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New planets from old pictures

R. Soummer / STScI / NASA / ESA

The left image shows Hubble's view of HR 8799p as seen in 1998, while the right image shows the view after state-of-the-art reprocessing, with three planets indicated within white circles.

New techniques for analyzing decade-old images from the Hubble Space Telescope are helping astronomers track planets that went undiscovered at the time. So far, the techniques have confirmed the existence of planets that were found in the meantime using other methods — but astronomers will be checking hundreds of stars in hopes of making brand-new discoveries.

Remi Soummer, an astronomer at the Space Telescope Science Institute in Baltimore who led the new study, compared the technique to a "time machine" for seeking out planets beyond our solar system.

The key to the time machine is a huge database of observations made in the '90s by the Hubble Space Telescope's Near Infrared Camera and Multi-Oblect Spectrometer, or NICMOS. The instrument was used back then to look for dusty planetary disks and brown dwarfs. NICMOS focused on the regions around hundreds of stars, using a coronagraphic disc to block out the glare of the stars themselves.

The images were then processed to remove any remaining glare and bring out dim details. But back then, astronomers "did not have the cleanup techniques that we have now," Soummer told me today. Now Soummer and other astronomers are taking a second look at the NICMOS targets with improved image-processing software, and they're finding objects that were missed the first time around.

The star HR 8799, which is 130 light-years away in the constellation Pegasus, serves a classic example. NICMOS took a look at the star in 1998, but the imaging software available at the time didn't pick up any planets. In 2008 and 2009, a team led by Christian Marois of Canada's National Research Council analyzed ground-based imagery of the star and spotted three planets. The same team detected a fourth planet in 2010.

R. Soummer / STScI / NASA / ESA

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 sizes of the dots are not to scale with the planet's true sizes. The three outermost planets, b, c and d, 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.

Spurred by the planet discoveries, the University of Montreal's David Lafrenière and his colleagues used upgraded software to find one of those four planets in the old NICMOS picture. Soummer, Marois and others followed up by locating two more of the planets. The fourth, innermost planet can't be seen in the NICMOS image because it's on the edge of the coronagraphic disc.

"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 in a Hubblesite news release. The results from his team are to be published in the Astrophysical Journal.

The three outermost planets make one orbit around HR 8799 roughly every 100, 200 and 400 years — so being able to see where the planets were a decade ago will give astronomers an extra data point for calculating the orbits more precisely. That's why the technique works like a time machine: It's as if you could go back to 1998 and see where the planets were back then. "It's 10 years of science for free," Soummer said.

But that's just the beginning. "What's really exciting now is that we're going to apply the same method to a bunch of other stars, and hopefully we'll make some discoveries of our own," said Brendan Hagan, a member of the research team who recently graduated from Goucher College in Baltimore.

Soummer said his team plans to analyze about 400 other stars in the NICMOS archive with upgraded image-processing software, which should improve image quality by a factor of 10.

"Once the code is ready, it's going to be a very intensive computing process," he told me. "It's going to take a few weeks to go through everything." Soummer plans to make several passes through the data, then compare the NICMOS results with other imagery to confirm the existence of new extrasolar planets.

The Extrasolar Planets Encyclopaedia currently lists 690 worlds that orbit other stars, and Soummer can hardly wait to add to the tally. "We have this huge wealth of data," he said, "and it's ready to be analyzed."

More about exoplanets:

In addition to Soummer, Marois and Hagan, the authors of "Orbital Motion of HR 8799 b, c, d Using Hubble Space Telescope Data From 1998: Constraints on Inclination, Eccentricity and Stability" include Laurent Pueyo, Adrien Thormann and Abhijith Rajan.

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