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Sub-stellar object’s
identity crisis

Posted: 7 April

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A mystery object that looks like a planet, but formed like a star, has been discovered orbiting a brown dwarf 450 light years away from Earth, throwing the definition of a planet into even more uncertainty.

Artwork depicting the two brown dwarfs of the 2M044144 system. The disc around the larger brown dwarf is unlikely to have contained enough mass to form the companion, which must have formed directly from a collapsing gas cloud. Image: Gemini Observatory, courtesy of Lynette Cook.

Using the resolving power of both the Hubble Space Telescope and the eight-metre Gemini North telescope in Hawaii, a team of astronomers from Pennsylvania State University directly imaged an anomalous companion orbiting a brown dwarf star called 2M044144 in the constellation of Taurus. The companion has a mass in the range of 5–10 times Jupiter’s mass, but appears to be only a million years old, which is far too young to have formed from the tried and trusted method of condensing out of a protoplanetary disc like normal planets. The only alternative is that this ‘planet’ formed directly from a collapsing cloud of molecular hydrogen gas, just like a star. Therefore, its discoverers are claiming that this object should be described as a brown dwarf, despite being far below the accepted brown dwarf mass limit.

Brown dwarfs are, essentially, failed stars. They are giant gas bags larger than a typical planet, but not quite massive enough to create the temperatures within their cores necessary to instigate hydrogen fusion like in a star. They are smaller than stars, with masses less than a tenth of the Sun; 2M044144 has twenty times the mass of Jupiter (one Jupiter mass is approximately 0.001 solar masses), and the lower limit for a brown dwarf is considered to be 13 Jupiter masses, which is set by the fact that above this limit they can still generate energy from the fusion of deuterium (rather than hydrogen), at least for a few million years. This is why 2M044144’s companion is causing such a problem.

The Hubble Space Telescope was able to image the suspected brown dwarf companion around 2M044144, which can be seen at the eight o’clock position. In the image on the right, the light of 2M044144 has been subtracted to better reveal the companion.

The ages of young stars and brown dwarfs can be deduced from how bright they are; a brown dwarf ten million years old will be fainter than one that is just a million years old. The Penn State team have been able to measure 2M044144’s age as about a million years, and therefore its companion must be the same age, or even younger. The companion orbits at a distance of 3.6 billion kilometres from the brown dwarf (for comparison Uranus is 2.88 billion kilometres from the Sun, and Neptune 4.5 billion kilometres).

The other way to define a brown dwarf, other than its mass, is to look at how it formed, says Dr Kevin Luhman of Penn State University. He tells Astronomy Now that he prefers to say whether something is a brown dwarf or not based on how it formed, because estimates of a brown dwarf’s mass can be uncertain. Furthermore, deuterium fusion has no bearing on how a brown dwarf evolves, unlike hydrogen fusion within a star, so it isn’t necessarily the most important characteristic by which to define brown dwarfs.

Three different models for planetary formation. The top two describe planets condensing from a protoplanetary disc of gas and dust. The bottom model, which is more akin to star formation, describes objects collapsing directly out of a gas cloud. Image: NASA/ESA/A Feild (STScI).

The story gets even more interesting when Luhman reveals that the observations also discovered a nearby binary system consisting or a red dwarf star (a low mass star that does create energy from hydrogen fusion) and a brown dwarf Luhman believes that this system is gravitationally connected to the 2M044144 system, making a quadruple system. “The most interesting implication of this result is that it shows that the process that makes binary stars extends all the way down to planetary masses,” he says.

The results are set to be published in the 1 May edition of The Astrophysical Journal.

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