Revealing the details of planetary construction zones

At the meeting of the American Association for the Advancement of Science, Rice University’s Andrea Isella began his talk with words that can start a brawl among astronomy fans: “Everybody knows we have these eight beautiful planets in our Solar System.” But he continued with words we can all agree on: we’d like to know how they got here and whether they represent a typical assortment we’d expect to see around other stars.

To answer both of these questions, we need to understand planet formation. And the best way to do that is to image as many systems as we can find that are in the process of forming planets. That process poses a number of challenges, however. In astronomical terms, planet formation is fast, taking place in 10 million years or so, and the process takes place in a diffuse, dusty disk that makes it difficult to do observations at visible wavelengths.

But we now have staggeringly precise images of these disks, thanks to the Atacama Large Millimeter/submillimeter Array, or ALMA. ALMA is capable of identifying the chemical composition of the disks, along with irregularities in their distribution and the motion of different parts of the disk. Isella and other researchers were on hand to tell what we’ve learned from this sort of information.

Isella talked about the gross features of the planet-forming disks. He started off by describing HL Tau, a very young (less than a million years) star that’s got a large planet-forming disk. The disk itself already has a number of grooves in it, carved out by forming planets—modeling suggests that you need three Saturn-sized planets in order to produce the sort of grooves seen here. Isella said that the gravitational interactions of the planets and the dust not only pulls dust into the forming bodies but also pushes it elsewhere in the disk. This process can create new areas of high density that are more likely to form further planets.

Planets are shaping the dust ring of HL Tau.

Credit: ALMA (ESO/NAOJ/NRAO), NSF

But the gravitational interactions don’t always involve planets. Isella’s talk moved on to HD 142527, a binary star system where the two stars are separated by 12 Astronomical Units (a bit farther than Saturn is from the Sun). A large disk surrounds the two stars (shown at top), and their gravitational influence is creating what Isella termed a “dust trap.” ALMA was able to spot that the dusty area of the ring doesn’t show signs of carbon monoxide, a common gas in these disks. The reason? It’s cold enough that the gas will freeze out onto the solid surface of the dust particles.

This sort of behavior is what interests Harvard’s David Wilner. Disks like these, he said, contain a lot of hydrogen and helium, but those elements are invisible at the wavelengths ALMA can image. The disks are rich with a variety of chemicals—in addition to carbon monoxide, Wilner said we’ve spotted cyanide, hydrogen cyanide, formaldehyde, carbon sulfide, ions such as HCO⁺, and even some complex molecules like methyl cyanide. If you look carefully, each of their emission lines are split into a double peak, caused by red and blue shifting as different sides of the disk rotate toward or away from us.

Smaller differences in these motions can help identify areas of the disk that are moving at different speeds, and it’s possible to image warm layers on the top and bottom of the disk that surround a frozen core.

The regions where different molecules freeze out—called snow lines—play a key role in planet formation. Carbon dioxide, water, and carbon monoxide all have snow lines at different distances from a star, and depending on where a planet forms, you can get very different carbon:oxygen ratios, and thus very different chemistries. There are also chemical reactions in the disk itself that depend on snow lines. The N₂H⁺ ion, for example, can only form where carbon monoxide is absent, or the CO will steal its hydrogen ion.