Planet hidden in Hubble archives

A new way to process images reveals an extrasolar planet that had been hiding in an 11-year-old Hubble picture

Like tiny jewels not yet uncovered, a trove of previously unknown extrasolar planets — perhaps as many as 100 — await discovery in a vast archive of images taken by the Hubble Space Telescope, the results of a new search technique suggest.

HUBBLE A new image processing technique reveals something not before seen in this Hubble Space Telescope image taken 11 years ago: A faint planet (arrows), the outermost of three discovered with ground-based telescopes last year around the young star HR 8799. D. Lafrenière et al., ApJ Letters

Using the new method, astronomers can more precisely model the amount and distribution of scattered light produced by young nearby stars suspected of spawning planets, and then subtract the light from images of those stars. Once the glare of the light from the parent stars is removed, young Jupiter-mass planets that emit faint but detectable amounts of heat may show up in images already taken by Hubble’s near-infrared camera.

That’s just what David Lafrenière of the University of Toronto and his colleagues found after examining old Hubble images to look for a planet known to exist around the star HR 8799. Last year, a team led by Christian Marois of the National Research Council of Canada’s Herzberg Institute of Astrophysics in Victoria,  which included Lafrenière, used ground-based telescopes to image three planets around that star (SN: 12/6/08, p. 5).

Alerted that another group of astronomers had used the Hubble camera in 1998 to image the same star but had come up empty-handed, Marois, Lafrenière and two collaborators reanalyzed the 11-year-old Hubble images of HR 8799. After subtracting the scattered starlight estimated from the new model, the astronomers recovered the outermost of the trio of planets recently imaged, the team reports online at arXiv.org (http://lanl.arxiv.org/abs/0902.3247) and in an upcoming Astrophysical Journal Letters. The other two planets, which lie closer to the star, still could not be seen in the Hubble images.

The new study “definitely indicates that we should reanalyze all the existing Hubble images of young stars with the new approach — there’s probably 100 to 200 stars where planets could be seen,” comments planet-hunter Bruce Macintosh of the Lawrence Livermore National Laboratory in California. Some of these stars, he notes, have already been examined by the high-resolution Keck telescopes atop Hawaii’s Mauna Kea. “But there’s quite a few that are outside Keck’s field of view or that have just never been looked at by anyone else. There’s definitely a chance of finding more.”

The archival image of the planet, which lies about 70 times the Earth-sun distance from HR 8799 and takes more than 400 years to complete a single orbit, supplies evidence that confirms the body has a circular path. It also adds to evidence that the planet’s atmosphere contains water vapor and clouds of dust, notes study coauthor Travis Barman of Lowell Observatory in Flagstaff, Ariz.

The full range of infrared wavelengths recorded by Hubble literally puts the planet in a new light, since absorption by Earth’s atmosphere prevents some of the radiation from reaching telescopes on the ground.

But the most important result, says Lafrenière, is that the new image processing technique reaches “sensitivities 10 times better than what people have been obtaining for the past 10 years. We have seen a planet that had gone unnoticed before despite careful looks.”

The new technique’s success, he adds, “will certainly have a bearing on all future space-based direct imaging searches for extrasolar planets” including those with Hubble, the future James Webb Space Telescope and the proposed Terrestrial Planet Finder.

“The first thing it tells you is how valuable maintaining long-term archives can be. Here is a major discovery that’s been lurking in the data for about 10 years!” comments Matt Mountain, director of the Space Telescope Science Institute in Baltimore, which operates Hubble “The second thing its tells you is having a well calibrated archive is necessary but not sufficient to make breakthroughs — it also takes a very innovative group of people to develop very smart extraction routines that can get rid of all the artifacts to reveal the planet hidden under all that telescope and detector structure.”

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