ALMA reveals workings of nearby planetary system

April 12, 2012
This image shows the narrow dust ring around Fomalhaut. Yellow at top is the ALMA image, and the blue at bottom is Hubble Space Telescope image. The star is at the location of the bright emission at the center of the ring. Credit: A.C. Boley (University of Florida, Sagan Fellow), M.J. Payne, E.B. Ford, MShabran (University of Florida), S. Corder (North American ALMA Science Center, National Radio Astronomy Observatory), and W. Dent (ALMA, Chile), NRAO/AUI/NSF; NASA, ESA, 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)

A new observatory still under construction has given astronomers a major breakthrough in understanding a nearby planetary system that can provide valuable clues about how such systems form and evolve. The scientists used the Atacama Large Millimeter/submillimeter Array (ALMA) to discover that planets orbiting the star Fomalhaut must be much smaller than originally thought.

The discovery, which helped resolve a controversy among earlier observers of the system, was made possible by high-resolution images of a disk, or ring, of dust orbiting the star, about 25 light-years from Earth. The ALMA images show that both the inner and outer edges of the thin, dusty disk have very sharp edges. That fact, combined with , led the scientists to conclude that the dust particles in the disk are kept within the disk by the gravitational effect of two planets -- one closer to the star than the disk and one more distant.

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. This is much smaller than astronomers had previously thought. In 2008, a (HST) image had revealed the inner planet, then thought to be larger than Saturn, the second-largest planet in our Solar System. However, later observations with failed to detect the planet.

That failure led some astronomers to doubt the existence of the planet in the HST image. Also, the HST visible-light image detected very small that are pushed outward by the star's radiation, thus blurring the structure of the . 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. This clearly revealed the disk's sharp edges, which indicate the gravitational effect of the two planets.

Particles in dust ring are kept inside the ring by gravitational interaction with planets. Faster-moving inner planet transfers energy to dust particles, moving them outward, deeper into the ring. Slower-moving outer planet removes energy from the particles, causing them to drop inward. Credit: Bill Saxton, NRAO/AUI/NSF

"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, 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.

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," Boley said.

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. ALMA's new capabilities, however, revealed the telltale structure that had eluded earlier millimeter-wave observers.

"ALMA may still be under construction, but it already has proven to be the world's most powerful telescope for observing the Universe at millimeter and submillimeter wavelengths," said Stuartt Corder, of the National Radio Astronomy Observatory, a member of the observing team. The scientists will report their findings in an upcoming edition of the Astrophysical Journal Letters.

The effect of planets or moons in keeping a dust ring's edges sharp was first seen when the Voyager 1 spacecraft flew by Saturn in 1980 and made detailed images of that planet's ring 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 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 , and its gravity decreases the energy of the particles, making them fall slightly inward.

Explore further: Subaru's sharp eye confirms signs of unseen planets in the dust ring of HR 4796 A

Related Stories

Earth's dust tail points to alien planets

November 15, 2010

Did you know that the Earth has a dust tail? The Spitzer Space Telescope sailed right through it a few months ago, giving researchers a clear idea of what it looks like. That could be a big help to planet hunters trying to ...

Building a new planet

January 4, 2011

( -- Astronomers over the past decade have made remarkable progress in the study of extrasolar planets; over 500 distant worlds are now confirmed. Meanwhile, as this active research community continues to discover ...

Hubble Reveals Two Dust Disks Around Nearby Star

June 27, 2006

NASA's Hubble Space Telescope has revealed two dust disks circling the nearby star Beta Pictoris. The images confirm a decade of scientific speculation that a warp in the young star's dust disk may actually be a second inclined ...

Unsettled Youth: Spitzer Observes a Chaotic Planetary System

November 4, 2009

( -- Before our planets found their way to the stable orbits they circle in today, they wiggled and jostled about like unsettled children. Now, NASA's Spitzer Space Telescope has found a young star with evidence ...

Recommended for you

NASA telescope studies quirky comet 45P

November 22, 2017

When comet 45P zipped past Earth early in 2017, researchers observing from NASA's Infrared Telescope Facility, or IRTF, in Hawai'i gave the long-time trekker a thorough astronomical checkup. The results help fill in crucial ...

Uncovering the origins of galaxies' halos

November 21, 2017

Using the Subaru Telescope atop Maunakea, researchers have identified 11 dwarf galaxies and two star-containing halos in the outer region of a large spiral galaxy 25 million light-years away from Earth. The findings, published ...

Cassini image mosaic: A farewell to Saturn

November 21, 2017

In a fitting farewell to the planet that had been its home for over 13 years, the Cassini spacecraft took one last, lingering look at Saturn and its splendid rings during the final leg of its journey and snapped a series ...


Adjust slider to filter visible comments by rank

Display comments: newest first

5 / 5 (1) Apr 12, 2012
The dust rings clock in at about 12 arc seconds wide. Pretty big as seen from Earth! Wonder if they will ever image planets inside the ring.
5 / 5 (3) Apr 12, 2012
New observatories and upgrades are coming online in the next 10 - 15 years that are meant to image and spectrograph planets.

Given the relatively diminutive size of these planets, I would expect maybe 10-15 years after that, or less, we could get an image.

25 light years is very close, as far as planetary systems are concerned.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.