The number of confirmed extra-solar planets (or exoplanets) is approaching 1000, with examples having masses and orbital characteristics far beyond the range present in the solar system. Most of the planets detected so far are gas giant planets like Jupiter or Neptune, but this is due to an observational bias, as these large planets are easier to detect. Technological advances are pushing inexorably towards the detection of Earth-like planets around Sun-like stars. As of December 2011, NASA’s Kepler mission has discovered several Earth-like planets: two planets Kepler 22-e and Kepler 22-f are the smallest exoplanets known so far, although too close to their star to be in its habitable zone, and Kepler 22-b which is about 2.4 times the size of the Earth and located in the habitable zone. The discovery of planets outside our solar system brings an opportunity to learn about the formation and evolution of planets in general, and improves our understanding of the planets in our own solar system by putting them into a larger context.
Artist's view of Kepler 22-b, a 2.4 Earth-size exoplanet orbiting in its star's habitable zone.
Artist's view of Kepler 20-e and Kepler 20-f, the smallest exoplanets known so far.
Studies
Although long-known from the Doppler reflex of their parent stars, direct imaging of planets has only recently become possible. High contrast imaging with the Gemini Planet Imager (GPI) is being used to image planets around other stars (Mike Fitzgerald).
Multi-channel spectroscopy of transiting planets is beginning to probe the upper atmospheres of planets that we will never directly see. The MOSFIRE infrared spectrometer is expected to make major advances in this area (Ian McLean, Brad Hansen, Ian Crossfield)
The atmospheres of white dwarf stars should be very pure, but some are contaminated by heavy elements that have recently fallen in from outside. UCLA researchers lead the investigation of these stars, using them to probe unseen disks of asteroids (Mike Jura, Siyi Xu, Shane Frewen)
Planets are even known to orbit neutron stars, presumably formed from debris left over after the supernova explosion of the central star (Brad Hansen)
Many of the known planets are “hot Jupiters”, whose extreme weather systems challenge and extend our ability to model circulation on gas giants (Jonathan Mitchell)