Hubble Has Been Watching This Planet Form for 13 Years

Hubble’s most remarkable feature might be its longevity. The Hubble has been operating for almost 32 years and has fed us a consistent diet of science—and eye candy—during that time. For 13 of its 32 years, it’s been checking in on a protoplanet forming in a young solar system about 530 light-years away.

Planet formation is always a messy process. But in this case, the planet’s formation is an “intense and violent process,” according to the authors of a new study.

When planetary scientists talk about planet formation, they mostly talk about the accretion process, or “core accretion,” as it’s called. In core accretion, planets are built from the ground up by accumulating smaller pieces. Everything from pebbles to boulders—and eventually planetesimals—collide and clump together to form a planet after millions of years.

For a gas giant like Jupiter, that means a rocky core forms, and the developing planet attracts a vast envelope of gas that it surrounds itself with.

But there’s another way that planets can form, and the Hubble has been keeping an eye on a protoplanet that’s following a different path to planet-hood.

A new study titled “Images of embedded Jovian planet formation at a wide separation around AB Aurigae” presents the observations in the journal Nature Astronomy. The lead author is Thayne Currie of the Subaru Telescope and Eureka Scientific.

The star at the center of this story is AB Aurigae, a young star only 2 million years old in the Auriga constellation. Our Solar System was the same age when planets started forming around the Sun. The nascent planet Hubble’s been watching is called AB Aurigae b. It’s a protoplanet similar to Jupiter but nine times more massive. And it orbits its star at an extreme distance. Its orbit carries it 13.8 billion km (8.6 billion miles) from the star, or twice as far from its star as Pluto is from the Sun. In other terms, it’s about 93 astronomical units from its star.

At such a great distance from its star, it’s unlikely to be forming due to core accretion. Instead, researchers think it’s forming through disk instability.

“Nature is clever; it can produce planets in a range of different ways,” said lead author Currie.

The Hubble played a vital role in these findings, and so did another telescope: Japan’s Subaru Telescope. It’s the Japanese National Astronomical Observatory’s premier telescope and is built around an 8.2-meter primary mirror. The Subaru Telescope also has an advanced instrument called the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO.) SCExAO is a state-of-the-art exoplanet imaging instrument. This new research is primarily based on data from SCExAO and from two of Hubble’s instruments: the Space Telescope Imaging Spectrograph (STIC) and the Near Infrared Camera and Multi-Object Spectrograph (NICMOS.)

Young solar systems are challenging to observe because of the glaring starlight and the veil of gas and dust. “Direct images of protoplanets embedded in disks around infant stars provide the key to understanding the formation of gas giant planets such as Jupiter,” the authors point out in their paper. But advanced spectrographs and coronagraphs are making inroads into this important area of astronomy.

Planet-forming disks around young stars also contain other complex disk features unrelated to planet formation. Discerning between planets and these other structures requires lots of data from space-based and ground-based telescopes. Along with Hubble and Gemini, additional facilities like ALMA also contributed data.

“Interpreting this system is extremely challenging,” Currie said. “This is one of the reasons why we needed Hubble for this project – a clean image to better separate the light from the disk and any planet.”

“We could not detect this motion on the order of a year or two years,” Currie said. “Hubble provided a time baseline, combined with Subaru data, of 13 years, which was sufficient to be able to detect orbital motion.”

Researchers were able to directly image newly forming exoplanet AB Aurigae b over 13 years using Hubble’s Space Telescope Imaging Spectrograph (STIS) and its Near Infrared Camera and Multi-Object Spectrograph (NICMOS). In the top right, Hubble’s NICMOS image captured in 2007 shows AB Aurigae b in a due south position compared to its host star, which is covered by the instrument’s coronagraph. The image captured in 2021 by STIS shows the protoplanet has moved in a counterclockwise motion over time. Credits: Science: NASA, ESA, Thayne Currie (Subaru Telescope, Eureka Scientific Inc.); Image Processing: Thayne Currie (Subaru Telescope, Eureka Scientific Inc.), Alyssa Pagan (STScI)

There’s more in this system than just the massive Jupiter-like planet at 93 au. The researchers also identified two other candidate planet-formation sites located at 430-580 au and spiral arms in the disk. The results show another way that Jupiter-like gas giants can form: through disk instability. “With at least one clump-like protoplanet and multiple spiral arms, the AB Aur system may also provide the evidence for a long-considered alternative to the canonical model for Jupiter’s formation: disk (gravitational) instability.”

Disk instability is when a massive circumstellar disk collapses into planet-sized, self-gravitational clumps. These clumps then evolve into planets. Theory shows that disk instability should form planets at greater distances from the star than core accretion. “Disk fragmentation is expected to more readily occur further out in the protoplanetary disk (several tens of AU) where the radiative cooling rates are higher,” according to exoplanet astronomer Paul Wilson, who’s not involved with this study.

These two images from the study give more detail. The image on the left combines ALMA data with data from the CHARIS instrument on the Gemini Telescope. The red region is a ring of pebble-sized dust, and AB Aurigae b lies just inside this ring. The image on the right shows Neptune’s orbit around the Sun for comparison. It also shows the faint spiral structure in the disk. Image Credit: Currie et al. 2022.

“This new discovery is strong evidence that some gas giant planets can form by the disk instability mechanism,” Alan Boss of the Carnegie Institution of Science in Washington, D.C., emphasized. “In the end, gravity is all that counts, as the leftovers of the star-formation process will end up being pulled together by gravity to form planets, one way or the other.”

The study points out differences between the planet formation routes. Not only between the two different models but how planets following these paths are detected differently.

“Almost all of the ~5000 known indirectly detected exoplanets orbit their host stars on solar system scales (a < 30 au),” the paper explains. “The core accretion model, where a young gas giant forms by slowly building up a multi-Earth mass core and then rapidly accreting protoplanetary disk gas, accounts for gas giants like Jupiter and Saturn at these locations. In contrast, directly-imaged exoplanets typically have wide, 50-300 au orbits and are over ~5 times more massive than Jupiter. Disk conditions might not allow in-situ formation for many of these planets by core accretion.”

Core accretion can seem like a calm and almost peaceful process. Over time, more material gathers together, growing more massive until no more matter is available and the planet takes its final form. Of course, collisions are part of this process, and they can be violent, as in the case of the Earth-Theia impact that created the Moon.

In contrast, disk instability is a more raucous process. It takes place much more rapidly than the almost-stately core accretion process. It can even be described as intense and violent.

“A plausible alternative model is disk instability: a violent and rapid process of gravitational collapse that is best suited for forming super-massive gas giant planets at ~100 au.” the authors write.

This study has implications for our overall understanding of solar systems architecture. Many exoplanet studies analyze fully-formed planets to try and constrain planet formation. But an exoplanet needn’t have formed in the location we find it in. After all, Jupiter migrated through the Solar System before settling into its current location. “AB Aur b provides direct evidence that planets more massive than Jupiter can form at separations approaching 100 au, more than double the distance from the Sun to Kuiper belt objects like Pluto, and in striking contrast to expectations of planet formation by the canonical core accretion model,” the authors write in their paper.

This figure from the study shows two Hubble images of Aur AB b. The left is a Hubble NICMOS image from 2007, and the right is a Hubble STIS image from 2021. The dashed magenta line shows a position angle of 180 degrees. In the earlier image on the left, Aur AB b is left of the line, but on the image on the right, it’s to the right of the line. Image Credit: Currie et al. 2022.

The AB Aurigae system is also significant because of its young age. At only two million years old, it’s only about half the age of another significant and well-studied young star, PDS 70. PDS 70b was the first directly-imaged exoplanet.

“Finally, this discovery has profound consequences for our understanding of how planets form,” the authors write. “AB Aur b provides a key direct look at protoplanets in the embedded
stage. Thus, it probes an earlier stage of planet formation than the PDS 70 system. AB Aur’s protoplanetary disk shows multiple spiral arms, and AB Aur b appears as a spatially resolved clump located in proximity to these arms.”

According to the authors, this is an “uncanny resemblance” to models of Jupiter forming via disk instability. It means that Aur AB b could be our first direct evidence for the disk instability mechanism of planet formation.

But it probably won’t be our last.

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Evan Gough

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