Extrasolar Planets = Exoplanets III.

Transcription

1 Extrasolar Planets = Exoplanets III

2 Outline Gravitational microlensing Direct detection Exoplanet atmospheres

3 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

4 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

5 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

6 Future Mission The Wide Field Infrared Survey Telescope (WFIRST) Wide-field nearinfrared imaging 2025???

7 WFIRST: Exoplanets via grav. lensing 2.4m mirror HST-like: 2.4-meter mirror but with a much wider field of view

8 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?

9 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)

10 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)

11 Direct Detection of Extrasolar Planets November 13, 2008: First exoplanet observed directly in visible light! Using Hubble Space Telescope Fomalhaut b: Jupiter masses ~115 A.U. from star ~18 A.U. closer than the debris disk

12 Direct Detection of Extrasolar Planets Fomalhaut b: Jupiter masses ~115 A.U. (18 A.U. closer than the debris disk) 872 yr orbital period!

13 Direct Detection from the Ground Adaptive optics: correct for the blurring effects of the Earth s atmosphere!

14 Direct Detection from the Ground Adaptive optics: correct for the blurring effects of the Earth s atmosphere!

15 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 10 7 ) as close as arcsecond Stay tuned!

16 Future: On the Ground Extremely Large Telescopes (ELTs) Diameter: meters! Using adaptive optics to deblur images and coronograph to block light from the bright stars 2020?

17 Future: In Space The James Webb Space Telescope HST on steroids: 6.5 meters (JWST) 2.4 meters (HST) 2018 launch

18 Zodiacal light will be a significant source of interference for imaging Earth-like planets A Pale Blue Dot: Earth seen by Voyager 1

19 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

20 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

21 Example: Spectrum of Extrasolar Planet

22 Venus Earth What should we be looking for? Comparison of atmospheric spectra in our Solar System Mars

23 Measuring Earth s transit absorption spectrum using a lunar eclipse

24 The transit spectrum of Earth Actual molecular oxygen (as opposed to ozone) has absorption in the visible and near-infrared

25 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 O 2 (or O 3 ). Without life, Earth s oxygen would be gone due to chemical reactions.

26 Plant Diagnostic? Living plants reflect brightly in near-infrared Needed to release heat Would unusual infrared brightness be a diagnostic of plants?

27 The red edge is a spectral biomarker for vegetation

28 The reflection spectrum (Earth as seen by the Messenger spacecraft) visible light near-infrared light

29 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

30 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)

31 Simulated detection of molecular oxygen in a transiting planet, using a 100-meter flux-collecting telescope à Detected! Snellen et al. 2013

32 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

33 Question Name two potential biomarkers in exoplanet atmospheres