“Super-Earths” and Neptune-sized planets could be forming in the cosmic dust around young stars in much greater numbers than previously estimated, a study has found. Observing a sampling of young stars in a star-forming region in the constellation Taurus, researchers from University of Arizona in the US found many of them to be surrounded by structures that can best be explained as traces created by invisible, young planets in the making.
The research, published in the Astrophysical Journal, helps scientists better understand how our own solar system came to be. Some 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 started to plow patterns of gaps and rings into 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 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 per cent – 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,” said Feng Long, a doctoral student at the Kavli Institute for Astronomy and Astrophysics at Peking University in Beijing, China.
“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,” said 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,” Pinilla said. 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 rivalling Jupiter, the largest planet in the solar system. “Since most of the current exoplanet surveys can’t penetrate the thick dust of protoplanetary disks, all exoplanets, with one exception, have been detected in more evolved systems where a disk is no longer present,” Pinilla said.