By Ron Cowen
BALTIMORE — Examining a dusty disk — a sign of planet formation — around a young star, astronomers appear to have found a new clue about how the youthful Earth acquired water to form oceans and provide a habitat for life.
The disk of debris surrounds the 10-million-year-old star EF Chamaeleontis, which lies about 310 light-years from Earth. Previous observations with NASA’s infrared Spitzer Space Telescope showed that the dust is relatively warm. That suggests the radiation is most likely generated by collisions between bits of material, or planetesimals, in the region around the stars where rocky, terrestrial planets could form.
In the new study, Thayne Currie of NASA’s Goddard Space Flight Center in Greenbelt, Md., and his colleagues used a spectrometer on Spitzer to separate the infrared emissions into individual wavelengths that would enable the scientists to identify the composition of material within the warm disk. Currie and his collaborators found that the emissions resemble those from a mixture of several minerals known as phyllosilicates, which can form only in the presence of liquid water. In the solar system, planetesimals laden with phyllosilicates are thought to have rained down on Earth from the asteroid belt or the more distant Kuiper belt of comets, delivering the water that became the planet’s oceans.
The presence of phyllosilicates in the terrestrial planet-forming zone of disks surrounding young stars may therefore trace water delivery to fledgling terrestrial planets, Currie notes. “To our knowledge, this is the first bona fide detection of phyllosilicates in a debris disk,” he says.
Currie presented the findings during a September 15 conference at the Space Telescope Science Institute in Baltimore on how terrestrial planets in the solar system and beyond acquired water and organic compounds.
Scientists have suggested that ancient regions of the Martian surface that are rich in phyllosilicates may be prime locations for finding fossils of ancient, primitive organisms on the Red Planet.
Phyllosilicates, a family of minerals that includes talc, have a layered, sheetlike structure like phyllo dough, with water molecules sandwiched between the layers. “Earth’s oceans may have started out as individual water molecules stashed between these rock layers, like the honey in Baklava,” comments Marc Kuchner of NASA-Goddard. “Now this team seems to have found this particular kind of water-bearing material orbiting another star,” he adds.
The new finding “makes me wonder whether this kind of process is going on in every planetary system — water-rich phyllosilicates flying around, delivering their juicy contents to the surfaces of young protoplanets,” Kuchner says.
Some researchers are cautious, however, about the identification of phyllosilicates in the debris disk. “I am generally skeptical of detailed mineralogical fits to infrared spectra,” notes Alycia Weinberger of the Carnegie Institution for Science in Washington, D.C. Some parameters, she says, most notably grain shape, make it difficult to match minerals with the spectra.
The fact that phyllosilicates have not previously been demonstrated to exist in debris disks also casts some doubt on the conclusion, says Steven Desch of Arizona State University in Tempe.
“Frankly, if we were sure of the existence of this mineral in interstellar space, or in other [planet-forming] disks, these spectra would be considered to clearly show the signature of [phyllosilicates],” Desch says. “But since it has not been seen anywhere else, and because its presence would be an important discovery with implications for water and planet formation, I think people are not going to naturally accept these spectra,” but instead focus on the limitations of the analysis.
Desch says that he would like the team to consider a wider range of phyllosilicates, especially those known to exist in solar system meteorites — fragments of asteroids that fall to Earth. Nonetheless, he adds, the researchers “have made a case for the presence of phyllosilicates that is, if not quite compelling, at least very strong. I think it should be taken seriously, and I hope it provokes other observers and modelers to consider and test this possibility.”
