Stellar Night-Goggles

Wide-field Infrared Survey Explorer.
Image Credit: NASA/JPL

A new NASA mission will scan the entire sky in infrared light in search of nearby cool stars, planetary construction zones and the brightest galaxies in the universe.

Called the Wide-field Infrared Survey Explorer, the mission has been approved to proceed into the preliminary design phase as the next in NASA’s Medium-class Explorer program of lower cost, highly focused, rapid-development scientific spacecraft. It is scheduled to launch in 2008.

Like a powerful set of night vision goggles, the new space-based telescope will survey the cosmos with infrared detectors up to 500,000 times more sensitive than previous survey missions. It will reveal hundreds of cool, or failed, stars, called brown dwarfs, some of which may lie closer to us than any known stars.

"Approximately two-thirds of nearby stars are too cool to be detected with visible light," said Principal Investigator Dr. Edward Wright of the University of California, Los Angeles, who proposed the new mission to NASA. "The Wide-field Infrared Survey Explorer will see most of them."

Brown dwarfs are a relatively new class of objects discovered in the mid-1990s that are too small to ignite hydrogen fusion and shine as stars, yet too big to be considered planets.

One possible explanation for the origin of brown dwarfs is that they are born in the same way as stars. Stars form in huge interstellar clouds in which gravity causes clumps of gas and dust to collapse into "seeds," which then steadily pull in more and more material until they grow to become stars. However, when this process is studied in detail by computer, many simulations fail to produce brown dwarfs. Instead, all the seeds grow into full-fledged stars. This result led some astronomers to wonder if brown dwarfs and stars are created in different ways.

Click here for larger image. Brown dwarfs lack sufficient mass (at least 75-80 Jupiters) to ignite core hydrogen fusion.
Credit: American Scientist/Linda Huff

Testing these ideas for the birth of brown dwarfs is hampered by the fact that brown dwarfs are normally extremely faint and hard to detect in the sky. For most of their lives, they are not hot enough to ignite hydrogen fusion, so they do not shine brightly like stars, and instead are relatively dim like planets. However, for a short time immediately following their birth, brown dwarfs are relatively bright due to the leftover heat from their formation.

As a result, brown dwarfs are easiest to find and study at an age of around 1 million years, which is newborn compared to the 4.5 billion year age of our Sun.

The telescope will also provide a complete inventory of dusty planet-forming discs around nearby stars, and find colliding galaxies that emit more light – specifically infrared light – than any other galaxies in the universe. In the end, the survey will consist of more than one million images, from which hundreds of millions of space objects will be catalogued.

"The mission will complete the basic reconnaissance of the universe in mid-infrared wavelengths, providing a vast storehouse of knowledge that will endure for decades," said Dr. Peter Eisenhardt, project scientist for the mission at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. "This catalogue of data will also provide NASA’s future James Webb Space Telescope with a comprehensive list of targets."

Since the discovery of brown dwarfs less than a decade ago, astronomers have come to think that they might be more numerous than the visible stars in the sky. In broad terms, 80 percent of the nearby stars are (red) dwarfs, 10 percent are solar-type stars, and 10 percent are more massive. There are probably around the same number of brown dwarfs as stars within the immediate solar neighborhood.

The true abundance of brown dwarfs, sub-brown dwarfs and extrasolar planets is not known, and large areas of the sky still need to be explored.