Catching stellar views
'Adaptive optics' lets scientists reduce the light cast by stars and see hidden planets
To anyone whose childhood echoed with the tune of "Twinkle, Twinkle, Little Star," the idea of taking the twinkle out of starlight might verge on the unromantic, if not the heretical.
Remove the twinkle, however, and you may find that two little stars actually are four, and one appears to host a nursery for planets. Look even closer, and you may find that the blurring twinkle masks the presence of a planet.
The technology to remove the twinkle has come of age, researchers say. Known as adaptive optics, the approach holds the promise of allowing astronomers using large ground-based telescopes to make digital images of an extrasolar planet long before space-based telescopes are launched to do the same task.
The ultimate goal is to use both sets of tools to gain a clearer understanding of the conditions that give rise to solar systems, the systems' intriguing diversity, and, perhaps, the presence of life elsewhere in the galaxy.
"Within the next five to 10 years, we should be able to answer the question" of whether
other solar systems like ours dot the sun's neighborhood," says Alan Boss, a astrophysicist at the Carnegie Institution of Washington. "The more planets we detect, the more we see that other solar systems have characteristics like our own. We're not a fluke."
Over the past decade, astronomers have discovered 78 planets orbiting other stars. The planets range in mass from 24 percent Jupiter's mass to nearly 17 times the Jovian planet's heft.
The newest member of the list was announced Jan. 8 during the winter meeting of the American Astronomical Society (AAS). A team led by Sabine Frink of the University of California at San Diego reported the discovery of a planet around the star Iota Dracona, in the constellation Draco.
The find is unique, the researchers say, because the host star has used up most of its hydrogen fuel and has expanded to a radius 13 times as large as the sun's.
"Until now, it was not known if planets existed around giant stars," notes Dr. Frink. "This provides the first evidence that planets at earthlike distances can survive the evolution of their host star into a giant."
Yet, researchers say, none of the planets discovered so far have been directly imaged using adaptive optics.
The problem is that a host star far outshines any planet it anchors, notes Ray Jayawardhana, an astronomer at the University of California at Berkeley. "Viewed from far outside the solar system, Jupiter would be one billion times fainter than the sun in the infrared," he says.
Thus, astronomers have had to content themselves with inferring a planet's presence by its influence on its host star.
The first generally accepted detection of an extrasolar planet was reported 10 years ago, when a pair of radio astronomers claimed to have found a pair of planets orbiting a pulsar. They detected the planets by their effect on the arrival time of radio signals from the rapidly spinning remnant of a supernova. The vast majority of extrasolar planets have been detected by the tug they exert on their parent star. If the star is close enough, in principle the tug can be detected visually by the star's wobble against stars that are so far away they appear fixed in the sky.
So far, however, this method has failed to yield a confirmed sighting of a new planet. Thus, astronomers must look for changes in a star's spectrum, which appears to alternately stretch and compress as a planet orbits in the line of sight of an observer on Earth, pushing and pulling on the star.
In some cases, a planet can be detected by the effect it has on a star's light as it passes between the star and the astronomer's telescope. Last November, astronomers announced that they had used a variation of this technique to detect an extrasolar planet's atmosphere.
A team led by David Charbonneau of the California Institute of Technology used the Hubble Space Telescope to track a planet's orbit around HD 209458, a sun-like star some 150 light-years from Earth in the constellation Pegasus. The planet, 220 times as massive as Earth, orbits the star once every 3.5 days at a distance of 4 million miles. At that distance, the planet is heated to a searing 1,100 degrees Celsius (2,000 degrees Fahrenheit).
Each time the planet passed in front of the star, the telescope's spectrograph detected sodium in excess of what the star alone contained, leading the team to conclude it had detected sodium in the planet's atmosphere.
"This opens up an exciting new phase of extrasolar planet exploration," Dr. Charbonneau explains, by opening the possibility of measuring and comparing the atmospheres of extrasolar planets.
The holy grail of planet hunting, however, is to directly image a planet outside our solar system. To that end, the National Aeronautics and Space Administration is planning to use space-based telescopes such as the Next-Generation Space Telescope and the Terrestrial Planet Finder to scan the heavens for extrasolar planets, free from the distorting effects of Earth's atmosphere.
As recently as two years ago, the thought of trying to do the same kind of science from the ground would have been laughable, says Dr. Jayawardhana. Even today, "it sounds a bit crazy," he adds. "But it's not."
Using adaptive optics on the largest telescopes, he explains, it's now possible to detect Jupiter-class planets by focusing on young stars only 10 million years old. In such systems, the parent star hasn't yet peaked in brightness and an adolescent Jupiter-like planet would be much hotter than a 5 billion-year-old graybeard.
A young Jupiter will still be faint compared with its parent star, but substantially brighter than today's Jupiter is in comparison with the sun, and so would be well within the capabilities of a 10-meter telescope with adaptive optics, Jayawardhana estimates. Such advanced adaptive-optics systems use a flexible mirror to cancel the distorting effects of the atmosphere on light that passed though it.
As light waves approach Earth from a distant source, their wave fronts are essentially flat and approach face on.
When the light waves enter the atmosphere, however, turbulence and temperature differences in effect twist, warp, and jumble the wave fronts, distorting the image.
To counter this effect, researchers have devised ways to measure the distortion and use that information to correct the image's wavefront as it passes through a telescope.
Some adaptive-optics systems use an artificial "star," produced by shining a laser on atoms high in the upper atmosphere. The excited atoms glow. Although the light is too dim to be seen by the naked eye, it is beacon-bright in a large telescope and provides the data on the atmosphere's distortion overhead. Those data are used to constantly alter the shape of the flexible mirror in ways that restore the wavefront to its original purity. Other systems forgo the artificial star and sample the light from the source itself. The effect on images is dramatic.
During the AAS winter meeting, Jayawardhana described how adaptive optics on the 8-meter Gemini North telescope and the 10-meter Keck telescopes on Hawaii's Mauna Kea allowed his team to discover the first protoplanetary dust disk around a member of a rare quadruple-star system.
Initially, he says, standard ground-based telescopes showed a gravitationally bound pair of young stars in a molecular cloud some 900 light-years away. When his team usedadaptive optics, they detected two new stars in the cloud. One sported a protoplanetary disk whose thinness may signal the earliest stages of planet formation.
Now, the team is hunting for planets around nearby stars with Gemini North, Keck, and two other behemoth telescopes. He says the team already has found three candidates, including one faint object that appears next to TWA-6, a small, sunlike star that is part of a 10 million-year-old group some 170 light-years from Earth in the constellation Hydra.
Yet "faint object" and "planet" are not synonymous, Jayawardhana acknowledges.
Such objects could merely be faint distant stars that happen to fall in the line of sight of the star thought to harbor a planet.
Or they could be brown dwarfs - stellar wannabes that failed to gain enough mass to trigger the fusion reactions that light up stars. As these age, they grow fainter. Depending on whose computer simulations one uses, the lower mass range for brown dwarfs merges with the upper ranges thought to characterize planets, potentially confusing the issue of exactly which object an astronomer has discovered.
Moreover, brown dwarfs are beginning to pop up at planetary distances from stars.
A team led by Michael Liu, an astronomer at the University of Hawaii, has discovered a brown dwarf orbiting a sun-like star, Sge A, some 58 light-years form Earth in the constellation Sagitta. As with Jayawardhana's work, Liu says his team's observations couldn't have been made from the ground without adaptive optics. Like Jayawardhana, Liu says his observations would have been impossible from the ground without adaptive optics.
The brown dwarf orbits Sge A roughly halfway between the orbits of Saturn and Uranus, at a distance of 14 astronomical units (14 a.u.). An astronomical unit is the distance from sun to Earth.
The object, which is between 55 and 78 times as massive as Jupiter, clearly falls into the brown-dwarf mass range. But this brown dwarf is the closest to its host star ever seen through direct imaging. Typically brown dwarfs have been seen as free-floating objects. When they are tied to a star, they often orbit at distances ranging from hundreds to thousands of a.u.
"The implication is that brown dwarfs exist at the same distances that giant planets in our solar system orbit the sun," Dr. Liu says. "It shows the diversity of other solar systems possible around stars like the sun."
He adds that while the brown dwarf's formation probably kept gas-giant planets from forming around Sge A, it doesn't rule out rocky planets. With the potential for misidentifying faint companions to stars as planets, Jayawardhana says that an object must pass an astronomical "duck test": It must share the same motion across the sky with its host star, and its spectrum must indicate a dim, cold body compared with the host. Signs of water or methane would help clinch the case.
Assessing the potential of adaptive optics in planet-hunting, Boss observes: "These are very tantalizing appetizers for what's to come."