This image shows the HR 8799 planets with starlight optically suppressed and data processing conducted to remove residual starlight. The star is at the center of the blackened circle in the image. The four spots indicated with the letters b through e are the planets. This is a composite image using 30 wavelengths of light and was obtained over a period of 1.25 hours on June 14 and 15, 2012. Image courtesy of Project 1640.
Gone are the days of being able to count the number of known planets on your fingers. Today, there are more than 800 confirmed exoplanets—planets that orbit stars beyond our sun—and more than 2,700 other candidates. What are these exotic planets made of? Unfortunately, you cannot stack them in a jar like marbles and take a closer look. Instead, researchers are coming up with advanced techniques for probing the planets’ makeup.
One breakthrough to come in recent years is direct imaging of exoplanets. Ground-based telescopes have begun taking infrared pictures of the planets posing near their stars in family portraits. But to astronomers, a picture is worth even more than a thousand words if its light can be broken apart into a rainbow of different wavelengths.
Those wishes are coming true as researchers are beginning to install infrared cameras on ground-based telescopes equipped with spectrographs. Spectrographs are instruments that spread an object’s light apart, revealing signatures of molecules. Project 1640, partly funded by NASA’s Jet Propulsion Laboratory, Pasadena, Calif., recently accomplished this goal using the Palomar Observatory near San Diego.
“In just one hour, we were able to get precise composition information about four planets around one overwhelmingly bright star,” said Gautam Vasisht of JPL, co-author of the new study appearing in the Astrophysical Journal. “The star is a hundred thousand times as bright as the planets, so we’ve developed ways to remove that starlight and isolate the extremely faint light of the planets.”
Along with ground-based infrared imaging, other strategies for combing through the atmospheres of giant planets are being actively pursued as well. For example, NASA’s Spitzer and Hubble space telescopes monitor planets as they cross in front of their stars, and then disappear behind. NASA’s upcoming James Webb Space Telescope will use a comparable strategy to study the atmospheres of planets only slightly larger than Earth.
In the new study, the researchers examined HR 8799, a large star orbited by at least four known giant, red planets. Three of the planets were among the first ever directly imaged around a star, thanks to observations from the Gemini and Keck telescopes on Mauna Kea, Hawaii, in 2008. The fourth planet, the closest to the star and the hardest to see, was revealed in images taken by the Keck telescope in 2010.