麻豆淫院

Is frozen water scattered in systems around other stars? Astronomers have long expected it is, partially based on previous detections of its gaseous form, water vapor, and its presence in our own solar system.

Now there is definitive evidence: Researchers confirmed the presence of crystalline water ice in a dusty debris disk that orbits a sun-like star 155 light-years away using detailed data known as spectra from NASA's James Webb Space Telescope. (The term water ice specifies its makeup, since many other frozen molecules are also observed in space, such as carbon dioxide ice, or "dry ice.") In 2008, data from NASA's retired Spitzer Space Telescope hinted at the possibility of frozen water in this system.

"Webb unambiguously detected not just water ice, but crystalline water ice, which is also found in locations like Saturn's rings and icy bodies in our solar system's Kuiper Belt," said Chen Xie, the lead author of the new paper and an assistant research scientist at Johns Hopkins University in Baltimore, Maryland.

All the frozen water Webb detected is paired with fine dust particles throughout the disk鈥攍ike itsy-bitsy "dirty snowballs." The results are today in the journal Nature.

Astronomers have been waiting for this definitive data for decades. "When I was a graduate student 25 years ago, my advisor told me there should be ice in debris disks, but prior to Webb, we didn't have instruments sensitive enough to make these observations," said Christine Chen, a co-author and associate astronomer at the Space Telescope Science Institute in Baltimore.

"What's most striking is that this data looks similar to the telescope's other recent observations of Kuiper Belt objects in our own solar system."

Water ice is a vital ingredient in disks around young stars鈥攊t heavily influences the formation of giant planets and may also be delivered by small bodies like comets and asteroids to fully formed rocky planets. Now that researchers have detected water ice with Webb, they have opened the door for all researchers to study how these processes play out in new ways in many other planetary systems.

Rocks, dust, ice rushing around

The star, cataloged HD 181327, is significantly younger than our sun. It's estimated to be 23 million years old, compared to the sun's more mature 4.6 billion years. The star is slightly more massive than the sun, and it's hotter, which led to the formation of a slightly larger system around it.

Webb's observations confirm a significant gap between the star and its debris disk鈥攁 wide area that is free of dust. Farther out, its debris disk is similar to our solar system's Kuiper Belt, where dwarf planets, comets, and other bits of ice and rock are found (and sometimes collide with one another). Billions of years ago, our Kuiper Belt was likely similar to this star's debris disk.

"HD 181327 is a very active system," Chen said. "There are regular, ongoing collisions in its debris disk. When those icy bodies collide, they release tiny particles of dusty water ice that are perfectly sized for Webb to detect."

Frozen water鈥攁lmost everywhere

Water ice isn't spread evenly throughout this system. The majority is found where it's coldest and farthest from the star. "The outer area of the debris disk consists of over 20% water ice," Xie said.

The closer in the researchers looked, the less water ice they found. Toward the middle of the debris disk, Webb detected about 8% water ice. Here, it's likely that frozen water particles are produced slightly faster than they are destroyed. In the area of the debris disk closest to the star, Webb detected almost none. It's likely that the star's ultraviolet light vaporizes the closest specks of water ice. It's also possible that rocks known as planetesimals have "locked up" frozen water in their interiors, which Webb can't detect.

This team and many more researchers will continue to search for鈥攁nd study鈥攚ater ice in debris disks and actively forming planetary systems throughout our Milky Way galaxy. "The presence of water ice helps facilitate planet formation," Xie said. "Icy materials may also ultimately be 'delivered' to terrestrial planets that may form over a couple hundred million years in systems like this."

The researchers observed HD 181327 with Webb's NIRSpec (Near-Infrared Spectrograph), which is super-sensitive to extremely faint dust particles that can only be detected from space.