Astronomy190 - Topics in Astronomy

Astronomy and Astrobiology

Lecture 19 : Extrasolar Planets

Ty Robinson

Questions of the Day

• What techniques are used to study extrasolar planets?

• What is the difference between direct and indirect detection of a planet?

• What are the known extrasolar planets like?

A Multitude of Worlds• 365 Planets• 308 Planetary

Systems• 39 Multiple Planet

Systems

• Most are Jovians• 10 SuperEarths (2-

10 Earth masses)

Not bad for not being able to see anything!

Indirect Detection of Extrasolar Planets

These techniques use changes in the position or brightness of a star to infer the existence of a planet

Planet and Star orbit around a common center of mass. This can make the star appear to ‘dance’, even when the planet can’t be seen.

The Doppler Technique

http://planetquest.jpl.nasa.gov

Detection of Radial Velocity via Spectral Shift

• Radial velocity changes in the star being observed are detected via the shift in the wavelengths of known absorption features in the spectrum of the star. Redshifted if the source is moving away from the observer, blueshifted if the source is moving towards the observer.

Obs

erve

r se

es B

lues

hift

Obs

erve

r se

es R

edsh

ift

Na @ 589.3nm

Na @ 591.5nm

Source not moving

Source redshifted

What We Can Observe

• If there is no planet, then there is no strong, periodic change in a star’s apparent radial velocity.

• Stars with planets show periodic changes in RV

No Planet: RV changes smallAnd random

Planet: Periodic RV changes

Period and Orbital Eccentricity

Period

Period

The Radial Velocity (Doppler) Technique

• uses spectroscopic measurements of starlight to detect the redshift or blueshift from the radial velocity component of a star’s motion

• most successful technique to date• most sensitive to big planets close to their star• can be used to infer planetary mass, albeit with a sine i

ambiguity due to the (usually) unknown inclination of the planetary system to the observer’s line of sight

• can determine planetary period and eccentricity

Transit

Change in the Star’s Brightness

From http://astro.u-strasbg.fr/goutelas/g2005/chap03-moutou.pdf

The Kepler Mission

Transit Telescope

T. Brown and D. Charbonneau

Measures stellar brightness changes caused by transiting terrestrial planets. Monitoring 100,000 stars for 4 years!

Launched 2009

Transit Technique

• detects the faint drop in a star’s brightness due to a planet passing across the face of the star

• sensitive to a large planet/star area ratio (so more sensitive to large planets overall, but has improved sensitivity to smaller planets around smaller stars)

• more sensitive to planets close to their star because these are more likely to be seen in transit

• can be used to determine planetary size, if the star’s size can be estimated (transit does not give mass!)

• provides planetary orbital period and distance• limitation: the probability of a transit is low for any given

planetary system, so transits are rare

Direct Detection of Extrasolar Planets

• Directly detects photons from the planet itself. • We can either suppress the light from the parent star• Or cleverly subtract the star’s light

106109

Direct Detection • allows us to directly

study a planet • Extremely difficult!

– star-planet contrast is large

– star and planet often can’t be separated on the sky (angular resolution)

• star-planet contrast is typically less in the infrared

Removing the Light from the Parent Star

• direct detection techniques also require some means to separate or remove the light from the parent star so that the light from the smaller, fainter planet can be seen– subtraction techniques: light from both

planet and star, and star alone, are measured and subtracted to reveal the planet’s radiation

– suppression or cancellation techniques: light from the parent star is cancelled (nulled) or blocked out (suppressed) using a special type of instrument

Extrasolar Planet Properties: Mass

• Rise towards lower masses (smallest are 3.5Mearth)

• In this sample, only 13/138 (10%) have Mp > 5Mj

Hot Jupiters

very easy to detect using transit, radial velocity techniquesform beyond frost line, migrate inwardshabitable moons?

Questions of the Day

• What techniques are used to study extrasolar planets?

• What is the difference between direct and indirect detection of a planet?

• What are the known extrasolar planets like?

Quiz

3 - What is one thing you did not understand from today’s lecture?

2 - Describe how “Hot Jupiters” form.

1 - Describe one way that we constrain the inner edge of the HZ around the Sun and one way that we constrain the outer edge.

Challenges: Angular Separation from Parent Star

• angular resolution improves with shorter wavelength and/or bigger optics (lens or mirror)