It's the University of Arizona's first discovery of an exoplanet and wouldn't you know - it's one that's breaking all the rules! An international team of astronomers led by a UA graduate student have picked up the signature of a planet orbiting a single, sun-like star. While that's nothing new, the planet's position most certainly is: it's more than 650 times the average Earth-Sun distance away from its host!
Weighing in at about eleven times the mass of Jupiter, this non-conformist planet cataloged as HD 106906 isn't like anything in our solar system and defies conventional planet formation theories. "This system is especially fascinating because no model of either planet or star formation fully explains what we see," said Vanessa Bailey, who led the research. Bailey is a fifth-year graduate student in the UA's Department of Astronomy.
Image: This is an artist's conception of a young planet in a distant orbit around its host star. The star still harbors a debris disk, remnant material from star and planet formation, interior to the planet's orbit (similar to the HD106906 system). (Image courtesy NASA/JPL-Caltech)
But just what are the "rules" of planet formation? For the most part, astronomers believe that planets similar to ours form relatively close to the parent star, taking shape from the disk of primordial gases and dust which envelope it. This formation is slow and doesn't lend itself well to giant planets forming at great distances. However, other theories postulate that giant planets could be the result of a quick, direct collapse of disk material. While this is a good notion, primordial disks aren't really known to house enough mass at their periphery to permit a planet like HD 106906 to form. In this instance, many alternative hypotheses have been suggested - including the formation of a "mini" binary star system.
"A binary star system can be formed when two adjacent clumps of gas collapse more or less independently to form stars, and these stars are close enough to each other to exert a mutual gravitation attraction and bind them together in an orbit," Bailey explained. "It is possible that in the case of the HD 106906 system the star and planet collapsed independently from clumps of gas, but for some reason the planet's progenitor clump was starved for material and never grew large enough to ignite and become a star."
According to Bailey, one problem with this scenario is that the mass ratio of the two stars in a binary system is typically no more than 10-to-1. "In our case, the mass ratio is more than 100-to-1," she explained. "This extreme mass ratio is not predicted from binary star formation theories -- just like planet formation theory predicts that we cannot form planets so far from the host star."
What really makes this rule-breaking discovery incredible is the fact that researchers can still detect the remnant debris disk of material left over from planet and star formation. "Systems like this one, where we have additional information about the environment in which the planet resides, have the potential to help us disentangle the various formation models," Bailey added. "Future observations of the planet's orbital motion and the primary star's debris disk may help answer that question."
Just how did the UA research team discover this unruly planet? Because it is only around thirteen million years old, it still glows red-hot with embers left over from its formation. It measures at approximately 2,700 degrees Fahrenheit - much cooler than its host star - and emits most of its energy in the infrared spectrum rather than in visible light. To get a direct image with the quality of the Hubble from the ground requires some very specialized equipment - adaptive optics. The team used the new Magellan Adaptive Optics (MagAO) system and Clio2 thermal infrared camera - both technologies developed at the UA - mounted on the 6.5-meter-diameter Magellan telescope in the Atacama Desert in Chile to take the discovery image.
UA astronomy professor and MagAO principal investigator Laird Close said: "MagAO was able to utilize its special adaptive secondary mirror, with 585 actuators, each moving 1,000 times a second, to remove the blurring of the atmosphere. The atmospheric correction enabled the detection of the weak heat emitted from this exotic exoplanet without confusion from the hotter parent star."
"Clio was optimized for thermal infrared wavelengths, where giant planets are brightest compared to their host stars, meaning planets are most easily imaged at these wavelengths," explained UA astronomy professor and Clio principal investigator Philip Hinz, who directs the UA Center for Astronomical Adaptive Optics.
Of course, all new discoveries require confirmation. In this case, the researchers were able to confirm the planet is gravitationally locked to its parent star by utilizing Hubble Space Telescope data taken eight years ago for another research program. Using the FIRE spectrograph, also installed at the Magellan telescope, the team confirmed the planetary nature of the companion. "Images tell us an object is there and some information about its properties but only a spectrum gives us detailed information about its nature and composition," explained co-investigator Megan Reiter, a graduate student in the UA Department of Astronomy. "Such detailed information is rarely available for directly imaged exoplanets, making HD 106906 b a valuable target for future study."
"Every new directly detected planet pushes our understanding of how and where planets can form," said co-investigator Tiffany Meshkat, a graduate student at Leiden Observatory in the Netherlands. "This planet discovery is particularly exciting because it is in orbit so far from its parent star. This leads to many intriguing questions about its formation history and composition. Discoveries like HD 106906 b provide us with a deeper understanding of the diversity of other planetary systems."
Original Story Source: University of Arizona Press Release
About Tammy Plotner - Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She's received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.