direct detection transit astrometry doppler spectroscopy detection of circumstellar disks

Extrasolar PlanetsThe Search For

Ever since humans first gazed into the night sky, the question of whether we are alone in the universe has remained unanswered.


Recent technological advances allow astronomers to address this question by searching for planets beyond our solar system.


• Direct Detection• Transit• Astrometry• Doppler Spectroscopy• Detection of Circumstellar

Disks

Astronomers use several techniques to search for extrasolar planets:


Direct Detection:

The most obvious way to find a planet outside our solar system is to see it through a telescope. Right?


The light of a star is 1 million to 10 billion times brighter than the light reflected by a planet, making it nearly impossible to see the planet. It’s like trying to see a firefly in front of a search light.

Wrong.


Since technology doesn’t yet allow astronomers to directly detect extrasolar planets, they look for the indirect effects of planets on their parent stars.


Transit:

Astronomers can measure the decrease in apparent brightness of a star as a planet eclipses (or transits) it.


Artist’s conception of a gas-giant planet orbiting star HD 209458.

In 2001, Hubble detected the atmosphere around an extrasolar planet. The planet itself was not directly visible. Instead, sodium was detected in light filtered through the planet's atmosphere when it transited its star, HD 209458.


In 2003, the planet OGLE-TR-56b was detected using the transit method. Based on a 1% dimming in the star's brightness during transit, astronomers believe the planet is about the size of Jupiter.

Artist’s conception of OGLE-TR-56b


Transit is a limited detection method, since only planets with side-on orbits will obscure our view of their stars. HD 209458 was first detected not by transit, but by measuring its gravitational pull on the parent star, called “wobble.”


• Bodies orbiting a star cause it to ‘wobble’ around the system’s center of mass.

• Precise measurement of this change in a star’s position (“astrometry”) may allow astronomers to detect a planet.

If alien astronomers 33 light-years away tracked the motion of our Sun relative to the center of mass of the solar system (CMSS), they would observe a measurable ‘wobble’ caused by planets in orbit. The green sphere shows the actual size of the Sun.

Astrometry:


A planet was discovered around star G1876 using astrometry. G1876 is close to Earth (15 light-years), and the planet is half its size, exaggerating the wobble. Few stars are close enough to be detected by this technique.


When the star moves away from us, the light shifts to the red end of the spectrum.

When the star moves toward Earth, we see a shift toward the blue end of the spectrum.

Most detections thus far have been achieved by the radial-velocity technique from ground-based telescopes, in which a star’s wobble is measured as a shift in the frequency of light emitted by the star.

Doppler Spectroscopy:


Artist’s conception of a giant planet close to its sun.

Over 100 extrasolar planets have been detected using Doppler Spectroscopy. Most are close to their parent stars and several times more massive than Jupiter. This technique cannot detect smaller worlds. With the best spectroscopes, astronomers can detect motions of about 15 meters/second. Earth only forces the Sun to move at 0.1 meters/second.


This chart compares sizes and orbits of known extrasolar planets to our own solar system.


Detection of Circumstellar Disks:

We also get clues about other planetary systems by looking for dust disks around stars. Disks may indicate locations where planets are forming or have already formed.

Infrared image of Beta Pictoris. The presence of a warp in this disk indicates the existence of a Jupiter-sized planet.


Vega 55 Cancri Fomalhaut

Irregularities in dust disks Irregularities in circumstellar dust disks indicate that the dust is concentrated in orbital resonances with a planet. Planets create “clumps” in the dust.

Infrared images of dust disks around 3 stars:


Spitzer infrared image. Fomalhaut is surrounded by a dust ring nearly five times larger than our own solar system.

NASA’s new Spitzer Space Telescope, launched in August 2003, can detect dust at various temperatures, allowing it to provide a full picture of the inner region of disks such as that around Fomalhaut, 25 light-years away.


Astronomers will apply advancing technology to the search for extrasolar planets. One approach, gravitational lensing, has been used to search for brown dwarf stars, and may be successful for planetary searches, as it can detect objects as small as the moon.

The Future of Planet Hunting:


Gravitational Lensing:When a massive object, such as a star or a planet, passes in front of a star, it acts as a lens, brightening the image of the star. Astronomers can calculate the mass of the object by the degree to which it affects the brightness of the star.


An additional “blip” in the light curve may indicate the presence of a planet around the lensing star, as shown in this animation of an actual microlensing event.


In 2004, a planet one-and-a-half times more massive than Jupiter was discovered orbiting a red dwarf star 17,000 light years away. This marks the first discovery of an extrasolar planet using gravitational microlensing.


First up in the future of extrasolar planet searches is NASA’s Kepler Mission, due to launch in 2007. Kepler will take the transit method to new heights, continuously monitoring over 100,000 stars from a heliocentric orbit slightly larger than Earth’s.

Artist’s concept. Kepler will utilize a specialized one-meter-diameter telescope called a photometer.


Previous studies have been limited to single transits of gas giants too close to their suns to support life. Kepler’s photometer, however, will measure multiple transits of planets similar in size – and possibly habitability – to Earth. Kepler will also support the science objectives of two future NASA missions, the Space Interferometry Mission and Terrestrial Planet Finder.


NASA is set to launch the Space Interferometry Mission (SIM) in 2009. Unlike Kepler, SIM will use astrometry to search for other planets.


SIM will measure stellar “wobble” several hundred times more accurately than current instruments do, detecting much smaller planets than previously possible.


NASA’s Terrestrial Planet Finder, set to launch in 2012, will detect Earthlike planets up to 45 light-years away with

imaging power

100x greater

than Hubble’s.

TPF’s concepts include a formation-flying interferometer, above, and visible-light coronagraph, left.


By combining high-sensitivity space telescopes with revolutionary imaging technologies, TPF will study planets as small as the Earth in the habitable zones of distant solar systems. TPF's spectroscopy will compare relative quantities of gases to determine whether a planet could potentially support life.

Artist’s concept of an Earthlike planet.


Will we discover other planets capable of supporting life? Time will tell. One thing is certain: however advanced technology becomes, human curiosity and ingenuity are the chief tools in the drive to solve our mysteries.

direct detection transit astrometry doppler spectroscopy detection of circumstellar disks

Documents