Other Planetary Systems

The first real observational evidence that other stars might produce planets came from infra‐red observations coming from cool dust around stars. Study of several nearby stars, such as Vega (the brightest star in the summer sky) and β Pictoris, suggests the surrounding dust is in the form of disks, which could be similar to the proto‐ planetary nebula that surrounded the Sun prior to the nebula's concentration into a few planets. The star‐forming regions in the direction of the Orion constellation show numerous young stars with surrounding material in the form of disks.

Present day technology does not allow astronomers to directly observe planets around other stars because the planets are lost in the glare of the central star. For instance, if Jupiter orbited the nearest star (other than the Sun), Jupiter's orbit would appear to an Earth observer only 3 arc seconds in radius. Jupiter itself would appear only 10 −9 as bright as that star and would therefore be invisible to existing ground‐based and satellite‐based telescopes. To detect the planet's existence, astronomers could look for the 12‐year oscillation in position (0.003”) of the central star as the star and planet orbited around a mutual center of mass. Alternatively, observers could look for a change in the radial velocity of the central star as it moves about the center of mass. Such velocities would be small — of the order of 13 m/s — but are detectable. At present, this latter technique has identified some fourteen other stars with planetary‐sized objects. (A planet is considered to have a mass less than about 13 times that of Jupiter. A mass greater than that would indicate an extremely faint, cool stellar object known as a brown dwarf, an object consisting of a body of gas that gives off a small amount of radiation but lacks sufficient mass to initiate the nuclear fusion that characterizes true stars.)

With such a limited number of planetary systems outside of our solar system to study, it is too early to draw major conclusions about the general nature of planetary systems and their process of origin. These discoveries of other planetary systems, however, do suggest that planets can form under unexpected circumstances. For example, two planetary systems have been found orbiting pulsars, the remnants of stars that exploded. Additionally, other planetary systems may display characteristics that differ substantially from the Sun's solar system, such as massive gas‐giant planets extremely close to the central star and planets with large orbital eccentricities.

Several nearby stars, such as Vega (the brightest star in the summer sky) and β Pictoris, show evidence of surrounding dusty disks, which could be similar to the proto‐planetary nebula that surrounded the Sun prior to the nebula's concentration into a few planets. The star‐forming regions in the direction of the Orion constellation show numerous young stars with surrounding material in the form of disks.