Imaging Other Worlds: The Gemini NICI Planet-Finding Campaign
by Michael C. Liu
NICI image of a young star near Earth with a candidate low-mass companion. The bright star in the center of the image is heavily dimmed by a barely transparent circular mask. The companion is at the 4 o'clock position. It would be undetectable by ordinary imagers, but is easily detected by NICI. Gemini NICI Planet-Finding Campaign
One of the most exciting developments in astronomy has been the discovery and characterization of exoplanets--planets orbiting stars other than our Sun. Our understanding of these objects has advanced by leaps and bounds over the past 15 years, since the discovery of the first exoplanet around the Sun-like star 51 Pegasi in 1995.
We now are entering a rich and special time for such studies. The current exoplanet census now exceeds 400 objects, most of them with masses comparable to the gas-giant planet Jupiter, which has a mass of one-thousandth that of the Sun, or 300 times that of Earth. Most exoplanets have been identified by the very subtle gravitational tug they exert on their host stars. While these discoveries have been groundbreaking, such planets are studied only indirectly. Other planets have been found as they transit across the face of their host stars, thereby diminishing the amount of light we detect from them. Such planets can be studied in great detail, but they orbit very close to their stars, making them unlike any planets in our solar system.
The next major advance is direct imaging, that is, taking actual digital images of planets around other stars. Directly detecting light from exoplanets opens the door to a host of new information about their properties (temperature, composition, etc.) and their formation. Direct detection is incredibly challenging. We see the planets in our own solar system because they reflect light from the Sun. Imaging the reflected light of exoplanets is currently impossible because the light reflected by the planets is swamped by the glare of their host stars, which are about a billion times brighter. However, when gas- giant planets are young, they also emit their own light at infrared wavelengths, by releasing the heat stored in their interiors at the time of formation. This makes young planets much easier to detect, since they are only(!) about one million times fainter than their parent star. In 2008, astronomers took the first direct images of young gas-giant exoplanets, thereby opening the door to this new way of learning about them.
My research group is leading an international effort known as the Gemini NICI Planet-Finding Campaign to expand the census of exoplanets detected by direct imaging. NICI (the Near-Infrared Coronagraphic Imager) is a powerful new instrument installed on the Gemini South 8.1-meter telescope in Chile, the twin of the Gemini North telescope on Mauna Kea. Most astronomical instruments can do multiple kinds of observations, but NICI was designed to do only one thing very well, namely, image exoplanets directly. NICI was built by Doug Toomey of Mauna Kea Infrared in Hilo, with heavy involvement by IfA faculty members Christ Ftaclas and Mark Chun, and funding from NASA. NICI is designed as a complete end-to-end system for exoplanet imaging. It is based on an advanced adaptive optics system that corrects for the blurring of astronomical images caused by Earth's turbulent atmosphere.
Since December 2008, we have been using NICI to obtain very sensitive images of 300 nearby young stars (within about 200 light-years) to search for gas-giant planets in emitted light. The campaign will take about three years and obtain a total of 50 nights of observations. This makes it the largest single program ever carried out at the Gemini telescopes. By having a major campaign, it is possible to assemble a large-scale coherent science program with a unified set of goals, observing methods, and data analysis techniques. While the observations are carried out in Chile, the planning and analysis are done here at the IfA, with postdoctoral fellow Zahed Wahhaj and Hubble Fellow Beth Biller being key contributors.
The campaign is about halfway finished. We have imaged about 180 stars so far, already making NICI the largest direct imaging search to date. However, in some sense, we are just getting started. Many of the stars we have imaged have very faint companions next to them, but we do not yet know what these are. They could be gas-giant planets in orbit around these stars, or they could be ordinary background stars that are much farther away but by chance are projected on the sky nearby. The way to check is to make a second set of observations about one year later to see if the faint candidates move with the target star, thereby proving that they are gravitationally bound together. We are in the process of doing that right now. Once candidates are confirmed, we can learn a great deal about the properties of young planets through dedicated follow-up observations to measure their brightnesses, colors, atmospheres, and orbits.