GAINESVILLE, Fla. — University of Florida astronomers have found compelling evidence for two low-mass planets orbiting the nearby star Fomalhaut, just 25 light years from Earth.
Twice as massive as the sun and 20 times brighter, Fomalhaut is surrounded by a ring of dust and debris, making it a favorite system for astronomers to study and a natural laboratory for testing planet formation theories.
In 2008, images of Fomalhaut taken by the Hubble Space Telescope led to the discovery of "Fomalhaut b," the first extra solar planet to be directly detected in visible light. At the time, astronomers believed it to be a giant planet, akin to Jupiter or Saturn, but later infrared images failed to detect the planet, meaning that it had to be smaller than Saturn.
UF astronomers, along with scientists from the new Atacama Large Millimeter Array in Chile, known as ALMA, and the National Radio Astronomy Observatory, used ALMA's superior resolution and sensitivity to study the system in unprecedented detail. Their results indicate that there are not one, but two planets, with masses between that of Mars and a few times larger than Earth, working together to shape the ring of dust.
The new study reveals that the ring is sharply truncated in the inner and outer edges and is only about 16 astronomical units, or AU, wide, or about 16 times the distance between the Earth and the sun. That may seem large, but the center of the ring is about 140 AU, making the ring relatively very narrow. It also finds that the ring is vertically thin, about one-seventh as tall as it is wide. Those properties give important clues to explain the planetary system of Fomalhaut.
The results are described in a paper to appear this month in the Astrophysical Journal Letters.
"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 Postdoctoral Fellow at UF and leader of the study. "The masses of the planets must be small so they do not destroy the ring, but their masses cannot be too low or they would not shape the ring."
Although Fomalhaut is a much hotter star than the sun, the planets are so far from their host star that they are among the coldest planets known around a normal star. They are thought to be low-mass bodies, but astronomers do not have enough data to tell whether they have a significant amount of hydrogen gas or are mostly rock and ice.
"ALMA observations show that Fomalhaut's ring is even more narrow and thinner than previously known," said Matthew Payne, an astronomer at the University of Florida who contributed to the study. "Fomalhaut b alone only explains the ring's sharp inner edge. Our analysis suggests that two planets, one interior and one exterior, are shepherding the ring, analogous to how Uranus' moons Cordelia and Ophelia confine Uranus' brightest ring."
The Atacama Large Millimeter/submillimeter Array, located in the Atacama Desert of northern Chile at an altitude of 16,400 feet is the largest astronomical project in existence. Still under construction, ALMA began scientific operations in September.
"ALMA may still be under construction, but it has already proved to be the world's most powerful telescope for observing the universe at millimeter and submillimeter wavelengths of light." said Stuartt Corder, an astronomer at the National Radio Astronomy Observatory and co-author of the study.
"Once ALMA is completed, we will be able to study systems like Fomalhaut with even greater detail, and see through the veil of dust that hides the early stages of planet formation," said co-author Bill Dent, an astronomer at ALMA.