Water Worlds

IRC+10216
In this illustration, we see IRC+10216 at the peak of both its radius and luminosity. The relatively rapid rise in its luminosity – over a few hundred thousand years – heats the previously undisturbed collection of orbiting icy bodies triggering the outwardly moving wave of evaporation depicted here. Such a collection of orbiting icy bodies is analogous to our Solar System’s Kuiper Belt.
Credit: Smithsonian Astrophysical Observatory

As an alien sun blazes through its death throes, it is apparently vaporizing a surrounding swarm of comets, releasing a huge cloud of water vapor. The discovery, reported in an article to be published tomorrow in the journal Nature, is the result of observations with the Submillimeter Wave Astronomy Satellite (SWAS), a small radio observatory NASA launched into space in December 1998.

The new SWAS observations provide the first evidence that extra-solar planetary systems contain water, a molecule that is an essential ingredient for known forms of life.

"Over the past two years, SWAS has detected water vapor from a wide variety of astronomical sources," said Dr. Gary Melnick of the Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, principal investigator on the SWAS mission. "What makes the results we are reporting today so unusual is that we have found a cloud of water vapor around a star where we would not ordinarily have expected to find water."

The star in question is an aging giant star designated by astronomers as IRC+10216, also known as CW Leonis, located 500 light-years (almost 3,000 trillion miles) from Earth in the direction of the constellation Leo.

"IRC+10216 is a carbon-rich star in which the concentration of carbon exceeds that of oxygen," Melnick said. "In such stars, we expect all the oxygen atoms to be bound up in the form of carbon monoxide (an oxygen atom and a carbon atom bound together), with almost nothing left over to form water (one oxygen atom bound to two hydrogen atoms). Yet we see substantial concentrations of water vapor around this star; the most plausible explanation for this water vapor is that it is being vaporized from the surfaces of orbiting comets, ‘dirty snowballs‘ that are composed primarily of water ice."

From its vantage point in orbit above the absorbing effects of water in Earth’s atmosphere, SWAS is capable of detecting the distinctive radiation emitted by water vapor in space. The observations of water vapor around IRC+10216 suggest that other stars may be surrounded by planetary systems similar to our own. Over the past decade, more than 50 stars have been shown to have large planets in orbit around them, but little is known about the composition of those planets.

In order to explain the water vapor concentration that SWAS has detected, several hundred billion comets would be needed at distances from the star between 75 and 300 times the distance of the Earth from the Sun.

SWAS Spacecraft
An artist’s conception of the SWAS spacecraft in low Earth orbit.
Credit: Smithsonian Astrophysical Observatory

"That sounds like a lot," said Saavik Ford, a graduate student at Johns Hopkins University in Baltimore who is a co-author of the article reporting the discovery. "But the total mass required of this swarm of orbiting comets is similar to the original mass of the Kuiper Belt, a collection of comets that orbits our own Sun beyond the orbit of Neptune. In our own solar system, these comets orbit the Sun quietly for the most part; occasionally a comet comes in close to the Sun, starts to vaporize, and displays the characteristic coma and tail that we are familiar with. But IRC+10216 is so much more luminous than the Sun that comets start to vaporize even at the distance of the Kuiper Belt. So one has several hundred billion comets all vaporizing at once."

The SWAS observations of IRC+10216 paint a picture of the future of our solar system.

"We think we are witnessing the type of apocalypse that will ultimately befall our own planetary system," said SWAS team member Dr. David Neufeld, a Johns Hopkins professor of physics and astronomy. "Several billion years from now, the Sun will become a giant star and its power output will increase five thousand fold. As the luminosity of the Sun increases, a wave of water vaporization will spread outwards through the solar system, starting with Earth’s oceans and extending well beyond the orbit of Neptune. Icy bodies as large as Pluto will be mostly vaporized, leaving a cinder of hot rock."

SWAS was built and operated by NASA with support from the German government and the participation of the Harvard-Smithsonian Center for Astrophysics; the University of Massachusetts at Amherst; Cornell University, Ithaca, NY; the Johns Hopkins University; the University of Cologne in Germany; Ball Aerospace, Boulder, CO; and Millitech (now Telaxis Communication Corp.), South Deerfield, MA.

In addition to Melnick, Neufeld and Ford, the other co-authors are Dr. David Hollenbach of NASA’s Ames Research Center, Moffett Field, CA, and Dr. Matthew Ashby of the Harvard-Smithsonian Center for Astrophysics.