A giant star passing into the final phases of its life has provided astronomers with their first-ever evidence of water in another solar system. Astronomers suspect the water comes from comets being vaporized by the aging star, which is expanding and growing brighter.
The new finding, published in the July 12 Nature, is doubly significant because water is thought to be a crucial prerequisite for life and because the results provide astronomers their first peek at the chemical contents of another solar system.
Astronomers already have evidence of other solar systems. In the past decade, they have detected planets around approximately 60 stars. But the technique they use to detect those planets--looking for the tiny gravitational wobbles large planets create in their stars--can tell them nothing about the makeup of the planets or any other bodies in those distant systems.
"What we've done could become the first example of a new and complementary technique that tells us more about the composition of these distant solar systems, particularly as new observatories come online in the future," says David Neufeld, professor of astronomy in the Krieger School of Arts and Sciences and an author of the Nature paper.
Neufeld was one of a group of astronomers who trained an orbiting NASA observatory, the Submillimeter Wave Astronomy Satellite, on a star located in the constellation Leo known as CW Leonis.
"SWAS was designed to allow us to look for two molecules that are hard to detect from the ground--water and molecular oxygen," says Neufeld, noting that a primary reason those molecules are so hard to detect from the ground is because Earth's atmosphere is full of them.
SWAS works by breaking down radio emissions into their component wavelengths, allowing astronomers to look for the spectroscopic "fingerprints" of water and molecular oxygen. The original intent was to use SWAS to hunt for those targets in interstellar clouds of gas and dust, but astronomers recently decided to try pointing the observatory at a star.
"We already knew that water is sometimes emitted by oxygen stars, stars with more oxygen than carbon," Neufeld says. "So we wanted to try a carbon star, which has more carbon than oxygen and shouldn't emit any water."
CW Leonis is the brightest and nearest giant carbon star, but it's still distant enough that astronomers had to observe it with SWAS for nearly 400 hours to get the data they needed. They were surprised to find a strong spectroscopic signature that suggested the presence of large amounts of water.
Nothing in astronomers' highly detailed models of the life cycle and chemistry of carbon stars could account for it.
"For example, there's a lot of carbon monoxide in carbon stars, and we tried to look at whether those molecules could be broken apart by shock waves inside or outside of the star, making the oxygen available to link up with hydrogen and form water," Neufeld says.
However, anything strong enough to break apart CO would have had an effect on hydrogen that should have been visible spectroscopically, Neufeld says. That effect wasn't seen.
"Generally, our models of these stars are very successful, but they failed spectacularly in the case of this water signal," Neufeld says. "And by spectacularly, I mean by a factor of about 10,000."
Neufeld credits Hopkins astronomy graduate student Saavik Ford with originating an offbeat and unexpected explanation that suddenly made the readings seem to click into place. She asked whether the signal they detected could come from ice in comets similar to those found in our own solar system. It was quickly realized that there would be one significant difference: Billions of comets in the CW Leonis system could have had their ice boiled into water vapor all at once by increased heat from the expanding, aging star.
Our own sun, which is younger than CW Leonis, will go through a similar transformation in about 6 billion years. If any life was present in the CW Leonis system, the changes in the star have more than likely brought about its end.
Based on the spectroscopic signal, Ford calculated the volume of water in the
vicinity of CW Leonis at about 10,000 times the water present in Earth's oceans. That estimate compares favorably with the amount of ice some astronomers estimate was present at the beginnings of our own solar system.
Astronomers have already begun using SWAS for follow-up observations of other carbon stars.
"This finding has definitely been one of the most interesting results we've had, and it's an area we never designed the SWAS instruments to study," Neufeld says.
Other authors on the Nature paper were Gary Melnick and Matthew Ashby, of the Harvard-Smithsonian Center for Astrophysics, and David Hollenbach, of NASA's Ames Research Center.
SWAS is operated by the Harvard-Smithsonian Center for Astrophysics through NASA's Goddard Space Flight Center.