In a planet-forming disc of gas and dust, the snow line divides the rocky and icy planetesimals (‘baby planets’). In our Solar System, this line separates the inner terrestrial planets (Mercury to Mars) from the outer gas giants (Jupiter to Neptune).
Credit: P. Huey / Science AAAS
SHROPSHIRE: A sprawling cloud of water vapour around a developing solar system, cold enough to form comets that could one day deliver vast oceans to dry planets, has been detected.
This finding, reported in Science today, provides new insight into how planetary systems form, as well as shedding light on the origins of the Earth's oceans.
"We find cold water vapour in the disc around a young star, which shows that there must be an underlying reservoir of water ice," said astronomer Michiel Hogerheijde, lead author of the study from the Leiden Observatory in the Netherlands. "This means that the mechanism proposed to deliver the oceans to Earth through cometary impacts is a viable one for other forming planetary systems."
Planet-forming discs
Around 4.5 billion years ago, however, the planets in our Solar System were yet to be born, and the Sun was a lonely star, encircled by a rotating disc of gas and dust.
Planets are formed in the great 'protoplanetary' discs that surround young stars. When these discs cool, rocky planets form close to the star, while gaseous giants form beyond the 'snow line' - where young planets are composed mainly of ice.
In order to understand how planets develop and evolve, astronomers study the distribution and properties of molecules within these discs. One of the most important disc constituents is water, crucial to the formation and ultimate surface composition of the planets.
Astronomers have so far detected water vapour only in the hot, inner regions of protoplanetary discs - observing cooler water vapour using ground-based telescopes is difficult, due to the water vapour in Earth's own atmosphere.
A distant, icy reservoir
Hogerheijde and colleagues overcame this problem by using the Herschel Space Observatory to look for water vapour in the disc around a young star called TW Hydrae, situated 175 light-years away from Earth.
The astronomers found cold water vapour in the outer regions of the disc, produced when ultraviolet light from the star hits ice grains near the disc surface.
Using a computer simulation of the protoplanetary disc, the researchers calculated the existence of a large reservoir of ice crystals, containing enough water to fill several thousand Earth oceans.
Oceans from outer space?
Studying the disc around a young star such as TW Hydrae also gives scientists insight into how our own Solar System might have looked in its infancy.
Because the newly-formed Earth was too hot to harbour liquid water, many astronomers believe that water arrived on earthbound, ice-covered comets or asteroids. This theory relies on there being a large number of comets and asteroids carrying water, as only a small fraction of the projectiles will hit the Earth.
"If the early Solar System had a similar amount of water [to the TW Hydrae disc], then a large comet or asteroid population with water could have been created," said Rachel Akeson, an astronomer at California's NASA Exoplanet Science Institute, who was not involved in the study.
Real-time planet formation
It's now hoped that planets will begin to form within the TW Hydrae disc, allowing astronomers to study planet genesis in real-time.
"TW Hydrae is one of the oldest young stars to still have substantial gas in the disc and if it is going to form planets, that is likely occurring now," said Akeson.
"One of the big remaining questions is how the structure and composition of the discs changes with time. The observations suggest that the gas disappears before the dust, but the gas is an essential component in forming planets since it makes up most of the mass and is also what any planetary atmospheres would form from."
