Mystery planet grew up quickly

Artist's concept of a binary system comprising a brown dwarf  and a 5- to 10-Jupiter-mass companion

Artist's concept of a binary system comprising a 20-Jupiter-mass brown dwarf (left) surrounded by cloud of gas and dust, and a 5- to 10-Jupiter-mass companion (right). The companion probably did not form our of the brown dwarf's cloud, but from the same larger, original cloud from which the brown dwarf itself formed.

  • Planet orbiting a brown dwarf
  • 5-10 times the mass of Jupiter
  • Probably formed in an unusual way

How big does a celestial body have to be before it is called a star? Conversely, how small does one have to be before it is considered a planet? And is there any size in between?

The answer to the in between question, is yes. They’re called brown dwarfs — objects that are typically tens of times the mass of Jupiter, but too small to sustain nuclear fusion to shine as stars do. They’re often called “failed stars”.

But even so, as telescopes grow more powerful, astronomers are finding other in-between objects that defy conventional wisdom.

The latest example is the discovery of a planet-like object circling a brown dwarf. It’s the right size for a planet — estimated to be 5-10 times the mass of Jupiter — but seems to have formed in less than 1 million years, much faster than the time it takes to build planets according to some models.

The mystery object orbits the brown dwarf at a distance of approximately 3.6 billion kilometres … which, in our Solar System, would put it halfway between Saturn and Uranus.

Diagram showing three ways a large planet might form.

Three scenarios for making a very large planet.

Kamen Todorov of Penn State University and co-investigators used the keen eyesight of the Hubble Space Telescope and the Gemini Observatory to directly image the object, which was uncovered in a survey of 32 young brown dwarfs in the Taurus star-forming region.

How to make a super planet

This mystery object is within the range of masses observed for planets orbiting other stars — that is, less than 15 times the mass of Jupiter. But should it be called a planet?

The answer depends on the mechanism by which it most likely formed.

Three scenarios have been proposed.

In the first, dust in a flattened cloud surrounding a young star slowly agglomerates to form a rocky planet 10 times larger than Earth, which then accumulates a large atmosphere.

The second idea is that a volume of gas in the cloud rapidly collapses to form an object the size of a gas giant planet.

The other idea is that, rather than forming in the cloud that surrounds the star, the body forms directly from the collapse of the same vast cloud of gas and dust from which the star (or brown dwarf) itself formed.

If the last scenario is correct, then the discovery of the strange object demonstrates that planetary-mass bodies can be made the same way that stars are made.

This is the likely solution because the mystery object is too young to have formed by the first scenario, which is very slow. The second scenario occurs more rapidly, but the cloud around the central brown dwarf probably did not contain enough material to make an object with a mass of 5-10 Jupiter masses.

Adapted from information issued by Gemini Observatory / NASA / ESA / A. Feild (STScI) / AURA artwork by Lynette Cook.

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