Gaps In Young Star’s Gas Disk Could Indicate Baby Planets

Two gaps in the gas disk around the young star, HL Tauri, are evidence of what might be newly-formed planets—challenging the belief that planets take millions of years to form.

AsianScientist (Jun. 3, 2016) – A new analysis of data from the Atacama Large Millimeter/submillimeter Array (ALMA) facility in Chile provides yet more firm evidence of baby planets around a young star, HL Tauri.

This discovery, published in Astrophysical Journal Letters, supports the idea that planets form in much shorter timescales than previously thought possible.

In November 2014, ALMA released a startling image of HL Tauri and its dust disk, which clearly depicts several gaps in the dust disk around the star. Some astronomers suggest that the dark gaps are carved by planets forming in the disk that attract or sweep away the dust along their orbits. But others doubt the planet explanation because HL Tauri is very young—estimated to be only about a million years old—and classical studies indicate that it takes more than tens of millions of years for planets to form from small dust.

To better understand the true nature of the disk, researchers led by Dr. Yen Hsi-Wei at Academia Sinica Institute of Astronomy and Astrophysics in Taiwan and Professor Shigehisa Takakuwa at Kagoshima University, Japan, focused on the distribution of gas in the disk.

If the dust gaps are caused by the variance of the dust’s properties, no gaps would be seen in the gas distribution. If on the other hand, the gaps in the dust are caused by the gravity of forming planets, we would expect gravity to create gaps in the gas as well.

The team extracted the emissions from HCO+ gas molecules in the publicly available 2014 ALMA Long Baseline Campaign data and summed up the emissions in rings around the star to increase the effective sensitivity. This novel data analysis technique revealed at least two gaps in the disk, at the radii of 28 and 69 astronomical units.

“To our surprise, these gaps in the gas overlap with the dust gaps,” said Yen. “This supports the idea that the gaps are the footprints left by baby planets.”

The fact that the gaps in the dust and the gas match up implies that the amount of material in the gaps likely decreased, supporting the planet formation theory, in spite of HL Tauri’s young age.

The team also found that the gas density is high enough to harbor an infant planet around the inner gap. Comparing the structure of the inner gap to theoretical models, the team estimates the planet has a mass 0.8 times that of Jupiter.

On the other hand, the origin of the outer gap is still unclear. The team suggested the possible existence of a planet 2.1 times more massive than Jupiter, but the present research cannot eliminate the possibility that the gap is made by the drag between the dust particles and the gas. To solve this question, the researchers say more data are needed.

The article can be found at: Yen et al. (2016) Gas Gaps in the Protoplanetary Disk around the Young Protostar HL Tau.


Source: National Astronomical Observatory of Japan; Photo: ALMA (ESO/NAOJ/NRAO).
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