- 5.1: Comparison with Other Planetary Systems
- The first planet circling a distant solar-type star was announced in 1995. Twenty years later, thousands of exoplanets have been identified, including planets with sizes and masses between Earth’s and Neptune’s, which we don’t have in our own solar system. A few percent of exoplanet systems have “hot Jupiters,” massive planets that orbit close to their stars, and many exoplanets are also in eccentric orbits.
- 5.2: Planets beyond the Solar System- Search and Discovery
- Several observational techniques have successfully detected planets orbiting other stars. These techniques fall into two general categories—direct and indirect detection. The Doppler and transit techniques are our most powerful indirect tools for finding exoplanets. Some planets are also being found by direct imaging.
- 5.3: Exoplanets Everywhere - What we are Learning
- Although the Kepler mission is finding thousands of new exoplanets, these are limited to orbital periods of less than 400 days and sizes larger than Mars. Still, we can use the Kepler discoveries to extrapolate the distribution of planets in our Galaxy. The data so far imply that planets like Earth are the most common type of planet, and that there may be 100 billion Earth-size planets around Sun-like stars in the Galaxy. About 2600 planetary systems have been discovered around other stars.
- 5.4: New Perspectives on Planet Formation
- The ensemble of exoplanets is incredibly diverse and has led to a revision in our understanding of planet formation that includes the possibility of vigorous, chaotic interactions, with planet migration and scattering. It is possible that the solar system is unusual (and not representative) in how its planets are arranged. Many systems seem to have rocky planets farther inward than we do, for example, and some even have “hot Jupiters” very close to their star.
Thumbnail: This painting, commissioned by NASA, conveys the idea that there may be many planets resembling Earth out there as our methods for finding them improve.