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  • Old Hubble Data Reveals New Planets
Oct 2011
BALTIMORE, Oct. 7, 2011 — A new analysis of data acquired by the Hubble Space Telescope in 1998 has unveiled evidence of two extrasolar planets that had gone undetected 13 years ago, but had since been found by sophisticated new ground telescopes. The modern work, achieved at the Space Telescope Science Institute, demonstrates a novel approach for hunting planets and analyzing their orbits.

Four giant planets are known to orbit the young, massive star HR 8799, which is 130 light-years away. In 2007 and 2008, the first three planets were discovered in near-infrared ground-based images acquired by the W.M. Keck Observatory and the Gemini North telescope by a team of astronomers led by Christian Marois of the National Research Council Canada. Marois and his colleagues then uncovered a fourth planet orbiting HR 8799 in 2010. This is the only multiple exoplanetary system for which astronomers have obtained direct snapshots.

Left: This is an image of the star HR 8799 taken by Hubble’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) in 1998. A mask within the camera (coronagraph) blocks most of the light from the star. In addition, software has been used to digitally subtract more starlight. Nevertheless, scattered light from HR 8799 dominates the image, obscuring any details. Center: Recent processing of the NICMOS data removes most of the scattered starlight to reveal three planets orbiting HR 8799. The positions of these planets coincide with orbits of planets observed by ground-based telescopes in 2007 and 2008. Right: This is an illustration of the HR 8799 exoplanet system based on the reanalysis of NICMOS data and ground-based observations. The positions of the star and the orbits of the four known planets are shown schematically. The size of the dots is not to scale with their true size. The three outermost planets (a, b, c) are detected in both the NICMOS and ground-based data. A fourth, inner planet (e) was detected in ground-based observations. The orbits appear elongated because of a slight tilt of the plane of the orbits relative to our line of sight. The size of the HR 8799 planetary system is comparable to our solar system, as indicated by the orbit of Neptune, shown to scale. (Images: NASA; European Space Agency; Space Telescope Science Institute, R. Soummer)

In 2009, David Lafreniere of the University of Montreal recovered hidden exoplanet data in Hubble images of HR 8799 acquired in 1998 with the telescope’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS). He identified the position of the outermost planet known to orbit the star, thus demonstrating the power of a new data-processing technique for retrieving faint planets buried in the glow of the central star.

A new analysis of the same archival NICMOS data by Remi Soummer of the Space Telescope Science Institute has recovered all three of the outer planets. The fourth, innermost planet is 1.5 billion miles from the star and cannot be seen because it is on the edge of the NICMOS coronagraphic spot that blocks the light from the central star.

By finding the planets in multiple images spaced over years of time, the orbits of the planets can be tracked. Knowing the orbits is critical to understanding the behavior of multiple-planet systems because massive planets can perturb each other's orbits.

“From the Hubble images, we can determine the shape of their orbits, which brings insight into the system stability, planet masses and eccentricities, and also the inclination of the system," Soummer said.

The three outer gas-giant planets have approximately 100-, 200- and 400-year orbits. This means that astronomers need to wait a very long time to see how the planets move along their paths. The added time span from the Hubble data helps enormously.

"The archive got us 10 years of science right now," Soummer said. "Without this data, we would have had to wait another decade. It’s 10 years of science for free."

Nevertheless, the slowest-moving, outermost planet has barely changed position in 10 years. "But if we go to the next inner planet, we see a little bit of an orbit, and the third inner planet we actually see a lot of motion," Soummer added.

The planets weren’t found in 1998 when the Hubble observations were first taken because the proper methods were not available yet. When astronomers subtracted the light from the central star to look for the residual glow of planets, the residual light scatter still overwhelmed the faint planets.

Lafreniere developed a way to improve this type of analysis by using a library of reference stars to more precisely remove the "fingerprint" glow of the central star. Soummer's team took Lafreniere's method a step further and used 466 images of reference stars taken from a library containing over 10 years of NICMOS observations assembled by Glenn Schneider of the University of Arizona.

Soummer's team further increased contrast and minimized residual starlight. They completely removed the diffraction spikes, which are artifacts common to telescope imaging systems. This allowed them to see two of the faint inner planets. The planets recovered in the NICMOS data are about 1/100,000th the brightness of the parent star when viewed in near-infrared.

Soummer next plans to analyze approximately 400 other stars in the NICMOS archive with the same technique, improving image quality by a factor of 10 over the imaging methods used when the data were obtained. From the NICMOS archive, Soummer's team will assemble a list of planetary candidates to be confirmed by ground-based telescopes. If new planets are discovered, they will once again have several years’ worth of orbital motion to measure.

The new results are to be published in the Astrophysical Journal.

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The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.
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