PTYS 214 – Spring 2011

Review Guide has been uploaded to class website

Review Session – Monday May 2nd? Thursday May 6th? – time??

Class website: http://www.lpl.arizona.edu/undergrad/classes/spring2011/Pierazzo_214/

Useful Reading: class website “Reading Material” http://en.wikipedia.org/wiki/Extrasolar_planet http://exoplanet.eu/catalog.php

http://www.solstation.com/stars/gl581.htm

Announcements

Planetary Sciences Graduate Teaching Assistant Excellence Award

Planetary Science Department initiative to promote, recognize and reward exemplary performance among graduate teaching

assistants assigned to PTYS undergraduate courses

If you think your PTYS-214 Teaching Assistant qualifies for the award, please fill out a nomination form describing:1) Why you are nominating the TA2) How the TA has contributed to your learning experience

Nomination forms and the drop box are located on the table outside your classroom, room 308

The Teaching Assistants for PTYS214-2 are:

Lissa Ong & Devin Schrader

Extrasolar Planets

Also called exoplanets, they are planets that orbit other

stars beyond our Sun

The existence of other solar systems has been suspected for centuries, but verified only

in the 1990s

Detecting Extra-Solar Planets

Problem #1: Planets are not bright objects!

Problem #2: Planets are relatively small and close to a bright star(DSun ~ 10 DJupiter ~ 100 DEarth)

Successful detection techniques: Stellar Radial Velocity, or Doppler Method Planetary Transit

Direct Imaging

Gravitational microlensing

Radial Velocity TechniqueMost exoplanets (>300) are detected

by radial velocity technique

Uses the Doppler Effect to measure changes in the radial velocity of a star caused by the small gravitational force of an unseen orbiting planet

….but it can only measure motion along the line of sight (edge on)

Allows to determine the planet’s mass plus shape and size of orbit

Radial Velocity Technique

Observe red-shifting and blue-shifting of the star’s spectral lines caused by Doppler effect

Amount of blue or red shift corresponds to the star’s radial velocity towards or away from us

Planetary Transit Technique

Measures dimming of star light as planet passes in front

of star

Star-light may dim by only 0.000001 (10-6)

Over 50 planets have been detected by the transit

technique

Planetary Transit Technique

Disadvantages: a) Bias towards large planets and in short period orbits b) False detections due to stellar variability c) Planet’s orbit must be seen edge-on from the observer point of view (so the planet passes in front of the star)

Advantages: a) Relatively cheap b) Can determine the size of the planet

Direct Imaging Direct detection of planets is extremely difficult

Rare cases when direct imaging can work are:− Planet is very large (considerably larger than Jupiter)− Planet is widely separated from its parent star − Planet is young (so that it is hot and emits intense infrared

radiation)

Few exoplanets are imaged directly

Image of a planet around GQ Lupi (early K-type star)

The planet is believed to be about twice the mass of Jupiter and to have an orbital radius of about 30 AU (similar to Neptune)

2005; European Southern Observ.

Gravitational Microlensing Use the gravitational effect of large objects that can bend light around them

If the object is a star with a planet, the planet can be detected by its effect on the microlensing

Less than 10 potential exoplanets detected by this technique

Hertzsprung-Russell

(HR) Diagram

Stellar Habitable Zone

Much less massive than Sun-like stars (G-type stars)Deeper transits, better RV detectionVery low luminosity Smaller HZs

Consider an M5 dwarf which has a luminosity L = 0.0055 x L(solar) [Recall L(Solar) = 3.84 x 1026 W]. At what distance (D) from the M5 dwarf would a planet receive the same total radiation flux of the Earth? [Recall S0 = 1370 W/m2]

Habitable Planets around M dwarf stars

So why looking at M dwarf stars?

More numerous than Sun-like stars– Constitute 20 of the 30 stars nearest to

Earth

Much longer life spans than Sun-like stars – Some can live for trillions of years!

Problems with M-dwarfsLimited Habitable Zone (too cold?)Tidal locking (only one face of planet facing star, becomes synchronously rotating)Stellar variability: lots of flares

HOWEVER: • A study by Joshi et al. 1997 has shown that atmospheric circulation should help reduce day-night temperature variations.• Heath et al. 1999 concluded that even during a flare, radiation received by a planet in the HZ of an M dwarf is comparable to that received by the Earth.

These planets could still be habitable!

Modeling the habitability of Earth-like exoplanets

Use atmospheric circulation models – General Circulation Models (GCMs)

GCMs for hot Jupiters are much less complicated than terrestrial GCMs

Why?Terrestrial GCMs must consider:

– Varying atmospheric compositions– Climate feedback systems!

MEarth Project Ground-based 2 year survey of

~2000 nearby M dwarfs with masses ~1/3 mass of Sun– Smaller M dwarfs — bigger transits,

less noise

Probability to find planets in HZ is only 10%, but the survey produced the discovery of super-Earth GJ1214b!

Observations from Space

ESA’s CoRoT (a French-European Space Agency mission) uses the transit method to detect planets around the size of Earth

It detected its first extrasolar planet in May 2007!

NASA’s Kepler Space Observatory was launched in March 2009 (similar to CoRoT)

Avoid distortion introduced by Earth’s atmosphere and provide for much more precise measurements

Advantage of avoiding Earth’s atmosphere

From the ground From space

SIM PlanetQuest will determine the positions of stars several hundred times more accurately

than anything previously possible, helping to pin-point Earth-sized planets

Currently under development, it will measure the wobble of stars against other stars in the background using optical interferometry (combining light from two or more telescopes)

NASA’s S(pace) I(nterferometry) M(ission) Lite

Other exoplanet searches

• HARPS (High Accuracy Radial Velocity Planet Searcher)

• TPF (Terrestrial Planet Finder)

• JWST (James Webb Space Telescope)

• Darwin

• TrES (Transatlantic Exoplanet Survey)

Detection Limits

Planet closest to Earth-size, discovered so far: − Planet around 2 Earth masses (Gliese 581e)

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How Do We Know if a Planet Can Support Life?

Look for evidence of oxygen

Look for liquid water

Analyze the reflected light from the planet to see if the planet has an atmosphere

Look for signs of biological activity (methane)

And Rule Out Other Explanations…

An Encouraging Experiment…

Galileo data inthe Near-IR

Simultaneous presence

of O2 or O3 and a reduced gas (CH4 or N2O) is the best

evidence for life

*Credit Joshua Lederburg and James Lovelock for the idea (1964)

Thermal IRspectra

Source:R. Hanel, Goddard Space Flight Center

Quiz Time !