THE first direct image of three extrasolar planets orbiting their host star was hailed as a milestone when it was unveiled late last year. Now it turns out that the had captured an image of one of them 10 years ago, but astronomers failed to spot it. This raises hope that more planets lie buried in Hubble鈥檚 vast archive.
In 1998, Hubble studied the star HR 8799 in the infrared, as part of a search for planets around young and relatively nearby stars. The search came up empty.
Last year, Christian Marois of the Herzberg Institute of Astrophysics in Victoria, British Columbia, Canada, and colleagues looked at the same star using the Gemini North telescope in Hawaii. They discovered three planets, each about 10 times as massive as Jupiter. They succeeded where the Hubble team failed mainly because of new strategies developed to carefully subtract the star鈥檚 glare, leaving only the faint infrared glow from its planets.
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Marois and David Lafreni猫re, of the University of Toronto in Ontario, Canada, decided to apply their new mathematical tools to the decade-old Hubble image. This involved digitally combining Hubble鈥檚 views of 23 similar stars that do not have planets to create a reference image nearly identical to that of HR 8799. When they subtracted the reference image from HR 8799鈥檚, the outermost of its three planets popped into view.
鈥淚 felt the same excitement I experienced when we discovered it the first time,鈥� says Lafreni猫re.
鈥淚 felt the same excitement as when we discovered the exoplanet for the first time鈥�
This opens the door for the discovery of exoplanets simply by reprocessing old Hubble images, says Bruce Macintosh of in California. He suggests that images of at least 200 stars that had been the target of Hubble鈥檚 exoplanet searches should be re-analysed using the new method.
Aspects of the HR 8799 solar system promise more riches. Daniel Fabrycky and Ruth Murray-Clay of the Harvard-Smithsonian Center for Astrophysics in Boston studied the dynamics of the three-planet system and found that the mutual gravitational pull of the massive planets should be enough to make the solar system unstable. They conclude that the planets have survived until now because they have slotted themselves into so-called resonance orbits: each time the outermost planet orbits the star once, they argue, the next one in must orbit twice and the innermost planet four times ().
As the planets trace their elliptical orbits, the 1:2:4 timings would mean that the three planets never gather closely enough as a group to gravitationally upset the system. If such resonances are common, it suggests there could be many more massive planets out there in extrasolar systems that would otherwise have been too unstable to persist.
