1992

First Extrasolar Planets

Are there planets around other stars? For most of the history of astronomy, the question has been either too heretical to ask (Giordano Bruno was burned at the stake in 1600 because of it) or too technologically impossible to address. Recently, though, astronomers have discovered that the answer is a resounding yes.

By the late twentieth century, telescope and observational technology had advanced to the level where astronomers could detect the presence of planets around other stars using a variety of methods. One method exploits the fact that planets make their parent stars “wobble” in the sky; Jupiter’s gravity, for example, makes our own Sun’s path appear to wobble slightly as it orbits the center of the galaxy.

In 1992, a team of astronomers discovered that such slight wobbles could also be detected as very slight changes in the rotational speed of rapidly spinning neutron stars known as pulsars. In 1990, astronomers using the Arecibo radio telescope discovered a millisecond pulsar named PSR B1257+12 in the constellation Virgo. Monitoring the pulses every 6.22 milliseconds (msec) from this collapsed neutron star’s supernova remnant revealed small regular variations in the pulse rate. In 1992 researchers explained that this was caused by the gravitational pull of at least three planets in orbit around the pulsar. Mathematical modeling showed that two of the planets were likely around four times Earth’s mass, and a third was around 2 percent of Earth’s mass; all appear to orbit within 0.5 astronomical units of the pulsar.

This first confirmed evidence for the existence of extrasolar planets came as a surprise to most astronomers, because the expectation had been that planets would be found around other normal, main sequence stars like the Sun rather than around exotic objects like neutron stars. There is much speculation, then, about the nature of these particular pulsar planets. Perhaps they are the rocky and metallic cores of previous gas or ice giants that had their outer volatile layers stripped away by the supernova explosion that created the pulsar. Or perhaps they represent the results of a second round of solar nebula planet formation using remnant materials ejected by the supernova explosion.

Whatever the origin of these worlds, their detection appears to be robust, and so astronomers and planetary scientists now discovering and characterizing planets around other suns must also consider an even wider range of ways that “extreme” extrasolar planets can form and evolve in a variety of environments.

Artist’s conception of the planetary system detected around the pulsar PSR B1257+12 (lower left).