In Search of New Worlds
NASA Exoplanet Exploration Program (ExEP) science and missions represent a voyage of unprecedented scope and ambition, promising insight into humankind's most timeless questions: Where did we come from? Are we alone?
The primary goal of these explorations is to discover and characterize planetary systems and Earth-like planets around nearby stars. The scientific investigations and the missions that carry them out are designed to build on each other's success, each providing an essential step forward toward the goal of discovering habitable planets and evidence of life beyond.
The first phase of exploration entails building an understanding of how many and what kinds of planetary systems nature has provided. Much of this work has been done with ground telescopes around the world, pushing the limits of their ability to detect smaller planets through Earth's turbulent atmosphere. The Kepler mission, in the stillness of space, is probing deeper into the galaxy to detect smaller and more Earth-like planets around other stars. Additional NASA and international missions, as well as larger and more sensitive ground observatories, will extend this exoplanetary census much farther by the end of the decade. At the same time, important investigations will tell us about the environments around stars with exoplanets, such as dust and debris in disks that could make further measurements of the planets more difficult.
Ultimately, the goal is to see whether there are exoplanets that show signs of possible life that we know how to interpret. The evidence will be primarily in the form of detailed spectroscopic studies of the atmospheres of extrasolar planets. For a planet to host life, our expectations are that the planet would require liquid water on the surface. We do not assume that the planet would necessarily resemble Earth itself. It would lie in an orbit that is neither too close nor too far from its star, so that liquid water could exist over geological timescales, and its atmosphere would contain the right balance of gases that could support life. Moreover, the atmosphere of the planet would be altered by the presence of life, such that only the existence of living organisms could account for the unusually high levels of gases in its atmosphere. (It's not that scientists reject any possibility of other life forms than what we know, but we don't yet know what other life forms could exist or how to look for them.)
The volume of space that would be explored for habitable planets would be limited to nearby stars. In this context "nearby" is understood to be stars that lie within approximately 20 parsecs (60 light-years) from our sun. This is roughly the distance that we can explore using technology available in the next couple of decades.
Breaking through technology barriers
The technological challenge of the ExEP program is essentially one of high dynamic range sensing coupled with high angular resolution imaging. This is true because extrasolar planets appear in the sky as extremely faint objects (10-8 - 10-10 times dimmer than the host star, depending on wavelength and type of planet) located in extreme close proximity to their host stars. The light from the planet must first be resolved separately from the starlight, and the glare of the starlight must be then be suppressed to allow atmospheric spectroscopy of extrasolar planets.
Large, light-weight mirror technology to achieve the required angular resolution in visible light is one key area. High-dynamic range imaging requires near perfect wavefronts so that speckles of starlight, created by subtle aberrations in the telescope optics, won't create a glare that would obscure the presence of faint planets. Wavefront control, and various techniques of coronagraphy to achieve starlight suppression are also being developed in the Exoplanet Exploration Program.
Optical/infrared interferometry is another key technology area of the ExEP program for the 2nd generation of missions. Traditional telescope designs have an achievable resolution that is limited by the diameter of the telescope's primary mirror. When telescopes are combined in an array, the achievable resolution is limited instead by the separation of the telescopes. In principle, arrays can be built that would have the resolution of a single telescope whose primary mirror was several hundred meters in diameter. When implemented in space, free from the distortions of the turbulent atmosphere, such interferometers hold the promise of resolving the planets around nearby stars.
Developing future explorers
The exploration of neighboring planetary systems will be a long-term project. The questions about life on other planets and around other stars have been recorded for over 3,000 years. (See more about the history of planet-hunting with the PlanetQuest Timeline) We may be able to answer those questions in just a few more decades. It is an exploration that excites a great many people.
In order to help develop future generations of scientific explorers, ExEP supports the Sagan Exoplanet Fellowship Program through the NASA Exoplanet Science Institute. The Sagan program supports typically six or seven young researchers for periods of three years, selecting new ones annually.
The ultimate stakeholder in the adventure of exoplanet exploration is the public who underwrites it. ExEP is especially interested in making materials and information available so that all can appreciate and understand the new scientific discoveries and the challenges ahead, and to engage and inspire students to take interest in technical and scientific matters. The PlanetQuest website serves as the portal to this part of the program.