On April 4, 2022, NASA’s Hubble Telescope, launched in 1990, detected a Jupiter-like protoplanet, which is otherwise known as a swirling, massive ball of gas and matter thought to eventually become a planet. Dubbed AB Aurigae b due to its location in the Auriga constellation, this protoplanet is located about 531 light years away from the sun. For context, one light year is equivalent to about 9.5 trillion kilometers, which is nearly an incomprehensible distance. Hubble has detected countless celestial bodies situated at a further distance than this, including Earendel, a recently-spotted star some 12.9 billion light years away from the sun. However, what is particularly striking about AB Aurigae b is the potential implications of its discovery for understanding planetary formation.
The core accretion model is commonly believed to be the way in which most planets formed. It involves the binding of small particles from the circumstellar disk around a collapsed star into larger particles, and it is a process primarily driven by gravity. This model is officially described as a “bottom-up” approach but can also be thought of as being an “inside-out” approach, as particles accumulate from the core outwards. The core accretion model works well to describe the formation of planets containing lots of mass and metals, such as Earth, but less so for lighter planets primarily consisting of gas and dust.
In contrast, the disk instability model is a much more unusual model for planetary formation, mostly due to its “intense and violent” nature, as described by NASA researchers. Whereas the core accretion model consists of more densely-packed particles binding together and building on one another, the disk instability model is characterized by the rapid binding of more loosely-packed gas and dust from larger chunks of the circumstellar disk around a collapsed star. It is described as a “top-down” approach, as the first particles to bind with one another are large to begin with.
The core accretion model describes a slow process, but AB Aurigae b, which is estimated to be about nine times larger than Jupiter, is thought to have been formed not more than 2 million years ago — very recent in star time! Additionally, it is 8.6 billion miles away from the star that it orbits around, which is more than two times further than Pluto is to our sun. Considering the age, size, and distance from its host star, AB Aurigae b cannot be described by the core accretion model.
Researchers compared data from the image of AB Aurigae b that was captured by Hubble with data from the SCExAO imaging instrument located at the Mauna Kea observatory in Hawaii. SCExAO is a massive device with the capability to transfer images of celestial bodies millions of light years away into clear resolution images. Hubble archival observations were also looked at to measure AB Aurigae b’s orbit, further confirming that it is, in fact, a planet. After a promising conclusion emerged from cross-analyzing data, Alan Boss of the Carnegie Institution of Science in Washington D.C. stated “This new discovery is strong evidence that some gas giants [including AB Aurigae b] can form by the disk instability mechanism.”
Such a discovery will be beneficial in expanding the understanding and historical knowledge of our own solar system. NASA’s James Webb Space Telescope, which was launched on Dec. 21, 2021 and will come on line later this year, will hopefully be able to confirm the disk instability theory and provide further observations about protoplanetary disks such as AB Aurigae b.