Extrasolar planets

Detection methods 1.Pulsar timing 2.Astrometric wobble 3.Radial velocities 4.Gravitational lensing 5.Transits 6.Dust disks 7.Direct detection

1. Pulsar timing Pulsars are rapidly rotating neutron stars, with extremely regular periods Anomalies in these periods indicate the gravitational influence of a companion.

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

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

Spectroscopic 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 Where i is the angle of inclination.

Radial 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.

Radial velocities: mass measurement E.g. the star HD73256: From Hipparcos data (and detailed stellar modelling) we know M star ~1.05 M sun From the light curve we measure P= days and v max =269.8 m/s. Sinusoidal shape means e~0 What is the mass of the planet, and the size of the orbit?

Radial velocities: difficulty The maximum velocity shift is only ~270 m/s. The Doppler shift is therefore: which is very small. For example the H  line is redshifted by only nm! The spectral resolution must therefore be very high. Detecting smaller planets, farther away from the star, is an even more difficult task.

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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.

Transits Detects a planet's shadow when it transits in front of its host star. Can be used to measure the radius of a planet.

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

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

7. Direct Detection The albedo of the Earth is about A V =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.

Atmospheres HD b: 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 searches As of Feb 2007, 217 planets have been detected outside our solar syste. See Most of these have a M Jupiter

Extrasolar planet searches Orbits tend to be quite eccentric

19 Future missions Keck Interferometer SIM PlanetQuest Kepler Large Binocular Telescope Interferometer Terrestrial Planet Finders

Space Interferometry mission Will search for terrestrial planets around the nearest ~250 stars, with astrometry accurate to 1  as.

Kepler 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 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.