Treasure of planets found hidden in dust



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"Super-Earths" and Neptune-sized planets could be forming around young stars in numbers much larger than scientists thought, new research by an international team of astronomers suggests.

Observing a sample of young stars in a star-forming region in the constellation Taurus, researchers have found that many of them are surrounded by structures that can be best explained as traces created by young and invisible planets in formation. The research, published in the Astrophysical Journal, helps scientists better understand how our own solar system came about.

About 4.6 billion years ago, our solar system was an undulating swirl of gas and dust around our newborn sun. In the early stages, the so-called protoplanetary disk had no discernible characteristics, but soon parts of it began to coalesce into clusters of matter – future planets. When they picked up new material along their journey around the Sun, they grew and began to plow patterns of gaps and rings on the disk from which they formed. Over time, the dusty disk gave way to the relatively orderly arrangement we know today, consisting of planets, moons, asteroids, and occasional comets.

Scientists base this scenario on how our solar system came to be in observations of protoplanetary disks around other stars that are young enough to currently be in the process of birth planets. Using the Atacama Large Millimeter Array, or ALMA, composed of 45 radio antennas in the Atacama Desert in Chile, the team conducted a survey of young stars in the star-forming region of Taurus, a vast cloud of gas and dust located 450 years -Light of Earth. When researchers imagined 32 stars surrounded by protoplanetary disks, they found that 12 of them – 40% – have rings and gaps, structures that, according to team measurements and calculations, can best be 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 Neptune are the most common type of planets, coincide with observations of protoplanetary disks," said lead study author Feng Long, a doctoral student. at the Kavli Institute for Astronomy and Astrophysics at Beijing University in Beijing, China.

While some proto-planetary disks appear as uniform, pancake-like objects without any features or patterns, bright concentric rings separated by gaps have been observed, but since previous research has focused on the brighter of these objects because they are easier to find, clear how common discs with ring structures and gaps really are in the universe. This study presents the results of the first unbiased research in which 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 research.

"Most of the previous observations were directed at detecting the presence of very large planets, which we know are rare, which had carved large internal holes or gaps in bright disks," said second author Paola Pinilla of the NASA Hubble Fellow. University of Arizona Steward Observatory. "While huge planets were inferred on some of these bright disks, little was known about the weakest disks."

The team, which also includes Nathan Hendler and Ilaria Pascucci in the AU Lunar and Planetary Laboratory, measured the properties of rings and gaps observed with ALMA and analyzed the data to evaluate possible mechanisms that could cause the rings and gaps observed. Although these structures can be sculpted by planets, previous research has suggested that they can 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 disc in response to the host star's distance and magnetic field create pressure variations on the disk. These effects can create variations on the disk, manifesting themselves as rings and gaps.

The researchers performed analyzes to test these alternative explanations and could not establish any correlations between the stellar properties and the patterns of gaps and rings observed.

"We can therefore rule out the commonly proposed idea of ​​ice lines causing the rings and gaps," Pinilla said. "Our discoveries leave the nascent planets as the most likely cause of the patterns we observe, though some other processes may also be at work."

Because detection of the individual planets directly is impossible because of the overwhelming glow of the host star, the team performed calculations to get an idea of ​​the types of planets that could be forming in the star-forming region of Taurus. According to the results, gaseous planets the size of Neptune or the 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 giants competing with Jupiter, the largest planet in the solar system.

"As most current exoplanet surveys can not penetrate the thick dust of protoplanetary disks, all of the exoplanets, with one exception, have been detected in more evolved systems where a disk is no longer present," Pinilla said.

Going forward, the research group plans to move the ALMA antennas further, which should increase the resolution of the array to about five astronomical units (one AU equals the average distance between Earth and the Sun) and make antennas sensitive other frequencies are sensitive to other types of dust.

"Our results are an exciting step in understanding this key phase of planetary formation," said Long, "and making those adjustments, we hope to better understand the origins of the rings and the gaps."

Reference: "Gaps and Rings in an ALMA Disk Survey in the Taurus Star Formation Region," Feng Long et al., 2018, to appear in the Astrophysical Journal [http://apj.aas.org, preprint: https://arxiv.org/abs/1810.06044].

This work was made possible by an international collaboration, including astronomers from the AU and LPL Steward Observatory. Funding for this project was provided by Beijing University, the National Science Foundation of China, the Hubble Scholarship Program, the National Science Foundation and the Earths in Other Solar Systems program Nexus for Exoplanetary Science.

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