Our solar system formed about 5 billion years ago from a cloud of interstellar dust and gas, which condensed first into a young proto-sun and surrounding (circumstellar) material, and later into the sun and planets we know today.
Extra-solar planetary systems, which we now know to be common, probably formed in the same way.
Although a theoretical picture of planet formation has been developed to explain these mature systems, astrophysicists have recently begun to observe the formation in action around young stars.
Building the giant planets in our solar system involved the formation of large rocky cores followed by the accretion of massive gaseous envelopes.
This process can be divided into three main stages:
1. Slow collisional growth of dust grains into rocks and eventually solid cores.
2. Slow accretion of a small amount of gas onto that core until the gaseous envelope and solid core have approximately equal masses.
3. Rapid, runaway accretion of a large amount of gas.
Although most of the giant planet’s mass is assembled during the rapid accretion phase, it cannot begin until the first two phases are complete.
The entire process is thought to have taken a few million years in our proto-solar system. To explain the existence of massive planets in other systems, the theory has been modified to try to speed up the process.
For example, if planets drift through the proto-solar system as they form, they have access to fresh food supplies and can hence grow faster.
Higher masses of circumstellar matter, that is, a larger food supply to begin with, can also accelerate the process. However, even under these more-optimistic scenarios, giant planets typically require more than 1 million years to grow to their final masses.
Recent research from University of Arizona astrophysicists has called this timescale, and hence the theoretical picture of giant planet formation, into question.
We recently discovered structure in the circumstellar matter around a young star, which suggests that giant planets may be able to form much more rapidly than previously thought.
We studied a star called WL 17, in the Ophiuchus constellation, about 400 light years away from our solar system. The star is young, probably 500,000 years old or less compared with our more mature solar system.
We used the Atacama Large Millimeter Array, the largest radio telescope in the world, located in the Atacama Desert in northern Chile. We were able to image the region around the star where we expected giant planets might be able to form one day.
What we discovered was that the circumstellar material has a large inner clearing around the central protostar. This clearing is probably caused by one or more giant planets, which can gravitationally sculpt the gas and dust in the vicinity.
Given the age of this system, the presence of one or more giant planets suggests rapid formation and argues for a revision of the basic theory of planet formation.
If giant planets can form on such timescales, this finding also indicates that Jupiter-like planets may be more common in young systems than has been previously realized.
Although the discovery of the structure in the WL 17 disk suggests new modes of giant planet formation, it remains unclear whether this is common around young stars.
Fortunately, we have an ongoing research program that uses the Atacama Large Millimeter Array to search for similar structure around other stars.
And we have already found another example. So, stay tuned.
