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Thayne Currie/University of Toronto

ROXs 42Bb image taken on Aug. 11, 2012. ROXs 42Bb appears to have properties of both planets and brown dwarf stars, blurring the line between them.

Is it a planet or is it a star?

by Laura L. Calderone
Jan 15, 2014


Thayne Currie/University of Toronto 

ROXs 42Bb image taken using the Keck Telescope in Hawaii in 2013 and released this month. ROXs 42Bb orbits its host star at 150 astronomical units, or from 13.9 billion miles away.  

Look out planets and stars – there’s a new kid in town.

A new celestial body that straddles the line between what differentiates planets from stars has been discovered by astrophysicists at the University of Toronto.

“It’s the kind of object that really blurs the lines between, or at least blurs very clean lines between what a planet is and what a star is,” said astrophysicist Thayne Currie, lead author of the research released this month and post-doctoral fellow at the University of Toronto.

The object, named ROXs 42Bb, is about nine times the mass of Jupiter, still more like a planet’s size but smaller than a brown dwarf star. Brown dwarfs are “failed” stars that are not massive enough to shine like regular stars.

Despite its planet-like mass, ROXs 42Bb is orbiting much farther from its host star, ROXs 42B, than usual for gas giant planets, such as Jupiter or Saturn. It is located 150 times farther from its host star than the Earth's 93-million mile distance from the sun.

Scientists have assumed there to be a clean break in differentiating between bona fide planets and brown dwarfs. The new object seems to stand in the middle of these two populations, Currie said.

Scientists discovered this body using data taken from NASA direct imaging surveys, a way of detecting planets that orbit stars other than the sun, according to the research published January 8 in the “Astrophysical Journal Letters.” The object is located nearly 500 light years away from our sun.

One theory is that this is a planet formed by a process called disk instability, also known as disk collapse.

“What happens is, if the whole disk of mass becomes massive [enough] the whole disk can collapse because of gravity. Gravity pulls the gas together” in such cases, said Yoram Lithwick, associate professor of the physics and astronomy department at Northwestern University.

Disk instability is a process that has not been observed before because it cannot occur close to the host star, like Jupiter’s distance from the sun.

Jupiter is thought to have evolved from a different method, known as core accretion, which is when gas forms around a solid core, Lithwick said.

“The theory behind this alternate model for planet formation is actually very, very old. It’s one of the very first theories for planet formation,” Currie said. “So this is an old theory, but this object could be the first that could possibly be formed by this very old theory that may be able to explain Jupiter and Saturn and other planets around our nearby star.”

It could be that Jupiter formed farther out in the solar system due to disk instability, like ROXs 42Bb, and was eventually pulled closer to the sun, Lithwick said.

Another possibility is that this could be a small brown dwarf that has some strange qualities - a star-like object orbiting another star.

“Two things would make [ROXs 42Bb] weird as a brown dwarf,” Lithwick said. “One is that it’s slightly less massive than most brown dwarfs. The second thing is maybe more weird than the first," he said. "It’s orbiting a star, which is unusual.”

Currie’s research began in May 2012 and he's not done yet. He said there are plans to study the object’s atmospheric properties and physical properties, such as gravity, temperature and radius.

More information is needed through analysis of other planets and stars before scientists can fully determine if ROXs 42Bb represents a new population of objects.

Currie looks forward to finding more information about the body and other planet masses that could be out there.

“I think it will be very interesting to find out what the answer is,” Currie said.