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PH507 Astrophysics Prof M Smith 1 2 EXOPLANET DETECTION: Prof Michael SMITH Detecting extrasolar planets (1) Direct imaging - difficult due to enormous star / planet flux ratio. Any optical image would have to be captured with starlight reflected by the planet's atmosphere or surface. This will depend of course on the albedo of the planet, Infrared. The light from the star will swamp that of the planet by a factor of 109 in the optical, so it seems that concentrating upon the infrared region would have the best chance of success. Detection may be possible when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and young (so that it is hot and emits intense infrared radiation). (2) Radial velocity • Observable: line of sight velocity of star orbiting centre of mass of star - planet binary system. (3) Astrometry • Observable: stellar motion in plane of sky (4) Transits: photometry • Observable: tiny drop in stellar flux as planet transits stellar disc • Requires favourable orbital inclination • Jupiter mass exoplanet observed from ground HD209458b • Earth mass planets detectable from space (Kepler (2007 launch. NASA Discovery mission), Eddington) (5) Gravitational lensing • Observable: light curve of a background star lensed by the gravitational influence of a foreground star. • Rare - requires monitoring millions of background stars, and also unrepeatable Each method has different sensitivity to planets at various orbital radii - complete census of planets requires use of several different techniques PH507 Astrophysics Prof M Smith 2 2.1 Direct Imaging Direct imaging of planets is difficult because of the enormous difference in brightness between the star and the planet, and the small angular separation between them. Direct detection: must be large and distant from star Circumstellar dust discs. (Circumstantial evidence.) Disc of material around the star Beta Pictoris bably connected with a planetary system. The disk does not start at the star. Rather, its inner edge begins around 25 AU away, farther than the average orbital distance of Uranus in the Solar System. Theoretically, this disk should have lasted for only around 10 million years. That it has persisted for the 20 to 200 million year lifetime of Beta Pictoris may be due to the presence of large disk bodies (i.e., planets) that collide with icy Edgeworth-Kuiper Belt type objects (dormant comets) to replenish the dust. Young stars are preferred because young planets are expected to be more luminous than older planets. In addition, direct imaging is based on detection of planet luminosity, which must be related to planet mass or size through uncertain theoretical models. Some stunning individual systems have been reported (Marois et al. 2010, Lagrange et al. 2010), but the surveys indicate that fewer planets are found than would be predicted by extrapolating the power-law (of Eqn. (1) – see next lecture) out to 10-100 AU. Circumstellar dust discs. (Circumstantial evidence.) Disc of material around the star Beta Pictoris – the image of the bright central star has been artificially blocked out by astronomers using a ‘Coronograph’ This disk around Beta Pictoris is probably connected with a planetary system. The disk does not start at the star. Rather, its inner edge begins around 25 AU away, farther than the average orbital distance of Uranus in the Solar System. Its outer edge appears to extend as far out as 550 AUs away from the star. PH507 Astrophysics Prof M Smith 3 Analysis of Hubble Space Telescope data indicated that planets were only beginning to form around Beta Pictoris, a very young star at between 20 million and 100 million years old. Most dust grains in the disk are not agglomerating to form larger bodies; instead, they are eroding and being moved away from the star by radiation pressure when their size goes below about 2-10 microns. Theoretically, this disk should have lasted for only around 10 million years. That it has persisted for the 20 to 200 million year lifetime of Beta Pictoris may be due to the presence of large disk bodies (i.e., planets) that collide with icy Edgeworth-Kuiper Belt type objects (dormant comets) to replenish the dust. PH507 Astrophysics Prof M Smith 4 Using high-contrast, near-infrared adaptive optics observations with the Keck and Gemini telescopes, the team of researchers were able to see three orbiting planetary companions to HR8799 Young stars are preferred because young planets are expected to be more luminous than older planets. In addition, direct imaging is based on detection of planet luminosity, which must be related to planet mass or size through uncertain theoretical models. Some stunning individual systems have been reported (Marois et al. 2010, Lagrange et al. 2010), but the surveys indicate that fewer planets are found than would be predicted by extrapolating the Direct Spectroscopic Detection? The starlight scattered from the planet can be distinguished from the direct starlight because the scattered light is Doppler shifted by virtue of the close-in planet's relatively fast orbital velocity (~ 150 km/sec). Superimposed on the pattern given by the planet's albedo changing slowly with wavelength, the spectrum of the planet's light PH507 Astrophysics Prof M Smith 5 will retain the same pattern of photospheric absorption lines as in the direct starlight. 2.2 Planet detection method : Radial velocity technique Also known as the "Doppler method". Variations in the speed with which the star moves towards or away from Earth — that is, variations in the radial velocity of the star with respect to Earth — can be deduced from the displacement in the parent star's spectral lines due to the Doppler effect. This has been by far the most productive technique used by planet hunters. We observe the star. So what can we say about the exoplanet? X a r1 r2 A planet in a circular orbit around star with semi-major axis a Assume that the star and planet both rotate around the centre of mass with an angular velocity: G(M * + m p ) W= a3 Using a1 M* = a2 mp and a = a1 + a2, then the stellar speed (v* = a ) in an inertial frame is: PH507 Astrophysics V* = Prof M Smith mp G(M * + m p ) M* a 6 (assuming mp << M*). i.e. the stellar orbital speed is small …. just metres per second. For a circular orbit, observe a sin-wave variation of the stellar radial velocity, with an amplitude that depends upon the inclination of the orbit to the line of sight: Vobs = V* sin(i) Hence, measurement of the radial velocity amplitude produces a constraint on: mp sin(i) This assumes stellar mass is well-known, as it will be since to measure radial velocity we need exceptionally high S/N spectra of the star. PH507 Astrophysics Prof M Smith 7 Observable yields a measure of mp sin(i). -> given vobs, we can obtain a lower limit to the planetary mass. In the absence of other constraints on the inclination, radial velocity searches provide lower limits on planetary masses Magnitude of radial velocity: Sun due to Jupiter: Sun due to Earth: i.e. extremely small running pace 12.5 m/s 0.09 m/s 10 m/s is Olympic 100m Spectrograph with a resolving power of 105 will have a pixel scale ~ 10-5 c ~ few km/s Therefore, specialized techniques that can measure radial velocity shifts of ~10-3 of a pixel stably over many years are required High sensitivity to small radial velocity shifts is achieved by: • comparing high S/N = 200-500 spectra with template stellar spectra • using a large number of lines in the spectrum to allow shifts of much less than one pixel to be determined. • correcting for Earth’s rotation and motion in orbit, accurate timing and position required Absolute wavelength calibration and stability over long timescales is achieved by: • passing stellar light through a cell containing iodine, imprinting large number of additional lines of known wavelength into the spectrum • with the calibrating data suffering identical instrumental distortions as the data PH507 Astrophysics Prof M Smith 8 Error sources: (1) Theoretical: photon noise limit • flux in a pixel that receives N photons uncertain by ~ N1/2 • implies absolute limit to measurement of radial velocity • depends upon spectral type - more lines improve signal • around 1 m/s for a G-type main sequence star with spectrum recorded at S/N=200 • practically, S/N=200 can be achieved for V=8 stars on a 3m class telescope in survey mode (2) Practical: • stellar activity - young or otherwise active stars are not stable at the m/s level and cannot be monitored with this technique • remaining systematic errors in the observations Currently, the best observations achieve: ~ 3 m/s ...in a single measurement. Thought this error can be reduced to around 1…0.6m/s with further refinements, but not substantially further. The very highest Doppler precisions of 1m/s are capable of detecting planets down to about 5 earth masses. Radial velocity monitoring detects massive planets, especially those at small a, but is not sensitive enough to detect Earth-like planets at ~ 1AU. Radial velocity measurement: Spectrograph with a resolving power of 105 will have a pixel scale ~ 10-5 c ~ few km/s Therefore, specialized techniques that can measure radial velocity shifts of ~10-3 of a pixel over many years are required For circular orbit: PH507 Astrophysics Prof M Smith 9 51 Peg b was the first known exoplanet With a 4 day, circular orbit: a hot Jupiter, lying close to the central star. Note the large radial velocity shift. Example of a planet with an eccentric orbit: e=0.67 where e = 1 – b2/a2 periastron = a (1-e) a = semi-major axis, b = semi-minor axis apastron = a (1+e) PH507 Astrophysics Prof M Smith 10 Note the shape, not sinusoidal at all. Summary: three parameters derived from observables (1) Planet mass, up to an uncertainty from the normally unknown inclination of the orbit. Measure mp sin(i) (2) Orbital period -> radius of the orbit given the stellar mass (3) Eccentricity of the orbit Summary: selection function Need to observe full orbit of the planet: zero sensitivity to planets with P > Psurvey For P < Psurvey, minimum mass planet detectable is one that produces a radial velocity signature of a few times the sensitivity of the experiment (this is a practical detection threshold) Which planets are detectable? Down to a fixed radial velocity PH507 Astrophysics m p sin i µ a Prof M Smith 11 1 2 Hence, inner massive planets are selected. 2.3 Planet detection method : Astrometry The gravitational perturbations of a star's position by an unseen companion provides a signature which can be detected through precision astrometry. Measure stellar motion in the plane of the sky due to presence of orbiting planet. Must account for parallax and proper motion of star. Magnitude of effect: amplitude of stellar wobble (half peak displacement) for an orbit in the plane of the sky is æ mp ö ÷÷ ´ a a1 = çç è M* ø In terms of the angle: æ m p öæ a ö ÷÷ç ÷ Dq = çç M è * øè d ø for a star at distance d. Note we have again used mp << M* PH507 Astrophysics Prof M Smith æ m p öæ a ö ÷÷ç ÷ Dq = çç radians M è * øè d ø The Wobble: Detection threshold as function of semi-major axis 12 PH507 Astrophysics Prof M Smith 13 2.4 Planet detection method : Transits Photometry TRANSITS Currently the most important class of exoplanets are those that transit the disk of their parent stars, allowing for a determination of planetary radii. SELECTION: Of course, while planets close to their parent stars will preferentially be found, due to their shorter orbital periods and greater likelihood to transit, planetary transits will be detected at all orbital separations. CONFIRMATION: In general, the detection of three successive transits will be necessary for a confirmed detection, which will limit confirmed planetary-radius objects to about 1.5 AU. DENSITIES: The first confirmed transiting planets observed were all more massive than Saturn, have orbital periods of only a few days, and orbit stars bright enough such that radial velocities can also be determined, allowing for a calculation of planetary masses and bulk densities. A planetary mass and radius allows us a window into planetary composition (Guillot 2005). The first transiting planets were mainly gas giants although one planet, HD 149026b, appears to be 2/3 heavy elements by mass (Sato et al. 2005; Fortney et al. 2006; Ikoma et al. 2006). Understanding how the transiting planet mass-radius relations change as a function of orbital distance, stellar mass, stellar metallicity, or UV flux, will provide insight into the fundamentals of planetary formation, migration, and evolution. The transit method of planet detection is biased toward finding planets that orbit relatively close to their parent stars. This means that radial velocity follow-up will be possible for some planets as the stellar "wobble" signal is larger for shorter period orbits. Drop in stellar flux due to a planet transiting across the stellar disc. PH507 Astrophysics Prof M Smith 14 Needs luck or wide-area surveys - transits only occur if the orbit is almost edge-on The photometric transit technique can determine the radius of a planet, but generally not the mass and hence does not immediately indicate if a transit signal is due to a planet or a binary star system. r i a o Probability. For a planet with radius rp << R*, probability of a transit- æ R* ö Ptransit = sin(q ) » ç ÷ èaø Close-in planets are more likely to be detected. P = 0.5 % at 1AU, P = 0.1 % at the orbital radius of Jupiter What can we measure from the light curve? PH507 Astrophysics Prof M Smith 15 (1) Depth of transit = fraction of stellar light blocked DF æ rp ö =ç ÷ Fo è R* ø 2 This is a measure of planetary radius! No dependence on distance from star. In practice, isolated planets with masses between ~ 0.1 MJ and 10 MJ, where MJ is the mass of Jupiter, should have almost the same radii (i.e. a flat mass-radius relation). -> Giant extrasolar planets transiting solar-type stars produce transits with a depth of around 1%. Close-in planets are strongly irradiated, so their radii can be (detectably) larger. But this heating-expansion effect is not generally observed for short-period planets. (2) (3) (4) Duration of transit plus duration of ingress, gives measure of the orbital radius and inclination Bottom of light curve is not actually flat, providing a measure of stellar limb-darkening Deviations from profile expected from a perfectly opaque disc could provide evidence for satellites, rings etc Transit depth for an Earth-like planet is: Photometric precision of ~ 10-5 seems achievable from space THE EFFECT OF A TRANSIT ON RADIAL VELOCITY PH507 Astrophysics Prof M Smith 16 . • We know that the observed radial velocity depends on the planetary system inclination to the line of sight () • What happens to the line shape when sin i = 1? • The planet moves across the face of the star, blocking some of the light (which is blue/red shifted by the rotation of the star) and changing the line shape – which makes it look as though the radial velocity is changing when it isn’t. This is known as the Rossiter-McLaughlin effect: it yields the direction of rotation of the star! Transit timing variation method (TTV) and transit duration variation method (TDV) If a planet has been detected by the transit method, then variations in the timing of the transit provide an extremely sensitive method which is capable of detecting additional planets in the system with sizes potentially as small as Earth-sized planets. Duration variations may be caused by an exomoon. Orbital phase reflected light variations Short period giant planets in close orbits around their stars will undergo reflected light variations changes because, like the Moon, they will go through phases from full to new and back again. Since telescopes cannot resolve the planet from the star, they see only the combined light, and the brightness of the host star seems to change over each orbit in a periodic manner. Although the effect is PH507 Astrophysics Prof M Smith 17 small — the photometric precision required is about the same as to detect an Earth-sized planet in transit across a solar-type star — such Jupiter-sized planets are detectable by space telescopes such as the Kepler Space Observatory. 3.5 Method : Gravitational microlensing Microlensing operates by a completely different principle, based on Einstein's General Theory of Relativity. According to Einstein, when the light emanating from a star passes very close to another star on its way to an observer on Earth, the gravity of the intermediary star will slightly bend the light rays from the source star, causing the two stars to appear farther apart than they normally would. This effect was used by Sir Arthur Eddington in 1919 to provide the first empirical evidence for General Relativity. In reality, even the most powerful Earth-bound telescope cannot resolve the separate images of the source star and the lensing star between them, seeing instead a single giant disk of light, known as the "Einstein disk," where a star had previously been. The resulting effect is a sudden dramatic increase in the brightness of the lensing star, by as much as 1,000 times. This typically lasts for a few weeks or months before the source star moves out of alignment with the lensing star and the brightness subsides. Light is deflected by gravitational field of stars, compact objects, clusters of galaxies, large-scale structure etc Simplest case to consider: a point mass M (the lens) lies along the line of sight to a more distant source PH507 Astrophysics Prof M Smith 18 DS θ Source DLS Lens (Mass M) Define: • Observer-lens distance • Observer-source distance • Lens-source distance DL Observer Dl Ds Dls Azimuthal symmetry -> light from the source appears as a ring R0 ...with radius R0 - the Einstein ring radius - in the lens plane relevant time scale is called the Einstein time and it's given by the time it takes the lens to traverse an Einstein radius. PH507 Astrophysics Prof M Smith 19 Timescales for sources in the Galactic bulge, lenses ~ halfway along the line of sight: • Solar mass star ~ 1 month (Einstein radius of order a few AU) • Jupiter mass planet ~ 1 day (0.1 AU) • Earth mass planet ~ 1 hour The dependence on M1/2 means that all these timescales are observationally feasible. However, lensing is a very rare event, all of the projects monitor millions of source stars to detect a handful of lensing events. • OGLE-2005-BLG-390Lb (Catchy! – OGLE-Year-PlaceNumber) • Detection of a 5 MEARTH mass planet. 2.6 Timing: Pulsar Planets PH507 Astrophysics Prof M Smith 20 In early 1992, the Polish astronomer Aleksander Wolszczan (with Dale Frail) announced the discovery of planets around another pulsar, PSR 1257+12.This discovery was quickly confirmed, and is generally considered to be the first definitive detection of exoplanets. Pulsar timing. Pulsars (the small, ultradense remnant of a star that has exploded as a supernova) emit radio waves extremely regularly as they rotate. Slight anomalies in the timing of its observed radio pulses can be used to track changes in the pulsar's motion caused by the presence of planets.