Hubble Detects How Planet Forms Through 'Intense And Violent' Process, Orbiting A Young Star

The discovery is that of a Jupiter-like protoplanet. Hubble's finding supports the theory of "disk instability."

NASA's Hubble Space Telescope has detected a planet forming through what scientists describe as an "intense and violent process." | Credit: NASA

New Delhi: NASA's Hubble Space Telescope has detected a planet forming through what scientists describe as an "intense and violent process." 

The discovery is that of a Jupiter-like protoplanet. A protoplanet is a hypothetical whirling gaseous mass within a giant cloud of gas and dust that rotates around a star and is believed to be developing into a planet. 

What Is “Disk Instability”?

It has long been debated that planets like Jupiter form through a process called "disk instability", which refers to a model for giant planet formation in which a region of a circumstellar disc becomes dense and cool enough to be unstable to gravitational collapse, resulting in the formation of a gaseous protoplanet. Circumstellar discs are discs of dust, gas, asteroids and other objects that rotate around a star. Circumstellar discs around newly formed stars are known as protoplanetary discs, according to the European Space Agency. 

Hubble's finding supports the theory of "disk instability."

The findings of the study, led by an international team of researchers, was published this week in the journal Nature Astronomy. 

Which Star Does The Protoplanet Orbit?

The Jupiter-like protoplanet under construction, called AB Aurigae b, is embedded in a protoplanetary disc of dust and gas with distinct spiral structure swirling around a young star estimated to be around two million years old, NASA said on its website.

This is about the age of our solar system when planet formation was taking place. The current age of the solar system is 4.6 billion years. 

In a statement issued by NASA, Thayne Currie, the lead author of the study, said "nature is very clever", and can produce planets in a range of different ways. 

What Are Jovian Planets?

The material with which planets are made originate in a circumstellar disc. The giant planets of the outer solar system are often referred to as Jovian planets. There are four Jovian planets, namely, Jupiter, Saturn, Uranus, and Neptune. "Core accretion" is the dominant theory for Jovian planets formation. 

What Is Core Accretion?

Core accretion occurs from the collision and coagulation of solid particles into gradually larger bodies until a massive enough planetary embryo is formed to accrete a gaseous envelope. It is a bottom-up approach where planets embedded in the circumstellar disc grow from small objects, and collide and stick together as they orbit a star. The objects' sizes can range from dust grains to boulders. Then, the core slowly accumulates gas from the disc. 

What Is The Difference Between Core Accretion And Disk Instability?

The disk instability approach, in contrast, is a top-down model. The massive disc around a star cools, and gravity causes the disc to rapidly break up into one or more planet-mass fragments. 

AB Aurigae B is probably about nine times more massive than Jupiter and orbits its host star at a distance of 8.6 billion miles, according to the study. This distance is more than two times the distance between Pluto and the Sun. 

How Did The Researchers Conclude That The Planet Is Forming Through Disk Instability?

A Jupiter-sized planet would take a very long time to form by core accretion at that distance. This means that the planet is not forming through the core accretion process, but is being formed through the disk instability approach, the researchers concluded. 

How Was The Planet Photographed?

The study authors were able to directly image the newly forming exoplanet AB Aurigae B over a 13-year span using Hubble instruments.  They combined data from two Hubble instruments: the Space Telescope Imaging Spectrograph and the Near Infrared Camera and Multi-Object Spectrograph. The scientists compared these data to those from a state-of-the-art planet imaging instrument called SCExAO on Japan's 8.2-metre Subaru Telescope located at the summit of Mauna Kea, Hawaii. With the help of these data, the scientists could distinguish between infant planets and complex disc features unrelated to planets. 

What Is The Significance Of Hubble In The Project?

Currie said interpreting this system is extremely difficult, and that this is one of the reasons why the researchers needed Hubble for the project. The telescope provided a clean image to better separate the light from the disc and any planet.

Currie also explained that Hubble's longevity played an important role in helping the scientists measure the protoplanet's orbit. Originally sceptical that AB Aurigae B was a planet, Currie changed his mind after the archival data from Hubble was combined with imaging from Subaru. 

He said that Hubble provided a time baseline, combined with Subaru data, of 13 years, which was sufficient to be able to detect orbital motion.

Olivier Guyon of the University of Arizona, Tucson, said the result "leverages ground and space observations" and that one gets to go back in time with Hubble archival observations. 

Guyon added that AB Aurigae B has now been looked at in multiple wavelengths, and a consistent picture has emerged.

How Is The Discovery Important?

Alan Boss of the Carnegie Institution of Science in Washington, DC, said the new discovery is strong evidence that some gas giants can form by the disk instability mechanism. The Jovian planets form mostly through the core accretion approach. Boss explained that 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.

What’s Next?

Astronomers can understand more about the history of our solar system by studying the early days of the formation of Jupiter-like planets. According to NASA, the chemical make-up of protoplanetary discs like AB Aurigae will be studied in the future using NASA's James Webb Space Telescope.