Over the last two centuries observations of our own solar system have refined our understanding of planet formation within the context of the nebular hypothesis proposed by Kant (1755) and Laplace (1796). In the last decade a revolution has unfolded as precision radial velocity measurements revealed the first exoplanets, and unveiled an unimagined diversity of planetary systems. The core accretion model for planet formation has been severely challenged by these observations, but remains the dominant paradigm. Direct imaging of exoplanet systems, which is now within our grasp, will pose fundamental new challenges to our understanding of the origin and evolution of planetary systems.
GPI will detect exoplanets in the outer regions (a > 5 AU) of the planetary systems of main sequence stars in the solar neighborhood. By probing large semimajor axis separations that are inaccessible to indirect methods, e.g., Doppler techniques, GPI will reveal the zone where the majority of gas giant planets are expected to reside. GPI surveys will:
- Establish directly the occurrence rate of planetary systems;
- Provide critical tests of the core accretion model, including a census of regions where gas giants can only form via gravitational instability;
- Shed light on the origin of hot Jupiters by finding planets that migrated outwards;
- Show whether or not the architecture of our own planetary system, with gas giants located between 5–10 AU is unique.
GPI spatially resolves exoplanets from their parent star and uses an integral field spectrometer to record their spectra at low resolution. About half of GPI detected exoplanets are expected to be cool enough for their atmospheres to contain water clouds. Thus, GPI spectra will provide our first view of the vast terra incognita of cool planetary atmospheres, with temperatures between that of Jupiter and the coolest T dwarfs. GPI may even see the ammonia cloud decks in the coolest exoplanets it detects.
Direct Detection of Exoplanets
In addition to the figure to the left showing a simulated planet detection with GPI, the figures below quantify some of the planet detection statistics with a survey of nearby field stars using GPI.
Spectra of Exoplanets
The direct detection of extrasolar planets enables observations of their chemical composition via spectroscopy. The spectrum of a Jupiter reveals the existence of water, ammonia, and methane. The depth and shape of each profile leads to an understanding of the temperature and gravity of planet.