Extrasolar Planets = Exoplanets III

http://www.astro.keele.ac.uk/~rdj/planets/images/TauGruisHydra2.jpg

Outline •  Gravitational microlensing •  Direct detection •  Exoplanet atmospheres

Detecting planets by microlensing: gravity bends starlight

Newton predicted that gravity bends starlight Einstein improved the theory and predicted twice the bend It was confirmed during a total eclipse of the Sun in 1919 by Arthur S. Eddington

One star + its planet(s) can bend and focus the light from a background star

Trick: Monitor many stars at once until an event is discovered

Example: Detection of a Neptune-mass planet!

Good: Lower mass planets produce brightness blips that are just as bright as giant planets, only shorter in duration Good: Also, can see planetary systems very far away

Future Mission

•  The Wide Field Infrared Survey Telescope (WFIRST)

•  Wide-field near-infrared imaging

•  2025???

http://www.thetechherald.com/media/images/201033/WFIRST_1.jpg

WFIRST: Exoplanets via grav. lensing

2.4m mirror

HST-like: 2.4-meter mirror but with a much wider field of view

Reminder: Direct Detection of Extrasolar Planets

•  First thought: just get a picture!

•  But angle is tiny and star is much brighter

•  Image at right: “star” is brown dwarf, 100,000 times dimmer than Sun. A planet would be even dimmer.

•  How much dimmer?

Contrast (Earth-to-Sun) vs. Wavelength

imaging at this contrast is extraordinarily difficult (= expensive)

Contrast is the term for the difference in brightness It’s somewhat more favorable in the Infrared (> 1 µm)

Direct Detection of Extrasolar Planets

•  A few tricks to make it easier: –  Observe from space where the Earth’s atmosphere

doesn’t blur the image (but space telescopes are expensive…)

–  Otherwise, try to correct for the blurring effect of the Earth’s atmosphere (using adaptive optics)

–  Block out the light from the bright star (using a coronograph)

Direct Detection of Extrasolar Planets

•  November 13, 2008: –  First exoplanet observed directly in visible light! –  Using Hubble Space Telescope

•  Fomalhaut b: –  0.5-2 Jupiter masses –  ~115 A.U. from star –  ~18 A.U. closer than

the debris disk

Direct Detection of Extrasolar Planets

•  Fomalhaut b: –  0.5-2 Jupiter masses –  ~115 A.U. (18 A.U.

closer than the debris disk)

–  872 yr orbital period!

Direct Detection from the Ground •  Adaptive optics: correct for the

blurring effects of the Earth’s atmosphere!

http://www.naoj.org/Pressrelease/2006/11/20/fig3_e.jpg http://www.ucolick.org/~max/max-web/images/History_AO_Max_img_6.jpg

Direct Detection from the Ground •  Adaptive optics: correct for the blurring effects of the

Earth’s atmosphere!

http://exoplanet.as.arizona.edu/~lclose/talks/ins/OFF_ON_nicepic_cap_gg.jpg

Direct Detection from the Ground •  Gemini 8-meter Telescopes •  Gemini Planet Imager (commissioned in 2014!) •  Uses (1) adaptive optics to deblur the images and

(2) a state-of-the-art coronograph to block the light from the star (by a factor of 107) as close as 0.2-1 arcsecond

•  Stay tuned…!

Future: On the Ground •  Extremely Large Telescopes (ELTs) •  Diameter: 20-40 meters! •  Using adaptive optics to deblur images and

coronograph to block light from the bright stars •  2020?

http://2.bp.blogspot.com/_eiXUzxkg3oI/S9cCpR27JlI/AAAAAAAAAIQ/1i3Dbh2CnwI/s1600/ELT.png

Future: In Space

•  The James Webb Space Telescope

•  HST on steroids: 6.5 meters (JWST) 2.4 meters (HST)

•  2018 launch

http://upload.wikimedia.org/wikipedia/commons/8/88/JWST.jpg

Zodiacal light will be a significant source of interference for imaging Earth-like planets…

A Pale Blue Dot: Earth seen by Voyager 1

Prospects for Spectra of Atmospheres?

•  If we saw oxygen, it might indicate life •  Same would be true for any molecules that

are short-lived in atmosphere without life to sustain them

•  Maybe, but very difficult at present •  MEarth and TESS should provide good

targets (smaller stars) for this type of study

What it takes to measure an exoplanet’s atmosphere using transit absorption

- Extended atmosphere à low average molecular weight helps (e.g. primitive atmospheres containing molecular hydrogen) - clear atmosphere – not totally cloudy - high precision measurements…

Example: Spectrum of Extrasolar Planet

http://www.scitizen.com/stories/Extrasolar-Planets/2006/01/Methods-to-search-for-extrasolar-planets/Hubble.gif

What should we be looking for? Comparison of atmospheric spectra in our Solar System

Venus

Earth

Mars

Measuring Earth’s transit absorption spectrum using a lunar eclipse

Actual molecular oxygen (as opposed to ozone) has absorption in the visible and near-infrared

The transit spectrum of Earth

Biomarkers: can we tell if a planet hosts life from its spectrum?

The classic biomarker is the presence of a highly reactive gas such as O2 – (or O3). Without life, Earth’s oxygen would be gone due to chemical reactions.

Plant Diagnostic?

•  Living plants reflect brightly in near-infrared Needed to release heat

•  Would unusual infrared brightness be a diagnostic of plants?

http://www.nelsontan.com/reviews/eir/ir7.jpg

The ‘red edge’ is a spectral biomarker for vegetation

The reflection spectrum (Earth as seen by the Messenger spacecraft)

visible light near-infrared light

Future: detection of molecular oxygen in a transiting planet?? Extremely Large Telescopes (ELTs) (both by Europe and the US in about 2020)

30-meter diameter

Large light-collecting gives high sensitivity to detect weak signal in transit

Dream: The “flux collector” telescope (it does not make extremely sharp images, it just collects a lot of light, so it’s cheaper to make)

Simulated detection of molecular oxygen in a transiting planet, using a 100-meter flux-collecting telescope à Detected!

Snellen et al. 2013

Summary •  Microlensing and direct detections are quickly

adding new exoplanets •  The spectroscopy of exoplanet atmospheres is still

in its infancy •  MEarth and eventually TESS will discover planets

around smaller stars that will be excellent targets for spectroscopy.

•  High-quality spectra of biomarkers in exoplanet atmospheres will require ELTs and beyond

Question

•  Name two potential biomarkers in exoplanet atmospheres