Proof that planets are more like stars than we thought

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Jun 4, 2017

Some 550 light-years away in the constellation Chameleon, OTS44 was just another lonely planet, void of a parent star and leading an isolated existence.

Then things started to get weird.

When astronomers observed it with the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, they saw something that should belong to a star—not a planet. OTS44 is haloed by the type of dusty, gaseous protoplanetary disk usually observed around nascent stars. Even stranger was evidence that this planet, which is 10 times bigger than Jupiter (still merely 1 percent the size of the sun), was fueling its growth by feeding off its accretion disc, which is also astral behavior.

OTS44 isn’t exactly cosplaying as a star. Both stars and planets are believed to emerge from clouds of dust and gas that fall apart within nebulae, and stars are born after gravity pulls in this collapsing material and compresses it enough to heat it up into the core of what will become a new blazing sphere in the night sky. Even as it falls apart, the collapsed cloud that first brought the star into being continues to swirl violently, flattening gradually as it compresses the star even further and setting off its rotation. Even scientists have compared this to making a pizza.

How one artist envisions planetary formation.

Planets like Earth develop out of these gargantuan gas clouds after the star sucks up all the gas in the disk and the remaining rocky refuse accumulates into micro-planets that eventually join forces into a something resembling a planet. Gas planets like Jupiter and Neptune are the product of any leftover gases and ice the star managed not to vaporize. OTS44 manages to defy even that theory. The exoplanet’s disk is a staggering ten times the mass of Earth, and as it continues to accrete material, it shows no signs of losing its appetite.

The more we know about OTS44, the greater its similarities with a young star,” said team lead Amelia Bayo. “But its mass is so low that theory tells us it cannot have formed like a star!”

What confounds Bayo and her research team, who recently published their findings in Astrophysical Journal Letters, is how such a low-mass object as OTS44 could contradict theory and give rise to the right conditions for infinitesimal specks of dust to accrete into grains as large as 1mm in diameter (pretty substantial in terms of planet formation). This alone makes the scientists suspicious that stars and planets may share some proverbial DNA. If the dust accretion process continues, this rebel exoplanet could end up with an unexpected moon.

"It is amazing how an observatory like ALMA allows us to see half an Earth mass worth of dust orbiting an object with ten times the mass of Jupiter at a distance of 500 light-years," said Thomas Henning of the Max Planck Institute for Astronomy. "But the new data also shows the limit of our understanding."