Are there life-sustaining planets around other stars?
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 the 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 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 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, or 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 of state-of-the-art optical and spectroscopic instruments that were limited in spectral and spatial resolution. But how quickly things changed.
In 1994, astronomers detected the first extrasolar planets orbiting 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 was not conducive to any of the planets bearing life, 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 Pegasus, is much more like our Sun with respect to its temperature, size, rotation speed and emitted radiation. The newly found planet orbiting 51 Pegasi had a size comparable to Jupiter or Saturn, but it was positioned extremely close to its parent star- closer than Mercury sits from our own Sun! Although not a good candidate for a life-bearing planet, it was the first evidence of an extrasolar planet around a Sun-like star.
Distribution of planets around nearby stars (Courtesy G. Marcy / E. Williams)
Since then, many other Jovian-sized planets have been found orbiting other Sun-like planets. Some of them are orbiting extremely close to their parent star, like the 51 Pegasi planetary system, while others are found at distances comparable to where Mars and Jupiter lie in our solar system. The accompanying chart shows distance and size relationships for several extrasolar planetary systems.
Plans for continued searches
The right size, the right distance, and 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 1,000 stars to our Sun hopes to find some evidence of planets very much like the Earth. "Earth-type" planets, the most conducive 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 that allow the planet's temperature and atmospheric pressure to support 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 the gravitational pull of planetary members, or changes in the apparent brightness of the host star due to transits and microlensing events. Each of these methods can indicate the presence of external bodies around nearby stars.