Planet Formation Theory Is Still Ensnared By A Chicken Or Egg Problem

Forbes

06-08-2025

In the rush to understand the formation history of solar systems like ours, one big conundrum still ensnares this field of research. That is, how protostellar disks form their very first planets.
We don't yet know how that first planet gets formed, Nienke van der Marel an astronomer at Leiden University in The Netherlands, tells me in her office. We understand how clumping can be caused by physical forces within the disk once there's a planet in that disk, van der Marel tells me, but the biggest question is still how that first planet formed.
Ongoing observations by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile's Northern Atacama Desert are largely responsible for the lion's share of our current knowledge about how protoplanetary disks form from millimeter-sized particles and grow into full-fledged planetary systems like ours.
In 2013, using some of the first ALMA observations van der Marel and colleagues detected the existence of a dust trap in a protoplanetary disk around the young A-type star Oph-IRS 48, located some 400 light-years away in the Northern constellation of Ophiuchus.
A dust trap is a location where millimeter-sized pebbles, are concentrated in one part of the disk, and where they can continue to grow all the way from planetesimals (planetary building blocks ranging from a few km to a few hundred km in diameter) to full-fledged planets.
That first planet will carve a gap along its orbit and then at the outer edge of that gap, you naturally get a maximum in the density and a bump in the gas density, says van der Marel. So now, the pebbles that come from the outer disk still drift inwards, but then at that maximum pressure location, they get trapped, she says.
But as soon as these particles reach typical pebble sizes (1 mm across), they will start to experience drag forces from the gas in the disk and rapidly move inwards toward the star, says van der Marel.
Disappearing Pebbles
So, within a hundred years, any pebble that you form out here in the outer region of the disk has moved all the way inwards and onto the star and is lost; it doesn't have time to continue growing all the way to planetesimals, says van der Marel. So, you need something that stops the pebbles from drifting inwards, she says. And the phenomenon that was proposed to stop it was a dust trap, says van der Marel.
Planetary Disk Pressure Bumps
'Pressure bumps' or 'dust traps' present in the disk will halt these inward moving pebbles and trap them, says van der Marel. This whole idea creates a major chicken or egg problem, because if you need a planet to create dust traps, then dust traps are necessary to form planetesimals and planets, she says.
Alternative Scenarios
If indeed planets are the only source of dust traps, then we do have a chicken or egg problem to form the first planet, Olja Panic, an astrophysicist at the University of Leeds in the U.K., tells me in Reykjavik. But recent research has been directed towards identifying possible scenarios under which these dust traps can arise without the need for planets to cause them, says Panic.
This could include various types of gravitational disk instabilities, whether generated from a passing star or during the disk's earliest formation.
It is also possible that there are other mechanisms that create the first dust traps, such as magnetic fields, ionization structures (which would arrange molecules so that they have a net electrical charge), or even planetary snowlines. That is, regions in a young planetary disk where temperatures are so cold that water, ammonia, carbon monoxide or even methane can freeze into ice grains. Some researchers posit that this increase in ice density might even trigger the formation of gas giant planets like Jupiter.
The idea is that these first dust traps would then concentrate these planetary pebbles until the first planet forms within the dust trap.
But that has yet to be confirmed observationally.
When Might This Problem Be Solved?
Although ALMA's array of 66 telescopes work have allowed astronomers to see incredible detail in these disks of gas and dust around young stars, future telescopes will reveal even more detail about how these stars spawn planets.
In the coming decades, a lot of new telescopes will teach us more about the composition of exoplanets which may also tell us how they form, says van der Marel. Just like ALMA, the Next Generation Very Large Array (NGVLA) in New Mexico is another up and coming radio array which will observe at even longer wavelengths where we can trace even larger dust grains, she says. That may help us to understand where most planet formation takes place, says van der Marel.
Is our solar system an anomaly?
We don't have the data right now to say for certain whether we are an anomaly or not, says van der Marel.