Planet Forming Supersonic Rain

Swirling disc of gas & dust surrounds a developing star (Illustration: NASA/JPL-Caltech)

Water from space is 'raining' onto a planet-forming disc at supersonic speeds, new observations from the Spitzer Space Telescope reveal. The unprecedented detail of the observations at this early stage of the disc's formation could help reveal which of two competing theories of planet formation is correct.

Planets form when matter clumps together in swirling discs of gas and dust, called protoplanetary discs, around infant stars. But many details of how this works are still not known. For example, some scientists think giant planets can form in just a few thousand years, while others argue it takes millions of years.

Now, astronomers led by Dan Watson of the University of Rochester in New York, have gained an unprecedented view of a protoplanetary disc at the young age of just a few hundred thousand years old.

They used the Spitzer Space Telescope to examine the spectrum of infrared light coming from the vicinity of an embryonic star called IRAS 4B, which lies about 1000 light years from Earth.
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Outer cocoon
At this very early stage, an outer cocoon of gas and dust called an envelope still surrounds the star and its swirling disc. Previous observations in the microwave portion of the spectrum suggested that this large cocoon is contracting and sending material onto the disc. But the inner region, where the disc meets the cocoon, could not be seen at these wavelengths.

The Spitzer observations probe this inner region and reveal infrared light emitted by massive amounts of water vapour – the equivalent of five times the content of the Earth's oceans.

The vapour is too hot to be explained by the embryonic star's radiation alone, suggesting another process must be heating it up.

Sonic boom
The team believes ice from the cocoon is pelting the disc at a rate faster than the speed of sound there, creating a shock front. "The sonic boom that it endures when it lands on the disc heats it up very efficiently" and vaporises it.

This supersonic shock "has been searched for and theorised about for decades", Watson says. It is a short-lived phenomenon that only occurs during the first few hundred thousand years of the star and disc formation, while the envelope is still feeding the disc.

The light emitted as the icy particles hit the disc can be used to learn more about the disc itself at this early stage, which could shed light on how planets form.

Turbulent birth
Most astronomers believe planets form according to a model known as "core accretion", in which small particles snowball into larger and larger objects over millions of years.

A competing idea, called "disc instability", is that turbulence in the disc can cause matter to collapse into planets extremely quickly, producing gas giants such as Jupiter in just a few thousand years.

"If you wanted to test between those scenarios, one of the most important places to look would be the stage we're looking at now." Future observations of such young discs could reveal how turbulent the discs are, and thus whether they boast the conditions required for disc instability. "The whole subject of the very beginnings of the development of solar systems is open to study now," Watson says.

Source: The development of a protoplanetary disk from its natal envelope

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