Extrasolar planetsExtrasolar planets

Detection methodsDetection methods

1. Pulsar Timing

• Pulsars are rapidly rotating neutron stars, with extremely regular periods

• Anomalies in these periods indicate the gravitational influence of a companion.

Detection methodsDetection methods

2. Astrometry

• the oldest method used in the search for extrasolar planets, used as early as 1943.

• Involves measuring the proper motion of a star in the search for an influence caused by its planets

• changes in proper motion are so small that the best current equipment cannot produce reliable enough measurements.

• This method requires that the planets' orbits be nearly perpendicular to our line of sight, and so planets detected by it could not be confirmed by other methods.

3. Radial motions: the Doppler shift3. Radial motions: the Doppler shift

Recall the Doppler shift of the wavelength of light due to the velocity of the source:

1 if

1)1(

1)1(

/1

/1

2

2

zz

z

z

c

v

z

cv

cv

r

restrest

restobs

r

rrestobs

Spectroscopic binariesSpectroscopic binaries

Single-line spectroscopic binary: the absorption lines are redshifted or blueshifted as the star moves in its orbit

Double-line spectroscopic binary: two sets of lines are visible

Java applet: http://instruct1.cit.cornell.edu/courses/astro101/java/binary/binary.htm

Spectroscopic binaries: circular orbitsSpectroscopic binaries: circular orbits

• If the orbit is in the plane of the sky (i=0) we observe no radial velocity.• Otherwise (if the orbit is inclined at an angle i relative to the plane of the

sky) the radial velocities are a sinusoidal function of time. The minimum and maximum velocities (about the centre of mass velocity) are given by

ivv

ivv

r

r

sin

sin

2max2

1max1

Extrasolar Planet searchesExtrasolar Planet searches

• A planet orbiting a distant star will behave like a single-lined binary system. In principle we can determine the mass from the Doppler shift of the star.

• If the star can be accurately classified (i.e. with a good spectral classification and a parallax distance) we can determine its mass independently of the orbit.

3/13/2max

3max3

2

3

2sin

2sin

G

Pmvim

G

vPi

mm

m

StarStarPlanet

Star

PlanetStar

Planet

Since the mass of the planet is generally much less than that of the star

Assuming circular orbits,

Extrasolar Planet searchesExtrasolar Planet searches

3/13/2max

2sin

G

Pmvim StarStarPlanet

E.g. the star HD73256:

From Hipparcos data (and detailed stellar modelling) we know Mstar~1.05 Msun

• From the light curve we measure P=2.54858 days and vmax=269.8 m/s.• Sinusoidal shape means e~0

So the planet is very massive (1.86 MJ) and very close to the star (0.037 AU: a tenth as big as Mercury’s semimajor axis of 0.3871 A.U.)

Why?

Extrasolar Planet searchesExtrasolar Planet searches

JupiterPlanet Mim 86.1sin

..037.0 UAa

The maximum velocity shift is only ~270 m/s. The Doppler shift is therefore:

7109

c

v

which is very small. For example the H line is redshifted by only 0.00059 nm! The spectral resolution must therefore be very high. Detecting smaller planets, farther away from the star, is an even more difficult task.

BreakBreak

Detection methodsDetection methods

4. Gravitational microlensing This effect occurs when the gravitational field of a planet and its

parent star act to magnify the light of a distant background star. The key advantage of gravitational microlensing is that it allows

low mass (i.e. Earth-mass) planets to be detected using available technology.

A notable disadvantage is that the lensing cannot be repeated because the chance alignment never occurs again.

5. Transit methods• Detects a planet's

shadow when it transits in front of its host star.

• Can be used to measure the radius of a planet.

TransitsTransits

•Imagine viewing the Earth-Sun system from a distant star. By how much will the Sun fade during a transit of the Earth? How about during a transit of Jupiter?

6. Circumstellar disks6. Circumstellar disks

Young main sequence stars often still have disks, even after the molecular cloud has been dispersed.

Infrared-emitting dust disk around -Pic. The central star has been subtracted.

The dust disk around Vega. At least one large planet is known to exist within this disk.

Circumstellar DisksCircumstellar Disks

7. Direct detection7. Direct detection

Infrared image of the star GQ Lupi orbited by a massive, young (therefore warm) planet at a distance of approximately 20 times the distance between Jupiter and our Sun.

2005 image of 2M1207 (blue) and its planetary companion, one of the first exoplanets to be directly imaged

7. Direct Detection7. Direct Detection

The albedo of the Earth is about AV=0.4. How bright is it in visible (reflected) light, relative to the Sun? How do they compare at infrared wavelengths, where Earth emits thermal radiation?

A picture of Earth, from the surface of Mars, just before sunrise.

HD 209458b was the first transiting planet discovered, the first extrasolar planet known to have an atmosphere, the first extrasolar planet observed to have an evaporating hydrogen atmosphere, and the first extrasolar planet found to have an atmosphere containing oxygen and carbon.

Extrasolar planet searchesExtrasolar planet searches

As of December 2005, 170 planets have been detected outside our solar system (in 146 systems).

See http://exoplanets.org/

Most of these have a<1 AU and masses >MJupiter

19

Future missions

Keck Interferometer

Spitzer Space Telescope

SIM PlanetQuest

KeplerLarge Binocular Telescope Interferometer

Terrestrial Planet Finders

Space Interferometry missionSpace Interferometry mission

http://planetquest.jpl.nasa.gov/SIM/sim_index.cfm

•Interferometer with 9m baseline•5 year mission; estimated launch in 2010•will determine the positions of stars several hundred times

more accurately than anything previously possible•Will search for terrestrial planets around the nearest ~250

stars, with astrometry accurate to 1 as.

KeplerKepler

•http://www.kepler.arc.nasa.gov/

•NASA mission to hunt for planets using a one-meter diameter telescope photometer to measure the small changes in brightness caused by transiting planets.

•Transits by terrestrial planets produce a periodic change in a star's brightness of about 1/10,000, lasting for 2 to 16 hours.

•Scheduled to launch 2008, 4 year mission

• sensitivity limits of radial velocity surveys, astrometric surveys, microlensing surveys, and space-based transit techniques.

• The shaded areas show the expected progress towards the detection of Earth-like planets by 2006 and 2010.

• The filled circles indicate the planets found by radial velocity surveys (blue), transit surveys (red), and microlensing surveys (yellow).

• The discovered extrasolar planets shown in this plot represent the reported findings up until 31 August 2004.

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