ALMA and the Beginnings of Planet Formation

Since 2012, I have been exploring the question of the chemistry associated with planet formation using the Atacama Large Millimeter Array (ALMA) in Chile. ALMA was built by an international consortium (USA, Europe, Japan and Chile) and is located in the high Atacama Plateau in the Andes Mountains. What makes ALMA special is that it is comprised of more than 50 individual telescopes operating together using a technique we call interferometry.

The Atacama Large Millimeter submillimeter Array (ALMA) by night under the Magellanic Clouds

Interferometers are powerful tools; they can peer deep inside astronomical objects and provide pictures or glimpses of these objects with much higher resolution than that of a single telescope. ALMA specializes in observing star- and planet-forming regions, and has the capability to provide Earth-Sun distance resolutions. By using ALMA, we have been able for the first time to see material at distances where analogs to Earth and the giant planets like Jupiter and Saturn are born. Since we know our solar system formed out of a disk of gas and solid particles (astronomers call it “dust,” but you can think of the solids as tiny grains of sand), we can use ALMA to look at disks surrounding other stars. Essentially, we are witnessing planet formation in action.

What I am trying to do is link the chemical composition of the disk to planets. One idea relates to icelines. Gas and dust in the disk surrounding a young star is hotter closer to the star and colder farther away. At some radius, there is a transition in which molecules such as water will be present as a gas (vapor) and then as an ice. We call this transition an iceline, and it has potential import for planets in many ways. First, it might influence planet composition. Gas giants are mostly made of gases and not solids, so if water is missing from the gases, it will also be missing from the atmosphere. Second, icelines might be favorable spots for planet formation. All of this points to a need to observe disks and detect this important chemical transition which will be different for different gas phase molecules, such as water and carbon monoxide. Kamber Schwarz and Coco Zhang of my U-M research group have studied the carbon monoxide iceline, while Fujun Du explored that of water.

Another important question concerns the overall chemistry of the disk itself. Our group has proposed that the main agent for powering disk chemistry, energetic cosmic rays from space, may not be present. The thesis work of Ilse Cleeves, who studied at U-M and is now a Hubble Postdoctoral Fellow at the Harvard-Smithsonian Center for Astrophysics, discussed how the chemistry of planet formation is impacted by having less ionization. One interesting result of that suggests that the water on Earth formed before the Sun itself – or the hydrogen atoms found in oxygen over 4.6 billion years ago. With ALMA, we have been able to explore this chemistry in greater detail, and have detected beautiful images of hydrocarbon rings. These images of C2H, a very simple hydrocarbon, show tremendous structure with rings of emission that we believe are related to the early stages of planet formation.

Images of C2H, a very simple hydrocarbon, show tremendous structure with rings of emission that we believe are related to the early stages of planet formation

This is an exciting time. With ALMA, we can for the first time really make connections between new planets and own birth. At the same time, we will be tracing our own origins – which is just plain fun.