But the team led by Throop still found evidence of dust grains that could evolve into smaller rocky planets. By pointing the Hubble Space Telescope at a giant dust disk called "114-426" in front of the nebula and observing how light filtered through its edges, they could infer the size of particles within.
"We've never seen dust in astrophysics that behaves like this," he said. "It's not direct visual evidence of planets but it's very direct evidence for large grains, which means these things are sticking together and growing and on their way to becoming planetesimals and planets."
Their results -- challenged by Karl Stapelfeldt at NASA's Jet Propulsion Laboratory, who has written a review of contrary findings with one of the Throop team members, Mark McCaughrean -- were published in this week's issue of the journal Science.
Astronomers have found some 50 planets orbiting stars beyond our solar system in the past five years, most of them gas giants that are inhospitable to life as we know it and none in the Orion Nebula -- a stellar nursery. The potential for solar systems with Earth-sized planets excites those who search or hope for extraterrestrial life.
Race against astronomical time
The challenge for would-be planets in the nearby Orion Nebula is to overcome the damaging forces of intense ultraviolet radiation produced by nearby bright stars which effectively rubs away at disks of dust and gas from which the tiniest of dust grains coalesce to form planets.
The model by Throop suggests that dust grains are in a race against astronomical time. If they lose a 10,000-year battle against UV radiation, they will fail to grow into the slightly larger grains (more than 5 microns across -- 20 times thinner than a human hair) that are needed to push them over the edge into forming planets.
"Our models are saying that it is really hard to build planets, especially large planets like Jupiter, in the Orion Nebula," Throop said.
Our solar system has three components -- rocky terrestrial planets (like Mars and Earth), gas giants (such as Jupiter and Saturn) and icy bodies (comets in the Kuiper Belt beyond Pluto). In the Orion Nebula, it is easier to form terrestrials but harder to form Jovian planets and Kuiper Belt Objects, Throop said. To collect enough gas to form a Jupiter-sized planet, it would take 1 million to 10 million years, he said, but disks in the nebula where such a planet would form are destroyed in 100,000 years.
The question then is, whether dust grains large enough to form smaller rocky planets can beat the clock. Throop's team said yes.
Contrary evidence
Stapelfeldt disagrees with the claim that dust grains can survive in this environment to form planets.
"It is clear from previous work that disk survival times will be short in an environment like the Orion Nebula," he said. "The outer parts of these disks will certainly not get the chance to form planets." Stapelfeldt said the Throop team failed to acknowledge contrary data that detected reflected starlight on the disk and found it behaved as it would if the grains had not grown at all.
Astronomers still lack techniques for looking inside protoplanetary disks, Stapelfeldt said, to see how fast grains are growing and assess whether planet formation is possible there.
"So it remains an open question if the Orion disks have enough time to form planets before they are destroyed by the ionizing radiation of their environment," he said.
Alan Boss, a Carnegie Institution astrophysicist who specializes on planetary formation, said these newest results are encouraging although they don't explain the whole story of how a dust grain turns into a planet.
"There is still a long way to go from 5 microns to 5,000 kilometers (3,000 miles)," he said, "but it is a start."
Blowtorch radiation
The quality of the light detected by Throop's team suggested that particles are starting to stick together, growing in mass to 1,000 times their initial size, with some as large as 0.04 inch (1 millimeter) across. Tiny that is, but it's enough to get a planet started, Throop said.
"If they are 1-millimeter size, then these are huge particles and planets are just around the corner, so to speak," he said.
The model suggests that grains grow mainly in the center of the disk where photo-evaporation fails to penetrate.
Only particles bigger than a millimeter across can resist the UV radiation in the long run. Particles that include ice (that could develop into comets) and gases (like our outer solar system planets Jupiter, Uranus, Neptune and Saturn) fail to endure.
If true, the implications for the formation of worlds beyond ours are significant because astronomers think that most stars in our Milky Way Galaxy form in UV-challenged, or photo-evaporative regions. One upshot is that smaller, rocky terrestrial planets could be abundant and typical rather than the exception in our galaxy, and such planets are far more hospitable to life than gas giants like Jupiter.
Jupiters: Abundant or rare?
The new finding flies in the face of dozens of Jupiter-sized planets that researchers have discovered recently. But Throop said Jupiter-sized planets are only found around 5 percent to 7 percent of stars when sought, and they can form in regions sheltered from bright stars and ultraviolet radiation.
The census of gas giant planets within 2 AUs of their central stars shows that 10 percent are Jupiter-sized, so Boss disagrees with claims that Jupiter-sized planets are rare. Instead, theorists need to model ways to explain those findings as well.
Another way to explain the formation of Jupiter giants, Boss told SPACE.com, is to accept a competing theory that Jupiter-sized planets form as a result of disk instability or gravitational collapse. That process only takes 1,000 years or less so the planet could build up before UV radiation ate it away.
"The upshot of all this is that for all we know, the Orion disks could produce planetary systems just like our own or something completely different or, most likely, a mixture of these two extremes," Boss said. "We can't really tell what will result based on what we know right now, but planet formation should still be possible in general."
The general tone among planet seeking astronomers is positive these days, with planet detection and models of planet formation churning out at a fast rate. And much remains to be learned, Boss said.
"The search for extrasolar planets must continue," he said. "We still do not know what the census results will be for our neighborhood of the galaxy, how many gas giants live here, how many ice giants and especially, how many Earth-like planets."
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