Worlds In Collision

Two terrestrial planets orbiting a mature sun-like star some 300 light-years from Earth recently suffered a violent collision, astronomers at UCLA, Tennessee State University and the California Institute of Technology will report in a December issue of the Astrophysical Journal, the premier journal of astronomy and astrophysics.
"It's as if Earth and Venus collided with each other," said Benjamin Zuckerman, UCLA professor of physics and astronomy and a co-author on the paper. "Astronomers have never seen anything like this before. Apparently, major catastrophic collisions can take place in a fully mature planetary system."
"If any life was present on either planet, the massive collision would have wiped out everything in a matter of minutes - the ultimate extinction event," said co-author Gregory Henry, an astronomer at Tennessee State University (TSU). "A massive disk of infrared-emitting dust circling the star provides silent testimony to this sad fate."
Zuckerman, Henry and Michael Muno, an astronomer at Caltech at the time of the research, were studying a star known as BD+20 307, which is surrounded by a shocking 1 million times more dust than is orbiting our sun.
The star is located in the constellation Aries. The astronomers gathered X-ray data using the orbiting Chandra X-ray Observatory and brightness data from one of TSU's automated telescopes in southern Arizona, hoping to measure the age of the star.
"We expected to find that BD+20 307 was relatively young, a few hundred million years old at most, with the massive dust ring signaling the final stages in the formation of the star's planetary system," Muno said.
Those expectations were shown to be premature, however, when Carnegie Institution of Washington astronomer Alycia Weinberger announced in the May 20, 2008, issue of the Astrophysical Journal that BD+20 307 is actually a close binary star - two stars orbiting around their common center of mass.
"That discovery radically revised the interpretation of the data and transformed the star into a unique and intriguing system," said TSU astronomer Francis Fekel who, along with TSU's Michael Williamson, was asked to provide additional spectroscopic data from another TSU automated telescope in Arizona to assist in comprehending this exceptional binary system.
The new spectroscopic data confirmed that BD+20 307 is composed of two stars, both very similar in mass, temperature and size to our own sun. They orbit about their common center of mass every 3.42 days.
"The patterns of element abundances in the stars show that they are much older than a few hundred million years, as originally thought," Fekel said. "Instead, the binary system appears to have an age of several billion years, comparable to our solar system."
"The planetary collision in BD+20 307 was not observed directly but rather was inferred from the extraordinary quantity of dust particles that orbit the binary pair at about the same distance as Earth and Venus are from our sun," Henry said. "If this dust does indeed point to the presence of terrestrial planets, then this represents the first known example of planets of any mass in orbit around a close binary star."
Zuckerman and colleagues first reported in the journal Nature in July 2005 that BD+20 307, then still thought to be a single star, was surrounded by more warm orbiting dust than any other sun-like star known to astronomers. The dust is orbiting the binary system very closely, where Earth-like planets are most likely to be and where dust typically cannot survive long.
Small dust particles get pushed away by stellar radiation, while larger pieces get reduced to dust in collisions within the disk and are then whisked away. Thus, the dust-forming collision near BD+20 307 must have taken place rather recently, probably within the past few hundred thousand years and perhaps much more recently, the astronomers said.
"This poses two very interesting questions," Fekel said. "How do planetary orbits become destabilized in such an old, mature system, and could such a collision happen in our own solar system?"
"The stability of planetary orbits in our own solar system has been considered for nearly two decades by astronomer Jacques Laskar in France and, more recently, by Konstantin Batygin and Greg Laughlin in the U.S.A.," Henry noted.
"Their computer models predict planetary motions into the distant future and they find a small probability for collisions of Mercury with Earth or Venus sometime in the next billion years or more. The small probability of this happening may be related to the rarity of very dusty planetary systems like BD+20 307."
"There is no question, however," Zuckerman said, "that major collisions have occurred in our solar system's past. Many astronomers believe our moon was formed from the grazing collision of two planetary embryos - the young Earth and a body about the size of Mars - a crash that created tremendous debris, some of which condensed to form the moon and some of which went into orbit around the young sun.
"By contrast with the massive crash in the BD+20 307 system, the collision of an asteroid with Earth 65 million years ago, the most favored explanation for the final demise of the dinosaurs, was a mere pipsqueak."
In their 1932 novel "When Worlds Collide," science fiction writers Philip Wylie and Edwin Balmer envisioned the destruction of Earth by a collision with a planet of a passing star. The 1951 classic movie based on the novel began a long line of adventure stories of space rocks apocalyptically plowing into Earth.
"But," Zuckerman noted, "there is no evidence near BD+20 307 of any such passing star."

A Strategy For Detecting Earth-Like Planets

The Exoplanet Task Force of the Astronomy and Astrophysics Advisory Committee has released its report detailing a strategy for obtaining the first detection and characterization of Earth-like planets around other stars within a time frame as short as 15 years.
Finding such planets would answer one of the great questions of modern science: are planets like Earth common or rare? We stand on a great divide in the detection and study of "exoplanets''-planets orbiting other stars. On one side of this divide are the hundreds of known massive exoplanets that we have discovered over the last 10 years.
We have expanded our knowledge of these extraordinary and unexpected planets by measuring the densities and atmospheric temperatures for some of the hottest exoplanets. On the other side of the divide lies the possibility, as yet unrealized, of detecting and characterizing a true Earth analog-a planet of one Earth mass or Earth radius orbiting a sun-like star at a distance similar to the Earth-Sun separation.
The Exoplanet Task Force Report describes potentially how to bridge this divide. Sixteen astronomers carried out the study, chartered by the National Aeronautics and Space Administration and the National Science Foundation. The larger astronomical community participated through a call for "white papers" that yielded 84 submissions on planet detection techniques.
The recommendations of the Task Force emphasize immediate investment in technology and space mission development that will lead to discovering and characterizing Earth analogs. Central to the strategy is a two-pronged approach that includes both ground- and space-based techniques.
The first prong uses ground-based and space-based systems in existence or under development to search for and characterize Earth-sized planets around the smallest stars-the "M-dwarfs."
This strategy provides short-term payback but won't find true Earth analogs since M-dwarfs are quite different from solar-like stars. The second prong develops more advanced techniques to search for Earths around stars like the Sun.
The strategy calls for a spaceborne astrometric telescope, specifically designed to identify nearby Sun-like stars with Earth-mass planets. Once we have the "addresses" of such stars and know where to look, an imaging system in space could determine the atmospheric composition of such bodies.
Although the latter step is extraordinarily challenging, such a capability would be a major step along the way to answering the question, "Are we alone?" The Executive Summary of the report is in press in the journal Astrobiology.
The report, with a list of Task Force Members, can be obtained from the NSF.

COROT Discovers Exotic Object

COROT has discovered a massive planet-sized object orbiting its parent star closely, unlike anything ever spotted before. It is so exotic, that scientists are unsure as to whether this oddity is actually a planet or a failed star.
The object, named COROT-exo-3b, is about the size of Jupiter, but packs more than 20 times the mass. It takes only 4 days and 6 hours to orbit its parent star, which is slightly larger than the Sun.
COROT-exo-3b was found as the satellite observed the drop in the brightness of the star each time the object (COROT-exo-3b) passed in front.
"We were taken by surprise when we found this massive object orbiting so close to its parent star", said Dr Magali Deleuil from the Laboratoire d'Astrophysique de Marseille (LAM), leader of the team that made the discovery. She added, "COROT-exo-3b is really unique - we're still debating its nature."
The search for planets with orbital periods less than 10 days orbiting close to the parent star has lasted almost 15 years. During this time, scientists have encountered planets with masses 12 times that of Jupiter, and stars with 70 times as massive as Jupiter, but none in between. This is why the 20-Jupiter-mass COROT-exo-3b was such a surprise.
This odd find does not fall into either conventional category of planets or brown dwarves. A brown dwarf is a 'failed star', a sub-stellar object that is not undergoing nuclear fusion at its core, but displays some stellar characteristics.
"COROT-exo-3b might turn out to be a rare object found by sheer luck", said Dr Francois Bouchy, from Institut d'Astrophysique de Paris (IAP), member of the team that made the discovery. "But it might just be a member of a new-found family of very massive planets that encircle stars more massive than our Sun. We're now beginning to think that the more massive the star, the more massive the planet," he said.
Team member Dr Hans Deeg, of the Instituto de Astrofisica de Canarias (IAC), explains why this new object is such an important find for planet hunters, "It has puzzled us; we're not sure where to draw the boundary between planets and brown dwarves."
As a planet, COROT-exo-3b would be the most massive and the densest found to date - more than twice as dense as lead. Studying it will help them better understand how to categorise such objects. The team also wants to understand how such a massive object formed so close to its parent.

NASA Supercomputer Shows How Dust Rings Point To Exo-Earths

Supercomputer simulations of dusty disks around sunlike stars show that planets nearly as small as Mars can create patterns that future telescopes may be able to detect. The research points to a new avenue in the search for habitable planets.
"It may be a while before we can directly image earthlike planets around other stars but, before then, we'll be able to detect the ornate and beautiful rings they carve in interplanetary dust," says Christopher Stark, the study's lead researcher at the University of Maryland, College Park.
Working with Marc Kuchner at NASA's Goddard Space Flight Center in Greenbelt, Md., Stark modeled how 25,000 dust particles responded to the presence of a single planet - ranging from the mass of Mars to five times Earth's - orbiting a sunlike star.
Using NASA's Thunderhead supercomputer at Goddard, the scientists ran 120 different simulations that varied the size of the dust particles and the planet's mass and orbital distance.
"Our models use ten times as many particles as previous simulations. This allows us to study the contrast and shapes of ring structures," Kuchner adds. From this data, the researchers mapped the density, brightness, and heat signature resulting from each set of parameters.
"It isn't widely appreciated that planetary systems - including our own - contain lots of dust," Stark adds. "We're going to put that dust to work for us."
Much of the dust in our solar system forms inward of Jupiter's orbit, as comets crumble near the sun and asteroids of all sizes collide. The dust reflects sunlight and sometimes can be seen as a wedge-shaped sky glow - called the zodiacal light - before sunrise or after sunset.
The computer models account for the dust's response to gravity and other forces, including the star's light. Starlight exerts a slight drag on small particles that makes them lose orbital energy and drift closer to the star.
"The particles spiral inward and then become temporarily trapped in resonances with the planet," Kuchner explains. A resonance occurs whenever a particle's orbital period is a small-number ratio - such as two-thirds or five-sixths - of the planet's.
For example, if a dust particle makes three orbits around its star every time the planet completes one, the particle repeatedly will feel an extra gravitational tug at the same point in its orbit. For a time, this extra nudge can offset the drag force from starlight and the dust can settle into subtle ring-like structures.
"The particles spiral in toward the star, get trapped in one resonance, fall out of it, spiral in some more, become trapped in another resonance, and so on," Kuchner says. Accounting for the complex interplay of forces on tens of thousands of particles required the mathematical horsepower of a supercomputer.
Some scientists note that the presence of large amounts of dust could present an obstacle to directly imaging earthlike planets. Future space missions - such as NASA's James Webb Space Telescope, now under construction and scheduled for launch in 2013, and the proposed Terrestrial Planet Finder - will study nearby stars with dusty disks.
The models created by Stark and Kuchner give astronomers a preview of dust structures that signal the presence of otherwise hidden worlds.
"Our catalog will help others infer a planet's mass and orbital distance, as well as the dominant particle sizes in the rings," Stark says.
Stark and Kuchner published their results in the October 10 issue of The Astrophysical Journal. Stark has made his atlas of exo-zodiacal dust simulations available online.

Astronomers searching for rocky planets that could support life in other solar systems should look outside, as well as within, the so-called "habitable zone," University of Arizona planetary scientists say.
Planets too close to their stars are roasted. Planets too far from their stars are frozen. In between, research models show, there's a habitable zone where planet temperatures approximate Earth's. Any rocky planets in this just-right Goldilocks zone could be awash in liquid water, a requisite for life as we know it, theorists say.
New research by Brian Jackson, Rory Barnes and Richard Greenberg of UA's Lunar and Planetary Laboratory shows that tides can play a major role in heating terrestrial planets, creating hellish conditions on rocky alien worlds that otherwise might be livable. And just the other way, tidal heat can also create conditions favorable to life on planets that would otherwise be unlivable.
Jackson presented the research Saturday at the 40th annual meeting of the Division of Planetary Sciences in Ithaca, N.Y. His talk is titled "Tidal Heating of Extrasolar Terrestrial-scale Planets and Constraints on Habitability." The research will be published soon in the Monthly Notices of the Royal Astronomical Society.
Our own solar system is something of an anomaly, in that its planets move in relatively quiescent, circular orbits around the sun. Most extrasolar planets found to date have highly elongated orbits. During each orbit, the planet is stretched most by tides when it is near the star, and less when the planet is farther from its star. The resulting friction generates internal heat, which drives the planet's geophysical processes.
If the recently discovered "super-Earths" ? Extrasolar planets only 2-to-10 times as massive as Earth ? Are indeed terrestrial, tidal heating may be great enough to melt them, or at least produce volcanism on par with Jupiter's moon, Io, "dimming their prospects for habitability," Jackson said.
So some of the recently discovered super-Earths may be more like "super-Ios," he said. The lo moon is the most volcanically active body in our solar system.
"Tidal heating scales with planet mass, so we expect that most easily detectable super-Earths will be dominated by volcanic activity," Jackson said.
"That's one of our first conclusions from this work, that the first Earth-like planets found are probably going to be strongly heated and have big volcanoes. Even if Earth-like planets are found within the habitable zone, they may not be habitable because they will be overwhelmed by this tidal heating."
Tidal heating may also create habitable conditions on planets that otherwise are too small or too cold to support life, Jackson said. Tidal heating can enhance outgassing of volatiles that contribute or replenish a planet's atmosphere through volcanism.
Tidal heating also can generate sub-surface liquid oceans on water-rich rocky planets that would otherwise be frozen, just as tidal heating is believed to warm a sub-surface liquid water ocean on Jupiter's moon Europa.
Also, tidal heating can drive plate tectonics, a mechanism that checks excessive carbon dioxide from accumulating in a planetary atmosphere, producing the kind of deadly greenhouse atmosphere found on Venus.
"Our study shows that tidal heating could produce enough heat to drive plate tectonics for billions of years, long enough for life to appear and flourish," Jackson said.

Exotic Weather On Distant Worlds

This artist's concept shows a cloudy, Jupiter-like planet that orbits very close to its fiery star. Spitzer Space Telescope observations for at least one such planet, called HD 189733b, showed the nightside temperatures are 1,300 degrees Fahrenheit hotter than they would be on a wind-free planet. readmore.....

Young Earthlike Planets May Glow Brightly Enough To Be Found

Hot, young planets may be easier to spot because they stay that way longer than astronomers have thought, according to new work by MIT planetary scientist Linda Elkins-Tanton.
For a few million years after their initial formation, planets like Earth may maintain a hot surface of molten rock that would glow brightly enough to make them stand out as they orbit neighboring stars.
Elkins-Tanton, Mitsui Career Development Professor of Geology in the Department of Earth, Atmospheric and Planetary Sciences, says the "magma ocean" stage for Earth-sized planets may last a few million years, much longer than previously estimated. "That means we may actually see them elsewhere, as detection systems get better," she said.
Elkins-Tanton presented her new findings this week at the annual meeting of the American Astronomical Society's Division for Planetary Sciences, being held this year in Ithaca, N.Y.
The research shows that even after the surface magma solidifies, within about five million years, it could stay hot enough to glow brightly in infrared light for tens of millions of years, providing a relatively long window for detectability.
The big problem for astronomers hoping to detect planets around other stars is the vast difference in brightness between the star and the planet, which shines only by reflecting light from its parent star. But the difference in brightness in infrared wavelengths for a glowing, molten planetary surface would be much less, making the detection more feasible.
The long duration of the molten stage turns out to be the result of a two-stage process, Elkins-Tanton explained. The initial heating, generated by a combination of radioactivity in the planet's interior and the heat generated by the collision of millions of chunks of rock crashing together to form the planet, actually is quite short-lived: The planet's surface is expected to solidify quickly, within a few hundred thousand years, as originally thought.
But then a secondary upheaval begins, in which heavier iron-rich material that has solidified at the surface begins to sink toward the core, causing other hotter material to rise to the surface.
This "overturn" process, it turns out, produces the much-longer-lived molten surface, lasting for millions of years, she said. Because the Earth's crust is so dynamic, there is no material left from that initial epoch that could be studied to test this modelling, she said, but on other planets such as Mars or Mercury there might be early remnant rocks that could be tested.
The analysis also leads to specific conclusions about the surface composition of planets, so detection of certain specific minerals on Mercury, for example, which the MESSENGER spacecraft may be able to carry out when it begins its study of the planet in 2011, might support the theory.
In addition, the detection of hot, young planets around other stars, which might become possible over the next several years, might provide another line of evidence to support this conclusion, she said.

Meteorites that are among the oldest rocks ever found have provided new clues about the conditions that existed at the beginning of the solar system, solving a longstanding mystery and overturning some accepted ideas about the way planets form.
The ancient meteorites, like disk drives salvaged from an ancient computer, still contain magnetic records about the very early history of planets, according to research by MIT planetary scientist Benjamin P. Weiss.
Weiss, the Victor P. Starr Career Development Assistant Professor of Planetary Sciences in the Department of Earth, Atmospheric and Planetary Sciences, and his five co-authors examined pieces of three meteorites called angrites, which are among the most ancient rocks known. The results of their study are being published in Science on Oct. 31.
The analysis showed that surprisingly, during the formation of the solar system, when dust and rubble in a disk around the sun collided and stuck together to form ever-larger rocks and eventually the planets we know today, even objects much smaller than planets - just 160 kilometers across or so - were large enough to melt almost completely.
This total melting of the planet-forming chunks of rock, called planetesimals, caused their constituents to separate out, with lighter materials including silicates floating to the surface and eventually forming a crust, while heavier iron-rich material sank down to the core, where it began swirling around to produce a magnetic dynamo.
The researchers were able to study traces of the magnetic fields produced by that dynamo, now recorded in the meteorites that fell to Earth.
"The magnetism in meteorites has been a longstanding mystery," Weiss said, and the realization that such small bodies could have melted and formed magnetic dynamos is a major step toward solving that riddle.
Until relatively recently, it was commonly thought that the planetesimals - similar to the asteroids seen in the solar system today - that came together to build planets were "just homogenous, unmelted rocky material, with no large-scale structure," Weiss said.
"Now we're realizing that many of the things that were forming planets were mini-planets themselves, with crusts and mantles and cores."
That could change theorists' picture of how the planets themselves took shape. If the smaller bodies were already molten as they slammed together to build up larger planet-sized bodies, that could "significantly change our understanding" of the processes that took place in the early years of the nascent planets, as their internal structures were forming, Weiss said.
This could have implications for how different minerals are distributed in the Earth's crust, mantle and core today, for example.
"In the last five or 10 years," Weiss said, "our understanding of the early history of the solar system has undergone a sort of mini-revolution, driven by analytical advances in geochemistry. In this study we used a geophysical technique to independently test many of these new ideas. "
"Events happened surprisingly fast at the beginning of the solar system," he said. Some of the angrite meteorites in this study formed just 3 million years after the birth of the solar system itself, 4,568 million years ago, and show signs that their parent body had a magnetic field that was 20 to 40 percent as strong as Earth's today.
"We are used to thinking of dynamo magnetic fields in rocky bodies as uncommon phenomena today. But it may be that short-lived planetesimal dynamos were widespread in the early solar system."
The paper was co-authored by Mitsui Career Development Assistant Professor of Geology Linda Elkins-Tanton, research scientist Eduardo
A. Lima, postdoctoral researcher Laurent Carpozen, student James S. Berdahl, and Sabine Stanley, assistant professor of physics at the University of Toronto. The work was supported by a grant from the National Science Foundation's Instrumentation and Facilities Program.

Dusty Shock Waves Generate Planet Ingredients

Shock waves around dusty, young stars might be creating the raw materials for planets, according to new observations from NASA's Spitzer Space Telescope.
The evidence comes in the form of tiny crystals. Spitzer detected crystals similar in make-up to quartz around young stars just beginning to form planets. The crystals, called cristobalite and tridymite, are known to reside in comets, in volcanic lava flows on Earth, and in some meteorites that land on Earth.
Astronomers already knew that crystallized dust grains stick together to form larger particles, which later lump together to form planets. But they were surprised to find cristobalite and tridymite. What's so special about these particular crystals? They require flash heating events, such as shock waves, to form.
The findings suggest that the same kinds of shock waves that cause sonic booms from speeding jets are responsible for creating the stuff of planets throughout the universe.
"By studying these other star systems, we can learn about the very beginnings of our own planets 4.6 billion years ago," said William Forrest of the University of Rochester, N.Y. "Spitzer has given us a better idea of how the raw materials of planets are produced very early on." Forrest and University of Rochester graduate student Ben Sargent led the research, to appear in the Astrophysical Journal.
Planets are born out of swirling pancake-like disks of dust and gas that surround young stars. They start out as mere grains of dust swimming around in a disk of gas and dust, before lumping together to form full-fledged planets. During the early stages of planet development, the dust grains crystallize and adhere together, while the disk itself starts to settle and flatten. This occurs in the first millions of years of a star's life.
When Forrest and his colleagues used Spitzer to examine five young planet-forming disks about 400 light-years away, they detected the signature of silica crystals. Silica is made of only silicon and oxygen and is the main ingredient in glass. When melted and crystallized, it can make the large hexagonal quartz crystals often sold as mystical tokens. When heated to even higher temperatures, it can also form small crystals like those commonly found around volcanoes.
It is this high-temperature form of silica crystals, specifically cristobalite and tridymite, that Forrest's team found in planet-forming disks around other stars for the first time. "Cristobalite and tridymite are essentially high-temperature forms of quartz," said Sargent. "If you heat quartz crystals, you'll get these compounds."
In fact, the crystals require temperatures as high as 1,220 Kelvin (about 1,740 degrees Fahrenheit) to form. But young planet-forming disks are only about 100 to 1,000 Kelvin (about minus 280 degrees Fahrenheit to 1,340 Fahrenheit) -- too cold to make the crystals. Because the crystals require heating followed by rapid cooling to form, astronomers theorized that shock waves could be the cause.
Shock waves, or supersonic waves of pressure, are thought to be created in planet-forming disks when clouds of gas swirling around at high speeds collide. Some theorists think that shock waves might also accompany the formation of giant planets.
The findings are in agreement with local evidence from our own solar system. Spherical pebbles, called chondrules, found in ancient meteorites that fell to Earth are also thought to have been crystallized by shock waves in our solar system's young planet-forming disk. In addition, NASA's Stardust mission found tridymite minerals in comet Wild 2.

Hubble Snaps Exoplanet Orbiting Nearby Star

After eight years and repeated photographs of a nearby star in hopes of finding planets, University of California, Berkeley, astronomer Paul Kalas finally has his prize: the first visible-light snapshot of a planet outside our solar system.
Only 25 light years from Earth, the planet - probably close to the mass of Jupiter - orbits the star Fomalhaut at a distance about four times that between Neptune and the sun. Formally known as Fomalhaut b, the planet could have a ring system about the dimension of Jupiter's early rings, before the dust and debris coalesced into the four Galilean moons.
The planet's existence was suspected in 2005, when images Kalas took with the Hubble Space Telescope's Advanced Camera for Surveys showed a sharply defined inner edge to the dust belt around Fomalhaut, in the southern constellation Piscus Austrinus (southern fish).
The sharp edge and off-center belt suggested to Kalas that a planet in an elliptical orbit around the star was shaping the inner edge of the belt, much like Saturn's moons groom the edges of its rings.
"The gravity of Fomalhaut b is the key reason that the vast dust belt surrounding Fomalhaut is cleanly sculpted into a ring and offset from the star," Kalas said. "We predicted this in 2005, and now we have the direct proof."
"It will be hard to argue that a Jupiter-mass object orbiting an A star like Fomalhaut is anything other than a planet," said coauthor James R. Graham, professor of astronomy at UC Berkeley. "That doesn't mean it's exactly what we expected when we went hunting for planets in this system."
The discovery will be reported Nov. 14 on Science Express, an online site that posts articles in advance of their print publication in the journal Science.
The Science paper is complemented by an article to appear in The Astrophysical Journal (ApJ) that analyzes the interaction between the planet and the dust belt surrounding Fomalhaut and cinches the estimation of the planet's mass.
"Every planet has a chaotic zone, which is basically a swath of space that encloses the planet's orbit and from which the planet ejects all particles," said Eugene Chiang, a UC Berkeley associate professor of astronomy and of earth and planetary science, and first author of the ApJ paper.
"This zone increases with the mass of the planet, so, given the size of the chaotic zone around Fomalhaut b, we can estimate that its likely mass is in the vicinity of one Jupiter mass."
Kalas, Graham, Chiang and UC Berkeley graduate student Edwin S. Kite are coauthors of both papers, along with Mark Clampin of the Goddard Space Flight Center in Greenbelt, Md. The other authors of the Science paper are Michael P. Fitzgerald of the Institute of Geophysics and Planetary Science at Lawrence Livermore National Laboratory, and John Krist and Karl Stapelfeldt of the Jet Propulsion Laboratory in Pasadena, Calif.
Kalas has focused on the star Fomalhaut since his days 15 years ago as a graduate student, when he used early optical CCD technology to look for dust surrounding Fomalhaut. In 1998, submillimeter-wavelength radio observations of the disk showed that cold dust was distributed in a ring around the central star, much like the ring of comets called the Kuiper Belt in our solar system.
In 2004, Kalas began using Hubble's Advanced Camera for Surveys to probe the dust belt and, three years ago, he showed that the disk's inner edge was sharply delineated, suggesting that an unseen planet orbits Fomalhaut and shapes the edge.
He now has two photographs of the planet, taken in 2004 and 2006, which show that its movement over a 21-month period exactly fits what would be expected from a planet orbiting Fomalhaut every 872 years at a distance of 119 astronomical units, or 11 billion miles. One astronomical unit (AU) is the average distance between the Earth and the sun, or 93 million miles.
"I nearly had a heart attack at the end of May when I confirmed that Fomalhaut b orbits its parent star," Kalas said. "It's a profound and overwhelming experience to lay eyes on a planet never before seen."
Because of the relatively low mass and distant orbit of the planet, it cannot be detected by today's standard technique, which is to measure the tiny wobble a planet induces in its star. Astronomers hoping to photograph extrasolar planets have taken a different approach, looking in the infrared at young stars in hopes of detecting the warm glow from cooling planets.
Fomalhaut b is not seen in the infrared, however. Instead, Kalas snapped photos in visible light, which for most planets would be impossible because of the bright glare of the star. Even with Hubble's coronagraph to block the star's light, the new planet would not have been detected if it had been much closer to its star, or much dimmer.
"To make this discovery at optical wavelengths is a complete surprise," he said. "If we're seeing light in reflection, then it must be because Fomalhaut b is surrounded by a planetary ring system so vast it would make Saturn's rings look pocket-sized by comparison. Fomalhaut b may actually show us what Jupiter and Saturn resembled when the solar system was about a hundred million years old."
Kalas noted that all other reported images of extrasolar planets have either been disproven or the planets have masses so uncertain they could be brown dwarfs, which are failed stars with masses above 13 Jupiter masses and that glow bright in the infrared.
The Fomalhaut science team, on the other hand, used not only the observed brightness of Fomalhaut b to estimate the mass, but also the fact that a massive planet or brown dwarf would push Fomalhaut's dust belt farther away than what is observed. Chiang thinks he can narrow the mass to between 0.3 and 2 Jupiter masses, most likely one Jupiter mass.
"Any more massive than that and its gravity would destroy the vast dust belt encircling the star," Kalas said.
The belt extends from 133 to about 200 AU from the star, and Chiang estimates that, in dust alone, it has a mass that exceeds three Earths.
"There's so much solid material in the belt that it could easily form the basis of a core for a Jupiter-mass planet. This is consistent with Fomalhaut b having formed in situ, at an unprecedentedly large distance from its host star," Chiang said.
Fomalhaut is about 200 million years old and will burn out in about a billion years, making it a short-lived star compared to our sun, which is now about 4.5 billion years old and expected to burn another 5 billion years.
Its short life is a result of being 16 times brighter than the sun, but this also makes the star appear from the planet's surface about as bright as our sun appears from Neptune, despite the fact that it lies four times farther from Fomalhaut than Neptune does from the sun, Kalas said. "Fomalhaut b sits in a frigid location, but it's not too different from that of Neptune in our solar system," he said.
Interestingly, the planet mysteriously dimmed by half a stellar magnitude between the 2004 and 2006 observations. Fomalhaut b could have a hot outer atmosphere heated by bubbling convection cells, or the brightness change could indicate there is hot gas at the inner boundary of the ring around the planet.
"This is not your theorist's ideal planet, it is obviously more complicated than we thought," Graham said.
The researchers are awaiting repair of Hubble's Advanced Camera for Surveys and the Near Infrared Camera to resume their observations to confirm the planet's orbit, discover the source of its unusual brightness in the visible, and perhaps find other planets.
"There is plenty of empty space between Fomalhaut b and the star for other planets to happily reside in stable orbits," Kalas said. "We'll probably have to wait for the James Webb Space Telescope to give us a clear view of the region closer to the star where a planet could host liquid water on the surface."
The research was supported by National Aeronautics and Space Administration and the National Science Foundation.

Seeing A Distant Planet

Astronomers for the first time have taken snapshots of a multi-planet solar system, much like ours, orbiting another star.
The new solar system orbits a dusty young star named HR8799, which is 140 light years away and about 1.5 times the size of our sun. Three planets, roughly 10, 10 and 7 times the mass of Jupiter, orbit the star. The size of the planets decreases with distance from the parent star, much like the giant planets do in our system.
And there may be more planets out there, but scientists say they just haven't seen them yet.
"Every extrasolar planet detected so far has been a wobble on a graph. These are the first pictures of an entire system," said Bruce Macintosh, an astrophysicist from Lawrence Livermore National Laboratory and one of the key authors of a paper appearing in the Nov. 13 issue of Science Express.
"We've been trying to image planets for eight years with no luck and now we have pictures of three planets at once."
Using high-contrast, near-infrared adaptive optics observations with the Keck and Gemini telescopes, the team of researchers from Livermore, the NRC Herzberg Institute of Astrophysics in Canada, Lowell Observatory, University of California Los Angeles, and several other institutions were able to see three orbiting planetary companions to HR8799.
Astronomers have known for a decade through indirect techniques that the sun was not the only star with orbiting planets.
"But we finally have an actual image of an entire system," Macintosh said. "This is a milestone in the search and characterization of planetary systems around stars."
During the past 10 years, various planet detection techniques have been used to find more than 200 exoplanets. But these methods all have limitations. Most infer the existence of a planet through its influence on the star that it orbits, but don't actually tell scientists anything about the planet other than its mass and orbit.
Second, the techniques are all limited to small to moderate planet-star separation, usually less than about 5 astronomical units (one AU is the average distance from the sun to Earth).
In the new findings, the planets are 24, 37 and 67 times the Earth-sun separation from the host star. The furthest planet in the new system orbits just inside a disk of dusty debris, similar to that produced by the comets of the Kuiper belt of our solar system (just beyond the orbit of Neptune at 30 times Earth-sun distance).
"HR8799's dust disk stands out as one of the most massive in orbit around any star within 300 light years of Earth" said UCLA's Ben Zuckerman.
In some ways, this planetary system seems to be a scaled-up version of our solar system orbiting a larger and brighter star, Macintosch said.
The host star is known as a bright, blue A-type star. These types of stars are usually ignored in ground and space-based direct imaging surveys since they offer a less favorable contrast between a bright star and a faint planet.
But they do have an advantage over our sun: Early in their life, they can retain heavy disks of planet-making material and therefore form more massive planets at wider separations that are easier to detect. In the recent study, the star also is young - less than 100 million years old - which means its planets are still glowing with heat from their formation.
"Seeing these planets directly - separating their light from the star - lets us study them as individuals, and use spectroscopy to study their properties, like temperature or composition," Macintosh said.
"Detailed comparison with theoretical model atmospheres confirms that all three planets possess complex atmospheres with dusty clouds partially trapping and re-radiating the escaping heat" said Lowell Observatory astronomer Travis Barman.
The planets have been extensively studied using adaptive optics on the giant Keck and Gemini telescopes on Mauna Kea, Hawaii. Adaptive optics enables astronomers to minimize the blurring effects of the Earth's atmosphere, producing images with unprecedented detail and resolution. LLNL helped build the original adaptive optics system for Keck, the world's largest optical telescope.
Christian Marois, a former LLNL postdoctoral researcher and the primary author of the paper who now works at NRC, developed an advanced computer processing technique that helps to extract the planets from the vastly brighter light of the star.
A team led by Macintosh is constructing a much more advanced adaptive optics system designed from the beginning to block the light of bright stars and reveal even fainter planets. Known as the Gemini Planet Imager, this new system will be up to 100 times more sensitive than current instruments and able to image planets similar to our own Jupiter around nearby stars.
"I think there's a very high probability that there are more planets in the system that we can't detect yet," Macintosh said.
"One of the things that distinguishes this system from most of the extrasolar planets that are already known is that HR8799 has its giant planets in the outer parts - like our solar system does - and so has 'room' for smaller terrestrial planets - far beyond our current ability to see - in the inner parts."