Witnessing the Birth of Planets
Peering into the previously unseen wombs of newly forming solar systems, astronomers believe they have caught two different stars in the act of creating planets.
“I think it’s the first real evidence of earthlike planets” elsewhere in the universe, said astronomer Wayne Holland of the Joint Astronomy Center in Hawaii, whose report on one of the stars appears Thursday in the journal Nature.
If confirmed, the discoveries will boost hopes that planetary families around stars are common--something astronomers have “been wanting to believe,” astronomer David Koerner said Tuesday at a news conference at NASA’s Jet Propulsion Laboratory in Pasadena.
In an impressive display of high-tech stargazing, three independent groups of scientists at UCLA/Hawaii, Caltech/JPL and Harvard/University of Florida released images Tuesday of doughnut-shaped holes in the dust rings surrounding two stars. They imaged two other stars with mysterious “blobs” that appeared to be near them.
The empty spaces appear to be the signature of newly congealing planets as they “sweep up” dust and other material in the ring, scientists said.
While the findings do not contradict astronomers’ notions of how Earth was formed billions of years ago, the images perhaps offer the earliest look yet at a solar system’s first years.
And they do offer a few rather startling surprises. Foremost among them are the mysterious, bright blobs hovering close to two stars, Beta Pictoris and Vega. Vega emitted fictional radio signals in the movie “Contact.”
Most exciting to astronomers is that the ability to see deep inside the gas-shrouded regions surrounding stars allows them to study the evolution of solar systems like our own, from infancy through adolescence to old age. “We’re starting to see the family tree,” said Michael Werner of JPL.
Our solar system was born, astronomers believe, out of a giant cloud of debris ejected by previous generations of exploded stars. As gravity sucked the matter in the cloud toward a common center, the dust spun itself into a flattened pancake, while the central, densest region lit a nuclear fire that ignited the sun.
Tens of millions of years later, the dust congealed into rocky globs, and eventually planets. The gravity of gas giants like Jupiter eventually swept out smaller rocky objects, making Earth safe from continual bombardment and a haven for life.
Detecting Planets Outside Solar System
Outside of our own solar system, planets are too dim to be seen directly, drowned in the light of their stars. Over the past five years, teams of astronomers have deduced the existence of nearly a dozen Jupiter-sized objects by observing that certain stars appear to wobble, apparently the result of gravitational tugs from orbiting bodies, which could be planets. Only a giant planet, however, would have the gravitational pull to make its influence felt on a star.
In contrast, the dust-free halos around the nearby star Fomalhaut--detected by the UCLA/Hawaii group--could be an area where newly formed rocky Earth-like planets are swirling about in a barrage of rocky material.
Although the evidence is indirect, the age of the stars, and the size of the halos, suggests that they could harbor solar systems much like our own, but much younger. Both stars are only tens to hundreds of millions of years old, compared to our 4.5-billion-year-old sun.
Still, said Holland, “I believe that the planets are already there.”
If they are, studying them could open a window to our own distant past. “These stars are very similar to our own sun,” he said.
Holland’s group viewed the disk around the stars in extremely cold, infrared light. In visible light, the cool disks would be drowned out by the hot light of the star. However, in the infrared, it is clear that the dust density drops off steeply toward the center of the disks.
Perhaps even more tantalizing, the images of Beta Pictoris and Vega reveal mysterious blobs of brightness off to the side of the stars. “We’re just mystified,” said UCLA astronomer Benjamin Zuckerman. “We have two mysterious objects.”
Chances are, the mysterious objects are distant galaxies that turned up in the photograph. But if both objects turn out to be part of the stars’ gravitational system, “it would be something totally new,” he said--new enough to compel astronomers to rescript their stories of planetary creation.
The more distant star imaged by the JPL and Harvard groups is much younger than the three other stars. The hole in this disk is suggestive of planet formation in part because of its size. “Our solar system would fit nicely into that hole,” said Michael Ressler of JPL, who built the MIRLIN detector that imaged the star.
This distant star, known only as HR 4796, also stands out because it is one of a pair of binary stars. Due to the tug of the second star’s gravity, HR 4796’s disk has been stretched into an elongated ellipse.
Unlike the results published in Nature, the JPL results have not yet been peer reviewed, but were submitted to a journal Friday.
Until recently, astronomers didn’t believe that planets could form around double stars. If there really are planets forming in the disk, said Koerner, it suggests that our solar system is not likely to be typical.
HR 4796 would give astronomers their earliest glimpse of a planetary system as it is coming into being, before the giant planets had completely cleared out the debris of rocks and dust. Admittedly “waxing poetic,” Werner said he could imagine a “family of planets and planets-to-be, some with moons forming in the material around them.”
Like the planets and moons in our solar system, these worlds would orbit in the same direction, in a flat disk. However, the system would still be in the “heavy bombardment” phase, with asteroid-like objects constantly colliding with the newborn planets in a violent version of cosmic bumper cars. “It would not be a very hospitable place to live,” Werner said.
The discoveries were made possible by new, highly sensitive instruments able to detect faint rays of radiation. The JPL-designed MIRLIN was fitted onto one of the twin eyes of the Keck Telescope on Mauna Kea, while the British SCUBA camera snapped images from the James Clerk Maxwell Telescope, also on Mauna Kea.
The new instruments allow astronomers to see disks in more detail than ever before. UCLA astronomer Michael Jura compared the feat to seeing a fetus in the womb. “If you examine a pregnant woman [with a stethoscope], you can get a heartbeat. But if you have a sonogram, you can [detect] much more.”
The new detectors, said Koerner, allow scientists to go from just speculating about how planets form to “actually watching them. . . . We think this is something like a snapshot” of Earth in its earliest, embryonic phase.
Of course, Koerner and others emphasized they haven’t actually seen any planets, “so we don’t know for sure. . . . We infer that there must be missing material at the center,” he said. “We are inferring that a solar system like ours is forming.”
There could be another explanation for the holes. For example, a smaller, dark companion star could be sweeping up the material, or the material could have evaporated into space.
“We don’t know exactly what’s there,” said Ressler. “We only have confidence that there is something there.”
If the guess is right, Jura said, it would be further evidence that planet-forming is “ubiquitous” in the universe. If so, more evidence for planetary families should be coming this way shortly. “We expect [this] to be followed by a family of similar discoveries,” Werner said.
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Three independent groups of astronomers have seen dark halos around young, sun-like stars--evidence that newborn solar systems are forming. If confirmed, the findings would be the earliest snapshots yet of infant, possibly earthlike, planets.
(1) A giant cloud of gas and dust left over from ancient stellar explosions contracts into a slowly rotating sphere.
(2) As the cloud collapses, it flattens, spinning faster, and a star ignites in the center.
(3) Planets congeal in the inner disk and sweep clear a doughnut-shaped hole. (Image of Fomalhaut, at left, is at this stage.)
Source: Jet Propulsion Laboratory