1995

Planets Around Other Suns

The discovery of the first extrasolar planets around pulsar B1257+12 in 1992 compelled astronomers to search even harder for evidence of extrasolar planets around “normal” main sequence or Sun-like stars. For decades it was known that stars in binary systems can show “wobbles” in their Proper Motion across the sky because both stars are actually orbiting the system’s center of mass. In theory, the same kinds of wobbles—though much smaller—should be seen if a giant Jupiter-like (or larger) planet is in orbit around a single star. A breakthrough came when astronomers realized that they didn’t need to measure the precise position of the star over time; instead, they could use the Doppler Shift of the star’s spectrum to deduce its wobbling motion from the star’s radial velocity—the part of that wobble directed toward or away from Earth.

In 1995, using this method, the first extrasolar planet was “found” orbiting the nearby Sun-like star 51 Pegasus (51 Peg). Based on the amount of wobble it induced in 51 Peg, and the timing of the Doppler-shifted spectral variations, planet 51 Peg b was inferred to be a gas giant many times the size of Jupiter, and orbiting very close—only 0.05 astronomical units—from its star. More than five hundred other planets around other nearby stars have been found since then using the radial velocity method. Most of these planets are known as “hot Jupiters” because they are large and they also orbit extremely close to their parent stars. Big planets, close in, are the easiest to find using the radial velocity method, so hot Jupiters may not be typical.

Other ways to find extrasolar planets, besides radial velocity and pulsar timing, are to watch for them passing in front of (transiting) their parent stars (the goal of NASA’s Kepler mission), to detect them by gravitational lensing, or to just directly image them through the glare of their parent stars. To date, most known extrasolar planets are still gas or ice giants. However, astronomers are now starting to find and catalog many Earth-size or super Earth-size worlds around nearby Sun-like stars using these methods. We are only at the tip of the iceberg of extrasolar planet discoveries!

Artist’s conception of the “hot Jupiter” planet HD189733b, which Hubble Space Telescope observations show has methane and water vapor in its atmosphere.