PTYS 214 – Spring 2011

Homework #10 DUE in class TODAY

Review Guide will be uploaded to class website soon

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

After April 25th, nomination forms and the drop box will be located on the table outside your classroom, room 308

The Teaching Assistants for PTYS214-2 are:

Lissa Ong & Devin Schrader

HW #9

Total Students: 24

Class Average: 8.2

Low: 3.5

High: 10

Homework is worth 30% of the grade

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

In 1995, a breakthrough:the first planet around another star

A Swiss team discovers a planet around 51 Pegasi – 50 light years from Earth

Artist's concept of an extrasolar planet (Greg Bacon, STScI) 7

Didier Queloz &Michel Mayor

And then the discoveries started rolling in:

“First new solar system discovered” USA TODAY - April 16, 1999

“10 More Planets Discovered” Washington Post - August 6, 2000

“New Planet Seen Outside Solar System”New York Times - April 19, 1996

You can even see some of the stars that have planets in the night sky…

…if you know where to look

Planet of 70 Virginis

=7.42 Jupiter masses

Planet of Tau Bootes

=4.41 Jupiter masses

Just how far are these new planets?

from Mars…it would take3-22 minutes

from the nearest extrasolar planet…

it would takeover ten years!

from the Moon… it would take<2 seconds

IF YOU WANTED TO RADIO HOME

FOR YOUR WORDS TO REACH EARTH

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

Most extrasolar planets orbit very close to their star—“hot Jupiters”

As of today (4-21-11)~544 mostly Jupiter-sized planets (~300 Earth masses) have been discovered(http://exoplanet.eu/catalog.php)

Radial Velocity TechniqueMost exoplanets (~499) 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)

About 127 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 ~21 candidates

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

About 12 potential exoplanets detected by this technique

Several extrasolar planets are Super-Earths, 1-10 times more massive than Earth

The smallest extrasolar planet discovered so far (in 2009) is Gliese 581e, estimated to be about 2 Earth masses

About 20 percent of extrasolar planets are within the Habitable Zone of their parent stars

Extrasolar Planets: discovered so far

Extrasolar Planets: prediction

Most Earth-like: Gliese 581d

Gliese 581 is a red dwarf star (about 1/3 the Sun’s mass),

located 20.3 light years from Earth

Planet Mass Distance(AU)

Orbit (days) Eccentr.

e 2ME

0.03 3.15 0

b 16ME

0.04 5.37 0

c 5ME

0.07 12.9 0.17

d 7ME

0.22 66.8 0.38

Venus-like?

Potential for life?

Gliese 581d animation

• http://www.youtube.com/watch?v=wJXSSYyIVqw

• http://www.youtube.com/watch?v=_kcquVBYbGw&feature=related

Prospects for finding habitable planets

Best candidates are F, G, and early K-type stars, i.e., stars not too different from the Sun

Early-type stars (blue stars)– High UV fluxes– Short life (not enough time for life to evolve)

Late-type stars (red dwarfs) – M-class, M dwarfs– Targets for many habitable, earth-sized planet

searches– Examples: GJ 436b, Gl 581c, OGLE-2005-BLG-

390L

Hertzsprung-Russell

(HR) Diagram

Activity

Searching for Habitable Planets

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!