Photometric Studies of Transiting Exoplanets

Norio Narita (NAOJ)

Transit of Venus on June 5, 2012 at Hawaii

Outline

• Introduction of Transit

• Current Status of Studies of Transiting Exoplanets

• Further Studies of Transiting Exoplanets

• How to Observe Transits by Youeself?

• Summary

What is Transit?

primary eclipse= transit

secondary eclipse

planetary orbit

How to Find Transiting Exoplanets

• We cannot spatially resolve a transiting exoplanet, but we can observe slight dimming due to transits

Methods to search for transiting exoplanets

1. Photometric follow-up of RV known planets

2. Transit survey and RV follow-up

The First Discovery of a Transiting Exoplanet

Charbonneau et al. (2000)Transits of “hot Jupiter” HD209458b

(Possible transit times were predicted by RV)

Transit Survey

One can search for periodic dimming from this kind of data

Some Characteristics of Transiting Planets

stellar radius ：

planetary radius :

Toward Earthsemi-major axis ：

orbital period ：

Transit Probability ：Transit Depth ：Transit Duration ：

～ Rs/a

～ (Rp/Rs)2

～ Rs P/a π

Science Importance of Transiting Planets

• One can learn precise size of planets

– One can also learn true mass and density of planets

when combined with RV

– The information is only available for transiting planets

• Also, transit observations enable us to infer

1. internal structure of planets

2. atmospheric composition of planets

3. orbital migration history (Talk by Hirano tomorrow)

What we can learn from transit light curve

stellar limb-darkeningplanet radius

ratio of planet/star size

stellar radius, orbital inclination, mid-transit time

Analytic transit light curve was modeled by Mandel & Agol (2002)

When combined with RVs

RVs provide minimum mass: Mp sin i

Transits provide planetary radius: Rp

orbital inclination: i

Combined information provides planetary mass: Mp

planetary density: ρ

Inferring Internal Structure of Planets

Charbonneau et al. (2009)

Solid line:H/He dominated

Dashed line:100% H2O

Dotted line:75% H2O, 22% Si, 3% Fe core

Dot-dashed line:Earth-like

Transits revealed varieties of planets’ structure

Transit Spectroscopy / Band Photometry

star

Transit depth depends on lines or bands

Inferring Atmospheric Composition of GJ1214b

Green: H dominated with solar metallicityRed: H dominated with sub-solar metallicity and cloud layer at 0.5 bar

Blue: Vapor dominated atmosphere

de Mooij et al. (2011)

Current Status of Studies of Transiting Exoplanets

The number of transiting planets is rapidly growing.

Dedicated Space Mission for Transiting Planets

CoRoTlaunched 2006/12/27

Keplerlaunched 2009/3/6

Kepler Field of View

TrES-2(Kepler-1)

HAT-P-7(Kepler-2)

HAT-P-11(Kepler-3)

Pre-Kepler Transiting Planets

<1.25 RE

First 4 Month Kepler Planet Candidates

1235 Planet Candidates

2326 Planet Candidates

54 candidates are in

possible habitable zone.

5 are terrestrial size.

Possible Habitable Planet Kepler-22b

(Sub-)Earth-sized Planets

Earth-sized planet Kepler-20f

Mars-sized planet KOI-961.03 (renamed as Kepler-42d)

Now Not a Science Fiction

Planetary Systems with Two Suns: Kepler-16, 34, 35, 47

→ 　 Tatooine-like (in Star Wars) planet

Summary of Current Status

• Various transiting exoplanets have been discovered– hot Jupiters– hot Nuptunes– super Earths– habitable planets– Tatooine-like planets

• We have learned that there are various interesting transiting exoplanets

• What’s next?

Kepler’s Weakness

• Kepler targets relatively faint and far stars

– Although over 1000 candidates discovered, RV follow-ups for all targets are difficult

– Further characterization studies are also difficult

Kepler is good for statistical studies, but not for detailed studies for each planet

Strategy of Future Transit Survey

• Future transit surveys will target nearby bright stars to detect terrestrial planets in habitable zone

• Space-based all-sky transit survey for bright stars– TESS (Transiting Exoplanet Survey Satellite) by MIT team

• Ground-based transit survey for nearby M dwarfs– MEarth lead by D. Charbonneau at Harvard

– Other teams all over the world

– Our IRD transit group (collaborating with UH etc)

What we would like to do

1. To search for new interesting transiting planets orbiting nearby bright stars

2. To characterize nature (internal structure, atmosphere, habitability) of those planets

• To do so, we need to observe transiting exoplanets with high precision!

How to Observe Transits by Yourself

• First you need to get observing time at your university or openuse facilities– not necessarily large telescope for bright targets– Let’s try submitting a proposal

• You can find information of transiting planets and transit ephemeris at– http://exoplanet.eu/catalog/ (Exoplanet.eu)– http://var2.astro.cz/ETD/ (Exoplanet Transit Database)

How to Achieve High Precision Transit Photometry

• High precision (~0.1% or ~1mmag) photometry was considered to be very difficult previously

• A good solution is1. to fix stellar position on the detector during observations

2. with image defocus

Example of Detector Image

Stars are defocused and kept off from bad pixels.

Field of View

~20 pix diameter

Example at Okayama in Japan (J band)

~1mmag is achieved for J~10 target (Fukui et al. in prep.)

Example at Okayama (Ks band)

~1mmag is achieved for Ks~10 target (Ohnuki et al. in prep.)

Example at South Africa / IRSF

~1 mmag is achieved for J < 10 target (Narita et al. 2013)

Example at Subaru Suprime-Cam (B band)

0.8 mmag is achieved for B = 16.4 target! (Narita et al. in prep.)

Let’s try transit observations

• Using the simple technique, we can observe exoplanetary transits with high precision1. to fix stellar position on the detector during observations

2. with image defocus

• If you are interested, you can submit a proposal in your country or other countries’ openuse time!

Summary

• Transits provide us useful information for properties of exoplanets

• Numbers of interesting transiting exoplanets have been discovered

• With the simple technique, you can achieve good precision for transit photometry

• So please try transit photometry by yourself!