Scientists Find ‘Framework’ of Planetary Formation
On a still, moonlit night earlier this week, UCLA astronomer Andrea Ghez was gazing at an image of star HR4796, made famous last month when astronomers found what they thought was evidence of planet formation around the star.
That evidence, though intriguing, consisted of what wasn’t there. Astronomers found an empty space or hole surrounding the star, suggesting that dust had been swept up by newly congealing planets.
Peering through the powerful Keck Telescope on Sunday night, however, Ghez was taken aback by the sight of something that was there: a dramatically warped structure in a disk surrounding HR4796.
The structure, she and her colleagues believe, may be the first direct evidence of planets as they are actually forming around a star other than our sun.
“This is much more exciting than the hole,” said University of Florida astronomer Charles Telesco. “The hole’s just sitting there. This is a mechanism [for planetary formation].”
Ghez, who said she was too excited to sleep before the next scheduled observation on Tuesday night, said it was a “very dramatic effect. It’s tremendously exciting. You’re looking at the construction site [for planetary formation]. You can begin to see some of the framework going up.”
Because the astronomers only discovered the warped structure Sunday night, they are cautious about drawing any final conclusions. Nevertheless, Telesco said, “it’s a good bet” that planets are forming. “It’s consistent with the evidence.”
It is clear from the early images that the disk surrounding the star is not flat like a pancake, but distorted in some way. It may be warped like a phonograph record left in the sun.
Alternatively, the disk may be striped with ridges of denser material, like the spiral arms in our galaxy. Since stars are generally born in the dense spiral arms of galaxies, planets also probably form in dense lanes running through stellar disks. Another possible explanation is that unseen planets are plowing through the dust and leaving a kind of wake.
The planets themselves would not be visible because they do not have enough of a surface to reflect the light from the star. Dust is easier to see because it is more spread out and therefore reflects more light and heat.
Either way, the warped disk will probably shed important light on the environment in which planets can condense out of disks surrounding stars.
“We’ve based our ideas of how planets form on very little evidence,” Ghez said. Because the planets in our solar system orbit in one plane, astronomers deduce that they clumped together from a disk of smaller particles encircling the young sun. And although astronomers have detected such dust disks around stars, “we’ve never really seen the structure that gives rise to the planets,” Ghez said.
The structure in the disk might be the first look at the mechanism that gives rise to planet formation, Telesco said. “This is the first real clue we’ve got.”
Using an infrared camera developed by the University of Florida, the astronomers were looking at room temperature dust surrounding the star. Because virtually everything on Earth and in the sky glows in infrared light, the astronomers have to subtract most of the light the detector picks up as extraneous “background.” Looking at the sky in infrared--even at night--is analogous to trying to see the stars during the day, when the sun drowns out all the other stars with its light.
However, the University of Florida detector, dubbed OSCIR for Observatory Spectrometer Camera for the Infrared, is so sensitive that it can pick up a signal that stands out from the background by only one part in a million. That sensitivity, combined with the resolving power of Keck--the world’s largest telescope--enabled them to see the unprecedented detail in the stellar disk.
At this point, the researchers are still speculating as to the nature of the structure they’re seeing. “Part of the fun of being at the telescope is speculating,” Ghez said.
Whatever the warpage turns out to be, however, it’s likely to be important, Telesco said. “It’s like being at a murder site and stumbling over a blunt object. It might not be connected to the murder, but it might be the clue you’ve been looking for.”
The structured disk is even more significant because HR4796 is part of a double star system, gravitationally bound to another star that orbits at about five times the diameter of our solar system. Until recently, many astronomers believed that disks--and therefore planets--were unlikely to form around double stars. Since most young stars are doubles, planets would be rare if they could form only around single stars like our sun. Planets are likely to be far more common if they can form around binary stars.
The likelihood of life elsewhere in the universe, in turn, rests on the likelihood of planets.
HR4796 is about 10 million years old, exactly the right age for beginning a planetary family, Ghez said. Our sun is 5 billion years old. Although HR4796 is considered to be in our solar neighborhood, it is still about 220 light-years away.
Although the star doesn’t yet have a proper name, its disk was first discovered a decade ago by UCLA astronomer Michael Jura.
On Monday night, the astronomers at Keck unofficially decided to call it Jura’s star.
