Planet-Forming Disks May Resemble Solar System 5 Billion Years Ago

The findings, published in the Astrophysical Journal, may provide insights into the birth of our own solar system.
To make a planet, you need stuff.
Protoplanetary disks — cosmic frisbees of gas and dust orbiting young stars across the galaxy — spin out new planets. But the size of those planets depends on just how much material these disks have to give.
A team of scientists led by the University of Arizona has imaged a cluster of protoplanetary disks in the Orion Nebula and discovered that they are smaller than those previously studied in closer, less-dense regions. The smallness of these newly imaged disks suggests that making giant planets such as Jupiter (which is 2.5 times more massive than all the other planets in our solar system combined) could be especially difficult.
What's more, the Orion Nebula looks a lot like other planet-forming regions in the Milky Way, meaning our own solar system likely formed in an Orion-like environment. The team's findings have been published in the Astrophysical Journal.
The scientists used the largest telescope in the world, an interferometric array of radio telescopes in Chile called ALMA, to observe about 110 protoplanetary disks in the Orion Nebula in the deepest survey of the region yet.
"The general motivation for the whole field is that we want to understand more about how planets are formed," says Josh Eisner, a UA professor of astronomy who led the study.
In their pursuit of that understanding, scientists have spent decades looking to star-forming regions such as Taurus, a mere 500 light-years away (as compared to Orion's 1,344). While its nearby location makes a slice of the universe such as Taurus easier to observe with less-powerful telescopes, it's not what one might call a "typical" planet-forming region.
Orion, on the other hand, with its many stars (and orbiting disks) clustered together in relatively small area, is typical. It requires a more powerful telescope to take sharp observations, but in terms of regions where planets — or entire solar systems — form, it's a better model.
"Orion is not at all an oddball region. The disks there look a lot like what we think our solar system looked like when it was a protoplanetary disk," Eisner says. "And now with the advent of ALMA, we can study regions like Orion well."
Based on the images, the team — which also included astronomy and astrophysics graduate student Ryan Boyden, Steward Observatory postdoctoral researchers Nicholas Ballering and Min Fang, Steward Observatory associate astronomer Jinyoung Kim, and Lunar and Planetary Laboratory associate professor Ilaria Pascucci — was able to calculate the mass of protoplanetary disks in the Orion Nebula.
"Disk mass tells you how much stuff there is in the disk and that gives you a budget for what you can build out of it," Eisner says. "And what we found was, in this region, mass is actually quite constraining."
Unlike those studied in nearby regions such as Taurus, planet-forming disks in the Orion Nebula don't have enough stuff to build large planets such as Jupiter, for which you would need tens of Earth masses. According to Eisner, this may mean that much of the stuff already has been used to make young planets. Disks in Orion also appear smaller in size than those in Taurus-like regions.
"It's pretty tantalizing that Orion looks so different from all these lower-density, closer regions but it's just one. We want to fill in the data with more of these high-density regions to see if they all look like Orion," says Eisner, who is already seeking grant funding and telescope observing time to do so.
The discovery also will be tantalizing for those interested in what our solar system looked like as it was cooking some 5 billion years ago.
"The initial conditions for planet formation can tell us a lot about the constraints and how the process really unfolds," Eisner says.
One theory about our solar system's formation, called the Nice Model, argues that, early on, the configuration of the planets within a disk was small and compact until resonance finally flung Neptune and Uranus onto longer orbits.
The fact that the small, compact systems Eisner's team observed in the Orion's disks match up so nicely with the initial planetary configuration in the Nice Model, Eisner says, is a compelling hint at the origins of our solar system.
"The solar system probably formed in an Orion-like environment," he says. "Now we've actually got an idea of what systems there look like."

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