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IfA-Built Instrument Finds Star's Cold Companion

by Louise Good


HiCIAO mounted on the Subaru Telescope. Photo by Klaus Hodapp.


A new instrument called the High-Contrast Coronographic Imager for Adaptive Optics  (HiCIAO for short) has made the first direct observation of a planetlike object orbiting a star similar to the Sun. The instrument was designed and built at the IfA and mounted on Subaru, one of the four very large optical/infrared telescopes on Mauna Kea.

"This infrared camera was designed specifically to obtain images of planets around other stars," explained IfA Associate Director Klaus Hodapp, who has served as the UH principal investigator for HiCIAO and is a member of the international team of astronomers that made the discovery. "Taking an image of an exoplanet is a very difficult task that astronomers are just learning to master," said Hodapp. Exoplanets--planets around stars other than our Sun--are faint objects that, relatively speaking, are located very close to their much brighter stars. "The first step in the detection process is to sharpen up the image by removing much of the blur caused by the turbulence of Earth's atmosphere. This is the adaptive optics part of the process. The second step is to mask off the image of the star itself, a technique commonly called coronography because it was first developed to study the Sun's corona," Hodapp explained.

After these two steps, there is still a lot of stray light from the star in the form of hundreds of small bright spots, or  "speckles," around the position of the star. The final step in the detection process uses techniques to distinguish the planet from these hundreds of speckles. HiCIAO offers three different techniques for doing this: The first is spectroscopy, because the planet may have a spectrum different from the speckles, which are scattered starlight.

The second is polarimetry, because the planet may have different polarization properties from the speckles, which have the same small polarization as the star. The third technique, called angular differential imaging, uses the fact that in alt-azimuth telescopes such as Subaru, which can move both horizontally and vertically, the field of view tends to rotate over the course of the night. Therefore, an exoplanet will appear to rotate around the image of the star over the course of the night, while speckles caused by telescope imperfections will remain stationary. This latter technique was used in the discovery of the companion object.

The planet around the star GJ 758 is called GJ 758 B, and was discovered during test runs of HiCIAO in May and August 2009 by the team responsible for the development of the angular differential imaging technique. "This early success illustrates HiCIAO's potential for many more discoveries, now that its main science mission has begun," stated Hodapp.

HiCIAO image
The August 2009 infrared discovery image of GJ758 B (marked with solid arrow). A possible second object is pointed out with dashed arrows. The white plus sign marks the location of the host star GJ 758. Reproduced by permission of the AAS.

GJ 758 B could be either a large planet or a failed star, also known as a "brown dwarf." It orbits the Sun-like star GJ 758, which is located 50 light-years from the Sun, and is estimated to be 10 to 40 times as massive as Jupiter. An object is generally considered to be a brown dwarf rather than a planet if its mass is between 13 and 75 times the mass of Jupiter. At a temperature of around 600 F, GJ 758 B is the coldest companion to a Sun-like star ever recorded in an image.

While astronomers have found over 400 exoplanets to date, most have been discovered using nonimaging techniques, such as measuring the gravitational effect the planet has on its star or measuring a decrease in light that occurs when the planet crosses in front of its star. Of the others that have been directly viewed, most have been farther from their star than GJ 758 B is, or around stars with temperatures far above the average temperature of GJ 758 or our Sun.

GJ 758 B is as far from its star as Neptune is from the Sun. A large planetlike object orbiting at this location challenges traditional thinking about planet formation. Theorists think that most larger planets form either closer to or farther from stars, but not in the location where GJ 758 is now. This discovery and the many that are likely to follow could help them refine their ideas about the formation of planetary systems.