Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have mapped out 18 organic and inorganic molecules in protoplanetary disks around five young stars: IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. The new ALMA maps reveal that the chemicals in protoplanetary disks are not located uniformly throughout each disk; instead, each disk is a different soup of planetary ingredients. The results suggest that planet formation occurs in diverse chemical environments and that as they form, each planet may be exposed to vastly different molecules depending on its location in a disk. A series of 20 papers detailing this project — named the Molecules with ALMA at Planet-forming Scales (MAPS) — will published in the Astrophysical Journal Supplement Series.
In this artist’s conception, planets form from the gas and dust in the protoplanetary disk surrounding a young star. Image credit: M. Weiss / Harvard & Smithsonian Center for Astrophysics.
Planets form and obtain their compositions in dust and gas-rich disks around young stars, and the outcome of this process is intimately linked to the disk chemical properties.
The distributions of molecules across disks regulate the elemental compositions of planets as well as access to water and prebiotically relevant organics.
Emission from molecules also encodes information on disk ionization levels, temperature structures, kinematics, and gas surface densities, which are all key ingredients of disk evolution and planet formation models.
The MAPS ALMA Large Program was designed to expand the understanding of the chemistry of planet formation by exploring disk chemical structures down to 10 AU scales.
The project focuses on five disks — around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480 — in which dust substructures are detected and planet formation appears to be ongoing.
“These planet-forming disks are teeming with organic molecules, some which are implicated in the origins of life here on Earth,” said Dr. Karin Öberg, an astronomer at the Harvard & Smithsonian Center for Astrophysics.
“Many of the chemicals in the disks are organic, and the distribution of these organics varies dramatically within a particular disk.”
“Two planets can form around the same star and have very different organic inventories, and therefore predispositions to life.”
In the research, the astronomers mapped out the specific locations of 18 molecules — including hydrogen cyanide, and other nitriles connected to the origins of life — in each of the five protoplanetary disks. They analyzed the images obtained by ALMA in 2018 and 2019.
“The chemistry occurring even in a single disk is much more complicated than we thought,” said Charles Law, a graduate student at the Harvard & Smithsonian Center for Astrophysics.
“Each individual disk appears quite different from the next one, with its own distinctive set of chemical substructures.”
“The planets forming in these disks are going to experience very different chemical environments.”
“Our team used these maps to show where some of the forming planets are located within disks, enabling scientists to connect the observed chemical soups with the future compositions of specific planets,” Dr. Öberg said.
Astronomers are confident that planets form in protoplanetary disks, but they can’t directly see them.
Dense gas and dust, which will last some three million years, shields young, developing planets from view.
In protoplanetary disks, gas and dust naturally rotate around a central star.
The speed of the moving material, which astronomers can measure, should remain consistent throughout the disk.
But if a planet is lurking beneath the surface, the authors believe it can slightly disturb the gas traveling around it, causing a small deviation in velocity or the spiraling gas to move in an unexpected way.
Using this tactic, they analyzed gas velocities in two of the five protoplanetary disks — around the young stars HD 163296 and MWC 480.
Small hiccups in velocity in certain portions of the disks revealed a young Jupiter-like planet embedded in each of the disks.
“As the planets grow, they will eventually carve open gaps in the structure of the disks so we can see them, but the process will take thousands of years,” said Dr. Richard Teague, an astronomer at the Harvard & Smithsonian Center for Astrophysics.
“We hope to confirm the discoveries sooner than that using the forthcoming James Webb Space Telescope. It should have the sensitivity to pinpoint the planets.”
