Lecture 34: Searching for Other Worlds
Astronomy 101/103
Terry Herter, Cornell University
Course Home Page

Lecture
Topics
  • Where to look for exosolar planets
  • Techniques
    • Direct Detection
    • Stellar Wiggle
    • Doppler Spectroscopy
    • Brightness variations
  • Are Exosolar planets habitable?

Finding other
Planets?

Four Possible Methods

  1. Direct observation
    • Reflected light from star
    • Intrinsic infrared radiation
  2. Search for brightness variations
  3. Stellar Wiggle
  4. Doppler spectroscopy

Where to
Look?

Nearby stars are the best because:

  • Planets are brighter.
  • The angular separation between planets and the star is larger.
  • Stars are brighter
    • Doppler shifts easier to measure
    • Motions on the sky easier to measure

Nearby Stars
(d < 10 pc)


Spectral Class
Mass (Msun)
Ntotal
Nsingle
A
1.8-3.4
2
..
F
1.1-1.8
11
5
G
0.8-1.1
26
13
K
0.5-0.8
42
18
M
<0.5
210
63

 


Main-Seq. Stars
with mv < 10



Class
rmax (pc)
# of M-S Stars
M5
4.4
~5
K5
24.7
~750
G5
95
>11,000
F5
208
~45,000

  • rmax = maximum distance a star of this class can be seen if mv < 10.

Analogy to
Binary
Stars
  • These techniques are similar to those used to detect binary stars.


    Search Technique
    Binary Type
    Direct observation
    Visual
    Brightness variations
    Eclipsing
    Wiggle on sky
    Visual
    Doppler shifts
    Spectroscopic

Direct
Observation
  • Very difficult because the star is very bright and the planet faint.
  • Hubble Space Telescope can detect objects to mv ~ 29.
    • See Jupiter to ~30 pc (near G2 star) with a separation of only 0.16"!

Relative Energy
Distributions


IR: May Detect
Planets
Better
  • Planets are much cooler than the stars so their spectrum peaks in the infrared.
  • Why not look in the infrared instead of in the visual? It's difficult.
    • Angular resolution of telescopes not as good in the infrared.

    • Can see much fainter objects in the visual.

What is
Being
Done?
  • Interferometers - provide high spatial resolution.
  • Infrared measurements are much better when done from space.
  • Programs are being considered which combine the two.

 

Steller Reflex
Motion
Illustration
  • As the planet moves around the star, the star has a reflex motion.

 


Reflex
Motion
  • The reflex motion can cause
    • the star to wiggle on the sky.
    • the stellar spectrum to show periodic Doppler shifts.
  • These effects get larger as the planet to star mass ratio increases
  • Consequently, for easier detection of planets:
    • large planets better
    • less massive stars better

Stellar
Wiggle

  • Star normally drifts in a straight line in the sky.
  • But an unseen companion causes the star to "wiggle."
  • "Wiggle" due to stellar reflex motion.

Wiggle
Size
  • If the sun were at 10 pc, the wiggle would be:


    Planet
    Period (years)
    Wiggle Amplitude (arcsec)
    Jupiter
    11.9
    0.5 x 10-3
    Uranus
    84.0
    84 x 10-6


  • Less massive stars display a larger motion but will have longer periods.
  • 10-4 arcsec possible with Keck

Doppler
Spectroscopy
Illustration


Doppler
Spectroscopy

  • Reflex motion of the star results in a Doppler shift which is best seen edge on.
  • Examples (~5 m/sec might be measurable):

    Planet
    Period (years)
    Doppler Shift (m/sec)
    Earth
    1.0
    0.09
    Jupiter
    11.9
    13
    Uranus
    84.0
    0.3


Finding
Extrasolar
Planets
  • We can't see the planet, only the star.
  • Measure periodic reflex motion of the star (to the planet).
  • The star's projected velocity changes as the planet and the star orbit one another.
  • The star's motion is very small.
  • We see the velocity change through a change in the Doppler shift of the spectral lines.

Planet Orbiting
51 Pegasi !!
  • First Exosolar planet detected
  • G5 main-sequence star, 42 lyr away
  • Astronomers of Geneva Observatory found:
    • 56 m/sec Doppler shift, P = 4.2 days
  • Planet parameters
    • Orbital period is very short (4.23 days)
    • 11.2x106 km (0.05 AU) from the star
    • 1/2 the mass of Jupiter
    • T ~ 1300 K

Plot from exoplanets web site. Go to link "Almanac of Planets" and the 51 Peg link there.

Some
Extrasolar
Planets


Star Data
Planet Data
Star
Spec. Type
Stellar Distance (lyr)
Distance from star (AU)
Mass (Mjupiter)
Period
(days)
47 UMaj
G0V
44
2.1
2.4
1090
51 Peg
G4V
42
0.05
0.47
4.23
55 Can
G8V
44
0.11
0.84
14.8
Tau Bootes
F7V
50
0.05
3.8
3.31
Upsilon And
F8V
54
0.05
0.68
4.61
70 Vir
G5V
78
0.43
6.6
117
HD 114762
F9V
91
0.3
10
84.1
16 Cyg B
G3V
85
0.6-2.7
1.7
804

Data from Extrasolar Visions web site (see link under "Interesting Web Sites") and referring links therein.


Brightness
Variations

1. Transits: (need edge-on orbital plane)

    • Planets pass in front of the star.
    • Last a few hours, variation < 0.1%

Eclipsing planet observed! It was known about previously from Doppler Spectroscopy.

  • Star HD 209458 in Pegasus.
  • This is a G-type main-sequence star 47 pc away.
  • The planet is ~ 0.7 the mass of Jupiter and 1.4 times the size of Jupiter.

There are now a number of eclipsing planets that have been detected.

 

2. Gravitational Microlensing

    • Star amplifies light a background star
    • Planet modifies light curve
    • One time event, need statistical studies

Possible detection of an event, but controversial. (Cannot be observed again!)

Habitable?
  • In general, they are much more massive than the earth.
  • And not the correct temperature.

    Tplanet ~ L1/4 / d1/2

    L = luminosity of the star,
    d = planet-star distance

Habitability of
Exosolar
Planets
  • The plot below shows where the currently discovered exosolar planets are located relative to the habitable zone of the star.

This is rough plot made from data at the exoplanets web site and the Hipparcos catalog. The equivalent solar distance is computed assuming that the stellar luminosity scales as M3.3. The "error bar" in the x-direction indicates the variation in distance of the planet from the star (some orbits are highly elliptical). The habitable zone is roughly computed by scaling from the Earth's temperature. The location of the solar system planets are indicated by vertical lines at the bottom of the plot. Plot last update October 2005.

Conclusions
  • Many exosolar planets are now known
    • Over 100 and counting
    • Some systems have multiple planets
    • All discovered by Doppler spectroscopy
    • One is an eclipsing planet
  • Some of these planets lie in the habitable zone
    • But planets much more massive than the Earth
    • More like Jupiter than the Earth
    • Current capabilites cannot detect Earth-like planets around normal stars
Course Home Page