Until recently, protoplanetary disks were believed to be smooth, like pancake-like objects. The results from this study show that some disks are more like doughnuts with holes, but even more often appear as a series of rings. The rings are likely carved by planets that are otherwise invisible to us. Image: Feng Long
4.6 billion years ago, our solar system was a roiling, billowing swirl of gas and dust surrounding our newborn sun. At the early stages, this so-called protoplanetary disk had no discernable features, but soon, parts of it began to coalesce into clumps of matter – the future planets. As they picked up new material along their trip around the sun, they grew and soon started to plow patterns of gaps and rings into the disk from which they formed, and over time, the dusty disk gave way to the (relatively) orderly arrangement we know today, consisting of planets, moons, asteroids and the occasional comet.
Scientists base this scenario of how our solar system came to be on observations of protoplanetary disks around other stars that are young enough to currently be in the process of birthing planets. Using the Atacama Large Millimeter Array, or ALMA, comprising 45 radio antennas in Chile's Atacama Desert, the team performed a survey of young stars in the Taurus star-forming region, a vast cloud of gas and dust located a modest 450 light-years from Earth. When the researchers imaged 32 stars surrounded by protoplanetary disks, they found that 12 of them – 40 percent – have rings and gaps, structures that according to the team's measurements and calculations can be best explained by the presence of nascent planets.
"This is fascinating because it is the first time that exoplanet statistics, which suggest that super-Earths and Neptunes are the most common type of planets, coincide with observations of protoplanetary disks," says the paper's lead author, Feng Long, a PhD student at the Kavli Institute for Astronomy and Astrophysics at Peking University in Bejing, China.
While some protoplanetary disks appear as uniform, pancake-like objects lacking any features or patterns, concentric bright rings separated by gaps have been observed, but since previous surveys have focused on the brightest of these objects because they are easier to find, it was unclear how common disks with ring and gap structures really are in the universe. This study presents the results of the first unbiased survey in that the target disks were selected independently of their brightness – in other words, the researchers did not know whether any of their targets had ring structures when they selected them for the survey.
"Most previous observations had been targeted to detect the presence of very massive planets, which we know are rare, that had carved out large inner holes or gaps in bright disks," says the paper's second author Paola Pinilla, a NASA Hubble Fellow at the University of Arizona's Steward Observatory. "While massive planets had been inferred in some of these bright disks, little had been known about the fainter disks."
The team, led by Gregory Herczeg, a professor at the Kavli Institute for Astronomy and Astrophysics at Peking University, measured the properties of rings and gaps observed with ALMA and analyzed the data to evaluate possible mechanisms that could cause the observed rings and gaps. While these structures may be carved by planets, previous research has suggested that they may also be created by other effects. In one commonly-suggested scenario, so-called ice lines caused by changes in the chemistry of the dust particles across the disc in response to the distance to the host star and its magnetic field create pressure variations across the disk. These effects can create variations in the disk manifesting as rings and gaps.
The researchers performed analyses to test these alternative explanations and could not establish any correlations between stellar properties and the patterns of gaps and rings they observed. "We can therefore rule out the commonly proposed idea of ice lines causing the rings and gaps," Pinilla says. "Our findings leave nascent planets as the most likely cause of the patterns we observed, although some other processes may also be at work."
ESA Herschel image: The Taurus Molecular Cloud, pictured here by ESA's Herschel Space Observatory, is a star-forming region about 450 light-years away. The image frame covers roughly 14 by 16 light-years and shows the glow of cosmic dust in the interstellar material that pervades the cloud, revealing an intricate pattern of filaments dotted with a few compact, bright cores — the seeds of future stars. Image: ESA/Herschel/PACS, SPIRE/Gould Belt survey Key Programme/Palmeirim et al.
Since detecting the individual planets directly is impossible because of the overwhelming brightness of the host star, the team performed calculations to get an idea of the kinds of planets that might be forming in the Taurus star-forming region. According to the findings, Neptune-sized gas planets or so-called super-Earths – terrestrial planets of up to 20 Earth masses – should be the most common. Only two of the observed disks could potentially harbor behemoths rivaling Jupiter, the largest planet in the solar system.
“We still do not know for sure whether these are fully-formed planets, or whether we are detecting an advanced stage in assembling the planets,” says Herczeg. To distinguish between fully-formed planets and planets that are still assembling, the research group plans to move ALMA's antennas farther apart, which should increase the array's resolution to around five astronomical units (one AU equals the average distance between the Earth and the sun), and to make the antennas sensitive to other frequencies that are sensitive to other types of dust.
"Our results are an exciting step in understanding this key phase of planet formation," Long says, "and by making these adjustments, we are hoping to better understand the origins of the rings and gaps.”
This work was made possible through an international collaboration, including astronomers at Peking University. For a complete list of authors and funding information, please see the paper, "Gaps and Rings in an ALMA Survey of Disks in the Taurus Star-forming Region." A preprint of the article is available at https://arxiv.org/abs/1810.06044 . Funding for this project was provided by Peking University and the National Science Foundation of China, the Hubble Fellowship Program, the National Science Foundation, and the Earths in Other Solar Systems Nexus for Exoplanetary System Science program.
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
Science Contact:
Feng Long
KIAA, Peking University
longfengshine@gmail.com
Gregory Herczeg
KIAA, Peking University
gherczeg1@gmail.com
