New Hubble Discovery Supports Alternative Model of Planetary Formation

There is no longer any doubt that the universe is teeming with planets — NASA recently announced that the total number of known exoplanets has crossed 5,000, and we’ve only just begun our search of the heavens. Upcoming projects leveraging the shiny new James Webb Space Telescope will certainly teach us more about these distant worlds, but Hubble still has its place. In fact, its longevity is a real asset. Astronomers recently leaned on archival Hubble data to study the formation of a large gas giant, and it stands out amongst exoplanet discoveries by challenging our ideas of planetary formation.

The proto-world is known as AB Aurigae b, and it sits some 531 light years away. Luckily for astronomers, this solar system is tilted face-on toward Earth. So, we can look “down” on the young star’s circumstellar disk, which we did in 2017 with the help of the Atacama Large Millimeter Array (ALMA). This ring of dust and gas will eventually coalesce into planets like AB Aurigae b — or maybe not like AB Aurigae b. This world is estimated to be around eight times the size of Jupiter, but it’s forming at the outer reaches of the AB Aurigae system. There’s not very much material out there, more than twice the distance from our sun to Pluto. It’s unlikely such an exoplanet would appear via the generally accepted model of planetary formation, known as core accretion. Instead, this lends credence to an alternative called “disk instability.”

The standard core accretion model is exactly what it sounds like — you start with a tiny core, which slowly picks up more material from the circumstellar disk until it becomes a planet. However, AB Aurigae b is enormous and at the edge of the disk. Current models predict such a world would take billions of years to form at the sparse edge of the disk, if it would ever form at all.

That leaves disk instability, which the team describes as an “intense and violent process.” This model says that the disk around a star can break up into clumps as it cools, and those fragments then collapse into planets in a relatively short time. This could account for the formation of a large gas giant at the edge of the solar system in just a few million years. And that’s what we see in AB Aurigae.

This work would not have been possible without Hubble’s long history of deep space observations. It was not possible to detect AB Aurigae b’s motion on a scale of a year or two — it wasn’t until the team looked at archival Hubble data that it became clear they were onto something. Combining Hubble data with observations from the state-of-the-art Subaru Telescope convinced the team they were watching the birth of a Jupiter-like exoplanet and not just a blob of diffuse gas.

This discovery will no doubt make the AB Aurigae system a target for future observations. The James Webb Space Telescope could be of particular use. Its ability to scan in the mid-infrared is ideal for peering through the sheath of dust and gas that surround young star systems like this one.

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