Astronomers using ALMA have discovered that planets orbiting the star Fomalhaut are much smaller than originally thought.
The discovery was made possible by exceptionally sharp ALMA images of a ring of dust orbiting Fomalhaut, which lies about 25 light-years from Earth. The ALMA images show that both the inner and outer edges of the thin, dusty disc have very sharp edges. That fact, combined with computer simulations, led the scientists to conclude that the dust particles in the disc are kept within the disc by the gravitational effect of two planets — one closer to the star than the disc and one more distant.
A new picture of the dust ring around the bright star Fomalhaut from the Atacama Large Millimeter/submillimeter Array (ALMA). The underlying blue picture shows an earlier picture obtained by the NASA/ESA Hubble Space Telescope. The new ALMA image has given astronomers a major breakthrough in understanding a nearby planetary system and provided valuable clues about how such systems form and evolve. Note that ALMA has so far only observed a part of the ring. Credit: ALMA (ESO/NAOJ/NRAO). Visible light image: the NASA/ESA Hubble Space Telescope.
Acknowledgement: A.C. Boley (University of Florida, Sagan Fellow), M.J. Payne, E.B. Ford, M. Shabran (University of Florida), S. Corder (North American ALMA Science Center, National Radio Astronomy Observatory), and W. Dent (ALMA, Chile), P. Kalas, J. Graham, E. Chiang, E. Kite (University of California, Berkeley), M. Clampin (NASA Goddard Space Flight Center), M. Fitzgerald (Lawrence Livermore National Laboratory), and K. Stapelfeldt and J. Krist (NASA Jet Propulsion Laboratory)
The effect of planets or moons in keeping a dust ring's edges sharp was first seen when the Voyager spacecraft flew by Saturn and made detailed images of that planet's ring system. In another example in our Solar System, one ring of the planet Uranus is confined sharply by the moons Cordelia and Ophelia, in exactly the manner the ALMA observers propose for the ring around Fomalhaut. The moons confining those planets' rings are dubbed "shepherding moons".
The moons or planets confining such dust rings do so through gravitational effects. A planet on the inside of the ring is orbiting the star more rapidly than the dust particles in the ring. Its gravity adds energy to the particles, pushing them outward. A planet on the ring's outside is moving more slowly than the dust particles, and its gravity decreases the energy of the particles, making them fall slightly inward.
Their calculations also indicated the probable size of the planets — larger than Mars but no larger than a few times the size of the Earth, much smaller than astronomers had previously thought. In 2008, a NASA/ESA Hubble Space Telescope image had revealed the inner planet, then thought to be larger than Saturn, the second largest planet in our Solar System, but later observations with infrared telescopes failed to detect the planet.
That failure led some astronomers to doubt the existence of the planet in the Hubble image. The Hubble visible-light image detected very small dust grains that are pushed outward by the star's radiation, thus blurring the structure of the dusty disc. The ALMA observations, at wavelengths longer than those of visible light, traced larger dust grains — about 1 millimeter in diameter — that are not moved by the star's radiation. They clearly reveal the disc's sharp edges and ringlike structure, which indicate the gravitational effect of two planets.
"Combining ALMA observations of the ring's shape with computer models, we can place very tight limits on the mass and orbit of any planet near the ring," said Aaron Boley, a Sagan Fellow at the University of Florida, who was leader of the study. "The masses of these planets must be small; otherwise the planets would destroy the ring," he added. The small sizes of the planets explain why the earlier infrared observations failed to detect them, the scientists said.
Fomalhaut in the constellation of Piscis Austrinus (The Southern Fish). This map shows most of the stars visible to the unaided eye under good conditions. Fomalhaut is the brightest star in the constellation and one of the brightest stars known to have an orbiting planet. It lies about 25 light-years from the Earth and is surrounded by a huge disc of dust. Credit: ESO, IAU and Sky&Telescope
The ALMA research shows that the ring's width is about 16 times the distance from the Sun to the Earth, and is only one-seventh as thick as it is wide. "The ring is even more narrow and thinner than previously thought," said Matthew Payne, also of the University of Florida.
The ring is about 140 times the Sun-Earth distance from the star. In our own Solar System, Pluto is about 40 times more distant from the Sun than the Earth. "Because of the small size of the planets near this ring and their large distance from their host star, they are among the coldest planets yet found orbiting a normal star," added Aaron Boley.
The scientists observed the Fomalhaut system in September and October of 2011, when only about a quarter of ALMA's planned 66 antennas were available. When construction is completed next year, the full system will be much more capable. Even in this Early Science phase, though, ALMA was powerful enough to reveal the telltale structure that had eluded earlier millimetre-wave observers.
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Wide-field view of the sky around the star Fomalhaut in the constellation of Piscis Austrinus (The Southern Fish). Fomalhaut is the brightest star in the constellation and one of the brightest stars known to have an orbiting planet. It lies about 25 light-years from the Earth and is surrounded by a huge disc of dust. The final view of this video shows a new ALMA image of the disc (orange) and the new results from ALMA have given astronomers a major breakthrough in understanding a nearby planetary system and provided valuable clues about how such systems form and evolve. Note that ALMA has so far only observed a part of the ring. The underlying blue picture shows an earlier picture obtained by the NASA/ESA Hubble Space Telescope. Credit:ALMA (ESO/NAOJ/NRAO). Visible light image: the NASA/ESA Hubble Space Telescope A. Fujii/Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble). Music: John Dyson (from the album Moonwind).
"ALMA may be still under construction, but it is already the most powerful telescope of its kind. This is just the beginning of an exciting new era in the study of discs and planet formation around other stars", concludes ESO astronomer and team member Bill Dent (ALMA, Chile).
“Constraining the Planetary System of Fomalhaut Using High-Resolution ALMA Observations” by A. Boley et al. to appear in Astrophysical Journal Letters. The team is composed of A. C. Boley (University of Florida, Gainesville, USA), M. J. Payne (University of Florida), S. Corder (North American ALMA Science Center, Charlottesville, USA), W. Dent (ALMA, Santiago, Chile), E. B. Ford (University of Florida) and M. Shabram (University of Florida).
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