New Worlds Imager
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The New Worlds Discoverer is a project funded by NASA Institute for Advanced Concepts (NIAC), headed by Dr. Webster Cash of the University of Colorado at Boulder in conjunction with Ball Aerospace & Technologies Corp., Northrop Grumman, Southwest Research Institute and others. The project plans to build a large occulter in space designed to block the light of nearby stars in order to observe their orbiting planets. The observations could be taken with an existing space telescope, possibly the James Webb Space Telescope when it launches, or a dedicated visible light telescope optimally designed for the task of finding exoplanets.
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[edit] Purpose
Currently, the direct detection of extrasolar planets (or exoplanets) is extremely difficult. This is primarily due to:
- Exoplanets appear extremely close to their host stars when observed at astronomical distances. Even the closest of stars are several light years away. This means that while looking for exoplanets, one would typically be observing very small angles from the star, on the order of several tens of milli-arcseconds. Angles this small are impossible to resolve from the ground due to Astronomical seeing.
- Exoplanets are incredibly dim compared to their host stars. Typically, the star will be approximately a billion times brighter than the orbiting planet. This makes it near-impossible to see planets against the star's glare.
The difficulty of observing such a dim planet so close to a bright star is the obstacle that has prevented astronomers from directly photographing exoplanets. To date, only one exoplanet has been photographed. This exoplanet, 2M1207b, is in orbit around a star called 2M1207. Astronomers were able to photograph this planet because it is a very unusual planet. Specifically it does not suffer from the two difficulties mention above. It is very far from the host star, approximately 55 astronomical units (about twice the distance of Neptune). Furthermore, the planet is orbiting a very dim star, known as a brown dwarf. Because the planet is so far from its dim host star, it is not lost in the glare. However, this is a very specific scenario, and would be unlikely to be useful in finding Earth-like planets capable of supporting life.
To overcome the difficulty of distinguishing a planet in the glare of a bright star, the New Worlds Discoverer would block the star's light with an occulter. The occulter would block all of the starlight from reaching the observing, while allowing the planet's light to pass undisturbed.
[edit] Design
Traditional methods of exoplanet detection all rely on indirect means of inferring the existence of orbiting bodies. These methods include:
- astrometry - watching a star move slightly due to the gravitational influence of a nearby planet
- Observing doppler shifts of the stars spectrum due to the star's movement
- Observing the amount of light from a star change as an extrasolar planet transits the star, preventing a portion of the light from reaching the observer.
- Pulsar timing
- Gravitational microlensing
- Observing radiation from Circumstellar disks in the infrared.
All of these methods provide convincing evidence for the existence of extrasolar planets, however none of them provide actual images of the planets.
The goal of the New Worlds Discoverer is to block the light coming from nearby stars with an occulter. This would allow the direct observation of orbiting planets. The occulter would be a large sheet disc flown thousands of kilometers along the line of sight. The disc would likely be several tens of meters in diameter and would fit inside existing expendable launch vehicles and be deployed after launch.
One difficulty with this concept is that light incoming from the target star would diffract around the disc and constructively interfere along the central axis. Thus the starlight would still be easily visible, making planet detection impossible. This concept was first famously theorized by Siméon Poisson in order to disprove the wave theory of light, as he believed the existence of a bright spot at the center of the shadow to be nonsensical. However Dominique Arago experimentally verified the existence of the spot of Arago. Fortunately this effect can be negated by specifically shaping the occulter. By adding specially shaped petals to the outer edge of the disc, the spot of Arago will disappear, allowing the suppression of the star's light.
This technique would make planetary detection possible for stars within approximately 10 parsecs (about 32 light years) of Earth. It is estimated that there could be as many as several thousands of exoplanets within that distance.
[edit] Possible Discoveries
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[edit] Mission Architecture
There are three possibilities for the New Worlds mission.
- New Worlds Discoverer would use either an existing space telescope (like the soon to be launched James Webb telescope), or a several meter diameter visible light telescope dedicated to finding exoplanets. The size of the starshade could be optimized for the observing telescope. The possibility of two starshades is also a consideration. One starshade to point towards the desired target while the other moves into position for the next target. This would eliminate the time delay in observing different systems and allow for many more targets to be observed in the same timespan.
- New Worlds Observer would use two spacecraft and two starshades increase the angular resolution and allow better analysis of the exoplanet's composition
- New Worlds Imager would use five spacecraft/starshades. This would allow observers to resolve the planet and obtain true planetary imaging.
[edit] Current Status
Dr. Cash was granted $400,000 US for initial research on this project by NIAC in October of 2005. A proposal was submitted to NASA in early 2006 for a discovery class mission. Three competing projects will be chosen for further study in late 2006.
