Exoplanets: Direct Search Methods 31 March 2016 © 2014 Pearson Education, Inc.

KEY IDEAS Planets around other stars are called Extrasolar planets (ESPs), or exoplanets They are common First direct evidence of an exoplanet was found in 1995 around 51 Pegasi Nearly 2000 ESPs have been found since then

ESP Detection Methods Direct methods: –Detection of visible or IR light from planet. Indirect methods: –Astrometry. –Doppler shift. –Transit. } to detect star’s “wobble”

© 2014 Pearson Education, Inc. Chapter Opener Images of 4 planets

Model spectrum of the sun and planets as seen from a distance comparable to that of a nearby star (10 pc), shown in physical units. Simple Planck emission and wavelength- independent albedo reflectance components are shown. For Earth, a pure molecular absorption spectrum is superimposed for reference. (DesMarais et al. 2002)

Detecting Extrasolar Planets Cannot yet use telescopes to take visible pictures of ESPs: –Planets are too close to their stars to image, even with the best telescopes. –Most planets are too dim to image in visible light: Sun-like star is ~10 9 x brighter than its planets. –Recent attempts to “null” out the starlight. In IR light, star is 10 6 x brighter than planet dwarf star, not planet

© 2014 Pearson Education, Inc. Unnumbered Figure Page 368 Another success for Newton’s Laws

Stellar "Wobble" A big planet gravitationally tugs on its star. A star and planet actually orbit around the center of mass of the two. The visible star appears to wobble slightly (“reflex motion”).

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Wobble of Our Sun Sun is tugged by the planets (mostly Jupiter & Saturn). Astrometry (measurement of star positions) is not precise enough to detect most stellar wobble ” (Hubble ≈ 0.1”) Sun from 30 light-years

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Stellar Wobble A big planet gravitationally tugs on its star. A star and planet actually orbit around the center of mass of the two. The visible star appears to wobble slightly (“reflex motion”).

Detecting Stellar "Wobble" Stellar wobble can be detected by observing the Doppler shift of light from the star. Doppler shift: Light appears bluer when star moves toward us, and redder when star moves away. Works best for edge- on orbits; does not work for face-on orbits.

The Doppler Shift Doppler shift.Doppler shift Sound waves.Sound waves

The Doppler Shift What we actually observe: –Absorption lines in star’s spectrum are shifted toward the red or blue.

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© 2014 Pearson Education, Inc. Kepler Spacecraft

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Kepler After 150,000 stars and 4 years of observation

Kepler (all planets) After 150,000 stars and 4 years of observation

Kepler (in “Habitable Zone”)

Kepler (extrapolation of “Earth-like planets”)

in our galaxy !

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© 2014 Pearson Education, Inc. Kepler 186f is like Earth

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There is a well-established correlation between a star’s metallicity and its likelihood to host hot Jupiters (Santos et al., 2000; Fischer & Valenti, 2005)

S. Seager (MIT)

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Summary: Direct Methods In principle, it is possible to see exoplanets by the light they reflect, but the brightness difference is too large: like a moth in a searchlight. Only a few cases found… Infra-red light (IR) is more promising, because the planets are only a million times dimmer Shades, coronagraphs, adaptive optics, interferometry improve this

Summary: Indirect Methods A planet causes the star to execute a small orbit, that can be detected by – measuring the star position (astrometry) or –by measuring its speed (Doppler spectroscopy of radial velocity A planet can be detected by occultation (transit method) Gravitational microlensing causes a background star to fade and brighten Each method has advantages for detecting some planets © 2014 Pearson Education, Inc.

Figure 13.17