Chasing Shadows: Discovering Planets around Distant Stars

[ILLUSTRATION OMITTED] The solar eclipse coming August 21 offers students the opportunity not only to explore the geometry of our solar system but also to learn about exoplanets transiting distant stars. Students can glimpse a great frontier of science: the search for other worlds and life (see "On the web"). An astronomical transit occurs when we see a smaller body cross the face of a larger one. From Earth, we can observe both Venus and Mercury transit the Sun with ground and space-based telescopes, and those transits can last for several hours. These events are fairly rare. The last transit of Venus occurred in 2012 (6 hours and 40 minutes), and the next will be in 2117. Mercury transits the Sun more often, with the next transit in November 2019 and nine more this century. The closer a planet is to its star, the more likely it will transit as viewed from a more distant planet. So it makes sense that Mercury, being the closest planet to the Sun, is most likely to transit the Sun as viewed from Earth. Both Venus and Mercury block only a tiny amount of sunlight during a transit. Likewise, planets transiting distant stars block only a tiny amount of the star's light. A special case of an astronomical transit is when the Moon and Sun--which are the same apparent size--align precisely, creating a solar eclipse, when the Moon fully or partially blocks the Sun. Searching for exoplanets Astronomers worldwide search for exoplanets. In 1992, using radio astronomy, Polish-born astronomer Aleksander Wolszczan and Canadian astronomer Dale Frail discovered a multi-planet system around a millisecond pulsar PSR 1257+12 (Wolszczan and Frail 1992). Pulsars are spinning neutron stars, the remains of huge stars that would have reduced their planets to cinders as they evolved. Before NASA's Kepler space observatory mission launched in 2009, radial velocity or "wobble" was the most productive method for finding exoplanets. In this method, astronomers look for periodic Doppler shifts in a star's spectrum produced by the changes in the star's apparent velocity as the star and its planet(s) orbit a common center of gravity. The star's spectral lines are red-shifted as it moves away from Earth and blue-shifted as it moves toward Earth, reflecting the orbital dance of the star and its planet(s). On October 6, 1995, Swiss astronomers Michel Mayor and Didier Queloz announced the discovery of 51 Pegasi, a planet about half the mass of Jupiter that orbits its star in just over four days (Mayor and Queloz 1995). "51-Peg" was the first "hot Jupiter" of many discovered. Hot Jupiters, also called "roaster planets," are Jupiter-size planets that orbit close to their stars (orbits of ~2 to 100 days), and are heated to high temperatures. These large, hot planets were thought to be the most common type of exoplanet because the radial velocity method is most sensitive to massive planets--like Jupiter and Saturn--that orbit close to their parent stars (Figure 1). The radial velocity signature of smaller exoplanets like Earth is lost in the observational noise. A different technique was needed to find Earth-size exoplanets where life might exist. In 1992, NASA Ames Research Center scientists Bill Borucki and David Koch led a team to propose a space telescope to search for transits of exoplanets orbiting distant stars. After eight years of experiments and additional proposals, NASA selected the Kepler spacecraft (Koch et al. 2010) for a mission to answer the question, "How common or rare are planets similar to our own Earth?" Development began in 2001, and the spacecraft launched in March 2009, 25 years after Borucki's first paper on using transits to find exoplanets (Borucki and Summers 1984). Often, great achievements in science require true persistence. The Kepler space telescope chases the shadows cast by exoplanets as they transit their stars. The telescope is a special-purpose reflecting telescope with a wide-field Schmidt camera with a 0. …