Planets Form in Just a Few Hundred Thousand Years

Astronomers like to observe young planets forming in circumstellar debris disks, the rotating rings of material around young stars. But when they measure the amount of material in those disks, they don’t contain enough material to form large planets. That discrepancy has puzzled astronomers.

The answer might come down to timing.

A new study suggests that planets form much quicker than astronomers think.

“We need to look earlier instead of looking for missing mass.”

Lukasz Tychoniec, Lead Author, Leiden Observatory.

The nebular hypothesis is the widely-held theory explaining how planets form.

It all starts with a star, which forms in a giant molecular cloud. When a star forms, left-over material forms a rotating disk around the star called the circumstellar disk. Planets form from the material in that disk and orbit the star.

In this artist’s conception, a possible newfound planet spins through a clearing in a nearby star’s dusty, planet-forming disc. Image Credit: NASA/JPL-CaltechThe original uploader was 1981willy at English Wikipedia.Later versions were uploaded by WilyD at en.wikipedia. – http://photojournal.jpl.nasa.gov/catalog/PIA05988Originally from en.wikipedia; description page is/was here., Public Domain, https://commons.wikimedia.org/w/index.php?curid=2640995

But a very young solar system is very difficult to see into. Thick lanes of dust block light, and that’s hindered our understanding of the planet formation process. There’s been some progress, though.

In recent years, improvements at observatories like ALMA and the VLT have let astronomers get a better look into these nascent solar systems and the young planets that form there. In one 2019 study, astronomers imaged the gaps in a protoplanetary disk around a nearby young star, and the young planets responsible for the gaps.

PDS 70 is only the second multi-planet system to be directly imaged. Through a combination of adaptive optics and data processing, astronomers were able to cancel out the light from the central star (marked by a white star) to reveal two orbiting exoplanets. PDS 70 b (lower left) weighs 4 to 17 times as much as Jupiter while PDS 70 c (upper right) weighs 1 to 10 times as much as Jupiter. Image Credit: ESO and S. Haffert (Leiden Observatory)

Some of that work allowed astronomers to measure the mass of material in these circumstellar disks. Studies of 1 to 3 million year old disks showed that disks don’t hold enough material to make even one planet as large as Jupiter, never mind the rest of the planets in a system like ours. Where was the mass?

A new study suggests that there is no missing mass, and astronomers need to look into even younger solar systems to find their answer.

The title of the new study is “Dust masses of young disks: constraining the initial solid reservoir for planet formation.” Lead author of the paper is Lukasz Tychoniec, a graduate student at Leiden Observatory in the Netherlands. The new paper will be published in the journal Astronomy and Astrophysics.

The SPHERE instrument on the ESO’s Very Large Telescope has captured a collection of images of circumstellar disks around young stars. The detail in the images is greater than any previous images, and reveals a stunning variety of shapes and sizes. Image: ESO/H. Avenhaus et al./E. Sissa et al./DARTT-S and SHINE collaborations

In the introduction of their paper the authors write that “…mature disks are lacking the solid material necessary to reproduce the observed exoplanetary systems, especially the massive ones.” The intent of the study is “…to determine if disks in the embedded stage of star formation contain enough dust to explain the solid content of the most massive exoplanets.”

To accomplish this, the researchers looked at very young solar systems. “We need to look earlier instead of looking for missing mass,” Tychoniec said in a press release.

Tychoniec and the team of researchers used images from the Atacama Large Millimeter/sub-millimeter Array (ALMA) and the Very Large Array (VLA) in their work. They looked at very young stars called protostars, stars so young that they’re still acquiring material from the molecular cloud they’re forming from. In this case, the young stars were in the Perseus Molecular Cloud, a gigantic region of star-formation that’a about 1,000 light years away from us.

The Perseus Molecular Cloud is about 500 light years across, and is about 1,000 light years away. It hosts an abundance of protostars. This image is from NASA’s recently retired Spitzer Space Telescope. Image Credit: By NASA/JPL-Caltech – https://photojournal.jpl.nasa.gov/figures/PIA23405.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=85073056

The young protostars they studied were each the center of their own equally young solar system, where planets were in the process of forming. The solar systems were estimated to be between 100,000 and 500,000 years old. That’s extremely young for any astronomical object.

The study looked at different ages of circumstellar disks around different ages of protostars called Class 0, Class 1, and Class 2. Each class of protostar is in a different stage of evolution. Class 0 is in the very earliest stages of planet formation. The disk around a Class 0 protostar is thicker, and has less material taken up by planet formation than a Class 1 or Class 2, etc.

The naming convention is based on the amount of infrared light emitted by the disk. The material in the disk is cooler than the surface of the young star at the center of it all. So the disk material radiates longer wavelengths of light, producing excess infrared energy. As more material is taken up by planet formation, the disk becomes less dense, and radiates less infrared energy. Observations with ALMA and the VLT measures the light as its emitted by dust grains in the disk. By measuring that light, the team is measuring the mass of dust in the disks.

An image of AB Aurigae from the ESO’s VLT and its SPHERE instrument, showing what scientists think is a baby planet forming. Image Credit: ESO/Boccaletti et al.

They found that the amount of dust in the very young systems they measured is indeed enough to account for giant planets. They took their disk dust measurements and compared them with the masses of more than 2,000 known exoplanet systems. In every single case, their measured dust masses were able to account for the assembly of the known exoplanets.

The team also compared the amount of dust around disks in the Perseus molecular cloud to disks in Orion. They found that disks in Perseus had median masses of 158 and 52 Earth masses, for Class 0 and Class 1 respectively. They found that Class 1 disks agree in both systems, while Class 0 disks in Perseus are more massive. This result “suggests that initial cloud conditions may lead to different masses of disks in the early phases,” the authors write.

An image of the Orion Nebula, which contains more than 180 proplyds, or ionized protoplanetary disks. Six of them are highlighted in this image. In this study, the authors compared disks in Perseus with disks in Orion. Image Credit: By ESA/Hubble, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=8778842

But the key result of this work revolves around planet formation, and when that process begins around a young star. “We find strong evidence that there is enough dust mass in young disks to make planet formation possible already in the first ? 0.5 Myr of star formation.”

“The implication of this discovery is profound,” says Alex Cridland, a Postdoctoral Researcher at Leiden Observatory and co-author of the paper. “For decades we’ve thought that planet formation should happen during the proto-planetary disk phase,” after the young star has aged a little. “…by pushing the beginning stage of planet formation back we have to rethink what the birthplace of planets actually looked like!” said Cridland.

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Evan Gough

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