The image above involves some artistic license. As suggested on Behind the Speculations, gas giants orbiting a sun-like star at distances similar to the Earth are likely to have clouds of water ice, with white a characteristic color. These water clouds may be obscured by higher layers of gas, primarily methane. Methane scatters blue light weakly, so these deeper cloud regions will have a slight bluish tinge.
John Whatmough, the author of the extrasolar.net page referenced above died in 2005 soon after it was finished. His heirs have kept the website. John's presentation on the science behind extrasolar speculations is still one of the clearest and most comprehensive on the web.

The first confirmed discovery of extrasolar planets (planets around another star - also called exoplanets) occured in 1992. Radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar. In 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi). This discovery was made at the Observatoire de Haute-Provence and ushered in the modern era of exoplanetary discovery.

Technological advances allowed astronomers to detect exoplanets indirectly by determining their gravitational influence on the motion of their parent stars. The planets are usually discovered by measuring the change in Doppler shift of the star's light, resulting from the star orbiting a common center of mass with a companion planet.

Several extrasolar planets were detected by observing the variation in a star's apparent luminosity as a planet passed or transited in front of it. The occasional transit of Venus across the sun is an example from our own Solar System.

With the announcement of 32 new exoplanets discovered by HARPS, by the fall of 2009, more than 400 exoplanets had been discovered. Most of them are huge and hot gas planets (similar or larger in mass to Saturn or Jupiter) orbiting very close to their sun.

Given the current early stage in planet searching technology, this preponderance of giant planet discoveries in close solar orbit should not be surprising. These would be the most noticeable planetary bodies given the degree of sensitivity of the available instruments.

As sensitivity increases in the future and new methodologies become common, the average range in planet distance from the parent star should increase and the average observed planet size should decrease. Astronomers have recently discovered that terrestrial planets might form around many, if not most, of the nearby sun-like stars in our galaxy.

If you want to keep apprised of the latest scientific findings and theories regarding exoplanets, check out the Exoplanet News. It is a monthly newsletter listing abstracts of the latest findings in the exoplanet field.

Planet Searchers

The California Planet Survey team is one of the most successful planet searchers, having discovered 167 planets by the beginning of 2010. These scientists use telescopes at the University of California's Lick Observatory near San Jose and the Keck Observatory in Hawaii. The Geneva Extrasolar Planet Search is a number of observational programs managed out of Versoix in France near Geneva, Switzerland. This team discovered 94 planets by July 2007. The Anglo-Australian Planet Search team uses the Anglo-Australian Observatory near Coonabarabran, NSW, Australia. As of July 2007 it had discovered some 25 planets. SuperWASP is the UK's extra-solar planet detection program involving a consortium of eight academic institutions monitoring the sky for planetary transit events. As of November 2007 it had discovered five planets. An international symposium on exoplanets was held in Suzhou, China in October 2007.

The Exoplanet Data Explorer is an interactive table and plotter for exploring and displaying data from the Exoplanet Orbit Database, a carefully constructed compilation of quality, spectroscopic orbital parameters of exoplanets orbiting normal stars. The Planetary Society presents current extrasolar planet discoveries with a good explanation of current methods for finding these planets. The Interactive Extra-solar Planets Catalog maintained by the Paris Observatory shows 342 planets discovered in total by March 2009. The NASA PlanetQuest site gives a good overview of the search for planets around other stars. The PlanetQuest site has a continually updated atlas of all exoplanets discovered. It even includes a 3-D map of stars with planets in our stellar neighborhood. The Guide to Extrasolar Planets contains a wealth of information on these discoveries. A German website (English version) focuses on some of the more interesting exoplanets.

Systemic is a collaboration of astronomers and interested individuals aimed at understanding the possible structure and composition of extrasolar planetary systems. The Tau Zero Foundation focuses on the subject of "practical starflight." Its associated Centauri Dreams is a news blog covering the latest developments in the hunt for planets beyond our solar system, progress in spaceflight technology, and the sociological implications of interstellar exploration.

'Alien Earths' enables you to build a hypothetical extrasolar planetary system and see how it functions over time. Click on 'planet families' under 'star & planet formation' to give it a try.

The table below summarizes known current and future instruments and missions participating in the search for planets around other stars:

Instruments & Space Missions Searching for Extrasolar Planets
COROT	Spitzer Space Telescope	Large Binocular Telescope Interferometer	EPOCh
Keck Interferometer	MOST	Hubble Space Telescope	Automated Planet Finder Telescope
Epoxi	HARPS	Kepler Mission	TEDI
Future Exoplanet Missions & Instruments
SIM PlanetQuest	Lyot	Terrestrial Planet Finder	GAIA
New Earths Facility	Darwin	European Extremely Large Telescope	James Webb Space Telescope
		Thirty Meter Telescope

Actual travel to the stars and landing on new Earths? Such a journey can't occur based on current* scientific knowledge. Faster than light travel requires a major jump into the unknown in technology and our understanding of the universe. Given the significant mysteries associated with our current knowledge (dark energy, dark matter, other dimensions and alternative universes, to name a few), a revolutionary discovery in space travel within 200 years may not be wildly unreasonable.

*Although at least one scientist had presented a theory that would enable such travel through 'hyperspace'. His ideas were not accepted by his peers.