Researchers have discovered water vapor in the disk surrounding a young star, precisely in the area where planets might form. Water is a key ingredient for life on Earth, and it's thought to play a significant role in the formation of planets as well.
Until now, we had never been able to map the distribution of water in a stable and cold disk - the type of disk that offers the most favorable conditions for planet formation around stars. These new findings were made possible thanks to the Atacama Large Millimeter/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner.
Water in the Disk of HL Tauri
"I never imagined that we could acquire an image of oceans of water vapor in the very region where a planet is likely to form," says Stefano Facchini, an astronomer at the University of Milan (Italy), who led the study published today in the journal Nature Astronomy. The observations reveal the presence of at least three times more water than in all of Earth's oceans in the inner disk of the young Sun-like star HL Tauri, located 450 light-years away in the Taurus constellation.
"It is truly remarkable that we can not only detect but also capture detailed images and spatially resolve the water vapor at a distance of 450 light-years from us," adds Leonardo Testi, an astronomer at the University of Bologna (Italy) and co-author of the study. The spatial resolution observations made with ALMA allow astronomers to determine the distribution of water in different regions of the disk. "Being part of such an important discovery in the iconic disk of HL Tauri went beyond my hopes for my first research experience in astronomy," adds Mathieu Vander Donckt from the University of Liège, Belgium, who was a master's student when he participated in this research.
A significant amount of water was found in the region where a known groove is located in the disk of HL Tauri. Radial grooves are carved in the gas and dust-rich disks by young planets in orbit, which accumulate matter and grow. "Our recent images reveal a significant amount of water vapor at different distances from the star, including a space where a planet could potentially be forming right now," explains Stefano Facchini. This suggests that this water vapor could affect the chemical composition of planets forming in these regions.
Observing water with a ground-based telescope is no easy feat, as the abundant water vapor in Earth's atmosphere degrades astronomical signals. ALMA, operated by ESO with its international partners, is a network of telescopes located in Chile's Atacama Desert, at about 16,404 feet (5,000 meters) above sea level. It was built in a high and dry environment, specifically to minimize this degradation and offer exceptional observation conditions. "To this day, ALMA is the only facility capable of spatially resolving the presence of water in a cold disk where planets are forming," explains co-author Wouter Vlemmings, a professor at the Chalmers University of Technology in Sweden
(1).
"It is really exciting to witness directly, in a photograph, the release of water molecules from icy dust particles," says Elizabeth Humphreys, an astronomer at ESO, who also participated in the study. The dust grains that make up a disk are the seeds of planet formation, as they collide and stick together to form increasingly larger bodies orbiting the star.
Astronomers believe that where it is cold enough for water to freeze onto dust particles, these particles stick together more efficiently - an ideal place for planet formation. "Our results show how the presence of water can influence the development of a planetary system, as was the case 4.5 billion years ago in our own solar system," adds Stefano Facchini.
Thanks to improvements to ALMA and the ESO's Extremely Large Telescope (ELT), which will come into service over the next decade, the formation of planets and the role that water plays in this process will be clearer than ever. In particular, METIS (for Mid-infrared ELT Imager and Spectrograph), will provide astronomers with unparalleled views of the inner regions of planet-forming disks, where Earth-like planets form.
Note:
(1) The new observations utilized ALMA's band 5 and band 7 detectors. Band 5 was developed by Chalmers with ESO's participation; it extended ALMA to a new frequency range specifically intended for the detection and imaging of water in the local Universe. In this study, the team observed three water spectral lines in the receiver's two frequency ranges to map the gas at different temperatures in the disk.