"Astrophysicists have detected the first signs of cold water vapor in the outer reaches of a baby star system. The discovery, announced today in Science, not only fills a gap in the convoluted question of how planets form, but also hints where the water that covers Earth-like planets is stored until the rocky bodies can receive and hold onto it as oceans.
The short version of how scientists believe the Earth formed goes like this: Roughly 4.5 billion years ago, the solar system was a spinning disk of gas and dust that looked something like a record, and one groove in that record collapsed into a molten orb that became our planet. About 700 million years later, when Earth was crusty and dried-out, comets, asteroids and other watery space wanderers bombarded the world. In just tens or hundreds of thousands of years, these impacts deposited our life-giving water.
The question is where all that water came from. For decades, astrophysicists have suspected that the water in these small icy bodies originated in the center of the freezing-cold outer zone of planet-forming disks. Yet the waterâ€™s temperatureâ€”just above absolute zeroâ€”made it virtually impossible to detect, preventing scientists from confirming their suspicion.
But now a team of researchers has seen the signs. Using the Herschel Space Observatory, scientists have spied faint signature of water on the surface of an expansive and chilly region of a planet-forming disk spinning around the star TW Hydrae. The extremely faint finding is probably the tip of a colossal celestial iceberg, as a store of water amounting to thousands of Earth oceans probably hides in the center of the disk.
"We now have a glimpse at a very early stage in planetary systems we had only hypothesized to exist," says space scientist Diane Wooden of NASA Ames Research Center, who was not involved in the study. "This has been an extremely difficult signature to find."
The Search for Ice
TW Hydrae, located 175 light-years away from Earth, is between 5 million and 10 million years old. Compared with the 4.5-billion-year-old sun of ours, itâ€™s practically an infant. The star is so young and so close to Earth that scientists look to it for a picture of what our own solar system looked like in its early years. Most captivating of all is TW Hydraeâ€™s spinning disk of gas, dust, water and other planet-building materials; it stretches 200 astronomical units (AU) from the star (one AU is the sun-to-Earth distance). By comparison, the dwarf planet Pluto at its farthest orbital distance is only 49 AU from the sun.
But itâ€™s not easy to find ice, even around a well-studied star. Water is easier to find when itâ€™s hot, because water vapor emits strong signals that instruments called spectrometers can detect. TW Hydrae is hot enough to thaw the ice in the part of its planet-forming disk thatâ€™s within three to five AU, so astronomers can see that easily. However, beyond TW Hydraeâ€™s three to five AU border, called the snow line, the signal fades because water freezes. Scientists whoâ€™d looked at the TW Hydrae system before couldnâ€™t detect that distant ice, and estimate how much of it might be around to form comets later in the star systemâ€™s life.
In May 2009, astronomers got a new tool when the European Space Agency (ESA) launched the Herschel Space Observatory, an orbiting telescope designed to pick up the faintest signals from the coldest objects in space. The team behind this study, led by astrophysicist Michiel Hogerheijde of Leiden University in the Netherlands, pointed Herschel at TW Hydrae and opened the shutter for 18 hours.
"Before Herschel, this was simply not feasible. You have to get outside Earthâ€™s atmosphere to see the water, so you go to space," Hogerheijde says. "The other space telescopes were not sensitive enough."
Although TW Hydraeâ€™s disk is extremely cold beyond 100 AU (just 20 degrees above absolute zero), a weak influx of ultraviolet and X-ray light both from TW Hydrae and nearby stars can form fleeting water vapor molecules. When a molecule of water ice absorbs one of these wavelengthsâ€™ photons, the moleculeâ€™s two hydrogen and one oxygen atoms split into one hydrogen and one oxygen-hydrogen molecule. They quickly recombine, but Herschel can see the faint infrared radiation they emit (if it stares long enough, that is).
"Itâ€™s like when you put an aluminum ball in the microwave. The microwave beam liberates electrons from the aluminum, which we see as sparks," Hogerheijde says.
To the teamâ€™s surprise, the cold water vapor signal was three to five times weaker than expected. Yet by comparing the result to a laboratory benchmark, they estimated the icy grains deep inside the disk harbor as much water as 6500 Earth oceans..."