Exoplanets: Indirect Search Methods 24 October 2016 © 2014 Pearson Education, Inc.

Nearly 4000 ESPs have been found since then 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 4000 ESPs have been found since then

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

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

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.

Detecting Extrasolar Planets dwarf star, not planet 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 ~109 x brighter than its planets. Recent attempts to “null” out the starlight. In IR light, star is 106 x brighter than planet

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

A big planet gravitationally tugs on its star. 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”).

Figure 13.1 © 2014 Pearson Education, Inc.

Sun is tugged by the planets (mostly Jupiter & Saturn). 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. 0.0002” Sun from 30 light-years (Hubble ≈ 0.1”)

Figure 13.3 © 2014 Pearson Education, Inc.

A big planet gravitationally tugs on its star. 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. Sound waves.

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

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Kepler Spacecraft © 2014 Pearson Education, Inc.

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Figure 13.9.1 © 2014 Pearson Education, Inc.

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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!

Figure 13.4b © 2014 Pearson Education, Inc.

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

© 2014 Pearson Education, Inc.

Hot Jupiters Giant planets near the star! Easy to find because their gravity pulls hard on the star and they orbit quickly Not like anything in our Solar System

There is a well-established correlation between a star’s metallicity and its likelihood to host hot Jupiters

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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) Each method has advantages for detecting some planets © 2014 Pearson Education, Inc.