Planets are forming around young stars far faster than scientists expected, arising in a cosmic eye blink of less than half a million years, according to a new study. That finding could inform models of planet formation and help resolve a problem plaguing astronomers since 2018, when data indicated that planetary nurseries contained far too little material to actually create planets.
Planets coalesce from massive disks of gas and dust that surround newborn stars. But detecting these embryonic worlds is difficult because both the star and the disk shine far brighter than any tiny planet.
To find out how much material is available for planet formation, researchers have used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to weigh the disks around young stars between 1 million and 3 million years old. Past studies found that some lacked the mass to form even a single Jupiter-size world. The results suggested astronomers were either overlooking some hidden reservoir of material or they were looking too late in the planet-forging process, after growing protoplanets had already vacuumed up much of the material.
Top of Form
Bottom of Form
The answer, says Łukasz Tychoniec, a graduate student at Leiden Observatory and lead author of the new paper, is that “we need to look earlier instead of [looking] for missing mass.”
Along with his colleagues, Tychoniec used images from ALMA and the Very Large Array (VLA) in New Mexico to study 77 protostars in the Perseus molecular cloud, a giant star-forming region roughly 1000 light-years away. These infant star systems are thought to be between 100,000 and 500,000 years old.
The dishes at the ALMA and VLA can gather far-infrared light, emitted by dust grains about 1 millimeter in wavelength, that can pass through obscuring gas clouds and reach Earth. Measuring the total amount of infrared light given off by the disks provides an estimate of their dust content and therefore their mass. The team found the young disks contained about an order of magnitude more material than disks observed just 1 or 2 million years later in their development, they report in a paper published on the preprint server arXiv, and accepted for publication at Astronomy & Astrophysics. That amount is more than enough to account for planet formation.
The large sample size and the use of two observatories, which see in slightly different wavelengths and tally up different dust populations, make the findings an important contribution, says astrophysicist Megan Ansdell at NASA headquarters. “But there are a lot of caveats,” she adds.
In particular, she says, it would be better to draw conclusions from star-forming regions across multiple molecular clouds, rather than just the Perseus cloud, which might have unique environmental conditions. Tychoniec says his team plans to look at more young stars in greater detail to see whether the result holds.
sciencemag.org, 16 June 2020