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, sophisticated software 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 Hubble 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, and 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.
Astronomers reviewing old images taken by the Hubble Space Telescope in 1998 have discovered evidence of two planets that were missed the first time around. The planets orbit a young star 130 light years from Earth. More recent observations in 2007 and 2008 led to the discovery of three gas-giant planets circling the star. Then, in 2010, researchers identified a fourth, innermost planet in the system. The star and its cadre of planets represent the only multiple exoplanetary system that has been identified to date with direct snapshots.
The more recent pictures were taken with near-infrared ground-based telescopes at the W.M. Keck Observatory and the Gemini North telescope, by Christian Marois of the National Research Council in Canada, and his team. Now, by meticulously reviewing old images taken by Hubble nearly a dozen years ago, astronomers have been able to identify the planets as Hubble captured them then, providing new insight into the nature of their orbits, mass and other characteristics. In 2009, David Lafreniere of the University of Montreal found two of the planets in Hubble's somewhat fuzzy old images. Now, Remi Soummer of the Space Telescope Science Institute in Baltimore, has identified the third planet buried in Hubble's image of the star system.
The discovery of multiple images, taken more than a decade apart, has allowed astronomers to calculate the planets' 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," says Soummer. The new results will be published in the Astrophysical Journal.
The three outer gas-giants have exceptionally long orbits, of 100-, 200-, and 400-years, respectively, which makes it difficult to glean information about them without waiting long enough for the planets to have progressed significantly in their orbits. The 12-year span between Hubble's archived old images and the newer data helped astronomer's perceive the planet's slow orbits around their distant star. "The archive got us ten years of science right now," says Soummer. "Without this data we would have had to wait another decade. It's ten years of science for free."
Even so, the outermost planet appears barely to have budged in ten 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," says Soummer.