What’s in that Dusty Gap?

Planet Formation is a Messy Process

ALMA Observatory image of the young star HL Tau and its protoplanetary disk. This best image ever of planet formation reveals multiple rings and gaps that herald the presence of emerging planets as they sweep their orbits clear of dust and gas. Credit: ALMA(ESO/NAOJ/NRAO); C. Brogan, B. Saxton (NRAO/AUI/NSF)

Tiny acorns make giant oak trees. But did you know that entire planets are born from pieces even smaller than acorns? Down to individual molecules themselves?

All the rocks and ices and gases of a planet’s layers and atmosphere come from tiny specks floating in a diffuse cloud of gas and dust, or nebula. When the cloud collapses to form a new star, it already contains the basic ingredients of all the planets, moons, asteroids, and comets in the stellar system. I should mention that astronomers use the word “dust” to refer to any particle or clump of matter that’s in a solid state—from simple silica to complex methanol.

As intense heat and pressure start the process of nuclear fusion in the newborn star, gas and dust, rotating around the star, flatten into a disk. Because some of the material swirling in these disks will eventually combine into planets, astronomers call them “protoplanetary disks.”

During a star’s formative years, dust particles are still minuscule—less than a millionth of a meter—and they are bound to bump into each other. When they do, electrostatic forces stick them together, similar to how a rubbed balloon sticks to the wall. After less than a thousand Earth-years, these sticking specks attain the grand size of one millimeter. The tiny clumps continue to collide, sticking and growing—to pebbles, to boulders, to asteroids.

At around 20 kilometers wide, their masses begin to exert gravity on their surroundings and are hereafter called planetesimals. Planetesimals’ gravity pulls in more and more stuff than before, enabling them to grow even faster. Collisions at this stage can cause chaos, breaking some planetesimals into pieces, but the most massive incorporate the collided matter into themselves. Planetesimals that survive this process can grow even larger, becoming protoplanets.

Protoplanets clear their orbits free of remaining dust and gas, and they can do this in two ways: either by plowing it away into other orbits or by incorporating it into their growing layers. Protoplanets emerging in the colder outer orbits contain solid ices—of methane, ammonia, or water. These bodies become massive enough to draw in hydrogen and helium and grow into gas giants.

Scientists began to understand how planets form from clues present in our own solar system. But the latest telescope technology now enables astronomers to see these protoplanetary disks in unprecedented clarity. The disks are not easily seen in visible light—but radio telescopes, like the Atacama Large Millimeter/submillimeter ArrayAtacama Large Millimeter/submillimeter Array (ALMA)Funded by the U.S. National Science Foundation and its international partners (NRAO/ESO/NAOJ), ALMA is among the most complex and powerful astronomical observatories on Earth or in space. The telescope is an array of 66 high-precision dish antennas in northern Chile. (ALMA) in Chile, can see the millimeter-wavelength light that glows from the gas and dust in these disks.

One of the most interesting examples of how this new technology has illuminated our view of planetary kindergarten is the star TW Hydrae. For starters, TW Hydrae is the closest star in a protoplanetary disk phase, about 10 million years old and only 175 light-years away. More significant, astronomers believe TW Hydrae may be an analog of our own solar system when it was at a similar age four and a half billion years ago. ALMA’s high-resolution image of this system clearly shows two concentric gaps amidst the nearly circular orbits of the glowing gas and dust. Those gaps may indicate two planets orbiting this star about the same distances as Uranus and Pluto are from the Sun. But, astronomers have caught something even more alluring—zooming in reveals a gap much closer to the star than the others, only 150 million kilometers away. That’s the same distance as the Sun to Earth.

These new discoveries only hint at what’s possible to find in this emerging field. In my next few posts, I’ll talk about more insights astronomers have gained about the formation of planets, and what they hope to discover in future studies.