Astronomers Discover Two Low-Mass Planets Orbiting Fomalhaut

An international team of astronomers has found compelling evidence for two low-mass planets orbiting the nearby star Fomalhaut, just 25 light years from Earth.

Fomalhaut and its dust ring (A. Boley / M. Payne / E. Ford / M. Shabran / S. Corder / W. Dent / NRAO / AUI / NSF / NASA / ESA / P. Kalas / J. Graham / E. Chiang / E. Kite / M. Clampin / M. Fitzgerald / K. Stapelfeldt / J. Krist )

Fomalhaut is twice as massive as the Sun and 20 times brighter. It 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.

Images of Fomalhaut returned by the NASA/ESO Hubble Space Telescope in 2008 led to the discovery of Fomalhaut b, the first extra solar planet to be directly detected in visible light. 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.

Now, the team using the new Atacama Large Millimeter Array (ALMA) in Chile has studied 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, which will be published this month in the Astrophysical Journal Letters, reveals that the ring is sharply truncated in the inner and outer edges and is only about 16 astronomical units (AU), wide, or about 16 times the distance between the Earth and the Sun.

An artist's impression how planets can shepherd a ring of dust around Fomalhaut (Bill Saxton / NRAO / AUI / NSF)

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.

“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 Dr. Aaron Boley, a Sagan Postdoctoral Fellow at the University of Florida and a lead author 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 Dr. Matthew Payne, an astronomer at the University of Florida and a co-author of 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.”