The Search for Extra-Solar Planets

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Methods for Detecting Exoplanets

   Imaging        Astrometric        Radial Velocity 
  Transits       Microlensing            Stellar Disk 
   Timing Methods                  Exoplanet Score

 

Picture of exoplanets

Three planets orbiting the star HB 8799, located some 120 light-years from earth. The planets were imaged in the infrared by NASA JPL astronomers using a small part (1.5 meters) of the 200 inch Halle telescope at Palomar Mountain. (Photo courtesy of NASA/JPL-Caltech/Palomar Observatory)

 

  Imaging

Obtaining direct images with ground telescopes of planets is rare because the intense light from their star hides the much dimmer light from the planets. However, by shifting to infrared wavelengths and blanking out the radiation from the star, NASA’s Jet Propulsion Laboratory used direct imaging to picture three HB 8799 planets that previously been detected by non-imaging techniques. This was significant in that it essentially used a 1.5 meter telescope, indicating that the technique could be used by much smaller telescopes than the European Southern Observatory Very Large Telescope Array in Chile, or the Keck and Gemini Observatories working together, which had previously imaged extrasolar planets.

The three planets are thought to be gas giants like Jupiter. They orbit their host star at roughly 24, 38 and 68 times the distance between our Earth and sun, respectively (Jupiter orbits at about five times the Earth-sun distance).

 

Astrometric

An initial indication of a possible extra-solar planet came from Barnard’s star. This star is an interesting subject for study as it is the second closest to the sun and moves across the sky faster than any other. It’s closeness opens the way for using  astrometric measurements to detect slight erratic movements in position that may be caused by the gravitational effect of a planet. Measurements of this type have been underway since the 19th century in searching for binary stars rotating around each other. Although a claim was made in 1969 that two Jupiter-sized planets were rotating around Barnard’s star, subsequent measurements have failed to confirm this. The star remains an important target in the search for small earth-like planets. Astrometric measurement led to the discovery of planet VB 10b in 2009. The technique remains difficult, although future improvements in technology may make its use more widespread.

 

Radial Velocity

The alternative of measuring changes in the velocity of a star as it rotates about the center of mass of the complete planetary system has been much more productive of extra-solar planets. The radial technique measures the velocity of the star along the radial direction from the Earth by means of the Doppler shift detected by precision spectroscopy. Together the radial and astrometric techniques have yielded 490 planets. Both suffer from increasing errors as the angle of the plane of the planets orbit becomes more perpendicular to the line of sight.

 

Transit

An optical technique that measures the slight dimming of a star as a planet passes across its face promises to become increasingly productive as more results come in from the COROT, Kepler and other spacecraft.

 transit

 

Transits across a star by terrestrial planets produce a change in a star's brightness of about 100 parts per million, lasting for 1 to 16 hours. This change is absolutely periodic and of repeatable brightness if it is caused by a planet. (Photo courtesy NASA)

 

The transit method has the advantage that the size of the planet can be estimated from the way the star light dims. Combining this with the radial detection method can provide an estimate of the density of the planet. The  planet's orbit can be calculated from the orbital period and the mass of the star. The size of the planet can be calculated from the size of the star and by how much its light is reduced by the planet. From the planet's orbit and the temperature of the star, the planet's characteristic temperature can be estimated.

A problem with the transit method is that it relies on the plane of the planets around a star being edge on to the line of sight from the earth. The probability of this happening varies from 0.5 percent for earth-sized planets to 10 percent for giant planets. This led to the design of the Kepler spacecraft so that it can observe over 100,000 stars at once and measure the light curves of all those that have planetary systems in a favorable alignment. From those curves a representative sample of the diversity of the different types of extrasolar planets is expected to be gathered. Currently the NASA Kepler spacecraft monitors in a fixed field of view the brightness of 156,000 stars continuously.  Many thousands of planets are expected to be detected and categorized over its minimum mission length of 3.5 years, including planets the size of earth.

The Kepler instrument is a photometer or light meter with a 0.95-meter aperture. The light of each star in its field of view is focused on a 95 megapixel detector made up of 21 modules. Each module has two 2200x1024 pixel CCDs. The array can handle stars between 9th and 16th magnitude.

The COROT space mission operated by the French Space Agency also carries out a search for planet transits and has been detecting planets since 2008.

Transits of planets in the habitable zone of solar-like stars occur about once a year and require three transits for verification. As a result, locating and verifying Earth-size planets orbiting in a habitable zone of a star like the sun star is expected to take three years.

 

Gravitational Microlensing

An alternative ground-based technique is to use the gravitational field of a star as a lens to focus the light of a more distant star. As stars are moving all the time, the technique of gravitational microlensing can only be used when an opportunity occurs and there it no possibility of going back later to confirm the results. It has been used successfully to detect several examples of extrasolar planets and promises to be of greater value when several robotic telescopes are combined to take greater advantage of opportunities as lensing occurs. The technique has the valuable advantage that it can detect earth-sized planets. Recent measurements of planets by this technique has led to the conclusion that the Milky Way galaxy contains a minimum of 100 billion planets [More . . .]

 

Diagram of Microlensing method

When one star passes precisely in front of another, the gravity of the foreground star bends the light from the background star. The foreground star thus acts like a giant lens amplifying the light from the background star. A planet around the foreground star can produce additional brightening of the background star to reveal a planet that is otherwise too faint to be seen by telescopes.(Image Credit: NASA, ESA, A.Feild (STScl)

    

Circumstellar Disk

Planets at their formation stage may be found by circumstellar disk technique, which observes the disk of dispersed material that surrounds many stars. The disks can be studied because the dust composing them radiates in the infrared at a much higher intensity than the central star. The Spitzer and Hubble space telescopes can both make infra-red measurements of disks, which appear to have small bodies and asteroids mixed in with the dust. Emerging planets may be detected by this means.

 

Timing Methods

Pulsar Timing

A special class of planets  orbit neutron stars that are operating as pulsars—emitting short pulses of radio waves at brief, very regular intervals. These planets can be detected by the timing method, which measures the slight changes in radio pulse timing. This technique originated in detecting pulsar companion stars and was later applied to detecting planets.

Transit timing variations

Transit observations can be taken a step further by measuring variations in the transit time of a planet. Such variations can reveal the presence of a second planet interacting gravitationally with the first to perturb its orbit in a regular way. It is a sensitive way of detecting additional planets orbiting a star.

Eclipse timing variations

If the orbits of stable binary stars circling about their common center of motion are lined up with Earth in the right way, the two stars eclipse each other in a regular fashion. The result is a regular dip in the light we receive from the stars. If a planet is orbiting the binary system its gravitational pull will modify the motion of the stars and cause variation in the time of minimum light. This variation is used to detect the presence of planets.

Pulsation timing variation

Stars that pulsate in size in a regular way can signal the presence of an orbiting planet by regular shifts in the rate of pulsation. The technique depends on a suitable and stable relationship between the pulsations of the star and the period of the planet.

Orbital brightness modulation

A heavy planet orbiting close to a star can distort the shape of the star in a regular way. When the shape change modifies the area of the star as seen from Earth, the brightness of the star undergoes a regular change, indicating the presence of a planet.