Orphan planet discovered

Scientists have discovered a huge planet all on its own out in space...
21 August 2018

Interview with 

Professor David Rothery, The Open University


This will surprise you - space is huge! This means that scientists are constantly finding new, strange things that confound what we know. Using radio astronomy, scientists have discovered what looks like an enormous planet all on its own in deep space. Adam Murphy found out more from The Open University's David Rothery...

Adam - Our planet, the seven others, all the planetoids, and countless other rocks are held together by our bright shining star - our Sun. But not every prospective star gets to shine and not every planet gets a sun. Scientists in California, Colorado, and Arizona have found one of these orphan planets 20 light years away.

It’s as large as 12 Jupiters, the largest planet in our solar system. It’s so large that it just falls short of becoming a star. But how could something like this come to be? To learn more about it I spoke with David Rothery, Professor of GeoPlanetary Sciences at the Open University…

David - How do you get a planet floating around on it’s own. Well, there are two ways: one way is you just form the planet in the same way that you’d form a star. A cloud of gas and dust and the interstellar medium starts to contract under its own gravity. And if it’s a big enough cloud you’ll get a star and maybe some planets around the star. If it’s a small cloud, all you can get is a brown dwarf, or even a smaller cloud and all you’ll get is a rogue isolated planet formed on its own.

Another way would be it could have begun as a planet round a normal star and been flung out from that star somehow. Orbits are very chaotic in the early stages of growth of a stellar planetary system because there’s still gas and dust affecting the object’s motions so you can get objects thrown out. So maybe this object began around a normal star and then was thrown out of that stellar system to wander space on its own.

Adam - But why is it a failed star stuck in darkness? What would be different if it had just that little bit more mass to it?

David - It’s glowing too faintly to be a proper star. In fact, it’s not even what we’d call a brown dwarf. The mass estimates come out at about 12 times the mass of Jupiter which is right on the boundary of the mass that you need for a body’s own gravity to compress its interior to sufficient pressures that deuterium, that’s the heavy isotope of hydrogen, can start undergoing nuclear fusion. Bodies from 12 to about 80 Jupiter masses are called brown dwarfs, their fusing deuterium which is much less common than proper hydrogen. And they’re never going to become normal stars like the Sun because you need to be fuzing hydrogen to undergo the processes which really we think of as going on in a star.

Adam - This orphan ball of gas has a few interesting things about it though. It has an aurora like our Northern Lights which has proven to be something of a mystery.

David - Well on Earth, the aurora are caused when charge particles streaming out from the Sun, mostly protons are channeled along Earth’s magnetic field lines and hit the Earth’s atmosphere above the Poles, and that ionises atmospheric gases high in the atmosphere and you get the glow.

Now what I’ve not been able to figure out is what they think is triggering the aurora in these objects because none of them is orbiting another star or a larger object, but something is impinging on their atmospheres and causing the auroral emissions.

Adam - So is this rogue orphan planet a quirk of a near infinite universe or are they something that could be hiding everywhere?

David - There haven’t been many detections of orphan planets wandering round space on their own, but you can estimate that given we’ve detected some there could easily be as many orphan planets blundering around the galaxy as there are stars in the galaxy. These could be very very abundant objects.





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