Extrasolar planetsExtrasolar planets

Detection methodsDetection methods

1. Pulsar timing2. Astrometric wobble3. Radial velocities4. Gravitational lensing5. Transits6. Dust disks7. Direct detection

1. Pulsar timing1. Pulsar timing

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

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

Astrometric wobbleAstrometric wobble

• 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:

restrest

restobsr

c

v

Spectroscopic binariesSpectroscopic binaries

The absorption lines are redshifted or blueshifted as the star moves in its orbit

Spectroscopic binaries: circular orbitsSpectroscopic binaries: circular orbits

• 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

• Where i is the angle of inclination.

Radial velocities: mass measurementRadial velocities: mass measurement

• 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

2sin

G

Pmvim StarStarPlanet

Radial velocities: mass measurementRadial velocities: mass measurement

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

What is the mass of the planet, and the size of the orbit?

Radial velocities: difficultyRadial velocities: difficulty

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

Gravitational microlensingGravitational 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.

TransitsTransits•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?

Circumstellar disksCircumstellar 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

• Orbiting planets can clear gaps in the dust disk This leads to a loss of orbital energy, so the planets “migrate”

inward

Direct detectionDirect 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.

AtmospheresAtmospheresHD 209458b: • the first transiting planet discovered• the first extrasolar planet known to

have an atmosphere: evaporating hydrogen contains oxygen and carbon.

• Recent Spitzer spectroscopy reveals: Less water vapour than expected Silicate dust clouds

Artist’s conception

Extrasolar planet searchesExtrasolar planet searches

As of Feb 2007, 217 planets have been detected outside our solar syste.

See http://exoplanets.org/

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

Extrasolar planet searchesExtrasolar planet searches

•Orbits tend to be quite eccentric

19

Future missions

Keck Interferometer

SIM PlanetQuest

KeplerLarge Binocular Telescope Interferometer

Terrestrial Planet Finders

Space Interferometry missionSpace Interferometry mission

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

•Will search for terrestrial planets around the nearest ~250 stars, with astrometry accurate to 1 as.

KeplerKepler

•http://www.kepler.arc.nasa.gov/

•Will search large numbers of stars for Earth-sized terrestrial planets using the transit method.

• 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.