Planet hunters have a new technique for finding planets orbiting very bright stars. Better yet, they can use this technique to clean up old images that were thought to contain only stars. The motion of planets over time can be determined by comparing the old images to the new images.
The first planet outside of our solar system was found in 1995. Since then, over 300 have been identified. The main difficulty with using images to find planets is that the light from the star the planets orbit dominates the image and the planets cannot be seen.
Rémi Soummer at the Space Science Telescope Institute has developed a new and improved method for subtracting out the light from telescope images of stars to reveal hidden planets. Using this method, he was able to make out three of the four planets that are known to orbit the star HR 8799.
HR 8799 was discovered in 1988 using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope. The images were taken using a coronagraph which blocks the light from the central star in an attempt to see things around the star. But the coronagraph was not completely effective — the star light still flooded the image, so that there was no way to tell whether or not the star had any planets around it.
New data since 2008 have shown that four large planets, seven to 10 times the mass of Jupiter, are in orbit around the star.
Soummer and his team analyzed the 12 year old Hubble data to try to discern the planets around HR 8799. "In this case we knew the planets were there, we just had to work very hard to clean up the image and find them," Soummer said.
The star has a large light flux, or output of light. The planets, however, are dim, about 100,000 times as dim as the star. To see the planets, scientists compare the target star to other stars that are known not to have planets. This is done by a method called Point Spread Function (PSF) subtraction. The generic light from a star of the same kind is determined from the reference star. This light is then subtracted from the image, leaving only the light from objects around the target star.
But using one reference star is not effective enough to completely eliminate the extra light, which is why the planets could not be found in 1998. Today's researchers have the advantage of a large library of stars to compare to.
"We had about 500 stars in our library and we found optimal ways of combining these images of stars to clean up the image even better than was possible before," Soummer said.
The team optimized the processing technique and found the three outer planets around HR 8799; the fourth was too close to the star and was blocked in the coronagraph image.
Having pictures of planets from 1998 is very useful. These large planets take 50 to 500 years to orbit their star once. Comparing the old images with current ones allows scientists to predict the periods, shape, and inclination of the orbits. "This data is so old, there's a 12 year time difference. You can see that the planet moved and you can develop the orbit," Soummer said.
Now that it has been proven effective, Soummer plans to use this method of processing old data to search for new planets around 400 likely candidate stars in the NICMOS archive. "Now that we have this experience . . . we know that if something is there we can see it," Soummer said. If any planets are found in the archival data, new observations can be noted and compared to old ones, giving scientists the ability to observe the motion of the planets.
