Search for Extra-Solar Planets

Background 1995 first discovered evidence that other stars have planets first discovered evidence that other stars have planets. As of April 2014, 1780 planets, including 460 multiple planetary systems As of April 2014, 1780 planets, including 460 multiple planetary systems Evidence suggests that a majority of sun- like stars possess them. Evidence suggests that a majority of sun- like stars possess them. Most of these stellar systems bear little resemblance to ours. Most of these stellar systems bear little resemblance to ours.

Difficulties with the Search Planets are very small and very dark compared to stars… Planets are very small and very dark compared to stars… Even stars appear as nothing more than pinpoints of light when viewed with even the largest telescopes Even stars appear as nothing more than pinpoints of light when viewed with even the largest telescopes Planets have only a fraction of the mass of a star, nuclear fusion reaction that makes stars “burn” does not take place. Planets have only a fraction of the mass of a star, nuclear fusion reaction that makes stars “burn” does not take place. Planets are found right next to the stars they orbit Planets are found right next to the stars they orbit

Methods Most planets cannot be observed directly, instead astronomers must observe stars and look for the minute effects that orbiting planets have upon them. Most planets cannot be observed directly, instead astronomers must observe stars and look for the minute effects that orbiting planets have upon them. Radial Velocity Radial Velocity Transit Photometry Transit Photometry Microlensing Microlensing Astrometry Astrometry Direct Imaging Direct Imaging

Radial Velocity Until the launch of the planet hunting spacecraft Kepler in 2009, radial velocity was the most effective method for locating extrasolar planets. Until the launch of the planet hunting spacecraft Kepler in 2009, radial velocity was the most effective method for locating extrasolar planets. The vast majority of Exoplanets detected from Earth were discovered by this method. The vast majority of Exoplanets detected from Earth were discovered by this method.

Radial Velocity also known as Doppler spectroscopy also known as Doppler spectroscopy when a star is orbited by a planet it responds to the gravitational tug of its smaller companion. when a star is orbited by a planet it responds to the gravitational tug of its smaller companion. these slight movements affect the star's normal light spectrum. these slight movements affect the star's normal light spectrum. If the star is moving towards the observer, its spectrum would appear slightly shifted towards the blue; if it is moving away, it will be shifted towards the red. If the star is moving towards the observer, its spectrum would appear slightly shifted towards the blue; if it is moving away, it will be shifted towards the red.

Radial Velocity Drawbacks: Drawbacks: Can only detect planets accurately that are “edge-on” Can only detect planets accurately that are “edge-on” Cannot accurately determine the mass of a distant planet. Cannot accurately determine the mass of a distant planet. This is a serious problem because mass is used for distinguishing between planets and small stars. This is a serious problem because mass is used for distinguishing between planets and small stars.

The sinusoid is the characteristic shape of the radial velocity graph of a star rocking to the tug of an orbiting planet. exoplanets.org

Transit Photometry Measures the minute dimming of a star as an orbiting planet passes between it and the Earth. Measures the minute dimming of a star as an orbiting planet passes between it and the Earth. The passage of a planet between a star and the Earth is called a "transit." The passage of a planet between a star and the Earth is called a "transit." Dimming detected at regular intervals and lasting a fixed length of time, indicates that a planet is orbiting the star. Dimming detected at regular intervals and lasting a fixed length of time, indicates that a planet is orbiting the star.

Transit Photometry The dimming directly reflects the size ratio between the star and the planet The dimming directly reflects the size ratio between the star and the planet A large planet transiting a small star will have a more noticeable effect. A large planet transiting a small star will have a more noticeable effect. The size of the host star can be known with from its spectrum, photometry therefore gives astronomers a good estimate of the orbiting planet's size. The size of the host star can be known with from its spectrum, photometry therefore gives astronomers a good estimate of the orbiting planet's size.

Transit Photometry Using both methods, scientists can calculate the planet's density, an important step towards assessing its composition. Using both methods, scientists can calculate the planet's density, an important step towards assessing its composition. Additionally, the light from the star passing through the planet's atmosphere is absorbed to different degrees at different wavelengths. Scientists can recreate the absorption spectrum and deduce the atmosphere's composition. Additionally, the light from the star passing through the planet's atmosphere is absorbed to different degrees at different wavelengths. Scientists can recreate the absorption spectrum and deduce the atmosphere's composition.

Transit Photometry The Kepler mission, launched in March of 2009, uses photometry to search for extrasolar planets from space. The Kepler mission, launched in March of 2009, uses photometry to search for extrasolar planets from space. The spacecraft's sensitivity is such that it has already detected thousands of planetary candidates, including several that are Earth-sized and orbiting in their star's habitable zone. The spacecraft's sensitivity is such that it has already detected thousands of planetary candidates, including several that are Earth-sized and orbiting in their star's habitable zone.

Transit Photometry Drawbacks: Drawbacks: The distant planet must pass directly between it's star and the Earth; the orbital plane must be almost exactly "edge-on" to the observer. The distant planet must pass directly between it's star and the Earth; the orbital plane must be almost exactly "edge-on" to the observer. Transits last only a tiny fraction of its total orbital period. Transits last only a tiny fraction of its total orbital period. A planet might take months or years to complete its orbit, but the transit would probably last only hours or days. A planet might take months or years to complete its orbit, but the transit would probably last only hours or days. Astronomers need to observe repeated transits occurring at regular intervals. Astronomers need to observe repeated transits occurring at regular intervals.

An artist's impression of a Jupiter size extrasolar planet passing in front of its parent star

Microlensing Microlensing is the only method capable of discovering planets at great distances from the Earth. Microlensing is the only method capable of discovering planets at great distances from the Earth. Microlensing can find planets orbiting stars near the center of the galaxy, thousands of light-years away. Microlensing can find planets orbiting stars near the center of the galaxy, thousands of light-years away. Microlensing, is most sensitive to planets that orbit in moderate to large distances from their star. Microlensing, is most sensitive to planets that orbit in moderate to large distances from their star.

Microlensing When the light emanating from a star passes very close to another star (“the lensing star”), the gravity of the lensing star will slightly bend the light rays from the source star. When the light emanating from a star passes very close to another star (“the lensing star”), the gravity of the lensing star will slightly bend the light rays from the source star. If the source star is positioned precisely behind the lensing star, this effect is multiplied. If the source star is positioned precisely behind the lensing star, this effect is multiplied. If a planet is positioned close enough to the lensing star, the planet's own gravity bends the light stream and temporarily produces a third image of the source star. If a planet is positioned close enough to the lensing star, the planet's own gravity bends the light stream and temporarily produces a third image of the source star.

Microlensing This effect appears as a temporary spike of brightness, lasting several hours to several days This effect appears as a temporary spike of brightness, lasting several hours to several days Such spikes indicate the presence of a planet. Such spikes indicate the presence of a planet. The precise characteristics of the microlensing light-curve, its intensity and length, allow scientists to deduce the planet’s total mass, orbit, and period. The precise characteristics of the microlensing light-curve, its intensity and length, allow scientists to deduce the planet’s total mass, orbit, and period.

Microlensing Drawbacks: Drawbacks: microlensing is dependent on rare and random events microlensing is dependent on rare and random events microlensing events do not repeat themselves microlensing events do not repeat themselves the distance of the detected planet and its star from the Earth is known only by rough approximation the distance of the detected planet and its star from the Earth is known only by rough approximation

The microlensing process. In the fourth image from the right the planet adds its own microlensing effect, creating the two characteristic spikes in the light curve.

Astrometry Astrometry is the science of precision measurement of stars' locations in the sky. Astrometry is the science of precision measurement of stars' locations in the sky. Planet hunters look for a minute but regular wobble in a star's position. If such a periodic shift is detected, it is almost certain that the star is being orbited by a companion planet. Planet hunters look for a minute but regular wobble in a star's position. If such a periodic shift is detected, it is almost certain that the star is being orbited by a companion planet.

Astrometry Until recently, the level of precision required to detect the slight shifts in a star's position was at the outer edge of technological feasibility Until recently, the level of precision required to detect the slight shifts in a star's position was at the outer edge of technological feasibility The Keck telescopes in Hawaii, the largest in the world, are being fitted for astrometrical measurements The Keck telescopes in Hawaii, the largest in the world, are being fitted for astrometrical measurements

Astrometry Astrometry is most effective when the orbital plane is "face on" (perpendicular) to an observer's line of sight Astrometry is most effective when the orbital plane is "face on" (perpendicular) to an observer's line of sight Excels in detecting planets of long periods, orbiting further away from their star. Excels in detecting planets of long periods, orbiting further away from their star.

Astrometry Drawbacks: Drawbacks: Atmospheric interference limits the accuracy of ground-based measurements Atmospheric interference limits the accuracy of ground-based measurements Can only detect the component of a star's wobble that moves it side to side Can only detect the component of a star's wobble that moves it side to side Can only be used for relatively close stars Can only be used for relatively close stars A star must be observed continuously for years or even decades A star must be observed continuously for years or even decades No confirmed planets discovered by this method, due to the precision required No confirmed planets discovered by this method, due to the precision required

An artist's conception of Gaia spacecraft. Launched Dec. 19, 2013.

Direct Imagining Direct imaging of exoplanets is extremely difficult, and in most cases impossible. Direct imaging of exoplanets is extremely difficult, and in most cases impossible. Being small and dim, planets are easily lost in the brilliant glare of the giant stars they orbit. Being small and dim, planets are easily lost in the brilliant glare of the giant stars they orbit. There are special circumstances in which a planet can be directly observed. There are special circumstances in which a planet can be directly observed.

Direct Imaging For humans “seeing is believing”, this is the only method that allows us to “see” For humans “seeing is believing”, this is the only method that allows us to “see” Works best for big, bright planets that orbit at a great distance from their stars. Works best for big, bright planets that orbit at a great distance from their stars.

Direct Imaging Drawbacks: Drawbacks: Only possible on very special circumstances. Only possible on very special circumstances.

Hubble Space Telescope image of planet Fomalhaut b orbiting the star Fomalhaut. (A coronagraph blocks out the star and accounts for the dark region at the center of the image).

Technology Radial Velocity = super sensitive spectrographs Radial Velocity = super sensitive spectrographs Transit Photometry = ground based photometers and Kepler space observatory Transit Photometry = ground based photometers and Kepler space observatory Microlensing = ground based observatories Microlensing = ground based observatories Astrometry = Keck Telescope, Gaia space observatory Astrometry = Keck Telescope, Gaia space observatory Direct Imaging = visible and infrared. ground based and space telescopes (Hubble, VLT, etc) Direct Imaging = visible and infrared. ground based and space telescopes (Hubble, VLT, etc)

Exoplanet Examples 51 Pegasi b: 51 Pegasi b: First exoplanet discovered around a “Sun- like” star First exoplanet discovered around a “Sun- like” star Announced: Oct 6, 1995 Announced: Oct 6, 1995 Method: Radial Velocity Method: Radial Velocity Has a mass about half of Jupiter and orbits much closer than Mercury. Has a mass about half of Jupiter and orbits much closer than Mercury. The discovery of other similar exoplanets forces scientists to re-examine theories of solar system formation. The discovery of other similar exoplanets forces scientists to re-examine theories of solar system formation.

Exoplanet Examples Fomalhaut b: Fomalhaut b: Orbits star Fomalhaut Orbits star Fomalhaut Announced: Confirmed: 2012 Announced: Confirmed: 2012 Method: direct imagining using Hubble telescope. Method: direct imagining using Hubble telescope. Star is surrounded by a thick disk of gas and dust. Located the planet in images of the disk. Star is surrounded by a thick disk of gas and dust. Located the planet in images of the disk. Very luminous; believe that it is surrounded by a ring system thicker than that of Saturn. Very luminous; believe that it is surrounded by a ring system thicker than that of Saturn.

Exoplanet Examples Alpha Centauri Bb: Alpha Centauri Bb: Closest exoplanet Closest exoplanet Orbits star Alpha Centauri B Orbits star Alpha Centauri B Announced: October 2012 Announced: October 2012 Method: radial velocity Method: radial velocity Great debate. Remains unconfirmed as a planet. Great debate. Remains unconfirmed as a planet.

News update mission-announces-a-planet-bonanza/ mission-announces-a-planet-bonanza/ mission-announces-a-planet-bonanza/ mission-announces-a-planet-bonanza/ five-years-in-space/ five-years-in-space/ five-years-in-space/ five-years-in-space/ mission-manager-update-loss-of-a-science- module/ mission-manager-update-loss-of-a-science- module/ mission-manager-update-loss-of-a-science- module/ mission-manager-update-loss-of-a-science- module/ mission-manager-update-k2-spacecraft- operation-tests-continue/ mission-manager-update-k2-spacecraft- operation-tests-continue/ mission-manager-update-k2-spacecraft- operation-tests-continue/ mission-manager-update-k2-spacecraft- operation-tests-continue/

references topics/exoplanets/ topics/exoplanets/ topics/exoplanets/ topics/exoplanets/