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69 Transiting planets

UCF 2010, G. Campanella

• Transit depth

• Total Transit Duration

Impact Parameter

• Ingress Duration

• We can calculate r, a, i.

• RV follow up observations

the sin i indetermination for

the mass disappears

2 2

1 cosT

PR r at i

a R R

cosa

b iR

21T

rt t b

R

2

off on

off

F F rF

F R

3Seager & Mallén-Ornelas 2002

Transit hunters

COROT (CNES/ESA)

launch Dec. 2006

Kepler (NASA)

launch Mar. 20094UCF 2010, G. Campanella

PLANET Transit depth (%)

HD 209458b 1.5

Earth-like 0.01

• A smaller, natural satellite that orbits an extrasolarplanet.

• There are no known exomoons, but their existence is theorized around many exoplanets.

Motivation Detection Methods Detectability with KCP


1. A novel detection and proof of principle.

2. Exomoons are likely to be < MEARTH and rocky.

3. Complex life may not form on exoplanets without large moons.

Lathe 2005

Waltham 2004Ward & Brownlee 2000Laskar et al. 1993






4. There may be more habitable exomoons than exoplanets.

5. Implications for planetary formation theory.

7



• Highly challenging!

• Transit timing variations (Sartoretti & Schneider 1999)

• Microlensing (Han et al. 2002)

• Planet-moon eclipses (Cabrera & Schneider 2007)

• Lightcurve distortions (Simon et al. 2007)

• Pulsar timing (Lewis et al. 2008)

• Transit duration variations (Kipping 2009)

• Rossiter-McClaughlin effect distortions (Simon et al. 2009, Szabo et al. 2009)



9



10



11



12

• Caused by the apparent motion of the planet along the projected diameter of its orbit around the barycentre of the planet-satellite system.

TTV is a positional effect, "astrometry"-like detection

Sartoretti & Schneider 1999


• TTV can also be caused by:

- gravitational influence of other planets,

- general relativistic precession of the orbit,

- tidal deformations of both the star and the planet.

• The period of an exomoon must always be much smaller than the period of the host exoplanet

TTV frequency > Nyquist frequency => harmonics

• TTV MMoon aMoon

• 1 measureable, 2 unknowns => Can’t solve!13



14

Kipping 2009

• TDV is a velocity effect (velocity vector of the planet and of the barycentreon opposite direction, the transit will last longer)

"radial velocity"-like detection


• TTV MMoon aMoon

• TTV and TDV are 90°out-of-phase.

• TTV and TDV provide the mass and orbital radius of the exomoon uniquely.

• TTV TDV 1s -150 seconds.

1 2

Moon MoonTDV M a

15


16

• Kepler is the most precise photometer

currently available: 20ppm/hour

• But the ground is catching

up fast!


For the purposes of TTV and TDV measurements we need equations for:

• with photon collection rate

• Uncertainty on transit depth

• Uncertainty on transit duration

• Uncertainty on mid-transit time

17

Exomoons detection

Carter et al. 2008

1 1d

ph

W dT

1 2T W T

1 2ct

W T

phW d T 8 1 0.4( 12)6.3 10 10 m

ph hr

In order to account for red noise, we modify W to W′:

18

The total noise

phW d T 2 21 1 1

' ph

I SW d T


Test of the modified formulation:

• Generate limb darkened lightcurves for a Neptune, Saturn and Jupiter-like planet in the habitable zone of a mKep = 12, G2V Sun-like star with i = 90°and e = 0.

• Generate 30,000 noisy lightcurves. The correlated noises and photon noise are randomly generated in all cases.

• Fit the noisy lightcurves.

• Obtain 10,000 estimates of T and tc in each case.

19

Lightcurve simulations

results for the Saturn-case T values

Jupiters vs Saturns vs Neptunes

20

• What is the optimum planet to search for moons around?

• Simulate the detectability of the TTV signal:

Calculate the TTV signal amplitude and mid-transit time uncertainty as a function of planetary orbital period

find the value for each case.

• Low-density planets offer largest SNR.

MS = 0.2M⊕

aS = 0.4895 RH

G2V star, mKep = 12

i = 90°

2

ξ-mKep parameter space

21

• Which exomoons are detectable?

• Consider an exomoon in the condition of a Saturn hosting a single satellite in the habitable zone of the host star.

• Acceptance criteria: 1) TTV confidence is ≥ 3-σ and TDV confidence is ≥ 8-σ

2) TTV confidence is ≥ 8-σ and TDV confidence is ≥ 3-σ

M0V star

MS = 1/3 M⊕

1 2

Moon MoonTDV M a

TTV MMoon aMoon

i = 90°

3

*

2( )

habhab

P S

aP

G M M M

*haba L L AU

1 3

2 PRoche P

S

r R

1 3

*3

PHill P

Mr a

M

Minimum detectable hab exomoons

22

• Assume the probability distribution of exomoons with respect to ξ is flat between ξmin and ξmax.

• Calculate the magnitude limits to detect 25%, 50% or 75% of the exomoons in the given sample, i.e. the quartile values.

• Determine the minimum detectable exomoon mass for various star types.

• With KCP habitable moons down to ~ 0.2 M⊕ are detectable.

• Consider a 0.2 M⊕ habitable moon with an M2V host star detected with Kepler. Expect RS = 0.65 R⊕ (Valencia et al. 2006).

• Collected multiple transit lightcurves of the planet-moon system.

• For each transit event subtract the planetary transit signal and then fold the residuals on the exomoon period.

• This produces a composite exomoon transit lightcurve:

( reducible to 3.2 ppm by observing 28 transits) RS can be estimated

• Transmission spectroscopy with JWST: rms scatter of 7.5 ppm per transit.

• Consider the moon with atmosphere (3 scale heights - 20km each)

transit depth of ∼ 146 ppm

• This difference (4 ppm) is detectable to 3-σ confidence by binning 32 transits together.

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What Next ?

142 17F ppm

F

Conclusions

• The search for exoplanets has imposed itself like one of the most dynamic research fields: 422 planets discovered in 14 years.• Attention has switched from finding planets to characterizing them.

• Exomoons may be detected through transit timing effects (TTV & TDV). • Our results highlight the promising opportunity of making the first exomoon detection using Kepler.• Saturn-like planets are the ideal host candidates for detection due to their large radius to mass ratio.• Kepler should be sensitive down to 0.2 M⊕ habitable exomoons.

• Transmission spectroscopy with JWST should be able to detect molecular species with 20-80 transit events, in the best cases (Type-M stars).

24UCF 2010, G. Campanella

422 Extrasolar planets

26

The lightest: Gliese 581 e (~ 1.9 M⊕)

PLANET Transit depth (%)

HD 209458b 1.5

Jupiter 1.01

Earth 0.0084



Simon et al. 2007

Sartoretti & Schneider 1999

Szabo et al. 2006

Kipping 2009





Valencia et al. 2006

Canup & Ward 2007

Belbruno & Gott 2005



1. A novel detection.

2. Exomoons are likely to be < MEARTH.


4. There may be more habitable exomoons than exoplanets.

Scharf 2008 Thommes et al. 2008



• Could we look for the dip in star light due to an exomoon’s shadow?

• Average position of moon results in lightcurves overlapping: indistinguishable.

• => Possible, but somewhat insensitive to low mass objects.

31

Cabrera & Schneider 2007


Kepler bandpass• A single broad bandpass in order to gather as many photons as

possible.

• Most of the throughput from Johnson V,R,I filters since we are not interested in early-type and active stars.

32

Lightcurve fitting• We fit for , i, the planet-star radii ratio k and tc:

1. The genetic algorithm PIKAIA gets close to a minimum in :

2. The solution is used as a starting point for a -minimisation performed with the AMOEBA routine:

transformations of the simplex (4 vertices) aimed to decrease the function values at its vertices

3. “prayer-bead” Monte Carlo simulation to obtain the parameter uncertainties:

33

*Pa R

2

assigned a fitness

crossovers & mutations

newgeneration

compared to observations

2

the set of residualsfrom the best-fit

solution is shifted by one data point and added to the

best-fit transit model

The new data set is fitted

purely random

set

global solution

Uncertaintiesestimated on

~ 2500 samples

Metcalfe & Charbonneau 2003

5.8 8 3.6

Star flux

At 3.6, 4.5, 5.8 and 8 mm,the planet shows different

transit depths: something is absorbing

in its atmosphere! ~ 0.09%

34

Submitted to MNRAS

G0 V star

P ~ 3.52 days

Mp ~ 0.69MJup

Rp ~ 1.4RJup


The Habitable Zone • Habitable Zone (HZ): the region around a star within which an Earth-like

planet can sustain liquid water on its surface.

• Continuously Habitable Zone (CHZ): the region that remains habitable for durations longer than 1 Gyr.

• Planets inside the HZ are not necessarily habitable.

• They can be too small, like Mars,

to maintain active geology and to

limit the gravitational escape of

their atmospheres.

• They can be too massive, like

HD69830d, and have accreted a

thick H2-He envelope below

which water cannot be liquid.

35

Biosignatures• We need to search for features that are specific to biological activities.

• The atmospheres of Mars and Venus are in chemical equilibrium, with an overwhelming abundance of carbon dioxide.

• A biosignature is a molecule which presence alters the chemical equilibrium of a planet atmosphere in a way that this disequilibrium cannot be explained solely by nonbiological processes (Lovelock 1960).

• The mid-IR spectrum of

Earth displays the 9.6 μm O3

band, the 15 μm CO2 band,

the 6.3 μm H2O band and

the H2O rotational band that

extends longward of 12 μm.

• To be able to characterize

terrestrial planet atmospheres

we have to wait for JWST (2014). 36

1 M⊕ Limit

38

• Compare the distance limit for detecting a 1 M⊕ habitable exomoon to the distance limit for a transiting 1 R⊕ habitable exoplanet.

• For moons the volume space is diminished by a factor of ∼4.

• With Kepler around 25,000 stars could be surveyed for habitable-zone exomoons.


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