The "Super-Earth" and Neptune planets could be formed around young stars in far greater numbers than scientists thought, suggests a new search for an international group of astronomers.
Observing a sampling of young stars in a region that forms stars in the constellation Taurus, researchers have found many of them surrounded by structures that can best be explained as traces created by invisible, young developing planets. The research, published in Astrophysical Journal helps scientists to better understand how our solar system was born.
About 4.6 billion years ago, our solar system was a swirl of gas and dust that surrounds our new sun. At the initial stages, this so-called protoplanetary disk had no distinguishable characteristics, but soon parts of it began to merge into groups of matter, the future planets. As they gathered new material on their journey around the sun, they grew and began to plow patterns of empty spaces and rings in the disc from which they formed. Over time, the dusty disk gave way to the relatively ordered arrangement we know today, consisting of planets, moons, asteroids, and occasional comets.
Scientists base this scenario on how our solar system was based on the observations of protoplanetary disks around other stars that are young enough to be currently in the process of birth planets. Using the Atacama Large Millimeter Array, or ALMA, which included 45 radio antennas in the Atacama desert in Chile, the team conducted a survey of young stars in the Taurus star formation region, a vast cloud of gas and powders located 450 light years from Earth. When the researchers imagined 32 stars surrounded by protoplanetary disks, they found that 12 of them – 40 percent – have rings and blanks, structures that according to the measurements and the calculations of the team can be better explained by the presence of nascent planets.
"This is fascinating because it is the first time that the statistics of extrasolar planets, which suggest that super-Earth and Neptunes are the most common type of planets, coincide with the observations of protoplanetary disks," he said. lead author of the document, Feng Long, a doctoral student in the Kavli Institute for Astronomy and the Astrophysics of the Beijing University in Bejing, China.
While some protoplanetary disks appear as uniforms, pancake-like objects lacking in features or patterns, concentric light rings separated by gaps have been observed, but previous surveys have focused on the brighter of these objects because they are easier to find, it is not clear how common disks with ring and gap structures really are in the universe. This study presents the results of the first impartial survey because the target disks were selected regardless of their brightness, in other words, the researchers did not know if any of their targets had ring structures when they selected them for the survey.
"Most of the previous observations were aimed at detecting the presence of very massive planets, which we know to be rare, that had dug large internal holes or empty spaces in light disks," said second author Paola Pinilla, a Hubble Fellow of NASA at the Steward Observatory of the University of Arizona. "While in some of these light disks huge planets had been deduced, little was known about the weaker disks."
The team, which also includes Nathan Hendler and Ilaria Pascucci at the UA lunar and planetary laboratory, measured the properties of the rings and spaces observed with ALMA and analyzed the data to evaluate possible mechanisms that could cause the observed rings and gaps. While these structures can be carved from the planets, previous research has suggested that they could also be created by other effects. In a commonly suggested scenario, so-called ice lines caused by changes in the chemistry of dust particles across the disk in response to distance from the host star and its magnetic field create pressure variations across the disk. These effects can create variations in the disk, manifesting itself as rings and gaps.
The researchers performed analyzes to test these alternative explanations and could not establish any correlation between the stellar properties and the patterns of empty spaces and rings observed.
"We can therefore rule out the commonly proposed idea of ice lines that cause rings and gaps," said Pinilla. "Our results leave nascent planets as the most probable cause of the models we have observed, although some other processes may even be at work."
Since detecting individual planets directly is impossible due to the overwhelming brightness of the host star, the team performed calculations to get an idea of the types of planets that could be formed in the Taurus star-forming region. According to the results, the Neptune gaseous planets or the so-called super terrestrial terrestrial planets of up to 20 land masses should be the most common. Only two of the observed disks could potentially host behemoth rivaling Jupiter, the largest planet in the solar system.
"Since most of the current exoplanet investigations can not penetrate the dense dust of protoplanetary disks, all exoplanets, with one exception, have been detected in more advanced systems where a disk is no longer present," he said Pinilla.
Going forward, the research group plans to move the more distant ALMA antennas, which should increase the array resolution to about five astronomical units (an AU equal to the average distance between the Earth and the sun) and to make the antennas sensitive to other frequencies sensitive to other types of dust.
"Our results are an exciting step to understand this key phase in the formation of the planets" Long said, "and by making these changes, we hope to better understand the origins of the rings and gaps".
Protoplanetary disk material found too poor to form populations of planets
Feng Long et al, Gaps and Rings in an ALMA survey of disks in the Taurus star formation region, The Astrophysical Journal (2018). DOI: 10.3847 / 1538-4357 / aae8e1