A rare snapshot of a planetary construction site

Contrary to expectations, the disk around the star HD 21997 appears to contain both primordial gas left over from the formation of the star itself and dust that appears to have been produced in collisions between planetesimals.
By | Published: October 28, 2013 | Last updated on May 18, 2023
disk around star HD 21997
ALMA images of the disk around HD 21997. The left image shows the emission of cold dust grains, situated in a ring around the central star. The middle image displays the emission from carbon monoxide and shows that gas can also be found closer to the star than dust. The right image depicts the velocity of the gas. The red-colored parts of the disk move away from us, while the blue-colored parts move towards us, indicating that the gas is rotating/orbiting around the central star.
Á. Kóspál (ESA)/A. Moór (Konkoly Observatory)
Planets are formed in disks of gas and dust around nascent stars. Now, combined observations with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Herschel Space Observatory have produced a rare view of a planetary construction site in an intermediate state of evolution. Contrary to expectations, the disk around the star HD 21997 appears to contain both primordial gas left over from the formation of the star itself and dust that appears to have been produced in collisions between planetesimals — small rocks that are the building blocks for the much larger planets. This is the first direct observation of such a “hybrid disk” and likely will require a revision of current models of planet formation.

When a star similar to our Sun is born, it is surrounded by a disk of dust and gas. Within that disk, the star’s planetary system begins to form. The dust grains stick together to build larger solid kilometer-sized bodies known as planetesimals. Those either survive in the form of asteroids and comets or clump together further to form solid planets like Earth or the cores of giant gas planets.

Current models of planet formation predict that as a star reaches the planetesimal stage, the original gas should quickly be depleted. Some of the gas falls into the star, some is caught up by what will later become giant gas planets like Jupiter, and the rest is dispersed into space, driven by the young star’s intense radiation. After 10 million years or so, all the original gas should be gone.

But now, a team of astronomers from the Netherlands, Hungary, Germany, and the United States has found what appears to be a rare hybrid disk, which contains plenty of original gas and also dust produced much later in the collision of planetesimals. As such, it qualifies as a link between an early and a late phase of disk evolution — the primordial disk and a later debris phase.

The astronomers used both the European Space Agency’s (ESA) Herschel Space Observatory and the compound telescope ALMA in Chile to study the disk around the star HD 21997, which lies in the southern constellation Fornax at a distance of 235 light-years from Earth. HD 21997 has 1.8 times the mass of our Sun and is around 30 million years old.

The Herschel and ALMA observations show a broad dust ring surrounding the star at distances between 55 and 150 astronomical units (1 AU is the average Earth-Sun distance). But the ALMA observations also show a gas ring; surprisingly, the two do not coincide. “The gas ring starts closer to the central star than the dust,” said Ágnes Kóspál from ESA. “If the dust and gas had been produced by the same physical mechanism, namely by the erosion of planetesimals, we would have expected them to be at the same location. This is clearly not the case in the inner disk.”

“Our observations also showed that previous studies had grossly underestimated the amount of gas present in the disk,” said Attila Moór from Konkoly Observatory in Hungary. “Using carbon monoxide as a tracer molecule, we find that the total gas mass is likely to amount to between 30 and 60 times the mass of the Earth.” That value is another indication that the gas disk is made of primordial material. Gas set free in collisions between planetesimals could never explain this substantial quantity.

“The presence of primordial gas around the 30-million-year-old star HD 21997 is puzzling,” said Thomas Henning from the Max Planck Institute for Astronomy in Heidelberg, Germany. “Both model predictions and previous observations show that the gas in this kind of disk around a young star should be depleted within about 10 million years.”

The team is currently working on finding more systems like HD 21997 for further studies of hybrid disks and to find out how they fit within the current paradigm of planet formation or the ways in which the models need to be changed.