For several months during the year, in the twilight hours of early evening or the hours before sunrise, one can catch a glimpse of one or more bright planets. Reflecting light from our Sun, the brightest of our nine planets can be easily seen amid the backdrop of stars and blackness of the sky.
A rare treat to backyard astronomers who view the skies, the planets in our solar system are especially well studied by astronomers and planetary geologists. The motions of the planets have been well established for centuries, the chemical abundances in their atmospheres have been analyzed more recently and the changes in their structure, atmospheres and physical appearance have been noted with repeated observations from ground- and space-based observatories. We, as a scientific community, know the planets quite well, enough to make predictions on the possibility of finding planets around other nearby stars.
Or do we?
The detection of planets outside our solar system
Until the 1990s, the nine planetary members of our own solar system were the only known planets. Astronomers did not believe that our Sun's environment was particularly unique to be the only planet producer in the universe. However, there had not yet been any evidence of other planets outside our solar system. This was a product of not only the length of time astronomers were trying to note changes in other stars due to the existence of orbiting planets, but by state-of-the-art optical and spectroscopic instruments that were limited in spectral and spatial resolution. But how quickly things changed.
In 1991 radio astronomers detected the first extrasolar planets orbiting none other than a dying pulsar star. This star was left over from a supernova explosion in the constellation Virgo. The pulsar's beam of radiation changed slightly due to the gravitational pull of three Earth-sized objects revolving around the host star, PSR B1257+12. Although the deadly radiation from the pulsar is not condusive to any of the planets being life-bearing, it was the first example of a star other than our Sun producing planets.
In 1995 Swiss astronomers found another extra-solar planetary candidate. It was discovered by noting a slight perturbation in the position of 51 Pegasi, a star in our nearby galactic neighborhood. This star, found in the constellation of Pegasus, is much more like our Sun with respect to its temperature, size, rotation speed and emitted radiation. The newly found planet orbiting 51 Peg had a size comparable to Jupiter or Saturn, however, it was positioned extremely close to its parent star- at a position closer than Mercury sits from our own Sun! Although not a good candidate for a life-bearing planet, it was the first ever evidence of an extrasolar planet around a Sun-like star.
Distribution of planets found around nearby stars (Courtesy G. Marcy / E. Williams)
Plans for Continued Searches
The right size, the right distance, the right temperature: we finally have evidence for the existence of extrasolar planets that may be candidates for life-bearing planets as well. Technology involved with many of the Origins Missions makes the objective of finding possible life-bearing extrasolar planets a soon-to-be reality. A search of the nearest 1000 stars to our Sun hopes to find some evidence of planets very much like the Earth. "Earth-type" planets, the most condusive to sustaining life, are required to be solid bodies (unlike the gaseous Jovian planets found in our outer solar system) with masses roughly between 0.5 - 10 Earth masses. These planets need to be found at distances from their parent star such that the planet's temperature and atmospheric pressure are supportive of the existence of liquid water.
Direct methods for examining stars in our nearby neighborhood for the existence of planets would involve the detection of starlight reflected by an orbiting planet or perhaps by the emitted thermal radiation from the planet itself. Optical reflected light and infrared thermal radiation could both be analyzed spectroscopically (provided astronomers could actually detect this gentle signal amid the powerful fury of its host star) to present information about the size, sunlight reflectivity (albedo) and temperature of a planet.
Indirect methods of planetary detection include measurements of radial velocities of nearby stars, measurements of pulsar rates, actual changes in the position of a host star based on gravitational mull of planetary members, or changes in the apparent brightness of the host star due to transits and microlensing events. Each of these methods is able to indicate the presence of external bodies around nearby stars.
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Reproduced from http://origins.stsci.edu