The Milky Way stretches above the telescopes of ALMA in Chile. Credit: ESO/José Francisco Salgado

The Milky Way stretches above the telescopes of ALMA in Chile. Credit: ESO/José Francisco Salgado

Sen—Europe’s new ALMA observatory, working with the Herschel space telescope, has discovered a new-forming solar system whose confused character has shed new light on current models of planetary formation.

Planets form in swirling disks of gas and dust around new-born stars. But to the surprise of astronomers, the disk around a star called HD 21997 is in a hybrid, intermediate state of evolution.

Contrary to what they expected, the disk contains both primordial gas left over from the star’s own birth and dust produced in collisions between the rocky building blocks called planetesimals that collect to form much larger planets.

Stars are born surrounded by disks of dust and gas. The dust grains stick together to build larger, solid, kilometer-sized bodies known as planetesimals which either survive as asteroids and comets, or join together further to form solid planets like our Earth, or the cores of giant gas planets.

However, according to current models of planet formation, as a star reaches the planetesimal stage, the original gas should swiftly be depleted, with some falling into the star, some helping form giant gas planets like Jupiter, and the rest blown away into space. All the original gas is expected to be lost after around 10 million years.

But now a team of astronomers from the Netherlands, Hungary, Germany, and the US has found what appears to be a rare hybrid disk, which contains plenty of original gas, but also dust produced much later in the collision of planetesimals. They are calling it a link between an early and a late phase of disk evolution - the primordial disk and a later debris phase.

ALMA images of HD 21997 show the emission of cold dust grains, left, in a ring around the central star, and the emission from carbon monoxide gas, right, lying closer to the star. Credit: Á. Kóspál (ESA) and A. Moór (Konkoly Observatory)

The astronomers used both ESA’s now-redundant Herschel Space Observatory and the array of telescope dishes called ALMA (the Atacama Large Millimetre Array) in Chile to study the disk around the star HD 21997.

The star lies in the Southern constellation Fornax, the furnace, 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 observations show a broad dust ring surrounding the star at distances between about 55 and 150 astronomical units (one astronomical unit is the average distance from the Earth to the Sun). But the ALMA observations also show a gas ring.

Surprisingly, the two do not coincide. Principal investigator Ágnes Kóspál from ESA said: “The gas ring starts closer to the central star than the dust. If the dust and the 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.”

Attila Moór from Konkoly Observatory, Hungary, said: “Our observations also showed that previous studies had grossly underestimated the amount of gas present in the disk. 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.”

The astronomers say that this value is another indication that the gas disk is made of primordial material because gas released in collisions between planetesimals could never explain this substantial quantity.

Thomas Henning from the Max-Planck Institute for Astronomy, in Germany, said: “The presence of primordial gas around the 30 million-year-old star is puzzling. 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 now trying to find more alien solar systems like HD 21997 so they can study further examples of hybrid disks, and to find out how they fit within the current picture of planet formation.

The Herschel space telescope which is no longer functioning. Credit: ESA