There is no longer any doubt that the universe is full of planets – NASA recently announced that the total number of known exoplanets has exceeded 5,000 and we have just begun our search in the skies. Upcoming projects using the glowing 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 advantage. Astronomers have recently relied on Hubble archival data to study the formation of a large gas giant and it stands out among the discoveries of exoplanets challenging our ideas of planet formation. The first world is known as AB Aurigae b and is about 531 light-years away. Fortunately for astronomers, this solar system is tilted toward the Earth. So we can look down at the young star’s peristaltic disk, something we did in 2017 with the help of the Atacama Large Millennium Array (ALMA). This dust and gas ring will eventually merge into planets like AB Aurigae b – or maybe not such as AB Aurigae b. This world is estimated to be about eight times the size of Jupiter, but forms at the outer limits of the AB Aurigae system. There is not much material out there, more than twice the distance from our sun to Pluto. It is unlikely that such an exoplanet would emerge through the generally accepted model of planetary formation, known as the augmentation nucleus. Instead, it gives credibility to an alternative called “disk instability”. The typical model of augmentation of the nucleus is exactly what it sounds like – you start with a tiny nucleus, which slowly collects more material from the periastric disc until it becomes a planet. However, the AB Aurigae b is huge and on the edge of the disc. Current models predict that such a world would take billions of years to form at the sparse end of the disk if it ever formed. This leaves the disk unstable, which the team describes as an “intense and violent process”. This model says that the disk around a star can break into clusters as it cools and these fragments then collapse into planets in a relatively short time. This could be responsible for the formation of a large gas giant at the edge of the solar system in just a few million years. And this is what we see in AB Aurigae. This work would not have been possible without Hubble’s long history of deep observations. It was not possible to detect the movement of AB Aurigae b on a scale of one or two years – only when the team examined the Hubble archives did it become clear that this was something. The combination of Hubble data with observations from the state-of-the-art Subaru telescope convinced the team that it was watching the birth of an exoplanet like Jupiter, and not just a mass of diffuse gas. This discovery will undoubtedly make the AB Aurigae system a target for future observations. The James Webb Space Telescope could be especially useful. Its ability to scan in mid-infrared is ideal for looking through the dust and gas casing surrounding young star systems like this. Read now:
