Astronomers know how planets like our own Earth were born. When the Sun was young, it was surrounded by a flat, rotating disk of gas and dust. Dust particles, pebbles, and small rocks in this protoplanetary disk slowly clumped together to form Earth-like planets.

But the origin of giant planets like Jupiter and Saturn is not so clear. These planets consist mainly of gas. According to one theory, icy particles first clumped together to form the giant planet’s core. At a later stage, the gravity of this core attracted huge amounts of gas from the surrounding disk. But another theory says that the giant planets were born in one fell swoop, from an instability in the disk that collapsed under its own gravity.

New ALMA observations of a young star now seem to support this second theory, at least in this case. The star, known as Elias 2-27, is surrounded by a disk of gas and dust. The disk is about the size of the orbit of the planet Neptune in our own solar system. Beyond the disk is a relatively empty gap – maybe it hints at the presence of a full-grown planet.

But beyond this gap, ALMA found two ‘spiral arms’ of dust, stretching out to some 10 billion kilometers from the star. The two spiral arms look a bit like the spiral flows of water from a rotating lawn sprinkler.

Spiral structures had earlier been found at the surfaces of some protoplanetary disks. But ALMA was able to peer all the way into the central plane of the dusty disk, where planet formation takes place. It turns out that the spiral structure is present there as well.

The spiral arms are so-called density waves. At some places in the disk, the gas and dust have a much higher density than in other regions. This suggests that gravitational instabilities may occur: if the density in a particular part of the disk becomes too high, it may collapse into a new giant planet.


 

What?
Elias 2-27 is a young star: it was born at most a few million years ago. It is surrounded by a thick, massive protoplanetary disk, from which planets may form. The star is located at some 450 light-years from Earth, in a star-forming region in the constellation Ophiuchus, the Serpent Bearer. It was first described by American astronomer Jonathan Elias, and named after him.

Who?
The ALMA observations of Elias 2-27 were carried out by a very large team of astronomers from Europe, the United States, and Chile. The team was led by Laura Perez of the Max Planck Institute for Radio Astronomy in Bonn, Germany. Laura and her colleagues have published their discovery in the weekly magazine Science.