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Extrasolar Planets = Exoplanets III
http://www.astro.keele.ac.uk/~rdj/planets/images/TauGruisHydra2.jpg
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
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 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