Astronomers have tracked two elusive extrasolar planets that went undetected in image data from NASA’s Hubble Space Telescope for 13 years.
They painstakingly conducted a thorough re-analysis of old data collected by the telescope in 1998, and found visual evidence for two alien planets that went undetected then.The finding will give astronomers an invaluable time machine for comparing much earlier planet orbital motion data to more recent observations.
It also demonstrates a novel approach for planet hunting in archival Hubble data.
There are four known planets circling the star HR 8799. This massive but young star is located approximately 130 light-years away from Earth in the constellation Pegasus.
The first three gas giant planets around HR 8799 were discovered in 2007 and 2008.
In 2009, David Lafreniere of the University of Montreal recovered hidden exoplanet data in Hubble images of the young, massive star, HR 8799, taken in 1998 with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS).
He identified the position of the outermost planet known to orbit the star.
This first demonstrated the power of a new data-processing technique for retrieving faint planets buried in the glow of the central star.
A new analysis of the same archival NICMOS data by Remi Soummer of the Space Telescope Science Institute in Baltimore has recovered all three of the outer planets.
The fourth, innermost planet is 1.5 billion miles from the star and cannot be seen because it is on the edge of the NICMOS coronagraphic spot that blocks the light from the central star.
By finding the planets in multiple images spaced over years of time, the orbits of the planets can be tracked. Knowing the orbits is critical to understanding the behavior of multiple-planet systems because massive planets can perturb each other’s 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,” said Soummer.
The planets weren’t found in 1998 when the Hubble observations were first taken because the methods used to detect them were not available at that time.
When astronomers subtracted the light from the central star to look for the residual glow of planets, the residual light scatter was still overwhelming the faint planets.
Lafreniere developed a way to improve this type of analysis by using a library of reference stars to more precisely remove the “fingerprint” glow of the central star.
Soummer’s team took Lafreniere’s method a step further and used 466 images of reference stars taken from a library containing over 10 years of NICMOS observations assembled by Glenn Schneider of the University of Arizona.
The findings are detailed in a yet-to-be published study in the Astrophysical Journal.