Post on 29-Mar-2018
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