209458b

Models I, C, and D are for isolated planets

Models A and B are for irradiated planets.

Evolution of the radius of HD 209458b and Boob

One hypothesis for the large radius is that the stellar radiation hinders the contraction of the planet (it is hotter than it should be) so that it takes longer to contract. Another is tidal heating of the core of the planet if you have nonzero eccentricity

Burrows et al. 2000

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

• 8 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

K = 510 170 m/s

i= 79.8 0.3

a= 0.0308

Mass = 4.5 MJ

Radius = 1.6 RJ

Spectral Type = F3 V

Vsini = 40 km/s

One reason I do not like OGLE transiting planets. These produce low quality transits, they are faint, and they take up a large amount of 8m telescope time.

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 Days

Atmospheric Temperature 1800 K

Mid-point of Transit 2453151.4860 HJD

WASP 12: Hottest (at the time) Transiting Planet

Discovery data

High quality light curve for accurate parameters

Doppler confirmation

Orbital Period: 1.09 d

Transit duration: 2.6 hrs

Planet Mass: 1.41 MJupiter

Planet Radius: 1.79 RJupiter

Planet Temperature: 2516 K

Spectral Type of Host Star: F7 V

Mass: 60 MJupiter

Radius: 1 RJupiter

Teff: ~ 2800 K

Planet Mass: 1.41 MJupiter

Planet Radius: 1.79 RJupiter

Planet Temperature: 2516 K

Comparison of WASP 12 to an M8 Main Sequence Star

WASP 12 has a smaller mass, larger radius, and comparable effective temperature than an M8 dwarf. Its atmosphere should look like an M9 dwarf or L0 brown dwarf

Mass – Radius Relationship

Stars

Planets

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

TrEs 1b

TrEs 2b

P = 2.47 d

M = 1.28 MJupiter

R = 1.24 RJupiter

i = 83.9 deg

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

• 12 Planets discovered

Follow-up with larger telescope

HAT-P-1b

HAT-P-1b has an anomolously large radius for its mass

HAT-P-12b

Star = K4 V

Planet Period = 3.2 days

Planet Radius = 0.96 RJup

Planet Mass = 0.21 MJup (~MSat)

= 0.3 gm cm–3

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)

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 for M2 star ≈ 4.3 gm/cm3

Period = 2.87 d

Rp = 0.7 RJup

Mp = 0.36 MJup

Sato et al. 2005

Special Transits: HD 149026

Mean density = 2.8 gm/cm3

So what do all of these transiting planets tell us?

Solar System Object (gm cm–3)

Mercury 5.43

Venus 5.24

Earth 5.52

Mars 3.94

Jupiter 1.33

Saturn 0.70

Uranus 1.30

Neptune 1.76

Pluto 2

Moon 3.34

Carbonaceous Meteorites

2–3.5

Iron Meteorites 7–8

Comets 0.06-0.6

The density is the first indication of the internal structure of the exoplanet

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

Mass Radius Relationship

HAT-12b

CoRoT-3

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

HD 149026 b provides strong support for the core accretion theory

Rp = 0.7 RJup

Mp = 0.36 MJup

Mean density = 2.8 gm/cm3

10-13 Mearth core

GJ436b (transiting Neptune) has a density comparable to Uranus and Neptune and thus we expect similar structure

Take your favorite composition and calculate the mass-radius relationship

5

4

3

2

1

1 10 100M/ME

R/REIces

Silicates

Fe

75%

H–2

5 % H

e U

The type of planet you have (rock versus iron, etc) is driven largely by the radius determination. E.g.: a silicate planet can have a mass that varies by more than 20%. A 20% error in the radius determines whether the planet is a rock or a piece of iron. This is because density ~ M, but R–3

N

GJ 436

To get a better estimate of the composition of an exoplanet, you need to know the radius of the planet to within 5% or better

• You need accurate transit depths → you should probably do this from space, or lots of measurements from the ground

• You only determine Rplanet/Rstar. This means you need to know the radius of the star to within a few percent. For accurate stellar radii you must use

- Interferometry

- Asteroseismology

The best fitting model for HAT-P-12b has a core mass ≤ 10 Mearth and is still dominated by H/He (i.e. like Saturn and Jupiter and not like Uranus and Neptune. It is the lowest mass H/He dominated gas giant planet.

HAT-P-12b

Global Properties

Planet Radius

Most transiting planets tend to be inflated

Mass-Radius Relationship

Radius is roughly independent of mass, until you get to small planets (rocks)

All Planets (mostly from Doppler surveys). 37% stars have mass greater than 1 Msun

Transiting Planets:

45% Stars have mass greater that 1 Msun

Mass of the Host Star

However, probability of a transit is ~ Rstar/a, so this might be a selection effect.

Planet Mass Distribution

It is rare to find close-in planets that have high mass, but they do exist. Some with masses up to 20 MJup (CoRoT-3)

Astronomer‘s

Metals

More Metals !

Even more Metals !!

The Planet-Metallicity Connection

Bracket Fe/H notation : [Fe/H]

This is the ratio of metal abundance to the solar value. Often it is for Iron lines, but sometimes other heavy elements are included. Sometimes [M/H] is used for metals:

[Fe/H] is calculated by taking the iron abundance, divide it by the abundance of iron in the sun, and then taking the logarithm base 10.

[Fe/H] = 0 → solar abundance

[Fe/H] = +0.5 →3.16 × solar

[Fe/H] = –1.0 →0.1 × solar

Most metal rich stars have [Fe/H] = +0.5, most metal poor have [Fe/H] ≈ –3

These are stars with metallicity [Fe/H] ~ +0.3 – +0.5

There is believed to be a connection between metallicity and planet formation. Stars with higher metalicity tend to have a higher frequency of planets. This has been used as evidence in support of the core accretion theory

Valenti & Fischer

The Planet-Metallicity Connection?

Transiting Planets

All Planets (mostly from Doppler surveys)

Comparison of Metalicity

Metallicity of Giant Stars also show no metallicity effect

[Fe/H]

%

Red line: Main sequence (1 solar mass)

Blue: Giants (~1.4 solar mass)

Most planets around giant stars and transiting planets are for 1.2-1.4 MSun host stars. And there seems to be no metallicity effect.

Endl et al. 2007: HD 155358 two planets and..

…[Fe/H] = –0.68. This certainly muddles the metallicity-planet connection

Hyades stars have [Fe/H] = 0.2 and according to V&F relationship 10% of the stars should have giant planets, but none have been found in a sample of 100 stars

More problems with Planet – Metallicity Connection

• Early indications are that the host stars of transiting planets have different properties than non-transiting planets.

• Most likely explanation: Transit searches are not as biased as radial velocity searches. One looks for transits around all stars in a field, these are not pre-selected. The only bias comes with which ones are followed up with Doppler measurements

• Caveat: Transit searches are biased against smaller stars. i.e. the larger the star the higher probability that it transits

Spectroscopic Transits:

The Rossiter-McClaughlin Effect1

1

0

+v

–v

2

3

4

2 3 4

The R-M effect occurs in eclipsing systems when the companion crosses in front of the star. This creates a distortion in the normal radial velocity of the star. This occurs at point 2 in the orbit.

From Holger Lehmann

The Rossiter-McLaughlin Effect in an Eclipsing Binary

Curves show Radial Velocity after removing the binary orbital motion

The effect was discovered in 1924 independently by Rossiter and McClaughlin

The Rossiter-McLaughlin Effect or „Rotation Effect“

For rapidly rotating stars you can „see“ the planet in the spectral line

For stars whose spectral line profiles are dominated by rotational broadening there is a one to one mapping between location on the star and location in the line profile:

V = –Vrot V = +Vrot

V = 0

For slowly rotationg stars you do not see the distortion, but you measure a radial velocity displacement due to the distortion.

A „Doppler Image“ of a Planet

The Rossiter-McClaughlin Effect

–v +v

0

As the companion crosses the star the observed radial velocity goes from + to – (as the planet moves towards you the star is moving away). The companion covers part of the star that is rotating towards you. You see more possitive velocities from the receeding portion of the star) you thus see a displacement to + RV.

–v

+v

When the companion covers the receeding portion of the star, you see more negatve velocities of the star rotating towards you. You thus see a displacement to negative RV.

Amplitude of the R-M effect:

ARV = m s–

1

Note:

1. The Magnitude of the R-M effect depends on the radius of the planet and not its mass.

2. The R-M effect is proportional to the rotational velocity of the star. If the star has little rotation, it will not show a R-M effect.

rRJup

( )2

RRּס

( )–2Vs

5 km s–1 ( )ARV is amplitude after removal of orbital mostion

Vs is rotational velocity of star in km s–1

r is radius of planet in Jupiter radii

R is stellar radius in solar radii

The Rossiter-McClaughlin Effect

What can the RM effect tell you?

1. The inclination of „impact parameter“

–v +v

–v +v Shorter duration and smaller amplitude

The Rossiter-McClaughlin Effect

What can the RM effect tell you?

2. Is the companion orbit in the same direction as the rotation of the star?

–v +v

–v

+v

What can the RM effect tell you?

3. Are the spin axes aligned?

Orbital plane

TrES-1

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

What about misalignment of the spin axis?

Results from the Rossiter-McClaughlin Effect

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

On the Origin of the High Eccentricities

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!

Fabricky & Winn, 2009, ApJ, 696, 1230

XO-3-b

Hebrard et al. 2008

= 70 degrees

Winn et al. (2009) recent R-M measurements for X0-3

= 37 degrees

Possible inclination changes in TrEs-2

Evidence that transit duration has decreased by 3.2 minutes. This might be caused by inclination changes induced by a third body

The biggest effect should be for grazing transits

Transit Timing Variations (TTVs)

Changes in the expected times of transits are indications of other bodies in the system:

• Another (outer) planet that influences the center of mass motion of the star and perturbs the transiting planets orbit

• A moon that influences the planet‘s center of mass

Transit delayed from previous time

TTVs: you need lots of transits!

Bean et al. 2009

Currently there is no evidence for TTVs in transiting planets, probably due to the difficulty in measuring these: these are too small.

1. 59 Transiting planets have been discovered most from the ground.

2. Most have radii that are larger than Jupiter (inflated)

3. Smallest has radius of Neptune (wait for CoRoT talks)

4. Mean density gives us our first estimate of the internal structure

5. R-M effect can tell us about spin alignment. Most transiting planets have aligned orbits

6. Transit searches may have a less biased sample than radial velocity searches

Summary