Astronomers Discover A Protoplanet Before Birth

An international research team has discovered a new planet so young that it has not yet emerged from the womb of matter where it is being formed. 

This is the youngest protoplanet discovered to date. Its location and surrounding matter patterns suggest that an alternative method of planet formation may be at work. 

This discovery, reported in Nature Astronomy, could help explain the histories and characteristics of extrasolar planets seen around other stars.

In the standard model of planet formation, a large gaseous Jupiter-like planet begins as a rocky core in a protoplanetary disk around a young star. This core then accumulates gas from the disk, growing into a giant planet.

Although this model works well for the planets of the Solar System, it has problems explaining the exoplanets that have been discovered around other stars at distances much greater than the orbit of Neptune, the outermost planet of the Solar System.

Theories about the formation of planets

All planets are made of material that originated in a circumstellar disk. The dominant theory for Jovian planet formation is called "core accretion", a bottom-up approach in which disk-embedded planets grow from small objects, ranging in size from dust grains to boulders. , which collide and come together while orbiting a star. This core then slowly accumulates gas from the disk. 

In contrast, the "disk instability" approach is a top-down model in which as a massive disk around a star cools, gravity causes the disk to rapidly break up into one or more planetary-mass fragments, i.e. outlying planets form near the central star and move outwards.

Image of the star AB Aurigae taken by the Subaru Telescope showing the spiral arms in the disk and the newly discovered protoplanet AB Aur b. The bright central star has been masked out and its location is indicated by the star mark (?). The size of Neptune's orbit in the Solar System is shown to provide scale./ Credit: T. Currie/Subaru Telescope

Supporting the latter theory, new observations using an extreme adaptive optics system that allows the Subaru Telescope to take direct images of faint objects near brighter stars show what appears to be a Jupiter-sized protoplanet in the process of forming at an enormous distance from its host star of 93 astronomical units: more than three times the distance between the Sun and Neptune.

At that distance, it would take a long time, if ever, for a Jupiter-sized planet to form by accretion from the core. This leads the researchers to conclude that disk instability has allowed this planet to form at such a great distance. And it contrasts sharply with expectations of planet formation under the widely accepted core accretion model.

The new world under construction is embedded in a protoplanetary disk of dust and gas with a distinctive spiral structure that revolves around a young star estimated to be around 2 million years old. That's about the age of our solar system when planet formation was underway. (The age of the solar system is currently 4.6 billion years.)

The new analysis of this protoplanet, named AB Aur b, combines data from two Hubble instruments: the Space Telescope Imaging Spectrograph and Near-Infrared Camera and Multi-Object Spectrograph. These data were compared to that from a state-of-the-art planet imaging instrument called SCExAO on Japan's Subaru 8.2-meter Telescope located on the summit of Mauna Kea, Hawaii. A large amount of data from space and ground-based telescopes was essential because it is very difficult to distinguish between young planets and complex features in the disk that is not related to planets.

Data from  AB Aur b  indicate that it is a protoplanet so young that it is still forming in a matrix of matter in the protoplanetary disk. Nearby spiral structures in the disk match models where a planet forms directly from the gravitational collapse of surrounding matter. This discovery has profound implications for explaining the many observed outlying exoplanets and the general theoretical model of planet formation. 

Reference(s): Nature Astronomy

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