ExoplanetsAn Introduction to Planetary BodiesOutside Our Solar SystemMike SwansonMay 2003

"With 10 to the 11th stars in our galaxy and 10 to the 9th other galaxies, there are at least 10 to the 20th stars in the universe. Most of them may be accompanied by solar systems. If there are 10 to the 20th solar systems in the universe, and the universe is 10 to the 10th years old -- and if, further, solar systems have formed roughly uniformly in time -- then one solar system is formed every 10 to the negative 10 yr = 3 x 10 to the negative 3 seconds. On the average, a million solar systems are formed in the universe each hour."

Carl Sagan and I.S. Shklovskii

Exoplanet BasicsThe existence of planets outside our solar system has been widely accepted by most astronomers for decades, but until recently no direct evidence existed.Initial (unsuccessful) attempts involved detection of a stars wobble by directly measuring the position of nearby stars in relation to more distant stationary stars.The first successful detections were based on Doppler shifts.

In 1995, the first confirmed exoplanet was detected around the star 51 Pegasi.More than 100 exoplanets have been discovered as of early 2003, most by Doppler-shift, the rest by dimming of the parent star as the planet transits in front.One recent estimate - 30 billion planets in our galaxy alone!

Detection by Doppler ShiftAs a planet circles its star, the star wobbles due to the gravitational tug of the planet.The wobble is greater for large planets, lesser for small planets.This wobble shifts the light coming from the star back and forth in wavelength (frequency) like the sound of the horn of a passing car the Doppler Effect.Indirect detection method requires several orbits of the planet to detect the pattern.Not possible to determine the precise mass of the planet due to the possible variations in the orbital plane.

Detection by TransitPlanet detected by the dimming of light from the star as the planet crosses in front of (transits) the star.Similar effect caused by transit of smaller companion star so only very slight dimming indicates a planet.Indirect detection method requires several orbits of the planet to detect the pattern.Combined with Doppler effect to determine the mass of the planet. High accuracy since the planet, its star and Earth are directly in line.Large amateur telescopes can actually measure the dips in luminosity of the stars/planets already discovered some efforts underway by amateurs to discover new exoplanets.Future space-based telescopes will attempt to detect small, Earth-like planets by transit method:NASAs Kepler Mission - approved for 2006ESAs Eddington Mission around 2008

Direct Visual DetectionNot yet successful none of our current instruments are capable of such detection.The light from the star overwhelms the reflected light from the planet.Telescopes with tiny opaque dots in the optics could block the light from the parent star.Arrays of multiple space telescopes can also provide the resolution required without blocking light from the parent star.Future space-based telescopes will attempt to make direct visual observations of exoplanets:NASAs Terrestrial Planet Finder Mission ESAs Darwin Mission

Free-floating ExoplanetsPlanetary mass objects (PMOs) that are not orbiting stars.Detected with large telescopes and the Hubble Space Telescope in infrared light (radiated heat).Very speculative at this point since determining their mass depends upon knowing their age they might be very old brown dwarves (the smallest star-like objects) cooled down to the infrared or they might appear cooler due to intervening gas or dust.Latest techniques combine studies of the temperature as well as the spectrum of radiation coming from the suspected PMO to determine how the gravity of the PMO is affecting atoms in its atmosphere.No agreement on how they might form as they arent massive enough to condense on their own from gas and dust.Perhaps they form in a normal star system and are flung out of orbit afterwards.

Detecting the Signature of LifeCurrent attempts based on studying the spectrum of light coming from stars when a planet transits.By studying spectrum, we can detect the existence of specific atoms and molecules.Does not guarantee life exists, just that some of the building blocks exist.Future direct imaging (space-based telescopes) will be able to detect the spectrum produced by the light reflecting from an exoplanet.

For More InformationThe Search for the Extrasolar Planets (Arizona State Univ.) - http://www.public.asu.edu/~sciref/ exoplnt.htmThe California and Carnegie Planet Search - http://exoplanets.org/ The STARE (STellar Astrophysics & Research on Exoplanets) Project - http://www.hao.ucar.edu/public/ research/stare/The Extrasolar Planets Encyclopaedia - http://cfa-www.harvard.edu/planets/ NASAs Kepler Mission - http://www.kepler.arc.nasa.gov/ European Space Agencys Eddington Mission - http://sci.esa.int/home/eddington NASAs Terrestrial Planet Finder Mission - http://planetquest.jpl.nasa.gov/TPF/ tpf_index.htmlEuropean Space Agencys Darwin Mission - http://sci.esa.int/home/darwin

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