Mysterious Planet-like Object Challenges Simple Definition

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April 6, 2010 by Ray Villard and Barbara Kennedy


This is an artist's conception of the binary system described in this story showing the primary brown dwarf (at left) and its orbiting planet-like object (at right). The disk of the brown dwarf likely never had enough material to make an orbiting object of this mass. As a result, this small companion probably formed like a binary star. In this illustration, both objects are presented at the same distance to show relative sizes. Not shown are two other nearby objects, a low-mass star and a brown dwarf that are probably both parts of this system. Science Credit: NASA, ESA, and K. Todorov and K. Luhman (Penn State University) Artwork Credit: Gemini Observatory, courtesy of L. Cook

(PhysOrg.com) -- A mysterious planet-like object orbiting a not-quite-starlike "brown dwarf" is the most recent enigma discovered by astronomers with their ever-more powerful telescopes. Kamen Todorov, a graduate student at Penn State University, and a team of co-investigators including Kevin Luhman, assistant professor of astronomy and astrophysics at Penn State, used the keen eyesight of the Hubble Space Telescope and the Gemini observatory to directly image the planet-like object. The team's discovery, which resulted from a survey of 32 brown dwarfs in the Taurus star-forming region, will be published in the Astrophysical Journal.

The astronomers estimate that the smaller orbiting object is five to 10 times the mass of Jupiter and that it orbits at roughly the distance from the Sun to Saturn or Uranus -- which makes it planet-like -- but it formed only 1 million years ago -- which is much faster than the time some theories predict is needed to build a planet. Their investigations of the nature of this mysterious object and its companion brown dwarf have revealed a new mechanism that Nature can use to make orbiting planetary-mass objects. "Our research demonstrates that nature can make planetary-mass bodies through the same mechanism that builds stars -- and that the mystery object has both planet-like and star-like characteristics," said Luhman, who is a researcher at the Penn State Center for Exoplanets and Habitable Worlds.

But what is the true nature of this object? Is it really a planet? To answer this question, the scientists considered the three possible ways such an object could form: 1) dust in a circumstellar disk slowly merges to form a rocky planet 10 times larger than the Earth, which then accumulates a large gaseous envelope; 2) a lump of gas in a circumstellar disk quickly collapses to form an object the size of a gas-giant planet; or 3) rather than forming from the dust in a circumstellar disk, a companion forms from the collapse of the vast cloud of gas and dust in the same way as a star or a brown dwarf forms.



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