First proof of planetary system under formation

Scientists have for the first time observed a planetary system under formation around a young star 450 light years from Earth in the constellation Taurus.
The researchers found that circular gaps in a disk of dust and gas swirling around the young star HL Tau are in fact made by forming planets.
"HL Tau likely represents the first image taken of the initial locations of planets during their formation. This could be an enormous step forward in our ability to understand how planets form," said Daniel Tamayo from the Centre for Planetary Science at University of Toronto.
The image of HL Tau, taken in October 2014 by the state-of-the-art Atacama Large Millimetre/submillimeter Array (ALMA) located in Chile's Atacama Desert, sparked a flurry of scientific debate.

While those who observed the original image claimed that planets were most likely responsible for carving the gaps, some remained skeptical.
It had been suggested that the gaps, especially the outer three, could not represent forming planets because they are so close together.

It was argued that planets massive enough to carve such gaps should be scattered violently by the force of gravity and ejected from the system early on in its development.
But Tamayo's study is the first to suggest the gaps are evidence of planetary formation because the gaps are separated by amounts consistent with what is called a special 'resonant configuration.'

In other words, these planets avoid violent collisions with each other by having specific orbital periods where they miss each other, similar to how Pluto has avoided Neptune for billions of years despite the two orbits crossing one another.
"The system can be much more stable in a resonant configuration and it's a natural state for planets in the HL Tau system to migrate to," said Tamayo.
The HL Tau system is less than a million years old, about 17.9 billion kilometres in radius and resides 450 light years from Earth in the constellation Taurus.
Since young systems like HL Tau are shrouded by a thick cloud of gas and dust, they can not be observed using visible light, researchers said.

ALMA resolves that issue by using a series - or an 'array' - of telescopes located 15 kilometres apart that use much longer wavelengths.
The result is unprecedented access to high resolution images that Tamayo says will continue to revolutionise the study of planetary formation.
"We've discovered thousands of planets around other stars and a big surprise is that many of the orbits are much more elliptical than those found in our solar system," said Tamayo.
"This and future ALMA discoveries may be the key to connecting these discovered planets to their original birth locations," Tamayo added.
The finding was published in the Astrophysical Journal.